Plant Soil (2009) 315:211–228 DOI 10.1007/s11104-008-9745-7
REGULAR ARTICLE
Carbon and nitrogen accumulation and microbial activity in Mount St. Helens pyroclastic substrates after 25 years Jonathan J. Halvorson & Jeffrey L. Smith
Received: 24 September 2007 / Accepted: 1 August 2008 / Published online: 29 August 2008 # Springer Science + Business Media B.V. 2008
Abstract Lupines (Lupinus lepidus var. lobbii) are important integrators of above and belowground development of Mount St. Helens (1980) pyroclastic substrates because they increase soil organic matter formation and microbial activity and influence other biotic processes. However, basic information is required to understand the unfolding pattern of soil development and to corroborate evidence for increasing rates of organic matter accumulation suggested by earlier work. Soil properties were measured in bare pyroclastic sites without lupines or other plants, under live and under dead L. lepidus. In 2005, pyroclastic substrates had low cation exchange capacity but appeared able to supply sufficient P for plant growth. Soil under live and dead lupines contained higher concentrations of soluble and total C and N, larger, more active microbial communities, more Bradford reactive soil protein and enzymes, and had higher mineralization potentials than bare soil.
Responsible Editor: Erik A. Hobbie. J. J. Halvorson (*) Appalachian Farming Systems Research Center, USDA-ARS, 1224 Airport Road, Beaver, WV 25813-9423, USA e-mail:
[email protected] J. L. Smith Land Management and Water Conservation Research Unit, USDA-ARS, Pullman, WA, USA
Comparatively, lupine soil was less dense and had lower C to N ratios and relative respiration rates. Soil microbial biomass-C, determined by substrate-induced respiration, had not increased under lupines since 1990, and was indistinguishable from hot water soluble-C, suggesting microorganisms were a predominant pool of labile-C in these developing soils. Evidence for an important soil glycoprotein, glomalin, in these pyroclastic substrates suggests it can form early during soil development. Concentrations of total C and N under lupines have risen to nearly 4 and 0.4 g kg−1 respectively since the 1980 eruption, but 2005 data indicate little change since 1997 and imply inputs from lupines into soil are in equilibrium with losses. Keywords Ecosystem engineer . Lupines . Soil organic matter . Mount St. Helens . Primary succession
Introduction Lupine (Lupinus lepidus var. lobbii) was the most important vascular plant colonizer of pyroclastic deposits during the 25 years following the May 18, 1980 eruption of Mount St. Helens, USA (del Moral and Rozzell 2005). As a legume, fixing nitrogen allowed it to establish on the infertile pyroclastic flows forming localized monospecific patches with high population growth rates and densities (Bishop et al. 2005; Halvorson et al. 1991a, 1992; Wood and Del Moral 1988).
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The role of N2-fixing colonists N like lupines in primary succession is complex and not yet completely understood (Vitousek et al. 2002). Lupines act as “ecosystem engineers” linking above and below processes and increasing biogeochemical heterogeneity (Halvorson et al. 1991b). These, in turn, can influence fauna and other plants (Gutiérrez and Jones 2006) and plant community structure (Holdaway and Sparrow 2006) by influencing productivity of subsequent colonists (Wright et al. 2004; Wright and Jones 2004). In general, lupines facilitate soil development and subsequent colonists (Gadgil 1971; Gosling 2005; Preisser et al. 2006). However, lupines may also negatively affect seedling establishment or growth (Morris and Wood 1989; Pickart et al. 1998); reduce biodiversity (Maron and Connors 1996); promote seed predators (Maron and Jefferies 1999), herbivores, or pathogens; or compete directly with plants (Bartelt-Ryser et al. 2005; De Deyn et al. 2004; Reynolds et al. 2003). Despite opposing mechanisms, L. lepidus is generally believed to enhance succession at Mount St Helens (del Moral 2007; del Moral and Rozzell 2005). Lupine ecology has been studied on Mount St. Helens pyroclastic flows to increase understanding of relationships between soil attributes and processes and ecosystem development. The direct effect of inputs of lupine fixed-C and N has been the accumulation of soil organic matter and more active and diverse soil microbial communities particularly after lupine death and subsequent decomposition (Halvorson et al. 2005; Ibekwe et al. 2007). However, others have linked lupines to ecosystem development through its ecophysiology, patterns of colonization and role on the subsequent floristic recovery, and interactions with herbivores and insects (e.g., Bishop et al. 2005; Braatne and Bliss 1999; del Moral et al. 2005). The objective of this study was to determine the cumulative impacts of lupines on biological, chemical, and physical properties in Mount St. Helens pyroclastic substrates, 25 years after the 1980 eruption. This information is needed to link early studies of soil development on the pyroclastic deposits to future studies of biotic and abiotic interactions that affect rates and patterns of soil processes, nutrient accumulation and soil ecosystem development. In comparison to earlier work (Halvorson and Smith 1995; Halvorson et al. 1991b), we hypothesized: (a) concentrations of soil C and N would still be significantly higher under lupines than in relatively bare locations; (b) strong distinctions
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between soil under living and dead lupines would be less evident because lupines have now undergone several population cycles of mass mortality and rapid reestablishment (Bishop et al. 2005); and (c) the rates of C and N accumulation and microbial activity have increased in pyroclastic substrates as suggested by recent studies (Halvorson et al. 2005).
Materials and methods Study sites and sample preparation Soil samples were collected in July 2005 on the Mount St. Helens pyroclastic flow deposits (N 46°14.8′ W 122° 9.9′), an area described in detail elsewhere (Swanson and Major 2005). Samples (n=10) were collected with a trowel at two depths, 0–5 (surface) and 5–10 cm (subsurface), in locations without lupines or other plants and from under living or dead L. lepidus (3 soil treatments×2 depths×10 reps=60 total samples). Although substantial fluctuations in the size, distribution, and turnover of lupine patches have occurred since L. lepidus colonists formed small dense colonies on bare pyroclastic substrates (del Moral and Rozzell 2005), these locations were selected to allow data to be compared with earlier studies that showed differences between live and dead lupines (Halvorson and Smith 1995; Halvorson et al. 1991b). Samples were sieved (2 mm), and a portion was maintained at field moisture levels for measurements of microbial biomass, aerobic mineralization, and enzyme activity. The remainder was dried at 55°C and stored in plastic bags at room temperature until analysis. Dried samples averaged less than 0.15% H20 by weight. Unconsolidated bulk density (BD) was estimated as the mass of air-dry soil needed to fill a container of known volume (25.225 cc). Samples were analyzed for selected chemical and biological properties thought to be important indicators of soil development and ecosystem functioning in Mount St Helens pyroclastic substrates. Composite samples were prepared for each soil typedepth by mixing an equal mass of each replicate together. Composite samples were assayed at the University of Kentucky, Soil Testing Lab for pH (1:1 soil: water), buffer pH (Sikora buffer, pH 7.5), Mehlich-III extractable-P (via ICP), Cation Exchange Capacity (ammonium acetate extraction), and particle size (macropipette
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method). Further details are available at http://soils.rs. uky.edu/index.php. Chemical properties Soil pH was determined for individual samples (1:1 soil: water). Total inorganic-N, or the sum of NH4+-N and NO3−N, was extracted from 10 g samples with 25 mL of 2 M KCl and analyzed using an auto-flow colorimetric procedure (Mulvaney 1996). Water soluble organic carbon (WSC), and nitrogen (WSN) were extracted using a sequential cool (23°C) and hot (80°C) water extraction procedure (Curtin et al. 2006; Ghani et al. 2003) and analyzed with a Shimadzu TOCVCPN analyzer equipped with a TNM-1 module (Shimadzu Scientific Instruments, Columbia, MD). Total soil-C and -N (TC, TN) were determined via dry combustion (Nelson and Sommers 1996). Natural abundance isotope ratios, δ13C and δ15N, were determined with an ANCA-GSL-HYDRA 20/20 mass spectrometer (PDZ Europa, Rudheath, UK) connected on-line to a CHN analyzer at the Stable Isotope Facility, University of California, Davis (http://stableisotopefacility. ucdavis.edu/). Carbon and N isotope results are reported in the standard delta (δ) notation as per mil deviations relative to PDB and air respectively. Biological properties Soil microbial biomass was estimated from substrateinduced respiration (see Bailey et al. 2002). Field moist soil samples (equivalent to 10 g of oven dry soil) were adjusted with water (15% by weight), and incubated in 40 ml glass vials equipped with septa for 6 days in the dark at 23°C. Samples were flushed with CO2-free air and amended with 600 μg of glucose (240 μg C) g−1 soil added as 0.5 ml of aqueous solution or with 0.5 ml of H20. Vials were resealed and incubated in the dark at 23°C. At 2 and 24 h, substrate induced respiration (SIR) was measured in the headspace of amended samples by gas chromatography with a Shimadzu 17A equipped with a TCD (Shimadzu Scientific Instruments, Columbia, MD). Basal respiration was similarly determined from the H20-amended samples. Soil microbial biomass-C (SIR-C) was estimated from 2 h SIR data following the approach of Anderson and Domsch (1978), and from 24 h SIR data, and the product formation equations developed by Smith et
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al. (1986), respectively. Cumulative basal respiration after 24 h of incubation were used together with the estimates of SIR-C to calculate metabolic quotients (qCO2) or the amount of CO2-C produced per unit of soil microbial biomass-C (Anderson and Domsch 1990; Insam and Haselwandter 1989). The microbial quotient, thought to be inversely correlated to the size and recalcitrance of soil organic pools, was calculated as the ratio of microbial biomass carbon to total soil carbon (Insam and Domsch 1988; Sparling 1992). Soil dehydrogenase and phosphatase activity were determined using the modified technique of Bolton et al. (1985). The substrate for dehydrogenase activity was 2,3,5-triphenyltetrazolium (3% w/v) and 25 mM p-nitrophenol phosphate for phosphatase activity. Enzyme assays were made in duplicate on field moist soil (10 g dry weight) and reported for dehydrogenase as mg triphenylformazan kg−1 soil (TPF) and for phosphatase as mg p-nitrophenol kg−1 soil (PNP). Bradford reactive soil protein (BRSP) was extracted from samples following the methods of Wright and Upadhyaya (1996, 1998) and used in subsequent studies as an operational definition of total glomalin (e.g., Lovelock et al. 2004b; Rillig et al. 2003). Glomalin, an important soil glycoprotein produced by arbuscular mycorrhizal fungi (AMF), is reported to promote soil aggregate formation and may represent a significant pool of stable soil organic matter (Nichols 2003; Rillig et al. 2001; Wander 2004). Soil samples (1 g) were autoclaved (127°C) for 60 min together with 8 ml of 50 mM sodium citrate buffer (adjusted to pH=8.0) centrifuged, and the supernatant containing glomalin was collected. The extraction process was repeated six times, and pooled extracts were centrifuged to remove soil particles, and protein concentration was determined by the Bradford assay using bovine serum albumin as standards (Bradford 1976). Short-term C and N mineralization Soil respiration and net aerobic nitrogen mineralization were measured during an 20-day static incubation following the methods outlined by Zibilske (1994). Moisture adjusted samples (10 g ODE, 15% w/w) in 40 mL glass vials equipped with septa were incubated in the dark at 23°C. After 4, 6, 11 and 20 days, CO2 in the headspace atmosphere was measured as above and samples were aerated and resealed.
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Carbon-mineralization potentials, the initial amount of readily decomposable carbon substrate in the soil (C0), and carbon-mineralization rate constants (k) were estimated from cumulative soil respiration curves. These were modeled assuming first order kinetics and using a nonlinear least-squares approach described by Smith et al. (1980). Mineralization potentials were evaluated as a percentage of total soil C and of cool water soluble-C. On days 10 and 20, a set of samples was sacrificed and extracted for inorganic-N as described above. Short-term net N mineralization was determined as the difference between 10 and 20-day total inorganic-N and expressed on a per day basis. We also measured NH4-N accumulation after a 8-day anaerobic incubation at 40°C (Bundy and Meisinger 1994; Waring and Bremner 1964). An index of potentially mineralizable N was determined as the difference in total soil inorganic-N before and after anaerobic incubations. Data analysis Data were analyzed by analysis of variance (ANOVA) using SAS 9.1 and PROC MIXED using a model that contained both fixed (soil type and depth) and random (sample location) effects (Littell et al. 1996; SAS 1999). The KR (Kenward-Roger) option was used to calculate degrees of freedom and the CS (compound symmetry) covariance structure was selected. Multiple pairwise comparisons of means from each depth were performed using protected LSD tests when main treatment comparisons were found statistically significant. An overall effect of lupines was determined by
comparing combined depth data from bare samples against the average of live and dead lupine samples with a single degree of freedom contrast. Assumptions of normality were confirmed by the Shapiro-Wilk test in PROC UNIVARIATE. Where appropriate, data were log10, square root or arcsine transformed to reduce heteroscedasticity before analysis. Similarities between selected variables were determined using paired t-tests and PROC TTEST. Homogeneity of linear models was determined using PROC REG. Mean values shown in the text and tables are arithmetic values followed by the standard error.
Results and discussion Chemical and physical properties Pyroclastic substrates were composed mainly of sand with a low cation exchange capacity (CEC) and a low percent base saturation indicating little capacity for holding cationic plant nutrients. Calcium and Mg were the most common cations (Table 1). Values of CEC were comparable to those reported for other primary succession sequences and should increase with time (He and Tang 2008; Kato et al. 2005). Buffer pH values were little changed from reference, >7.2, indicating pyroclastic substrates were low in total exchangeable or potential acidity (data not shown). These correlate to very low estimates of buffering capacity reported earlier (Halvorson 1989). Recent studies by Gill et al. (2006) suggest plant communities growing on Mount St. Helens pyroclas-
Table 1 Selected soil properties for composite samples of 2005 Mount St. Helens substrate Soil type
Depth CECa Particle size Exchangeable cations Base Pc b (cm) (cmolc/kg) (%) (mg/kg) K Ca Mg Na Sand Silt Clay (cmolc/kg)b (cmolc/kg)b (cmolc/kg)b (cmolc/kg)b (%) (%) (%)
Bare sites
0–5 5–10 0–5 5–10 0–5 5–10
Live L. lepidus Dead L. lepidus a
3.6 3.4 3.8 3.8 3.6 3.3
0.04 0.05 0.07 0.06 0.06 0.06
0.41 0.48 0.61 0.53 0.71 0.44
Cation exchange capacity
b
CEC units are cmolc/kg or centimols of charge per kilogram soil
c
Mehlich III extractable-P
0.12 0.15 0.17 0.19 0.19 0.15
0.06 0.07 0.09 0.07 0.08 0.06
17.4 21.8 24.1 22.8 29.0 21.6
41 37 58 38 59 51
84.8 88.1 76.8 80.4 81.3 74.7
8.4 7.2 20.3 16.4 17.4 22.0
6.8 4.7 2.9 3.2 1.4 3.3
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tic substrates, like other primary succession sites, could be limited by the availability of P (Walker and Syers 1976). However, plant available P in pyroclastic soil, determined by the Mehlich III method (Table 1), averaged about 42 mg kg−1, close to the range of 45–50 mg P kg−1 soil considered optimal for plant growth (Sims 2000a). Evidence for non-limiting concentrations soil P is consistent with early reports of appreciable amounts of P in new pyroclastic deposits as apatite (Kuntz et al. 1981; Nuhn 1987) or P2O5 (Lipman et al. 1981). While estimates of plantavailable phosphorus vary with methodology, nonlimiting quantities of plant-available P in pyroclastic soil were also indicated by average Bray-P values (17.3±1.1 mg kg−1, n=60) observed by Halvorson et al. (unpublished data). In contrast, Fagan et al. (2004) reported Olsen-P values, near 5 mg P kg−1 soil, suggesting low P availability for plants. However, the Olsen method is thought to be best suited for use in calcareous soils and may extract less P from most soils than other methods (Sims 2000b). Lupines colonizing the pyroclastic sites have been linked with increased soil pH (Ugolini and Dahlgren 2002), but pH did not differ among soil types in 2005. Average pH increased with depth (P2 mm and do not reflect compaction. Consequently we did not use them to convert gravimetric data into volumetric data before analysis (see Reganold and Palmer 1995). In addition, conversion to volumetric measurements implies a spatial uniformity that is misleading if associated with a larger area or volume of soil than the actual phenomena of interest. After more than 20 years, Bishop et al. (2005) found lupines directly affected relatively little of the pyroclastic area, thus their effects on soil development are likely to be localized at the spatial scale of the individual plant or patch (Halvorson et al. 1992, 2005) and local variations in BD may be important. While the overall average of BD was 1.26± 0.02 g/cm3, individual values ranged from 0.88 to 1.48. Perhaps more importantly, conversion to volumetric estimates provided little addition insight into the data since the average differences in BD between lupine and bare soil and the two depths were modest (0.2 g/cc). We found no change in our conclusions when we compared statistics calculated with volumetric or gravimetric forms of the data. Soil C and N Total inorganic-N did not vary among soil types but decreased with depth (P90%) in the NH4+ form (data not shown). These values correspond to about 2% and 4% of the total soil N pool respectively, slightly less than those reported earlier (Halvorson et al. 2005). Pools of inorganic-N in the pyroclastic sites
Patterns of accumulation of soil C and N since 1980
ANOVA calculated with log10-transformed data
Overall lupine effect as determined by a single degree of freedom contrast
have historically been small, less than 3 mg kg−1 soil, and similar for bare locations and under lupines (Fagan et al. 2004; Halvorson et al. 1991b). Lupine effects on patterns of soil organic matter remain evident. Significant interactions between soil type and depth were observed for WSC and WSN (P≤0.05). Surface concentrations of both were lower in bare soil than under live or dead lupines (Table 2). Conversely, there were no differences among soil types for either WSC or WSN at 5–10 cm. Both WSC and WSN decreased with depth under living and dead lupines but did not vary in bare soil. Main effects of soil type and depth were significant (P≤0.005) for total soil C and N. Average TC was higher in samples from under living (3,015±534) and dead lupines (2,249±531) than in bare soil (1,016± 240 mg kg−1) (Table 2). Total N was highest in soil from under dead lupines, about two times greater than under live lupines, and more than four times that in bare soil, 404±57>222±52>92±23 mg kg−1, respectively. Concentrations of TC and TN decreased with depth from 2,791±495 and 305±50 mg kg−1, respectively, to 1,396±203 and 173±34 mg kg−1. An interaction was found between soil type and depth for C to N ratios (P≤0.05). Ratios did not vary among soil types in surface samples, averaging about 10. However, subsurface ratios varied for each soil type, being highest in bare soil and lowest in soil from under dead lupines (Table 2). Carbon to N ratios increased with depth in bare soil but did not vary in soil under living or dead lupines.
b
BD Bulk density, TIN total inorganic N, WSC water-soluble C, WSN water-soluble N, TC total soil C, TN total soil N
a
Overall lupine effecta
5–10
Values represent means and standard errors of the means (n=10). Within each depth, differences among soil types are denoted by letters (capitalized when only main effects are significant). For each soil type, differences among depths are denoted by numbers (protected LSD; P≤0.05)
−0.7±1.1a,1 −0.7±0.5a,2 −0.9±0.8a,2 −0.1±1.1y,1 3.2±1.1x,1 2.6±0.8x,1 NS −27.1±0.2a,2 −26.9±0.2a,1 −27.3±0.1a,1 −26.7±0.2x,1 −27.1±0.1xy,1 −27.3±0.1y,1 NS 10±2b,1 21±5a,1 28±6a,1 7±3x,1 7±1x,2 12±2x2 Higher P≤0.05 Bare sites Live L. lepidus Dead L. lepidus Bare sites Live L. lepidus Dead L. lepidus 0–5
1.37±0.04a,1 1.18±0.04b,1 1.18±0.05b,1 1.37±0.05x,1 1.30±0.04x,2 1.16±0.05y,1 Lower P≤0.005
102±21b,1 234±46a,1 277±55a,1 80±24x,1 94±14x,2 145±23x,2 Higher P≤0.05 2.5±0.2A,1 2.8±0.3A,1 4.7±1.0A,1 2.0±0.2A,2 1.8±0.1A,2 2.5±0.4A,2 NS
1144±325B,1 3206±957A,1 4024±926A,1 889±366B,2 1292±270A,2 2006±345A,2 Higher P≤0.0005
110±31C,1 321±90B,1 483±83A,1 74±35C,2 122±31B,2 324±72A,2 Higher P≤0.0001
11±0a,1 10±1a,1 8±1a,1 16±2x,2 12±1y,1 9±2z,1 Lower P≤0.005
δ15N (‰) WSN (mg kg−1) Soil type
BD (g cm−3)
TIN WSC (mg kg−1)b (mg kg−1)
TC (mg kg−1)b
TN (mg kg−1)b
C/N (mg kg−1)
δ13C (‰)
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Depth (cm)
Table 2 Soil particle bulk density, total inorganic N, water-soluble C and N, total soil C and N, C:N ratios, and C (δ13C) and N (δ15N) isotope composition
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Developing volcanic soils are characterized, in part, by accumulation of organic matter (Shoji et al. 1993). Total C and N have increased from below detection limits in uncolonized substrates soon after the eruption (Engle 1983) to concentrations of more than 4,000 and 480 mg kg−1 respectively in surface soil under dead lupines (Table 2). After 25 years, total C and N increased in bare surface soil at an average annual rate of about 46 and 4.4 mg kg−1 soil respectively compared to 128 and 12.8 mg kg−1 soil under live lupines, and 161 and 19.3 mg kg−1 soil under dead lupines. These values correspond to annual C and N accumulation rates of 29±7 and 3±1 kg ha−1 in bare soil; 70±17 and 7±2 kg ha−1 under live lupines; and 88±17 and 11±2 kg ha−1 under dead
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lupines. In contrast to earlier trends reported by Halvorson et al. (2005), neither concentrations or volumetric estimates of total soil C and N have increased in pyroclastic sites since 1997 (Fig. 1). This lack of change could result from several mechanisms that decrease the quantity of inputs of C and N from lupines, increase losses associated with
microbial activity or change patterns of nutrient allocation from below to above ground (Cannell and Thornley 2000; Chapin et al. 1986; Field and Kaduk 2004). Reductions in inputs may be the result of insect herbivory or competition among lupines for resources that lower productivity (Bishop 2002; Fagan and Bishop 2000). Nitrogen fixation in lupines
Fig. 1 Trends of a gravimetric and b volumetric total soil carbon and c gravimetric and d volumetric soil nitrogen in lupineinfluenced (from under live or dead lupines) and bare pyroclastic soil. Values are averages with 95% confidence intervals. For bare samples n=30, 30, 11, 18, 30, and 10 for 1987, 1988, 1990, 1997, 2000, and 2005 respectively. For lupine-influenced
samples n=30, 30, 22, 28, 29, and 20 for 1987, 1988, 1990, 1997, 2000, and 2005 respectively. Data are derived from (Halvorson 1989; Halvorson and Smith 1995; Halvorson et al. 1991b, 2005). Volumetric calculations assume a bulk density of 1.2 g cm−3 for 1987, 1988, 1990; 1.3 g cm−3 for 1997 and 2000 data; and used values determined for individual samples for 2005
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may be diminished from rates recorded earlier in response to increased soil N or a shortage of cofactors (e.g., Mo, Fe or P) (Gill et al. 2006; Vitousek et al. 2002). Lupine inputs may be removed directly by herbivores (Bishop et al. 2005) or indirectly through linkages among herbivores, plants and soil biota that increase losses of C and N such as plant die back, increased root exudation or sloughing, or denitrification (Adema et al. 2005; Bardgett and Wardle 2003; Wardle et al. 2004). The ability for soil microorganisms to decompose organic inputs has increased under lupines as their numbers and diversity have developed in response to the quantity and quality of organic matter inputs (Halvorson and Smith 1995; Halvorson et al. 2005; Ibekwe et al. 2007). Accumulation of soil organic matter in developing ecosystems is linked to inputs of comparatively stable pools of C and N (Kaye et al. 2003), but decomposition in pyroclastic soils is likely to be proportional to the quantity of inputs because lupine biomass is not highly lignified (Gill et al. 2006; Halvorson and Smith 1995). Net migration of nutrients from soil to aboveground standing pools of plant biomass may indicate a shift in progressive stages of vegetative succession, from assembly to interaction, suggested by del Moral et al. (2005) or result from stochastic forcings thought to predominate at the pyroclastic site (Bishop et al. 2005; del Moral 2007). Unpredictable events such as weather patterns may have linked plant community dynamics, herbivory and microbial activity to create a pulse of C and N losses from the soil. Slowing accumulations of total soil C and N, shown in Fig. 1, commenced during a period of unusually wet conditions recorded for the pyroclastic site from 1995–1999 (http://www.climate. washington.edu/). These conditions foreshadowed a massive die-off of lupine stands in 2000 due to totricid root borers and noctuid cutworms followed by explosive regrowth of lupine populations (Bishop et al. 2005), presumably accompanied by a strong demand for soil N. C and N isotopes Generally, values of δ13C were consistent with inputs from C3 species (Hobbie and Werner 2004; O’Leary 1981) while δ15N values were low and comparable to nearby Lyman glacier, located at 48°10′ N, 120°53′ W, in the Cascade Mountains of Washington, USA
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(Hobbie et al. 2005) and to other volcanic sites (Schlesinger et al. 1998). Interactions between soil type and depth were observed for both δ13C (P≤0.01) and δ15N (P≤0.05). In surface soil, δ13C and δ15N did not vary among the soil types, averaging −27.1±0.1 and −0.78±0.4‰ (Table 2). In bare soil, δ13C was enriched with depth, a pattern consistent with other studies and thought related to isotope fractionation during decomposition or the accumulation of enriched microbial products (Boström et al. 2007; Ehleringer et al. 2000; Nadelhoffer and Fry 1988). However, there was no change in δ13C with depth under lupines suggesting inputs from lupine inhibit or obscure changes in the isotopic signature due to fractionation by soil microorganisms. The δ15N did not vary with depth in bare soil but was enriched under live or dead lupines by about 3.7‰. In subsurface soil, δ13C was enriched in bare soil compared dead lupines but δ15N was enriched under lupines compared to bare soil by about 3.0‰. In all soil types, δ13C varied inversely with the concentration of total soil C (Fig. 2a). Slightly enriched values observed in samples with the low organic matter, suggest non-lupine contributions to soil organic matter such as air deposition or development of refractory organic matter predominate in bare soil. Generally, δ15N decreased with increasing concentrations of total soil N (Fig. 2b). However, at very low soil N, the δ15N was depleted in several bare samples, in the range associated with atmospheric inputs, −10‰ to 0‰ (Nadelhoffer and Fry 1994). Nitrogen derived from N2 fixation by lupines would be expected to exhibit little or even slightly negative isotopic discrimination (−1‰ to −2‰) (BedardHaughn et al. 2003; Shearer and Kohl 1993; West et al. 2005). Thus, enriched δ15N values observed under the lupines at concentrations of N less than about 100 mg kg−1 are likely a legacy of microbial processes like nitrification and denitrification that increase δ15N in an open soil ecosystem with depth and age (Hogberg 1997; Martinelli et al. 1999; Nadelhoffer and Fry 1988). At higher concentrations of organic matter, values for δ13C and δ15N reflect the composition of lupines (Hobbie et al. 2005; O’Leary 1981; Shearer and Kohl 1993) and imply a more closed ecosystem in which the proportion of lupine inputs greatly exceeds the proportion of losses from the soil.
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a
b
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lupines, 120±27 and 160±20 mg kg−1, greater than the 60±11 mg kg−1 in bare soil (Table 3). Average SIR-C at the surface, 147±22 mg kg−1, was nearly twice that in subsurface soil. Metabolic quotient (qCO2) did not vary with soil type (P≤0.09) or depth but the average qCO2 was higher under living and dead lupines, 31.2±2.3, than in bare soil 25.1±3.2 as determined by single degree of freedom contrast (P≤ 0.05) (Table 3). These estimates of soil microbial biomass C were low compared to other ecosystems (Smith and Paul 1990) but comparable to values reported for other early successional sites such as comparably aged forefronts of receding glaciers (Ohtonen et al. 1999; Tscherko et al. 2003) and other volcanic sites (Schipper et al. 2001). There was little indication of change in soil microbial biomass since 1990 except in bare soil (Fig. 3). Surprisingly, concentrations of hot water soluble-C were just as suitable for estimating soil microbial biomass in pyroclastic soil as substrate induced respiration methods (Fig. 3) suggesting soil microorganisms were a predominant pool of labile-C in soil. Soil C and N recovered with cool water is believed to correspond to recent inputs into soil such as fertilizer, lime, manure, or soluble plant
Fig. 2 Isotopic composition for soil a carbon and b nitrogen in relation to log10 transformed total pool size
Soil microbial biomass-C Estimates of SIR-C derived from conceptually different approaches of Anderson and Domsch (1978) and Smith et al. (1986) were similar (paired t-test, P≤ 0.15) therefore data were averaged before ANOVA (See Fig. 3 below). Soil microbial biomass-C was found to vary with soil type (P=0.0006) and depth (P≤0.0001) with values observed under live and dead
Fig. 3 Comparisons of 2005 hot water extractable-C, soil microbial biomass-C determined by the equations of Anderson and Domsch (1978) and Smith et al. (1986), and the 1990 SIRC determined average of both methods (Halvorson and Smith 1995). Values represent averages and standard errors (n=11 for 1990 and 10 for 2005)
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Table 3 Soil microbial biomass-C, metabolic quotient, dehydrogenase activity, phosphatase activity and Bradford reactive soil protein Depth (cm)
Soil type
SIR-Cb (mg kg−1)
qCO2b,c
Dehydrogenase (mg TPF kg−1)
Phosphatase (mg PNP kg−1)
BRSP (mg kg−1)
0–5
Bare sites Live L. lepidus Dead L. lepidus Bare sites Live L. lepidus Dead L. lepidus
67±14B,1 175±47A,1 200±33A,1 53±18B,2 65±12A,2 120±16A,2 Higher P≤0.0005
23.9±2.2 33.5±3.5 28.9±2.4 26.3±6.3 26.8±1.7 35.7±7.8 Higher P≤0.05
4.6±0.9b,1 14.2±4.2a,1 15.5±4.5a,1 3.7±0.8x,1 3.0±0.4x,2 3.0±0.7x,2 Higher P
