biogeochemistry in a shortgrass landscape ... - Wiley Online Library

Ecology, 81(10), 2000, pp. 2686–2703 q 2000 by the Ecological Society of America

BIOGEOCHEMISTRY IN A SHORTGRASS LANDSCAPE: CONTROL BY TOPOGRAPHY, SOIL TEXTURE, AND MICROCLIMATE PAUL B. HOOK1,3 1

AND

INGRID C. BURKE2

Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana 59717 USA 2Department of Forest Sciences and Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523 USA

Abstract. Biogeochemistry of terrestrial ecosystems is controlled by interactions among factors operating at several spatial and temporal scales. The purpose of this study was to evaluate the relative importance and interaction of relatively static landscape factors and more dynamic factors in a shortgrass steppe landscape. The landscape factors examined were topographic position, and soil texture. The dynamic factors studied were seasonal climate and the localized effects of individual plants on soils. Patterns were evaluated by sampling soil between and under individual Bouteloua gracilis plants in paired upland (erosional) and lowland (depositional) plots at eight locations at the Central Plains Experimental Range (CPER), Colorado. We quantified five organic C and N pools (total, fine and coarse particulate organic matter [POM], mineral-associated organic matter [MAOM], and potentially mineralizable C and N), and we estimated seasonal patterns of in situ N dynamics with three methods (extractable inorganic N, net N mineralization in uncovered cores, and N adsorbed on ion exchange resin [IER] bags). Topographic position and soil texture each explained much of the landscape-scale variation of C and N pools and vegetation structure. Most lowland plots were enriched in silt, clay, C, and N relative to adjacent upland plots, and topographic position affected most pools significantly. Most vegetation and biogeochemical variables were strongly correlated with soil sand content. Across the range of sand content encountered (40–83%), the fraction of area in bare soil openings .5 cm across increased sevenfold, and most C and N fractions increased by 2–4.5 times. Plant-induced, microscale heterogeneity of soil C and N was comparable in magnitude to landscape-scale heterogeneity for pools with more rapid turnover (POM and mineralizable C and N), but presence or absence of plants did not affect more stable, mineral-associated organic matter. Plant-induced heterogeneity was significant in all locations, but its importance likely decreases with decreasing sand content as cover becomes more continuous, particularly in lowlands. Total extractable inorganic N and nitrate, N adsorbed on resin bags, and the proportion of mineralized N that was nitrified during incubations increased with increasing soil water content or precipitation, but net N mineralization did not vary systematically with precipitation. Inorganic N availability was greatest during relatively moist spring periods, and these were the only times when indices of in situ N availability followed the spatial patterns expected from laboratory assays of C and N pool distribution. The relatively weak spatial patterns for N dynamics contrast with the substantial landscape and individual-plant scale variation in C and N pools. Landscape patterns of N mineralization and availability may be tied to N pools, such as POM N, that are not strongly related to topography or texture. Particulate organic matter appears to be especially important to N retention and availability on sandy soils, which typify most sites at the CPER; the proportion of total N residing in POM was high and POM C:N ratios were low in sandy upland soils. We suggest that soil texture is a key proximal control over biogeochemical processes and is largely responsible for observed landscape-scale patterns, including topographic differences. Models that integrate effects of texture and topography have the potential to link landscape biogeochemical patterns to short-term processes that directly influence SOM dynamics and to long-term geomorphic processes that influence soil distribution, including eolian redistribution of soil materials, which is important in many dry regions. Key words: biogeochemistry; catena model; landscape ecology; microclimate; nitrogen mineralization; semiarid ecosystems; shortgrass steppe; soil carbon; soil nitrogen; soil organic matter; soil texture; topography.

Manuscript received 24 March 1999; revised and accepted 26 July 1999; final version received 23 August 1999. E-mail: [email protected]

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SHORTGRASS STEPPE LANDSCAPE BIOGEOCHEMISTRY

INTRODUCTION Carbon and nitrogen cycling in semiarid ecosystems is controlled by interactions among physical and biological processes that operate across a wide range of spatial and temporal scales. Topography, soil texture, and plant cover influence spatial patterns of soil organic matter (SOM) accumulation in landscapes and affect variation in SOM quality (Jenny 1980, Schimel et al. 1985, Burke 1989, Burke et al. 1989, Hook et al. 1991). Daily to seasonal changes in soil water and temperature are important controls of C and N mineralization in semiarid soils (Schimel and Parton 1986). Microclimate interacts with distribution and quality of SOM to determine C and N turnover and availability (Burke 1989). Although each of these controls has been shown to be important in specific cases, few studies have evaluated their relative importance and interaction in semiarid landscapes. The purpose of this paper is to explore the roles of topography, soil texture, plant cover, and microclimate in controlling biogeochemistry in a shortgrass steppe landscape. Because the effects of soils and landforms on ecological processes are of broad interest (Swanson et al. 1988), we focus primarily on landscape-scale factors and how they influence the transient effects of microclimate and plant distribution. Our objectives were (1) to evaluate variation of soil C and N pools between uplands and lowlands and in relation to soil texture, (2) to compare individual-plant scale variation of soil C and N pools to landscape-scale variation, and (3) to characterize seasonal patterns of N mineralization and availability and the effects of topography, soil texture, and microsite on seasonal N dynamics. In contrast to most previous biogeochemical research in semiarid grasslands, this study used extensive spatial replication representing diverse locations within a landscape. Hypotheses related to each objective are developed below.

Landscape-scale controls Explanations of landscape-scale biogeochemical patterns must account for the long-term evolution of landforms and soil parent materials as well as short-term controls of biological processes. The catena model, which has been used to interpret semiarid soil landscapes for several decades (Aandahl 1948, Ruhe and Walker 1968, Muhs 1982, Aguilar and Heil 1988), has become an important framework for analyzing spatial patterns of biogeochemistry and ecology partly because it integrates long- and short-term processes (Woodmansee and Adamsen 1983, Schimel et al. 1985, Schimel 1986, Lajtha and Schlesinger 1988, Milchunas et al. 1989, Burke et al. 1995a). The catena model has many variations, but generally assumes that within geologically and climatically similar areas, hydrologic and geomorphic processes generate more or less consistent patterns of soil development, biogeochemistry, and

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ecology along hillslopes. Biogeochemical differences between erosional or stable uplands and depositional lowland sites are thought to reflect long-term redistribution of soil materials, modification of soil water availability by soil texture and runoff, and preferential grazing on lowlands; all of these may influence vegetation structure, SOM accumulation, and nutrient cycling. Colorado shortgrass steppe has been presented as an ecosystem in which catenas are strongly developed (Schimel et al. 1985, Swanson et al. 1988), but available evidence is contradictory. Soil landscapes reflect bedrock geology and Quaternary fluvial processes as well as topography, and movement of wind-blown sand during the Holocene may have limited or masked topographic patterns (Davidson 1988, Yonker et al. 1988, Madole 1994). Redistribution of soil particles by runoff is one of the main processes of catena development, typically resulting in downslope increases in silt, clay, organic matter, and nutrients (Ruhe and Walker 1968, Schimel et al. 1985, Aguilar and Heil 1988, Burke et al. 1995a). Runoff is infrequent in shortgrass steppe, however (Parton et al. 1981), and a recent analysis of long-term soil water data (Singh et al. 1998) suggests that there is not a consistent pattern in soil water availability along hillslopes. Contemporary and Holocene droughts have promoted eolian redistribution of surficial materials (Muhs 1985, Yonker et al. 1988, Madole 1994), potentially counteracting hydrologic sorting of soil materials along hillslopes and altering landscape patterns of soil texture. Studies of paleosols show truncated and buried soils resulting from periods of stability and erosion (Blecker et al. 1997). Although intensive studies involving one or a few, subjectively selected hillslopes have generally supported the catena model (e.g., Schimel et al. 1985; cf. global review by Gerrard 1992), extensive sampling in a shortgrass steppe landscape did not show consistent catenary patterns for soil C (Yonker et al. 1988). In this study, we tested the hypothesis that soil C and N pools and N mineralization rates are consistently greater in depositional lowlands than erosional uplands in shortgrass steppe. We applied the catena concept in this simple, highly generalized form and sampled replicate hillslopes across our study area to evaluate topographic patterns over a range of parent materials and landforms. We also evaluated the relationship between soil texture and landscape-scale variation of C and N because soil texture is an important control of SOM content in the central grasslands region of the United States (Parton et al. 1987, Burke et al. 1989). Soil texture directly influences organic matter accumulation (Tisdall and Oades 1982, Hassink 1996) and the dynamics of soil water, the resource that most frequently limits biological processes in shortgrass steppe (Lauenroth et al. 1978, Lauenroth and Milchunas 1992). Therefore, soil texture may influence distribution of soil C and N across the landscape, either in association

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with topography or independently. Spatial patterns of texture are controlled by bedrock and surficial geology, recent alluvial and eolian erosion and deposition, and the hillslope processes that generate catenas.

Plant-induced heterogeneity Carbon and nitrogen distribution is often patchy in dry regions because plant cover is discontinuous and belowground organic matter inputs are heterogeneous (Charley and West 1975, 1977, Burke 1989, Jackson and Caldwell 1993, Schlesinger et al. 1996). In shortgrass steppe, soil under individual bunchgrass plants has much higher C and N concentrations than adjacent openings (Hook et al. 1991, Burke et al. 1995b, Vinton and Burke 1995, Kelly et al. 1996, Burke et al. 1998). Hook et al. (1991) suggested that plant-induced heterogeneity may be as great as landscape-scale variability, especially for biologically active C and N pools that turn over rapidly. Such fine-scale heterogeneity may increase from lowlands to uplands in shortgrass steppe because production is lower and cover is more patchy in uplands (Milchunas and Lauenroth 1989, Milchunas et al. 1989). We hypothesized that soil C and N concentrations are greater under plants than in openings between plants regardless of landscape position. However, we predicted that the relative importance of plant-induced heterogeneity would be greater for biologically active pools than for passive pools, with distribution of passive pools being controlled mainly by landscape-scale factors.

Seasonal nitrogen dynamics The seasonal dynamics of soil N in semiarid grasslands are not well studied. In research on a shortgrass steppe catena, extractable inorganic N varied seasonally and N mineralization increased downslope, but N mineralization was not clearly related to precipitation (Schimel and Parton 1986). It is not known if topographic influences on N availability in semiarid grasslands are consistent among locations or through time. Furthermore, little is known about the influence of plant-induced soil heterogeneity on in situ N dynamics in semiarid grasslands because most relevant research has used only laboratory indices of potential N mineralization. Research on N dynamics in sagebrush steppe found that spatial variation at both large and small scales was seen mainly during seasons when microclimate favors biological activity (Burke 1989). Following Burke (1989), we hypothesized that net N mineralization and N availability are greatest during warm, wet periods, and that spatial differences in N mineralization and availability are greater during such periods than during cold or dry periods. METHODS

Study area The Central Plains Experimental Range (CPER) in Colorado, USA (408499 N, 1048469 W) is typical of the

Ecology, Vol. 81, No. 10

northern portion of the shortgrass steppe region (Lauenroth and Milchunas 1992, Burke and Lauenroth 1993). Mean annual precipitation at the CPER is 321 mm, with 83% of precipitation falling during the April through September growing season, and mean annual temperature is 8.68C. Plant cover and production are limited most frequently by water deficits, but N may be limiting during relatively wet periods (Lauenroth et al. 1978). Vegetation is strongly dominated by the perennial bunchgrass Bouteloua gracilis (blue grama), which makes up ;90% of basal cover (Milchunas et al. 1989). Most pastures are grazed by cattle with ;40% removal of forage annually. Cattle graze lowlands preferentially (Senft et al. 1985, Milchunas et al. 1989). The landscape consists mainly of gently rolling uplands and terraces shaped by Quaternary fluvial activity. Modern alluvial surfaces that border ephemeral stream courses make up a small part of the area. Runoff rarely generates stream flow, and the drainage network is weakly developed. Gently sloping, concave swales occur within the rolling uplands and on slopes of terraces; swales receive runoff but do not have stream channels. Closed drainages make up a small proportion of the area but were not sampled. Elevations in the study area are 1600–1690 m. Surficial materials include poorly consolidated shale and sandstone, coarse Pleistocene alluvial deposits, and fine Holocene and modern alluvium, all of which may be covered by an extensive but discontinuous mantle of Holocene eolian sand deposits (Davidson 1988). Typical soils are Aridic Argiustolls, Ustollic Haplargids, and Ustic Torriorthents (Yonker et al. 1988).

Sampling design We evaluated variation in N dynamics and surface soil characteristics at landscape and individual plant scales. Paired upland and lowland plots were selected at eight replicate locations in a 4600-ha study area. Locations were separated by 0.3 to 8 km. Each location was in a different pasture, and all pastures were moderately grazed. At each location, paired 400-m2 plots were separated by 50 to 200 m. Upland plots were 1.4 to 11.1 m higher than lowland plots, and on the same hillslope. Average slopes between paired plots were 2– 9%. Plots represented common geomorphic and pedologic settings. Lowlands were gently concave and included minor swales, toeslopes, and alluvial surfaces of ephemeral streams. Uplands included gently convex ridges and hillslopes. Slopes were 1–8% (mean 5 3%) in lowland plots and 2–6% (mean 5 4%) in upland plots. Lowland plots were in settings judged to be depositional using topographic criteria discussed by Gerrard (1992), whereas upland plots were in settings judged to be erosional. The watershed area with potential to contribute water or sediment to each lowland plot ranged from 1.3 to 6600 ha; contributing areas

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were ,60 ha for six locations. Parent materials included shale, sandstone, alluvium, and eolian material. Aerial photographs from the 1930s through the 1980s were used to verify that plots were in native range that had never been cultivated. Within each plot, soil characteristics and N dynamics in the 0–15 cm layer were analyzed for two types of microsites: (1) soil directly under B. gracilis plants (‘‘plant’’), and (2) soil in openings .5 cm across, between plants (‘‘opening’’). Previous research indicated that openings .5 cm across are about one-third of the area on sandy loam sites (Hook et al. 1994), and that C and N pools differ significantly between soil under plants and in openings (Hook et al. 1991, Vinton and Burke 1995). Plots were sampled once (15–20 July 1993) to quantify soil physical properties and C and N pools. Exceptions were soil bulk density, which was estimated from all soil cores collected in seasonal sampling, and root biomass, which was sampled during two summers. Vegetation and surface cover patterns were sampled and plot slopes and elevations were surveyed in May 1994. Seasonal patterns of N mineralization and N availability were quantified using in situ incubations, ion exchange resins, and soil extractions between 26 May 1993, and 24 May 1994. Incubations were done monthly during the growing season. One overwinter incubation spanned from 23 September 1993 through 17 April 1994. All soil samples and field incubations were repeated in triplicate for each plot, microsite, and date. Soil samples were 5 cm diameter by 15 cm deep cores, and all analyses were for soil passing a 2-mm sieve. Particles .2 mm were usually ,2% of sample mass.

Site characteristics Sand content was estimated by sieving, clay content by the hydrometer method, and silt by difference. Field capacity was estimated by shallowly wetting sieved soil in a column, allowing water to redistribute over 24 h while protected from evaporation, and determining the water content of soil above the wetting front. Precipitation and air temperature were recorded at a standard weather station located centrally in the study area. Plot slopes and elevations were surveyed with an electronic total station. The area and size of aboveground openings between plants were estimated in each plot using a line-intercept technique. Openings were measured on five parallel 20m transects by recording the length of each transect segment that crossed an opening. Openings were identified as any area without live plants that could contain a 5-cm diameter disk. Basal cover was estimated with a 10-point frame placed at 30 random locations on the transects. Contacts were identified as B. gracilis and Buchloe dactyloides, other grasses and sedges, bare soil and litter, cacti, forbs, or subshrubs.

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Carbon and nitrogen pools Five C and N pools were quantified: total (all soil ,2 mm), coarse and fine particulate organic matter (0.5–2 mm and 0.05–0.5 mm, respectively), mineralassociated organic matter (the silt- and clay-sized fraction, ,0.05 mm), and potentially mineralizable C and N. Particulate organic matter (POM), which consists of partially decomposed roots and other organic matter, is thought to be important to C and N cycling (Cambardella and Elliott 1992) and may correspond to SOM that has an intermediate turnover time (Kelly et al. 1996). Mineral-associated organic matter is more decomposed and amorphous and may correspond to passive SOM with a long turnover time (Parton et al. 1987, Cambardella and Elliott 1992, Delgado et al. 1996). The two POM fractions were separated by wet-sieving 50 g of soil dispersed in sodium hexametaphosphate solution following methods modified from Cambardella and Elliott (1992). Total and POM C and N were determined with a Carlo-Erba NA1500 C/N Analyzer (Soil Biology Laboratory, University of Georgia). Percentages of C and N in POM were converted to percentages in bulk soil based on the proportion of soil mass in each fraction, which included sand particles as well as POM. Because results for fine and coarse POM followed similar patterns, results are presented only for total POM except where noted. Mineral-associated C and N (MAOC and MAON) were estimated by subtracting POM C and N from total C and N. Potential C and N mineralization were estimated with 30-d aerobic incubations of 25 g of soil at field capacity and 258C, as described by Hook et al. (1991). Respired CO2 was trapped in NaOH and measured titrimetrically. Net N mineralized was calculated as the difference between initial and final NO31NO2 and NH4, which were measured colorimetrically in 2 mol/L KCl extracts. Data were converted to an area basis (mass per square meter in the top 15 cm of soil) using bulk density estimated for each plot and microsite. Plot-scale estimates were calculated as the average of both microsites weighted by their relative area. Mass of roots retained on the 2-mm sieve was measured after drying at 558C for 48 h and corrected for ash content, estimated by incinerating subsamples at 5508C.

In situ N dynamics In the field, net N mineralization was estimated using an uncovered core incubation technique (Hook and Burke 1995). Aluminum tubes (5 cm inside diameter, 17.5 cm long) were driven 15 cm into the soil and removed temporarily. Soil was pushed 2 cm upwards to allow insertion of a 5 cm diameter by 2 cm thick nylon mesh bag containing mixed cation and anion exchange resins to adsorb ions from any solution leaching from the 15-cm cores. Cores were replaced without further disturbance, with tops uncovered. Grass shoots were clipped from roots and removed before driving


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TABLE 1. Characteristics of vegetation and soils of upland and lowland sites at the Central Plains Experimental Range, Colorado, USA. Uplands Characteristic Openings . 5 cm Area (%) Mean size (cm) Basal cover (%) B. gracilis 1 B. dactyloides Other grasses 1 sedges Cacti Forbs Subshrubs Total plant cover Soil properties Most frequent texture Sand (%) Silt (%) Clay (%) Bulk density (g/cm3) Field capacity (%)

Lowlands

Mean

SE

Mean

SE

Ptopo

Ptopo 3 loc†

22 11.9

2 0.7

11 8.8

2 0.6

** **

*** *

37 3.5 3.8 0.6 0.8 46

2 0.7 0.9 0.2 0.3 2

49 8.4 1.1 0.3 0.3 59

3 1.4 0.6 0.2 0.2 3

** ** *

nt nt nt nt nt nt

Sandy loam 68 3 14 1 18 2 1.30 0.02 21 1

Sandy clay loam 52 3 21 1 27 2 1.19 0.02 26 1

NS NS

** * * * ** *

*** *** *** NS

nt

Note: For all means, n 5 8 replicate locations. * P , 0.05; ** P , 0.01; *** P , 0.001, NS 5 not significant (P values are for significance of main effects of topographic position and interactions of topographic position and replicate location). † nt 5 not tested because absence of within-plot subsamples prevented estimation of appropriate error term.

tubes under plants. Soil was left in the field for 28–38 d during the growing season, and for 186 d during the cold, dry months. When incubations were initiated, soil adjacent to each incubation core was collected to estimate initial NH4 and NO31NO2 concentrations; these values were also used to characterize ambient N availability. Bagged samples were placed on ice in coolers for transport to the laboratory the same day. Samples were stored at 48 C and extracted with 2 mol/L KCl within 24–48 h. Soil samples were mixed thoroughly, and a portion was sieved through a 2-mm screen before extracting inorganic N. NH4 and NO31NO2 were extracted from 10-g soil subsamples and from whole resin bags in 50 mL of 2 mol/L KCl and measured colorimetrically. Net N mineralization was calculated as the change in NH4 plus NO31NO2 in soil and on exchange resins. Net nitrification was calculated as the change in NO31NO2 content. Ion-exchange resins (IERs) were used to estimate N availability in the presence of functioning plant roots (Binkley and Matson 1983). Mixed cation/anion exchange resin bags identical to those placed under incubation cores were buried adjacent to incubation cores. IER bags were inserted 5 cm below the surface using a trowel to open a small slot with minimal soil disturbance. IER bags were installed and collected at the same time as incubations, and they were extracted and analyzed using the same procedures. IER N was converted to an area basis assuming that each bag intercepted solution in a 5 cm diameter area.

Statistical analysis Data were analyzed by ANOVA using a significance level of 0.05. Analyses of spatial patterns focused on two main factors, topographic position (upland, low-

land) and microsite (plant, opening), and their interaction. Replicate locations were treated as blocks. Treatment-block interaction mean squares were used as error terms in tests of topographic and microsite effects. The consistency of topographic and microsite differences across locations was evaluated by testing significance of block-by-microsite and block-by-topographic-position interactions; the residual mean square, which reflects variation among triplicate subsamples within plots, was used as the error term. In situ N data for each date were analyzed separately rather than in a repeated-measures analysis of variance because residuals of pooled data were highly nonnormal, variances were heterogeneous across dates, these distributions were not corrected by transformations, and strong interactions between date and spatial factors prevented direct interpretations of main effects. Separate analyses for each date yield relatively conservative tests of topographic and microsite effects due to reduced degrees of freedom. Relationships between physical variables (soil texture, soil water content, and precipitation) and C and N variables were evaluated using regression analysis; plot-scale estimates were used in these analyses. RESULTS

Site characteristics Soil properties and plant cover patterns differed significantly between uplands and lowlands (Table 1). Compared to lowlands, the average upland had sandier soil with lower field capacity, more and larger openings, lower total plant cover, lower cover of grasses and sedges, and more cacti. Variability within each topographic position was substantial, however, and differences between paired upland and lowland plots

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ranged from large to absent. Variation in topographic patterns was shown by significant interactions between topographic position and replicate location for most variables for which data allowed tests of interaction. Mean clay content was 4% lower under plants than in openings in uplands (P , 0.0001), but did not differ significantly between microsites in lowlands (significant microsite-by-topographic position interaction, P , 0.005). Bulk density was 5% lower under plants than in openings (P 5 0.035). Otherwise, microsite did not affect soil physical properties. Differences in sand content, or equivalently, siltplus-clay content, explained much of the variation in gross vegetation structure across topographic positions (Fig. 1). As sand content increased from 40 to 83%, plant basal cover decreased by almost one-half, the area of openings increased sevenfold, and average size of openings doubled; field capacity, a correlate of potential soil water availability, doubled. Typical upland and lowland plots differed in texture (59–83% and 40–55% sand, respectively), with corresponding differences in vegetation. Because texture and topographic position were largely confounded, we performed separate regressions of texture against vegetation response variables for uplands and lowlands. In uplands, variation of opening area and size was significantly related to sand content, but total plant cover was not. Sand content did not explain variation of vegetation among lowland plots, which differed little in texture except for one atypical plot with 70% sand. However, vegetation in that atypical lowland plot closely resembled upland plots with similar textures. Regressions of field capacity against sand content were significant for both topographic positions and did not differ significantly with topographic position.

Landscape-scale patterns About two-thirds to three-fourths of total C and N were in mineral-associated organic matter, with the remainder in POM. Mineralizable C and N were 2–3% and 0.3–0.5% of total C and N, respectively. Most C and N pools were influenced by both topographic position and microsite (Fig. 2). All pools except for N in coarse POM were significantly greater in lowlands than uplands. Topographic differences were large (28–64%) except for POM N, and differences were not related to relative turnover rates. Average proportions of total C and N in each pool generally differed little between topographic positions. Topography and location interacted significantly for all C and N pools. Soil texture explained much of the variation in C and N pools, with most pool sizes decreasing dramatically as sand content increased (Fig. 3). POM C and total, mineral-associated, and mineralizable C and N varied by factors of 2–4.5 over the range of sand contents encountered. POM N varied by 45% and was only weakly related to sand content. Because total N and MAON decreased much more than POM N as sand

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FIG. 1. Relationships between sand content, gross vegetation structure, and field capacity. Each symbol represents one lowland (v) or upland (□) plot (n 516). Regressions were significant at P , 0.0001. Mean size of openings (cm; not shown) was also significantly related to sand content (size of opening 5 20.016 1 0.17[% sand], R2 5 0.71, P , 0.0001). Linear regressions performed separately for each topographic position were significant for area (%) and size of openings in uplands (area of opening 5 221.1 1 0.64[% sand], R2 5 0.66, P 5 0.014; size of opening 5 3.057 1 0.22[% sand], R2 5 0.66, P 5 0.014) and for field capacity (FC) in uplands and lowlands (FC 5 0.48 2 0.0041[% sand], R2 5 0.64, P 5 0.017, and FC 5 0.52 2 0.0051[% sand], R2 5 0.74, P 5 0.006, respectively).

content increased, the proportion of total N residing in the POM fraction increased from 26% to 43% (% of N in POM 5 10.5 1 0.39 3 % sand, R2 5 0.46, P 5 0.004). This relationship was due entirely to variation in the proportion of N in fine POM (% of N in fine POM 5 4.2 1 0.37 3 % sand, R2 5 0.59, P 5 0.0005); coarse POM N averaged 7% of N across the range of texture. The proportion of total C in POM was not related to texture. Carbon to nitrogen ratios of total SOM decreased slightly but significantly with texture

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FIG. 2. Variation of soil C and N pools between topographic positions and microsites (O 5 openings, P 5 under B. gracilis plants) (mean 1 1 SE). Significance of main effects of topographic position (T) and microsite (M) are noted as: * P , 0.05; ** P , 0.01; *** P , 0.001; NS not significant. Interactions between microsite and topographic position were significant only for N in particulate organic matter (P 5 0.03). All interactions between topographic position and location were significant (P , 0.0001 to P 5 0.0004), but no interactions between microsite and location were significant. MAOC 5 mineral-associated organic carbon; MAON 5 mineral-associated organic nitrogen; POM 5 particulate organic matter.

across the range sampled (11.4 to 9.6, C:N 5 13.2– 0.044[% sand], R2 5 0.76, P 5 0.005). This reflected differences in POM rather than MAOM. C:N ratios for mineral-associated organic matter increased slightly with increasing sand content (MAOM C:N 5 7.7 1 0.056[% sand], R2 5 0.45, P 5 0.005), whereas POM C:N decreased from 14 to 6 with increasing sand content (POM C:N 5 22.6 2 0.21[% sand], R2 5 0.86, P , 0.0001). The C:N ratio of coarse POM was relatively high (15–21), while that of fine POM was low (5–10), as found by Kelly et al. (1996). Regression results for lowlands were consistent with those presented above, except that sand content was not significantly related to mineral-associated organic matter C or C:N. For uplands, only POM C:N and the proportion of N in fine POM were significantly related to sand content.

Plant-induced heterogeneity Differences between plant and opening microsites were greatest for pools expected to turn over most quickly (Fig. 2). POM and mineralizable fractions were elevated under plants compared to openings, with greater enrichment for coarse POM and mineralizable fractions (59–88%) than for fine POM fractions (16– 44%). In contrast, mineral-associated organic C and N did not differ between microsites. Total C, which comprises the other fractions, was enriched 11% under plants, but total N was not significantly enriched. Root biomass was 4.2 times greater under plants than in openings. Pools that were enriched under plants showed this effect consistently across the landscape; microsite-by-topographic-position and microsite-by-

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FIG. 3. Relationships between soil C and N pools and soil sand content. Each symbol represents one lowland (●) or upland (▫) plot (n 516); each value is the average of three subsamples per plot. All regressions are highly significant with significance levels ranging from P , 0.0001 to P 5 0.008, except for N in particulate organic matter (P 5 0.047). See Fig. 2 for abbreviations.

location interactions were not significant. Proportions of C and N in mineralizable and POM pools were significantly elevated under plants. Mean total C:N was less than one unit higher under plants than in openings, which was due to slight elevation of POM C:N under plants. Microsite did not affect mineral-associated organic matter C:N.

Seasonal nitrogen dynamics Three annually integrated estimates of in situ N availability did not show consistent spatial patterns for either topographic position or microsite, and spatial differences were less than found in laboratory incubations (Fig. 4). IER N was significantly greater under plants than in openings. Otherwise main effects of to-

pographic position and microsite were not significant, although N trapped on ion exchange resins, net N mineralization, and net nitrification tended to be greater in lowlands than uplands. Extractable inorganic N in initial soil samples did not differ between topographic positions, and was greater in openings than under plants in uplands but not in lowlands. Nitrate was the main N ion for all three indices. Eighty percent of IER N and 57% of extractable soil N were NO3. Annually, net nitrification equaled 85% of net N mineralization. Overall, indices of N dynamics varied more within than between topographic positions. For example, net N mineralization ranged from 1.5 to 7.5 g·m22·yr21 in upland plots and from 2.8 to 9.1 g·m22·yr21 in lowland plots (Fig. 5). Soil texture did not account for this

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FIG. 4. Variation of indices of in situ N mineralization and availability between topographic positions and microsites (O 5 openings, P 5 under B. gracilis plants; mean 1 1 SE). Estimates of ion-exchange resin N and net N mineralization are sums of values for all incubation periods. Estimates of initial N are averaged over sample dates. Asterisks indicate significance of main effects of topographic position (T) and microsite (M) (see Fig. 2 caption). The effect of topographic position on net nitrification and the effect of microsite on extractable N approached significance (P 5 0.07 and 0.08, respectively). No interactions between microsite and topographic position were significant. IER N refers to N adsorbed on ion exchange resin bags.


variation. Although mean soil water content and all C and N pools were inversely related to sand content (Figs. 3 and 5), annual in situ net N mineralization and IER N were not correlated with soil texture (Fig. 5). Annual estimates of net N mineralization and IER N showed consistent patterns of variation among plots (r 5 0.76), although IER N was only 10–25% as much as net N mineralization. In situ N indices were not consistently related to most N pool sizes. Annual net N mineralization was significantly correlated with POM N (r 5 0.64), and specifically with fine POM N (r 5 0.67), but was not correlated with other pools. Annual IER N was significantly correlated with potential net N mineralization (r 5 0.53), POM N (r 5 0.71), and total N (r 5 0.66). Extractable N was not significantly correlated with laboratory estimates of any N pool. Total precipitation during the year was 290 mm, 10% below the long-term average. Mean daily precipitation varied greatly among incubation periods (Fig. 6), allowing evaluation of relationships of N dynamics to water availability. Consistent with differences in soil texture and field capacity, soil water content was greater in lowland plots than upland plots, except on the two driest dates, but water content did not differ between microsites. Extractable inorganic N was highest during relatively moist periods, and varied in parallel with soil water content. Extractable N did not vary significantly between topographic positions, but was greater in openings than under plants on two sampling dates. Seasonal patterns of IER N, net N mineralization, and net nitrification were generally consistent with each other and with precipitation. For these indices, rates of N accumulation increased and decreased in parallel in apparent association with precipitation. Nitrogen accumulation rates were highest during relatively moist periods during the growing season. Topographic differences were also greatest during wet periods, and were usually nonsignificant at other times. Net N mineralization, net nitrification, and IER N were greater under plants than in openings only during one spring incubation period each. Net N mineralization and net nitrification were significantly lower under plants than in openings in one summer period. Field results matched patterns of potential net N mineralization (significantly elevated rates in lowlands and under plants) only twice; this occurred with IER N in May of 1993 and with in situ net N mineralization and nitrification during April 1994. Relationships between N dynamics and availability of water were confirmed by regression analyses using average N indices for each topographic position and period, which were produced by averaging plot-scale estimates across replicate locations (Fig. 7). Increases in soil water content or precipitation resulted in significant increases in extractable N, NO3, and IER N, but not extractable NH4, net N mineralization, or net nitrification. Although net nitrification rates did not in-

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crease significantly with increasing precipitation, net ammonification decreased significantly, and the proportion of mineralized N that was nitrified therefore increased. Thus, amounts of NO3 and NH4 were similar under dry conditions, but NO3 was the main form of N under wetter conditions. Plot-scale estimates of IER N, net N mineralization, and nitrification in lowlands were greater than or equal to estimates for uplands during almost all periods. DISCUSSION

Landscape-scale controls

FIG. 5. Relationships between soil sand content and annual indices of in situ N mineralization and availability, and relationship between in situ net N mineralization and N adsorbed on ion exchange resins. Each symbol represents one lowland (v) or upland (□) plot (n 516). Each value is the average (soil water content) or sum (net N mineralization and IER N) of data for all sampling periods and both microsites. Regression lines and equations are shown only for significant relationships (P , 0.0001 for soil water content vs. sand, 0.0007 for IER N vs. net N mineralization). See Fig 4 for abbreviations.

Both topography and soil texture appear to influence vegetation and soil C and N pools in shortgrass steppe. Across diverse uplands and lowlands representing a wide range of parent materials, textures, geomorphic settings, and watershed sizes, erosional and depositional sites usually differed as expected under the catena model. Plant cover and virtually all C and N pools were greater in lowlands than uplands. Soil C and N pools and vegetation structure also varied with soil texture. Because soil texture and topography covary, it is difficult to separate their effects on soils and ecology and to assess their relative influence. We argue below that texture contributes directly to topographic patterns of biogeochemistry, and that texture also influences C and N distribution independently from topography. Fig. 8 summarizes our observations and interpretations as a conceptual model of patterns of N and C pools in shortgrass steppe; we hypothesize that soil texture and topography largely determine distribution of N across the landscape, between openings and microsites occupied by plants, and among soil organic matter fractions. Most paired plots in this study showed downslope increases in silt and clay content. This is consistent with the interpretation that soil materials have been sorted hydrologically in spite of arid Holocene periods with major eolian activity (Muhs 1985, Madole 1994, Blecker et al. 1997) and the dry present climate. Runoff is infrequent in the shortgrass steppe because the dominant sandy loam soils have high infiltration capacities, soils are usually dry, and precipitation is limited. Microwatershed studies in the 1970s and 1980s (Shortgrass Steppe Long Term Ecological Research Project, unpublished data) and our own observations indicate that runoff is virtually absent in some years and occurs infrequently in other years. Nonetheless, runoff is apparently great enough in the long run to concentrate silt and clay in swales, toe slope locations, and alluvial sites. Organic matter and nutrients probably also have been redistributed by runoff. Thus, even in locations with limited runoff, the catena model remains a useful framework for analyzing grassland biogeochemistry, at least at the broad level of distinctions between erosional and depositional sites (Schimel et al. 1985, Schimel and Parton 1986, Burke et al. 1995a).

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FIG. 6. Seasonal patterns of soil water content and nitrogen dynamics. Significant main effects during any period are noted by the letters T (topographic position) and M (microsite). Lines are included to facilitate comparisons among variables and do not represent interpolations. See Fig. 4 for abbreviations.

This study, research by Yonker et al. (1988), and studies reviewed by Gerrard (1992) do show, however, that topographic patterns can be highly variable within a landscape. Although upland and lowland soils in shortgrass steppe differ on average and in many specific locations, such differences are not consistent. Soil texture and SOM vary widely within topographic categories (Yonker et al. 1988, this study). Yonker et al. (1988) sampled soil organic carbon along 24 geologically and topographically diverse hillslopes and seven plains sites at the CPER. They estimated that on average lowlands contained 40% more organic carbon per unit area than slopes and 10% more C than level uplands and hillslope summits; organic C distribution did

not follow expected catenary patterns when data were stratified according to a commonly used, five-segment catena model (Ruhe and Walker 1968). Our results also demonstrate that in locations where soil texture does not correspond with landscape position, such as those reported on by Yonker et al. (1988), the catena model does not explain patterns in biogeochemical pools and processes (Fig. 3). Soil distribution reflects the combined effects of parent material, water, and wind (Yonker et al. 1988), only sometimes resulting in classic catenas (e.g., Schimel et al. 1985). Our sampling design focused on topographic differences and did not allow strong tests of the independent effects of texture, but for several reasons we speculate

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FIG. 7. Relationships between seasonal variation in availability of water and nitrogen concentrations or process rates. Each symbol represents the average value for all lowland (v) or upland (V) plots for one sampling period. Values are averages of eight plots. See Fig. 4 for abbreviations.

that variation in soil texture is central to landscapescale variation of vegetation and biogeochemistry. First, variation in vegetation and soil C and N pools among plots was correlated with variation in soil sand content. Although much of this variation appeared to be driven by differences between depositional and erosional areas, relationships often appeared to be consistent across topographic positions, and separate tests

of relationships within uplands or lowlands showed that some variation within each topographic position was related to sand content. Rigorous tests of the effects of texture would require sampling more sites within one or more topographic positions and deliberate sampling of sites with diverse textures. Few studies have attempted to separate effects of topography and soil texture, but work by Raghubanshi (1992) suggests that

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FIG. 8. Our conceptual model of controls of patterns of nitrogen pools in shortgrass steppe hypothesizes a pervasive role for soil texture and topography in determining distribution of N across the landscape, between openings and microsites occupied by plants, and among soil organic matter fractions. (a) Landscape patterns of surficial soil parent materials and topography are dictated primarily by long-term geomorphic processes and reflect geology, alluvial processes at hillslope and watershed scales, and eolian redistribution of soil materials at local and regional scales. (b) Landscape patterns of soil texture and topography influence soil water dynamics, which largely determine the degree of continuity or heterogeneity of plant cover, as well as levels of primary production. On sandy upland sites with patchy cover, plant presence creates islands of soil stability and organic matter accumulation in a matrix of relatively impoverished erodible soils. On fine-textured lowland sites with continuous cover, SOM accumulation is greater and more uniform. Runoff, grazing, and gap-forming disturbances by animals can reinforce these landscape patterns. (c) Texture directly influences SOM stabilization by aggregation and association with mineral surfaces. Because C and N pools respond differently to texture, the relative importance of particulate organic matter to biologically active N pools is greatest on sandy upland sites. Although total C and N pools and mineralassociated C decline rapidly with increasing sand content and from lowlands to uplands, POM N decreases only slightly. As a result, POM contains a large proportion of total N and has a low C:N ratio on sandy upland sites with relatively infertile soils. Thus, partially decomposed plant detritus is a particularly important reservoir of active N on these sites. Positive and negative relationships are indicated by 1 and 2; 0 indicates a weak relationship or no relationship.

patterns of soil texture may sometimes account for patterns of biogeochemistry that differ from conventional catena models. On a dry tropical forest hillslope, Raghubanshi (1992) found that sand content increased downslope, while soil C, N, and P decreased from the top to the base of the hill; organic C was strongly correlated with soil silt and clay content. Raghubanshi (1992) cited similar results from other tropical landscapes, hinting at possible widespread control of landscape biogeochemistry by soil texture. Second, biological processes that respond to soil water status (Lauenroth et al. 1978, Lauenroth and Milchunas 1992) are more likely to be affected by the

direct influence of texture on soil water dynamics than by differences in water supply due to runoff. As noted above, runoff is infrequent at the CPER. On the other hand, texture directly affects infiltration, retention, and release of water. Third, our results are consistent with research that demonstrates the importance of texture to C and N accumulation, soil water dynamics, primary production, and plant community structure. Texture may influence soil C and N accumulation in several ways, including effects of plant cover on erosion and effects of silt and clay on SOM stabilization (Fig 8). Potential for erosion generally increases as plant cover decreas-

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es. Consequently, sandy soils with low plant cover are likely to be more prone to erosion and SOM loss than heavy soils with high plant cover. The effects of sparse cover are likely compounded by the high susceptibility of unprotected, dry, sandy soils to wind erosion (Skidmore 1994), and by the tendency of disturbance gaps to persist longer on sandy than fine-textured soils due to lower soil water availability (Coffin and Lauenroth 1994). The high infiltration capacities typical of sandy soils may limit runoff and water erosion, however. Regional analyses show that organic C and N increase with increasing silt and clay content in U.S. grassland soils (Jenny 1941, Burke et al. 1989). This relationship may reflect positive effects of silt on soil water availability and plant production as well as positive effects of clay on SOM protection (Burke et al. 1989). Laboratory studies of decomposition, SOM accumulation, and carbon isotope distribution in soil physical fractions have demonstrated that silt and clay promote SOM accumulation by aggregate formation and adsorption on mineral surfaces (Jenkinson 1977, Tisdall and Oades 1982, Hassink 1996). Hassink (1996) recently proposed a direct, quantitative relationship between silt plus clay content and the capacity of soils to stabilize organic matter. A regional analysis of forest ecosystems found that annual net N mineralization increased with soil silt plus clay content, which varied with parent material and soil development (Reich et al. 1997). Our data are consistent with the idea that texture affects C and N accumulation partly through stabilization of mineral-associated SOM (Delgado et al. 1996), which was the majority of SOM. Texture can affect both the amount of organic C and N and their distribution among different pools. Total N pools and the proportion of total N residing in mineral-associated organic matter increased substantially with silt and clay content, but POM N varied little with texture. Consequently, the proportion of N in POM was much greater in sandy than fine-textured soils. Apparently, as sand content increases, the relative importance of N storage in partially decomposed roots and associated microbial biomass and decomposition products increases, whereas the importance of SOM protection by adsorption and aggregation decreases. Considering the direct effects of texture on soil water and organic matter, it is logical to expect soil texture to be an important control of soil organic matter, nutrients, and vegetation in shortgrass steppe. Other research in shortgrass steppe indicates that soil texture influences many ecological patterns and processes. In addition to controlling C and N pools, soil water, and plant cover patterns, the distribution of sandy soils also influences plant establishment and the abundance and effects of disturbance gaps created by harvester ants, root-feeding beetles, and small mammals (Coffin and Lauenroth 1988 and 1994, Hook et al. 1994; P. B. Hook and I. C. Burke, unpublished observations).

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Plant-induced heterogeneity Strong small-scale heterogeneity of SOM appears to result wherever openings interrupt plant cover in shortgrass steppe, but the importance of such differences varies among SOM fractions and across the landscape. As proposed by Hook et al. (1991) and Kelly et al. (1996), individual plant effects were greatest for pools that turn over rapidly or reflect incomplete decomposition and relatively recent inputs of plant detritus (POM and mineralizable fractions), and these pools were a larger proportion of total C and N under plants than in openings. Mineralizable and POM C and N fractions were affected about equally by topography and by the mosaic of individual plants and openings. In contrast, individual plants did not have a detectable effect on C and N in mineral-associated organic matter, which was presumably well decomposed or protected SOM with a long turnover time. Mineral-associated organic matter integrates long-term pedogenic processes that are affected by topography and texture, but apparently not by the transient effects of individual plants on organic inputs (Kelly et al. 1996). Enrichment of SOM under plants relative to openings was consistent across the landscape, in all replicate locations, and in both uplands and lowlands. Nonetheless, the ecological and biogeochemical importance of plant-induced soil heterogeneity must vary with soil texture because openings are a large proportion of the surface on sandy sites, but only a minor proportion on fine-textured sites (Fig. 8b). Though measurable, differences between microsites are relatively unimportant on fine-textured soils because cover is nearly continuous. This effect of soil texture on the importance of plant cover patterns to biogeochemistry in the CPER landscape is comparable to the effect of variation of precipitation across the central grasslands region of the United States. As precipitation increases from west to east, plant cover becomes continuous and individualplant-scale soil heterogeneity decreases (Vinton and Burke 1997, Burke et al. 1998).

In situ nitrogen dynamics Seasonal N dynamics were apparently controlled largely by precipitation and soil water, and temporal variation was greater than variation between topographic positions or microsites. Nitrogen availability was highest during moist periods in the growing season, consistent with other studies in semiarid ecosystems (Gosz and White 1986, Burke 1989). Plant available N, NO3, N adsorbed on ion exchange resins, and nitrification of mineralized N increased significantly with increasing availability of water. The relatively short period of observation in this study limits close evaluation of the influence of microclimate, but soil water and precipitation accounted for between one-half and two-thirds of the variation in these indices of N dynamics. Surprisingly, net N mineralization did not

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increase with availability of water. While many laboratory studies show strong relationships between microclimate and N mineralization (e.g., Kladivko and Keeney 1987), relationships are generally weaker in field studies and are complicated by N immobilization and loss (e.g., Schimel and Parton 1986). Several researchers working in grasslands, shrublands, and dry tropical forests have reported that N mineralization is greatest in moist periods (Gokceoglu 1988, Burke 1989, Holland and Detling 1990, Mazzarino et al. 1991, Raghubanshi 1992). However, cycles of drying and wetting may be as important to N mineralization as total precipitation, and wetting may even decrease N availability in some desert soils, possibly due to accelerated mineralization and subsequent loss (Schimel and Parton 1986, Fisher et al. 1987). Effects of topography and microsite on N dynamics were also greatest early in the growing season when soils were relatively warm and moist. Although topographic and microsite differences in potential N mineralization were large in laboratory incubations, they were generally expressed in the field only when microclimate was most favorable. Burke (1989) reported similar results in a Wyoming sagebrush steppe. There the effects of topography and vegetation type on N mineralization were most important in spring and early summer, when 90% of annual N mineralization occurred. Viewed over the entire study period, spatial patterns of in situ N dynamics did not correspond well to spatial patterns of N pools. Annual estimates of N availability were not significantly greater in lowlands than uplands, were not related to soil sand content, and generally were not elevated under plants. For all field indices of N dynamics, differences among replicate locations were greater than average topographic and microsite differences. Although seasonal data provided some evidence that N availability was greater in lowlands than uplands and greater under plants than in openings, spatial patterns were not consistent among incubation periods or among indices of N availability. Resin bag data provided stronger support for topographic and microsite effects on N availability than other measurements. Indices of N availability are often inconsistent. Each method has different problems (Hook and Burke 1995), principally transient effects on N mineralization and immobilization in incubations, dependence on soil solution movement to resin bags, and extreme temporal variability of N extracted from soil. To reduce such problems, we tested incubation and IER techniques prior to this study (Hook and Burke 1995). We cannot exclude methodological problems that frequently influence measurements of in situ N dynamics, but the fact that annual IER N and in situ net N mineralization in cores were reasonably well correlated suggests that the unexplained variation among plots was not spurious.


Our results contrast with observations showing general correspondence between in situ N dynamics and the distribution of organic N pools in landscapes (Schimel et al. 1985, Burke 1989, Raghubanshi 1992). Results of research on topographic patterns of N mineralization have been inconsistent. In one Minnesota forest, topography controlled net N mineralization indirectly through the effects of soil water, organic matter, leaf biomass, and fire frequency (Clark 1990), but in another, N mineralization was not related to slope position (Zak et al. 1991). One possible explanation for the lack of strong landscape-scale patterns for in situ N dynamics is that N mineralization may be associated with an organic N pool that is not controlled strongly by the landscape factors we studied. Annual net N mineralization was correlated only with POM N, not with total, mineralassociated, or potentially mineralizable N. Of all the N pools studied, POM N had the weakest relationship with topography and sand content. In contrast to mineral-associated C and N and POM C, POM N levels were relatively uniform across plots. As a result, the proportion of total N in POM increased and POM C: N ratios decreased with increasing sand content. Thus, particulate organic matter appears to be important to N retention and availability in sandy soils, which typify the CPER (Fig. 8c). If so, storage and mineralization may be linked less tightly with topography and soil texture for N than for C. Based on patterns of N mineralization from different organic matter fractions, Cambardella and Elliott (1992) proposed that POM may be an important source of mineralizable N. In contrast, Sollins et al. (1984) reported that net N mineralization was greater for heavy soil fractions than light fractions. Their density fractions were broadly similar to our particulate and mineral-associated fractions, but C:N ratios were wider, especially for the light fraction, and probably contributed to N immobilization. While a relationship between N mineralization and partially decomposed root detritus makes sense where C:N ratios are low, this does not account for the surprising lack of correlation between annual net N mineralization and potential net N mineralization. One reason for low correlation may be that we estimated potentially mineralizable N using samples collected on one date, but this pool can vary considerably during a year (e.g., Fisher et al. 1987). Other research has documented topographic differences in N mineralization in shortgrass steppe (Schimel et al. 1985, Schimel and Parton 1986) and other ecosystems (e.g., Zak et al. 1986, Burke 1989, Giblin et al. 1991, Schimel et al. 1991), arguing for caution in dismissing topographic effects or interpreting N dynamics primarily in terms of particulate organic matter.

Conclusions In shortgrass landscapes soil organic matter distribution is strongly related to topography and soil tex-

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ture. Although the CPER landscape is more complex than envisioned in many catena models, differences between adjacent uplands and lowlands were broadly consistent with the catena concept, a cornerstone of many analyses of landscape-scale controls of soils, plant ecology, and biogeochemistry (Schimel et al. 1985, Swanson et al. 1988). Catena models vary in their details, but they generally assume that hydrologic and geomorphic processes generate orderly, repeatable topographic patterns of soil morphology and biogeochemistry (Birkeland 1984, Gerrard 1992). Our study area has diverse geologic parent materials, infrequent runoff, and a complex geomorphic history that includes extensive eolian deposition and repeated episodes of soil development and erosion (Muhs 1985, Davidson 1988, Yonker et al. 1988, Madole 1994, Blecker et al. 1997). The strength and frequency of topographic differences we observed suggest that the catena concept is quite robust and applies, at least in a very general form that contrasts erosional and depositional areas, even in complex, semiarid landscapes. To understand the structure of shortgrass landscapes more fully, however, future research must address the interactions of long-term climate variation, hydrology, and eolian processes as they affect soil development and biogeochemistry (Yonker et al. 1988). Even without a full understanding of landscape evolution, biogeochemical research in semiarid regions and other areas can benefit from focusing more on the role of soil texture and processes directly involved in SOM dynamics (Fig. 8). Correlation of vegetation patterns and most soil C and N pools to sand content, and process-level studies of SOM accumulation (Tisdall and Oades 1982, Hassink 1996) indicate that soil texture may be an important control of C and N dynamics. The mechanisms by which texture influences soils and plants are more direct than the effects of topography, particularly in landscapes where runoff is limited and topographic patterns of soil water are inconsistent (Singh et al. 1998). Texture appears to influence soil C and N both in association with topography and independently (Raghubanshi 1992). Clark (1990) demonstrated that landscape-scale analyses of ecosystem processes can be more powerful when direct and indirect relations at several scales are considered. Localized enrichment of biologically active organic matter under individual B. gracilis plants is superimposed on the landscape patterns summarized above. We previously speculated that individual plants’ effects on C and N pools might be as important as the effects of topography in ecosystems with discontinuous plant cover (Hook et al. 1991). In this study, the size of C and N pools that turn over rapidly appeared to be influenced about equally by topography, soil texture, and fine-scale heterogeneity of plant cover and production. In contrast, stable mineral-associated organic C and N were strongly related to landscape position and soil texture, but were not elevated under plants. Thus, the

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relative importance of vegetation structure and physical characteristics of landscapes in controlling biogeochemistry depends strongly on the time scale of biogeochemical processes considered. Because vegetation is influenced by soil sand content and topography, the effects of discontinuous plant cover on active SOM can also be interpreted as indirect consequences of soil texture and topography (Fig. 8b). On sandy, erosional uplands, sparse plant cover exacerbates site instability and low silt and clay content limits accumulation of mineral-associated organic matter; consequently, the localized stability and organic matter inputs provided by bunchgrasses are important to maintaining active C and N pools. We hypothesized that dynamics of inorganic N would reflect the interaction of seasonal microclimate with spatial patterns of C and N pools. As expected, temporal variation in indices of N availability and net N mineralization was high and corresponded largely to seasonal changes in soil water and precipitation. Although N availability and turnover tended to be elevated in lowlands and under plants, these effects varied through time, were smaller than suggested by laboratory analyses, and frequently were absent. Even during relatively warm, wet periods when spatial patterns of N dynamics were most apparent, patterns were inconsistent among N indices. Most annual indices of N dynamics were not significantly related to topography, texture, or microsite. These relatively weak spatial patterns for N dynamics contrast sharply with the substantial landscape or individual-plant scale variation observed for all C pools and most N pools. The poor relationships between N dynamics and sand content or topography are puzzling because most of the biogeochemical pools studied varied strongly with soil texture and topography. Landscape patterns of N mineralization and availability may be related to N pools, such as fine POM N, that are not strongly related to topography or texture (Fig. 8c). Although correlations between annual indices of in situ N dynamics and biologically active N pools were not consistent, both IER N and net N mineralization were related to POM N, which varied only slightly with soil texture and topographic position. If POM is central to N mineralization and availability, the relative uniformity of POM across the landscape may reduce systematic spatial variation of inorganic N compared to other C and N pools. Topography and soil texture appear to control many features of the shortgrass steppe ecosystem, defining strong gradients of vegetation structure and biogeochemistry. We suggest that soil texture is a key proximal control over biogeochemical processes and their patterns at the landscape scale, and is largely responsible for observed topographic differences. Biogeochemical models based on texture can link landscapescale studies to research showing that soil texture influences SOM dynamics directly. Such models may help to explain hillslope biogeochemical patterns both

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where soil texture varies systematically with slope position and where soils deviate from typical catenas. They may also provide a basis for relating current landscape patterns to past eolian processes that are important to soil development and distribution in many dry regions. Linking soil geomorphic models to models of proximate controls of biogeochemistry by soil texture, vegetation structure, and microclimate can provide a powerful framework to analyze effects of long-term landscape development on biogeochemistry. ACKNOWLEDGMENTS Many individuals assisted in this work; we thank Amy Rupp and Becky Riggle in particular for overseeing much of the field and laboratory work. A National Science Foundation grant to the Shortgrass Steppe Long Term Ecological Research Project (DEB-9632852) supported this research. Field and laboratory work were performed at Colorado State University while Paul Hook was a postdoctoral researcher in the Department of Forest Sciences; support was provided by the Montana Agricultural Experiment Station during data analysis and manuscript preparation. Comments from Peter Reich and anonymous reviewers enhanced the manuscript significantly. Journal Series No. 2000-44 MAES, MSU-Bozeman. LITERATURE CITED Aandahl, A. R. 1948. The characterization of slope positions and their influence on total nitrogen content of a few virgin soils of western Iowa. Soil Science Society of America Proceedings 13:449–454. Aguilar, R., and R. D. Heil. 1988. Soil organic carbon, nitrogen and phosphorus quantities in Northern Great Plains rangeland. Soil Science Society of America Journal 52: 1076–1081. Binkley, D., and P. A. Matson. 1983. Ion exchange resin bag method for assessing forest soil nitrogen availability. Soil Science Society of America Journal 47:1050–1052. Birkeland, P. W. 1984. Soils and geomorphology. Oxford University Press, New York, New York, USA. Blecker, S. W., C. M. Yonker, C. G. Olsen, and E. F. Kelly. 1997. Paleopedologic and geomorphic evidence for Holocene climate variation, Shortgrass Steppe, Colorado, USA. Geoderma 76:113–130. Burke, I. C. 1989. Control of nitrogen mineralization in a sagebrush steppe landscape. Ecology 70:1115–1126. Burke, I. C., E. T. Elliott, and C. V. Cole. 1995a. Influence of macroclimate, landscape position, and management on soil organic matter in agroecosystems. Ecological Applications 5:124–131. Burke, I. C., and W. K. Lauenroth. 1993. What do LTER results mean? Extrapolating from site to region and decade to century. Ecological Modelling 67:19–35. Burke, I. C., W. K. Lauenroth, and D. P. Coffin. 1995b. Soil organic matter recovery in semiarid grasslands: implications for the conservation reserve program. Ecological Applications 5:793–801. Burke, I. C., W. K. Lauenroth, M. A. Vinton, P. B. Hook, R. H. Kelly, H. E. Epstein, M. R. Aguiar, M. D. Robles, M. O. Aguilera, K. L. Murphy, and R. A. Gill. 1998. Plant– soil interactions in temperate grasslands. Biogeochemistry 42:121–143. Burke, I. C., C. M. Yonker, W. J. Parton, C. V. Cole, K. Flach, and D. S. Schimel. 1989. Texture, climate, and cultivation effects on soil organic matter content in U.S. grassland soils. Soil Science Society of America Journal 53:800–805. Cambardella, C. A., and E. T. Elliott. 1992. Particulate soil organic-matter changes across a grassland cultivation sequence. Soil Science Society of America Journal 56:777– 783.


Charley, J. L., and N. E. West. 1975. Plant induced soil chemical patterns in some shrub-dominated semi-desert ecosystems of Utah. Journal of Ecology 63:945–964. Charley, J. L., and N. E. West. 1977. Micro-patterns of nitrogen mineralization activity in soils of some shrub-dominated semi-desert ecosystems of Utah. Soil Biology and Biochemistry 9:357–365. Clark, J. S. 1990. Landscape interactions among nitrogen mineralization, species composition, and long-term fire frequency. Biogeochemistry 11:1–22. Coffin, D. P., and W. K. Lauenroth. 1988. The effects of disturbance size and frequency on a shortgrass plant community. Ecology 69:1609–1617. Coffin, D. P., and W. K. Lauenroth. 1994. Successional dynamics of a semiarid grassland: effects of soil texture and disturbance size. Vegetatio 110:67–82. Davidson, J. M. 1988. Surficial geology and Quaternary history of the Central Plains Experimental Range, Colorado. Thesis. Colorado State University, Fort Collins, Colorado, USA. Delgado, J. A., A. R. Mosier, D. W. Valentine, D. S. Schimel, and W. J. Parton. 1996. Long term 15N studies in a catena of the shortgrass steppe. Biogeochemistry 32:41–52. Fisher, F. M., L. W. Parker, J. P. Anderson, and W. G. Whitford. 1987. Nitrogen mineralization in a desert soil: interacting effects of soil moisture and nitrogen fertilizer. Soil Science Society of America Journal 51:1033–1041. Gerrard, J. 1992. Soil geomorphology: an integration of pedology and geomorphology. Chapman and Hall, London, UK. Giblin, A. E., K. J. Nadelhoffer, G. R. Shaver, J. A. Laundre, and A. J. McKerrow. 1991. Biogeochemical diversity along a riverside toposequence in arctic Alaska. Ecological Monographs 61:415–435. Gokceoglu, M. 1988. Nitrogen mineralization in volcanic soil under grassland, shrub and forest vegetation in the Aegean region of Turkey. Oecologia 77:242–249. Gosz, J. R., and C. S. White. 1986. Seasonal and annual variation in nitrogen mineralization and nitrification along an elevational gradient in New Mexico. Biogeochemistry 2:281–298. Hassink, J. 1996. Preservation of plant residues in soils differing in unsaturated protective capacity. Soil Science Society of America Journal 60:487–491. Holland, E. A., and J. K. Detling. 1990. Plant response to herbivory and belowground nitrogen cycling. Ecology 71: 1040–1049. Hook, P. B., and I. C. Burke. 1995. Evaluation of methods for estimating net nitrogen mineralization in a semiarid grassland. Soil Science Society of America Journal 59:831– 837. Hook, P. B., I. C. Burke, and W. K. Lauenroth. 1991. Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe. Plant and Soil 138:247–256. Hook, P. B., W. K. Lauenroth, and I. C. Burke. 1994. Spatial patterns of roots in a semiarid grassland: abundance of canopy openings and regeneration gaps. Journal of Ecology 82:485–494. Jackson, R. B., and M. M. Caldwell. 1993. The scale of nutrient heterogeneity around individual plants and its quantification with geostatistics. Ecology 74:612–614. Jenkinson, D. S. 1977. Studies on the decomposition of plant material on soil. V. The effects of plant cover and soil type on the loss of carbon from 14C-labeled rye grass decomposing under field conditions. Journal of Soil Science 28: 424–494. Jenny, H. 1941. Factors of soil formation: a system of quantitative pedology. Dover, Mineola, New York, USA.

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Jenny, H. 1980. The soil resource. Ecological Studies 37. Springer-Verlag, New York, New York, USA. Kelly, R. H., I. C. Burke, and W. K. Lauenroth. 1996. Soil organic matter and nutrient availability responses to reduced plant inputs in shortgrass steppe. Ecology 77:2516– 2527. Kladivko, E. J., and D. R. Keeney. 1987. Soil nitrogen mineralization as affected by water and temperature interactions. Biology and Fertility of Soils 5:248–252. Lajtha, K., and W. H. Schlesinger. 1988. The biogeochemistry of phosphorus cycling and phosphorus availability along a desert soil chronosequence. Ecology 69:24–39. Lauenroth, W. K., J. L. Dodd, and P. L. Sims. 1978. The effects of water- and nitrogen-induced stresses on plant community structure in a semiarid grassland. Oecologia 36: 211–222. Lauenroth, W. K., and D. G. Milchunas. 1992. Short-grass steppe. Pages 183–226 in R. T. Coupland, editor. Natural grasslands I. Introduction and Western Hemisphere. Elsevier, New York, New York, USA. Madole, R. F. 1994. Stratigraphic evidence of desertification in the west-central Great Plains within the last 1000 yr. Geology 22:483–486. Mazzarino, M. J., L. Oliva, A. Nun˜ez, G. Nun˜ez, and E. Buffa. 1991. Nitrogen mineralization and soil fertility in the Dry Chaco ecosystem (Argentina). Soil Science Society of America Journal 55:515–522. Milchunas, D. G., and W. K. Lauenroth. 1989. Three-dimensional distribution of plant biomass in relation to grazing and topography in shortgrass steppe. Oikos 55:82–86. Milchunas, D. G., W. K. Lauenroth, P. L. Chapman, and M. K. Kazempour. 1989. Effects of grazing, topography, and precipitation on the structure of a semiarid grassland. Vegetatio 80:11–23. Muhs, D. R. 1982. The influence of topography on the spatial variability of soils in Mediterranean climates. Pages 269– 284 in C. E. Thorn, editor. Space and time in geomorphology. George Allen and Unwin, Boston, Massachusetts, USA. Muhs, D. R. 1985. Age and paleoclimatic significance of Holocene sand dunes in northeastern Colorado. Annals of the Association of American Geographers 75:566–582. Parton, W. J., W. K. Lauenroth, and F. M. Smith. 1981. Water loss from a shortgrass steppe. Agricultural Meteorology 24: 97–109. Parton, W. J., D. S. Schimel, C. V. Cole, and D. S. Ojima. 1987. Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Science Society of America Journal 51:1173–1179. Raghubanshi, A. S. 1992. Effect of topography on selected soil properties and nitrogen mineralization in a dry tropical forest. Soil Biology and Biochemistry 24:145–150. Reich, P. B., D. F. Grigal, J. D. Aber, and S. T. Gower. 1997. Nitrogen mineralization and productivity in 50 hardwood and conifer stands on diverse soils. Ecology 78:335–347. Ruhe, R. V., and P. H. Walker. 1968. Hillslope models and soil formation. I. Open systems. Pages 551–560 in Trans-

2703

actions of the Ninth International Congress of Soil Science, Adelaide, Australia. Angus and Robertson, London, UK. Schimel, D. S. 1986. Carbon and nitrogen turnover in adjacent grassland and cropland ecosystems. Biogeochemistry 2:345–357. Schimel, D. S., T. G. F. Kittel, A. K. Knapp, T. R. Seastedt, W. J. Parton, and V. B. Brown. 1991. Physiological interactions along resource gradients in a tallgrass prairie. Ecology 72:672–684. Schimel, D. S., and W. J. Parton. 1986. Microclimatic controls of nitrogen mineralization and nitrification in shortgrass steppe soils. Plant and Soil 93:347–357. Schimel, D. S., M. A. Stillwell, and R. G. Woodmansee. 1985. Biogeochemistry of C, N, and P in a soil catena of the shortgrass steppe. Ecology 66:276–282. Schlesinger, W. H., J. A. Raikes, A. E. Hartley, and A. F. Cross. 1996. On the spatial pattern of soil nutrients in desert ecosystems. Ecology 77:364–374. Senft, R. L., L. R. Rittenhouse, and R. G. Woodmansee. 1985. Factors influencing patterns of cattle grazing behavior on shortgrass steppe. Journal of Range Management 38:82– 87. Singh, J. S., D. G. Milchunas, and W. K. Lauenroth. 1998. Soil water dynamics and vegetation patterns in a semiarid grassland. Plant Ecology 134:77–89. Skidmore, E. L. 1994. Wind erosion. Pages 265–293 in R. Lal, editor. Soil erosion research methods. Second edition. St. Lucie, Delray Beach, Florida, USA. Sollins, P., G. Spycher, and C. A. Glassman. 1984. Net nitrogen mineralization from light- and heavy-fraction forest soil organic matter. Soil Biology and Biochemistry 16:31– 37. Swanson, F. J., T. K. Kratz, N. Caine, and R. G. Woodmansee. 1988. Landform effects on ecosystem patterns and processes. BioScience 38:92–98. Tisdall, J. M., and J. M. Oades. 1982. Organic matter and water-stable aggregates in soils. Journal of Soil Science 33: 141–163. Vinton, M. A., and I. C. Burke. 1995. Interactions between individual plant species and soil nutrient status in shortgrass steppe. Ecology 76:1116–1133. Vinton, M. A., and I. C. Burke. 1997. Plant effects on soil nutrient dynamics along a precipitation gradient in Great Plains grasslands. Oecologia 110:393–402. Woodmansee, R. G., and F. J. Adamsen. 1983. Biogeochemical cycles and ecological hierarchies. Pages 497–516 in R. R. Lowrance, R. L. Todd, L. F. Asmussen, and R. A. Leonard, editors. Nutrient cycling in agricultural ecosystems. University of Georgia College of Agriculture Experiment Station Special Publication Number 23. Yonker, C. M., D. S. Schimel, E. Paroussis, and R. D. Heil. 1988. Patterns of organic carbon accumulation in a semiarid shortgrass steppe, Colorado. Soil Science Society of America Journal 52:478–483. Zak, D. R., A. Hairston, and D. F. Grigal. 1991. Topographic influences on nitrogen cycling within an upland pin oak ecosystem. Forest Science 37:45–52. Zak, D. R., K. S. Pregitzer, and G. E. Host. 1986. Landscape variation in nitrogen mineralization and nitrification. Canadian Journal of Forest Research 16:1258–1263.