Soil Texture and Carbon Dynamics in Savannah ...

Forest, Range & Wildland Soils

Soil Texture and Carbon Dynamics in Savannah Vegetation Patches of Central Argentina Adriana A. Gili Román Trucco Selene Niveyro Facultad de Agronomía Universidad Nacional de La Pampa CC 300 Ruta Nac. 35 Km. 334 Santa Rosa, L.P. Argentina

Mónica Balzarini Conicet Facultad de Ciencias Agropecuarias Univ. Nacional de Córdoba C.P. 5000 Avenida Valparaíso y Rogelio Martínez Córdoba, Argentina

Daniel Estelrich Alberto Quiroga Elke Noellemeyer* Facultad de Agronomía Universidad Nacional de La Pampa CC 300 Ruta Nac. 35 Km. 334 Santa Rosa, L.P. Argentina

This study was intended to contribute information on the relation between soil texture and plant community composition, and the effect of interactions between texture and vegetation patch on soil C dynamics. Six sampling sites were chosen and a vegetation census and soil sampling were performed for between-canopy (BC) and undercanopy (UC) patches at each site, Shannon–Weaver diversity index (DI) was determined for herbaceous species, an relative abundance of forage species and avoided grasses were calculated. Soil data included texture, bulk density, total organic C (TOC), particulate organic C (POC), intermediate organic C (IOC), and fine organic C (FOC). Correspondence analysis showed that sandy and loamy sites did not share the same herbaceous species. We also found an interaction between vegetation patches and texture, with distinct vegetation composition in the same type of patch in different textures. The DI was lower in UC than BC patches (0.86 vs. 1.7), but no effect of texture was found. The TOC showed a positive relation with texture; the extremes were 9.0 vs. 16.3 g kg−1 for a loamy sand and loam, respectively. Carbon fractions varied in their response to texture: while POC and IOC showed no clear trend with texture, FOC contents followed a textural gradient. Particulate organic C showed higher values in UC than BC (10.8 vs. 4.0 g kg−1). Differences between patches diminished with increasing clay contents. Intermediate organic C was similar to POC, while FOC showed no effect of patch but was related to texture. The high proportion of POC in sandy soils (50 vs. 35% in a loam) makes these more susceptible to changes in C balance and might thus render them less resilient under grazing. Abbreviations: BC, between canopy; DI, Shannon–Weaver diversity index; FOC, fine organic carbon; FS, relative abundance of forage species; IOC, intermediate organic carbon; POC, particulate organic carbon; TOC, total organic carbon; UC, under canopy.

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egetation patches or differences in plant community structures are probably natural features of grassland ecosystems (Teague et al., 2004) that can be reinforced by anthropogenic factors. In areas where soil conditions are very homogeneous, such as South African savannas along the Kalahari transect, recent studies have shown a climatic control on the patchiness of vegetation. Caylor et al. (2003) found that tree spacing and vegetation patterns varied according to rainfall regime. Spatial heterogeneity with respect to vegetation and soil nutrient contents increased toward the more arid regions of this climosequence (Okin et al., 2008). The patchiness of vegetation is reflected by a heterogeneous distribution of soil resources that tend to accumulate beneath woody plant canopies (Wang et al., 2007; Feral et al., 2003; Hibbard et al., 2003). The accumulation of soil nutrients and C stocks in woody patches has been described as “islands of fertility” (Reynolds et al., 1999; Schlesinger and Pilmanis, 1998; Schlesinger et al., 1996; Hook et al., 1991). Biotic factors such as the uptake of soil nutrients and deposition of litter, and higher microbial, invertebrate, and vertebrate activities under shrub canopies are known to reinforce these islands of fertility (Herman et al., 1995). On the other hand, abiotic processes such as wind and water erosion, alluvial and fluvial processes, as well as environmental factors such as topography and climate create spatial heterogeneity of soil Soil Sci. Soc. Am. J. 74:647–657 Published online 29 Dec. 2009 doi:10.2136/sssaj2009.0053 Received 10 Feb. 2009. *Corresponding author ([email protected]). © Soil Science Society of America, 677 S. Segoe Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

SSSAJ: Volume 74: Number 2 • March–April 2010

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properties (Solon et al., 2007; Fisk et al., 1998; Parsons et al., 1992; Coppinger et al., 1991). Other studies have identified soil heterogeneity as the cause of the invasion of woody shrubs (Schlesinger et al., 1990). Skarpe (1990) suggested a hydraulic and physical control of woody species distribution, since grasses take water from shallower depths than deeper rooted trees, thus restraining the establishment of woody vegetation in areas with dense grass cover. Ringrose et al. (1998) and Feral et al. (2003) pointed to a combined effect of moisture conditions and grazing pressure on the development of woody weed patches. These findings suggest that the moisture regime is one of the determining factors that govern vegetation patch dynamics in arid grasslands. The effect of soil properties on vegetation composition has recently been studied by Solon et al. (2007), who found that properties such as elevation, moisture, pH, and nutrient contents affected species richness and vegetation composition in catenae of the Wiegirski National Park (northeast Poland). It would therefore be reasonable to assume that soil properties also have an effect on vegetation composition and species diversity in natural environments of semiarid grasslands. Considering the evidence of the effect of moisture regime on vegetation patch dynamics in savannas, specifically those soil properties that affect water availability could act as a driving force for vegetation dynam-

ics in these arid lands. Recent studies in agricultural systems in semiarid central Argentina have shown that the most outstanding features of soil variability in this environment are texture and profile depth, which determine the soil’s water storage capacity and thus directly affect plant-available water (Fernandez et al., 2008). Wheat (Triticum aestivum L.) yields were directly related to soil depth and, in soils with similar profile development, finer textures produced higher yields than sands (Fontana et al., 2006). Therefore our hypothesis was that soil texture would have a strong effect on vegetation species composition and diversity in natural grasslands of the same region, where environmental variability is more associated with the heterogeneity of soil properties than with climate. Texture also has a strong effect on the soil’s capacity to accrue C; it has been shown by numerous studies in rangelands (Noellemeyer et al., 2006; Hibbard et al., 2003; Neufeldt et al., 2002; Bird et al., 2000) that C stocks are positively related to the soil’s clay contents. We therefore hypothesized that texture would affect the interaction between vegetation structure and soil C dynamics, through differences in C fractions between vegetation patches in different soil textures. Fractionation of soil C has shown to be a valuable tool to assess C dynamics under land use change and under different soil management (von Lützow et al., 2007; John et al., 2005; Cambardella and Elliott, 1993). While the most labile fraction, POC, is understood to reflect recent C inputs to the soil with a mean residence time of a few years (Zach et al., 2006), C associated with finer mineral soil fractions is chemically and physically stabilized organic material with a very slow turnover rate of around 400 yr ( Jastrow, 1996). The relative proportions of these types of organic matter reflect a soil’s C budget in terms of vegetation residue inputs and organic matter losses through mineralization, but also reveal information about chemical and physical stabilization mechanisms (von Lützow et al., 2007; Six et al., 2002). Therefore these fractions are useful as early indicators for C stock changes (Leifeld and KögelKnabner, 2005) and as an appraisal of the resilience of a soil–plant system. The present study was intended to contribute knowledge on the effect of soil texture on plant community composition, and on the effects of interactions between texture and vegetation patch on soil C dynamics.

MATERIALS AND METHODS Study Area The Calden savanna in the province of La Pampa extends between 63° and 66° W and 35° to 39° S, and covers approximately 40 km2 (Fig. 1). Slightly undulated plains that are interrupted by ample valleys in the southwest to Fig. 1. Map of La Pampa. The whole shaded area comprises the Caldenal savanna. Black northeast direction characterize the topography. The soils indicates dune areas, light gray corresponds to low brush in valleys, and intermediate developed on loess sediments that were deposited by the gray represents Calden savanna on plains. 648


prevailing southwest winds. The most important soil taxonomic Great Groups found in this region are Typic and Entic Haplustolls in the plains and Typic Ustipsamments in dune areas in the valleys. Haplustolls are typically limited by a CaCO3 layer at depths varying between 2 m in depressions. Rainfall is highest in the northeast, with an annual average of 600 mm, and decreases toward the southwest to 450 mm yr−1. The mean annual temperature is 16°C, ranging from monthly means of 8°C during the winter to 25°C in summer (Instituto Nacional de Tecnologia Agropecuaria, 1980). Six rangeland sites under grazing were selected according to textural and topographic differences and vegetation structure. The dune sites have predominantly herbaceous vegetation [Poa lanuginosa Poir., Digitaria californica (Benth.) Henrard, Bothriochloa springfieldii (Gould) Parodi, Hyalis argentea D. Don ex Hook. & Arn., Stipa tenuis Phil., and Schismus barbatus (L.) Thell.], while at the plains sites, both grasses [Hordeum stenostachys Godr., Bromus brevis Steud., Poa ligularis Nees ex Steud., Piptochaetium napostaense (Speg.) Hack., Stipa brachychaeta Godr., Stipa ichu (Ruiz & Pav.) Kunth and Stipa tenuissima Trin.] as well as trees and shrubs (Prosopis caldenia Burkart, Prosopis flexuosa DC., and Condalia microphylla Cav.) coexist. The Caldenal savanna evolved without the presence of any large ungulates until the introduction of bovine (Bos spp.) and ovine (Ovis spp.) cattle at the turn of the 19th to the 20th century. At present, bovine cattle are the most important grazers, while the numbers of sheep or goats are insignificant; and despite the presence of introduced red deer (Cervus elaphus) and wild boars (Sus scrofa), their grazing impact is negligible, except on game farms. All six selected sites are used for cow and calf ranching, where calves are sold at weaning. The grazing intensity is approximately one cattle unit per 5 ha, with a rotational grazing scheme in fields of 250- to 500-ha size; no fertilization or other improvements of natural vegetation have been used in these sites. The soil profile depth at the plains sites (Sites 3, 4, 5, and 6) was >1.50 and 2.00 m deep; A horizons were about 0.15 to 0.18 m deep and no difference between patches was observed.

sus was performed, specifying the relative abundance and cover of the different herbaceous species and of bare soil and litter; trees and shrubs were not taken into account.

Vegetation Data The obtained values for herbaceous species abundance and cover, bare soil, and litter were transformed into percentages and the proportions of each species as well as its relative importance (ni/Ni) were calculated. Species were separated according to preferred (forage) and avoided species, and the relative abundance of forage species (FS) was determined. The DI was determined according to the following equation: S

DI = −∑ pi log 2 pi

[1]

i=1

where pi is the relative abundance of the ith species, calculated as the ratio of individuals of a given species to the total number of individuals in the community (ni/N), ni is the number of individuals in the ith species, S is the number of species, and N is the total number of all individuals.

Soil Analyses After drying, the bulk density was determined on all soil samples before grinding them to pass a 2-mm sieve. The soil texture of each sample was determined by the hygrometer method of Bouyoucos (Gee and Bauder, 1986). Fractionation was performed by the method developed by Cambardella and Elliott (1993), as modified by Noellemeyer et al. (2006), which consists in shaking a 50-g soil sample in 200 mL of distilled water with three plastic beads for 6 h and subsequently wet sieving through a battery of sieves passing particles with 100- and 56-μm diameters. The C contents of the bulk soil (TOC) and size fractions were determined by oxidation with potassium dichromate in an acid medium at 120°C and colorimetric valuation (Soon and Abboud, 1991). Particulate organic C (>100 μm) was determined for all soil samples,

Sampling Scheme Sampling of Sites 1, 3, and 6 was performed during 2006. Two fields were selected at each site; in each field, vegetation and soil sampling was performed randomly at three points along three transects of approximately 100 m to create pseudo-replicates for statistical analysis. Thus, 18 sampling points each were established at every site for BC and UC patches, for a total of 36 points for each site. During 2007, Sites 2, 4, and 5 were sampled in a similar scheme, but at each site only one field was chosen, therefore totaling only nine sampling points each for BC and UC. A typical view of these vegetation patches is shown in Fig. 2. At each point, soil core samples were obtained at two depth intervals (0–6 and 6–12 cm) in bulk density cylinders of 471.2-cm3 volume; a vegetation cen- Fig. 2. Typical view of between canopy and under canopy vegetation patches in the Caldenal savanna. The photograph was taken at Site 3.


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Table 1. Shannon–Weaver diversity index (DI) and relative abundance of forage species (FS) of between-canopy (BC) and undercanopy (UC) vegetation patches at the six sites. Parameter

1 BC

2 UC

BC

3 UC

BC

4 UC

DI 1.9 ab† 1.5 c 1.6 c 0.56 e 1.9 ab 1.5 c FS 0.54 c 0.16 e 0.41 d 0.00 fg 0.21 e 0.01 fg † Different letters indicate statistically significant differences (P ≤ 0.05).

while IOC (100–50 μm) and FOC (70%, and in many cases >95%, representation in the plane. The analysis of this graph clearly showed a correspondence of plant species with sites. Site 1 corresponded with the following species: Acantholippia seriphioides (A. Gray) Moldenke, Aristida subulata Henrard, Aristida adscensionis L., Bromus brevis, Centaurea solstitialis L., Cynodon hirsutus Stent, Elionurus muticus (Spreng.) Kuntze, Gaillardia megapotamica (Spreng.) Baker, Halimolobus montanus O.E. Schulz, Melica bonariensis Parodi, Morrenia odorata (Hook. & Arn.) Lindl., Panicum urvilleanum Kunth, Parietaria debilis G. Forst, Poa lanuginosa, Schyzachyrium spicatum (Spreng.) Herter, Taraxacum officinale F.H. Wigg. aggr., Nicotiana noctiflora Hook., Verbena intermedia Gillies & Hook., Setaria leucopila (Scribn. & Merr.) K. Schum., Sporobolus cryptandrus (Torr.) A. Gray, Hyalis argentea, Solanum pyrethrifolium Griseb., and Solanum elaeagnifolium Cav., all of which are predominantly found in sandy areas. On the other hand, Site 6 showed correspondence with the following species: Ximenia americana L., Asclepias mellodora A. St.-Hil., Carduus nutans L., Acaena myriophylla Lindl., Euphorbia serpens Kunth, Setaria leiantha Hack., Chenopodium multifidum L., Trixis papillosa D. Don, Marrubium vulgare L., Gaya gaudichaudiana A. St.-Hil., Mionandra camarioides Gris., Evolvulus sericeus Sw., Schisnus fasciculatus Beauv., Lecanophora ecristata (A. Gray) Krapov., Portulaca oleracea L., Glandularia hookeriana Covas & Schnack, Turnera pinnatifida Juss. ex Poir., and Baccharis pingraea DC. Between both groups, we found the following species, which apparently corresponded with sites that had intermediate sand and clay contents: Digitaria californica, Bidens pilosa L., Eupatorium patens Hook. & Arn., Eragrostis cilianensis (All.) Vignolo ex Janch., Chenopodium album L., Nierembergia aristata D. Don, Ephedra triandra Tul., Stipa ichu, Stipa longiglumis Phil., and Piptochaetium napostaense. Sites 4, 5, and 2 showed specific correspondence with Baccharis gilliesii A. Gray, Baccharis ulicina Hook. & Arn., Stipa brachychaeta, Conyza bonariensis (L.) Cronquist, Lycium chilense Bertero, and Condalia microphylla Cav. To analyze how species were distributed according to vegetation patch, we chose Sites 1 and 6 as representative of textural extremes and due to the fact that the previous correspondence analysis had shown that the frequency of species differed markedly among them. We thus performed a multiple correspondence SSSAJ: Volume 74: Number 2 • March–April 2010


Fig. 3. Biplot of Axes 1 and 2 of the correspondence analysis.

analysis taking into account texture as well as vegetation patch. Axes 1 and 2 were chosen to present auto-values relatively proximate to 1 (0.76 and 0.71, respectively). These values represented the proportion of information explained by each dimension. Axis 1 was determined by the sand content of the site and therefore opposes Sites 1 and 6 (Fig. 4). The second axis was defined by vegetation patch, thus species that appear in the upper quadrants corresponded to shadow patches while those found in the lower quadrants belonged to BC patches. When the three variables were analyzed jointly, we found that the following species have higher frequencies in soils with high sand contents and in UC patches: Morrenia odorata, Parietaria debilis, Taraxacum officinale, Panicum urvilleanum, Melica bonariensis, Amaranthus crispus (Lesp. & Thévenau) A. Braun ex J. M. Coult. & S. Watson, Solanum pyrethrifolium, Solanum elaeagnifolium, Setaria leucopila, Conyza bonariensis, Lycium chilense, Baccharis ulicina, Halimolobus montanus, Oxalis cordobensis R. Knuth, Cynodon hirsutus, Lepidium bonariense L., Hyalis argentea, and Bromus brevis. The following species, in turn, corresponded with BC areas of the same texture: Gaillardia megapotamica, Stipa tenuis, Acantholippia seriphioides, Elionurus muticus, Sporobolus cryptandrus, Aristida adscensionis, Aristida subulata, Verbena intermedia, Schyzachyrium spicatum, Poa lanuginosa, Poa ligularis, Panicum urvilleanum, Nicotiana noctiflora, Carex bonariensis Desf. ex Poir., Relbunium richardianum (Gillies ex Hook. & Arn.) Hicken, Physalis mendocina Phil., Centaurea solstitialis, Baccharis crispa Spreng., Bothriochloa springfieldii, and Digitaria californica. On the other hand, in soils with low sand contents UC patches corresponded with Solanum meloncillo Parodi, Mionandra camarioides, Rhynchosia senna Gillies ex Hook. & Arn., Stipa brachychaeta, Stipa

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Fig. 4. Biplot of the multiple correspondence analyses, where Axis 1 represents the sand content of the site and Axis 2 the vegetation patch. 652

ichu, Pfaffia lanata Gibert., Chenopodium album, Gaya gaudichaudiana, Baccharis pingraea, Schisnus fasciculatus, Setaria leiantha, Setaria leucophila, Setaria pampeana Parodi ex Nicora, Ximenia americana, Marrubiun vulgare L., Alchemilla parodii I.M. Johnston, Salsola kali L., and Bidens pilosa; their BC counterparts were Baccharis gilliesii, Baccharis artemisioides Hook. & Arn., Ephedra triandra, Evolvulus sericeus, Glandularia hookeriana, Stipa longiglumis, Stipa tenuissima, Portulaca oleracea, Euphorbia serpens, Trixis papillosa, Lecanophora ecristata, Carduus nutans, Asclepias mellodora, Acaena myriophylla, Hordeum stenostachys, and Turnera pinnatifida. Intermediate species that showed an average frequency in both sites were Nierembergia aristata, Piptochaetium napostaense, Eragrostis cilianensis, Sphaeralcea crispa Hook. ex Baker, Trichloris crinita (Lag.) Parodi, Chenopodium multifidum, and Eupatorium patens.

Soil Properties The six sites represented a gradient of soil texture ranging from a minimum of 14.8% to a maximum of 60.2% clay + silt (Table 2) and, with the exception of Sites 5 and 6, all sites were significantly different with respect to this parameter. Site 3 had an intermediate clay + silt content of 45.2%; however, its clay content (14.5%) was higher than that of Sites 4 and 5 (9.5 and 11.6%, respectively). The latter two sites had the highest silt contents, with values of 42.3 and 47.5%, respectively. Sites 3, 5, and 6 showed differences between sampling depths, where the surface soil (0–6 cm) had lower clay + silt contents than samples taken at 6 to 12 cm (Table 3). This was related to higher clay contents in samples from the 6- to 12-cm depth. The clay contents were 9.6% (0–6 cm) and 13.6% (6–12 cm), and 17.3 and 19.8% for Sites 5 and 6, respectively, whereas the difference observed for Site 3 was not significant. Only at Site 4 did we find a difference of clay + silt contents between BC and UC patches; in this case, BC had a slightly finer texture than UC (Table 4) due to a higher clay content. A similar but not statistically significant trend was observed in Site 3. When analyzing the clay contents, how-


ever, we found significant differences between BC and Table 2. Soil texture, total organic C (TOC), and particulate organic C (POC) at the different sites; values are averaged across sampling depths UC patches at Sites 4, 5, and 6. In each case, BC patches and vegetation patches (n = 36). had higher clay contents than UC patches. At these Site Clay + silt Clay Silt Sand Textural class TOC POC sites, UC values were comparable with surface sample ——————— % ——————— — g kg−1 — clay contents, while BC values were similar to the 6- to 1 14.8 e† 4.3 e 10.5 e 85.2 a loamy sand 9.0 a 3.3 c 12-cm samples. 2 31.2 d 5.6 e 25.7 d 68.8 b sandy loam 16.8 abc 6.2 a Bulk density showed a tendency to decrease with 3 45.2 c 14.5 b 30.7 c 54.8 c sandy loam 14.0 c 3.7 c increasing clay + silt contents, especially in 0- to 6-cm 4 51.7 b 9.5 d 42.3 b 48.3 d loam 17.3 ab 4.9 b soil samples, while at 6 to 12 cm no clear pattern was ob- 5 59.0 a 11.6 c 47.5 a 41.0 e loam 17.7 a 4.9 b 6 60.2 a 18.6 a 41.6 b 39.8 e loam 16.3 bc 3.8 c served (data not shown). Large differences in bulk den† Different letters in the same column indicate statistically signifi cant differences sity were found between BC and UC patches, with much (P ≤ 0.05). higher values in BC samples (P ≤ 0.05). The mean values for BC surface samples were 1.16, compared with 0.95 Under-canopy patches had higher POC contents, but no for UC patches; at the 6- to 12-cm depth, these values were 1.24 strong relation with site or texture was observed. Mean values and 1.18 for BC and UC, respectively. for surface samples were 10.0 and 4.0 g kg−1 in UC and BC Differences in TOC among sites, patches and soil depths, patches, respectively, while at 6 to 12 cm these averages were 2.3 and interactions between site and patch, and site and depth were and 1.6 g kg−1, respectively. In the UC patches of the two sandidetected (P < 0.0001 for all cases). Mean TOC contents for all est sites, POC made up >40% of TOC (Table 4) and although sites are presented in Table 2. The highest TOC content was in UC patches the proportion of POC was higher than in BC found at Site 5 (17.7 g kg−1) and the lowest value corresponded patches at any site, it decreased with increasing clay + silt conto Site 1 (9.0 g kg−1). The correlation between TOC and clay tents. A strong positive relation between POC and TOC (r = + silt contents (n = 319) for all samples was significant (P < 0.83, P < 0.001) was found for BC and UC patches at the dif0.0001) but showed a very low regression coefficient (R2 = 0.25). ferent sites. In samples from the 0- to 6-cm depth, TOC contents ranged For both IOC and FOC, significant differences among from 23.1 to 11.3 g kg−1, and at 6 to 12 cm the maximum and sites, soil depths, and vegetation patches were found. In all minimum values were 14.3 and 6.7 g kg−1, respectively (Table three sites, FOC was considerably higher than IOC and lower 3). The range of difference between depths was between 1.4 and than POC in the sandy site, but similar to POC values in the 11.7 g kg−1, at Sites 3 and 2, respectively. finest texture (Table 5). The range of IOC was from 1.9 g kg−1 Under-canopy patches had the highest TOC contents, rangin Site 2 to 3.9 g kg−1 in Site 5 for surface samples, while the ing from 22.5 to 10.9 g kg−1, whereas for the BC patches this 6- to 12-cm depth samples had approximately half this amount. range was from 15.4 to 7.1 g kg−1 (Table 4). With the exception of This strong difference between sampling depths was not obSites 1 and 3, the difference in TOC contents between vegetation served in the case of FOC: in the sandy site no difference bepatches was significant. Under-canopy patches at Site 2 had twice tween depths was observed, and the greatest difference was the amount of TOC as BC patches, but in the soils with higher found in Site 5. There were significant differences among sites clay + silt contents these differences tended to diminish. in the surface samples, whereas at the 6- to 12-cm depth this On average, surface UC soils had 22.8 g kg−1 compared with did not occur. Vegetation patches had an inverse effect on these 14.6 g kg−1 in surface BC samples (P ≤ 0.05). At the 6- to 12-cm depth. the difference in TOC Table 3. Soil texture, total organic C (TOC), and particulate organic C (POC) for contents (12.3 and 11.1 g kg−1 for UC and BC, samples from 0- to 6- and 6- to 12-cm depths at the different sites; values are averaged across vegetation patches (n = 18). respectively) was not statistically significant. POC Particulate organic C showed a clear effect Site Depth Clay + silt Clay Silt Sand TOC POC proportion of vegetation patch and soil depth (data not cm —————— % —————— —— g kg−1 —— % of TOC shown). Marked differences between sites were 0–6 15.2 h† 4.4 e 10.8 e 84.8 a 11.3 c 5.3 c 47 found at the 0- to 6-cm depth, whereas at 6 to 12 1 6–12 14.4 h 4.1 e 10.3 e 85.6 a 6.7 d 1.3 d 19 cm the POC contents were not significantly dif0–6 31.6 g 5.5 e 26.2 d 68.4 b 22.6 a 10.8 a 48 ferent and ranged from 1.3 to 2.4 g kg−1 (Table 2 6–12 30.8 g 5.7 e 25.2 d 69.2 b 10.9 c 1.6 d 15 3). In the surface soil, this range was from 4.9 to 0–6 44.1 f 13.7 c 30.4 c 55.9 c 14.7 c 4.9 c 33 8.0 g kg−1, but no clear trend according to tex- 3 6–12 46.3 e 15.3 c 30.9 c 53.7 d 13.3 c 2.4 d 18 ture could be observed; however, a decrease in 0–6 51.7 d 9.6 d 42.1 b 48.3 e 22.0 a 8.0 b 36 4 the proportion of POC with respect to TOC in 6–12 51.7 d 9.4 d 42.5 b 48.3 e 12.7 c 1.9 d 15 0–6 57.5 c 9.6 d 47.9 a 42.5 f 23.1 a 7.9 b 34 finer textures was observed. While POC made 5 6–12 60.5 ab 13.6 c 47.0 a 39.5 gh 12.4 c 2.0 d 16 up almost half of TOC in the sandy sites, in the 0–6 58.8 bc 17.3b 41.4 b 41.2 fg 18.4 b 5.1 c 28 finer soils of Sites 5 and 6 this proportion was 6 6–12 61.6 a 19.8 a 41.8 b 38.4 h 14.3 c 2.4 d 17 around 30% in surface samples. † Different letters in the same column indicate statistically significant differences (P ≤ 0.05).


653

Table 4. Soil texture, total organic C (TOC), and particulate organic C (POC) in dynamics in Texas rangelands. Other studies between-canopy (BC) and under-canopy (UC) vegetation patches at the different showed that plant community responded princisites; values are averaged across the two sampling depths (n = 18).

pally to soil moisture regime and fertility (Solon et al., 2007). Some studies in the Kalahari transect (Caylor and Shugart, 2004; Ringrose et al., −1 ——————— % ——————— —— g kg —— % of TOC 1998; Skarpe, 1990) showed that the relative dis1 BC 14.8 f† 4.3 h 10.5 e 85.2 a 7.1 e 1.7 e 24 tribution of grasses and trees is directly related to UC 14.7 f 4.2 h 10.5 e 85.3 a 10.9 e 4.9 c 45 rainfall in an area with very homogeneous soils. 2 BC 31.2 e 6.2 h 25.1 d 70.1 b 11.1 de 3.0 cde 27 UC 31.3 e 5.1 h 26.2 d 70.0 b 22.5 a 9.5 a 42 Our study was concerned with herbaceous species 3 BC 46.1 cd 15.2 c 30.8 c 54.0 c 13.6 d 2.7 de 20 distribution in an area where the rainfall gradient UC 44.3 d 13.8 c 30.5 c 55.7 c 14.5 cd 4.6 c 32 was, at most, 150 mm yr−1 between the north4 BC 54.0 b 11.2 de 42.8 b 47.3 e 14.6 cd 2.8 de 19 east and southwest extremes, but with edaphic UC 49.5 c 7.7 fg 41.8 b 51.8 d 20.1 a 7.1 ab 35 variability among sites. Our results support the 5 BC 60.1 a 13.2 cd 46.9 a 41.2 fg 15.3 c 3.5 cd 23 hypothesis that in a semiarid environment with UC 58.0 a 10.0 ef 48.1 a 43.3 f 20.1 a 6.3 b 31 homogeneous rainfall distribution, soil texture 6 BC 61.4 a 20.1 a 41.3 b 38.6 g 15.4 c 3.0 cd 19 is the main factor that determines the floristic UC 59.0 a 17.1 b 41.9 b 41.1 fg 17.2 b 4.5 c 26 † Different letters in the same column indicate statistically significant differences (P ≤ 0.05). composition of any site. The textural control on herbaceous species distribution can be attributed C fractions in surface samples (Table 6): while IOC was higher to the soil’s hydraulic dynamics, which are determined by texture in UC than in BC (3.7 and 1.9 g kg−1, respectively), the higher (Quiroga et al., 1998, 1999). Sandy soils hold little water, but value of FOC was found in BC (10.8 vs. 8.0 g kg−1 in UC). this water is stored at higher matric potentials; the opposite ocNeither fraction showed differences between patches at the curs in finer loamy textures. In a semiarid environment with long depth of 6 to 12 cm. A gradient of FOC contents was observed, dry spells, this implies that sandy soils are often completely dry with the highest value at Site 5 with the lowest sand contents while loamy ones still hold some available water. This fact has and the lowest in the sandiest site (Site 2). been used as a possible explanation for root depth differences between trees and grasses, with peaks at 0.20 to 0.50 and 0 to 0.30 DISCUSSION m, respectively (Wang et al., 2007). Skarpe (1990) modeled the Effect of Texture on Vegetation and Species Diversity hydraulic behavior of Kalahari sands and found that most water The sites we selected for this study represented a textural was used in the upper 0.50 m, which was depleted by grasses a few gradient from loamy sand to loam. We found a clear association weeks after saturation. They postulated that climatic factors that between soil texture, specifically clay and sand contents, which condition water availability to roots at different soil depths affect the defined the principal axes of the correspondence analysis and distribution among herbaceous and woody vegetation, whereas our herbaceous species distribution. Sandy and loamy sites did not results showed that water availability as determined by soil texture share the same species, whereas another group of species was asalso affected the relative abundance of different herbaceous species. sociated with intermediate textures (Fig. 3). We also found an A common phenomenon that was observed in all sites was interaction between vegetation patches and texture (Fig. 4); spea lower diversity in UC patches (Table 1); on average, these had cies abundance was clearly distinct between vegetation patches a DI of 0.86, compared with 1.7 in BC. This could be associated in different textures. with a lower degree of disturbance by grazing in these patches, These results coincide with the concepts of Glasscock et al. which also leads to less bare soil (range 0–3% in UC compared (2005), who defined soil texture as the key determinant in dewith 11–40% in BC patches). Pelaez et al. (1992) described the fining plant community submodels for simulating vegetation interaction between woody and herbaceous species and found that higher densities of woody species and higher soil cover inTable 5. Carbon contents of 100- to 50- (IOC) and