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DIVISION S-7—FOREST & RANGE SOILS

Tree Species and Soil Textural Controls on Carbon and Nitrogen Mineralization Rates

^a Dep. Natural Resources and Environmental Management, Univ. of Hawaii-Manoa, 1910 East-West Rd., Honolulu, HI 96822
^b USDA-Forest Service, Rocky Mountain Research Station, 240 West Prospect St., Ft. Collins, CO 80526, and Graduate Degree Program in Ecology and Dep. Forest Science, Colorado State Univ., Ft. Collins, CO 80526
^c USDA-Forest Service, Rocky Mountain Research Station, 240 West Prospect St., Ft. Collins, CO 80526, and Graduate Degree Program in ecology, Colorado State Univ., Ft. Collins, CO 80526
^d Graduate Degree Program in Ecology and Dep. Forest Science, Colorado State Univ., Ft. Collins, CO 80523

* Corresponding author (giardina{at}hawaii.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Terrestrial ecosystem models assume that high quality litter leads to the formation of high quality organic C and N in mineral soil, and that increased soil clay content decreases soil C and N mineralization rates. Few studies in forests, however, have examined the effects of initial litter quality and clay content on C mineralization rates (g C kg^-1 soil C) and net N mineralization rates (g N kg^-1 soil N) in soil. We used 16-mo laboratory incubations of mineral soil sampled from stands of lodgepole pine (Pinus contorta Dougl. ex loud ssp. latifolia Englem. ex S. Wats.) and aspen (Populus tremuloides Michx.) that varied in clay content (70 to 390 g kg^-1 soil) to examine how soil C and N mineralization rates relate to initial litter quality and soil texture. Aspen litter quality (C/N = 52–71; lignin/N = 26) was higher than pine litter quality (C/N = 82–111; lignin/N = 40–57), but pine soils released an average of 238 g C kg^-1 soil C over 16 mo compared with 103 g C kg^-1 soil C for aspen soils. Higher microbial biomass (mg kg^-1 soil C) under pine also indicates that pine soil C was of higher quality than aspen soil C. Net N mineralization rates did not relate to species or to soil C mineralization rates, and neither C nor N mineralization rates were related to soil clay content.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
THE MINERALIZATION OF SOIL ORGANIC C and N are important processes regulating the functioning of natural and managed ecosystems (Tiessen et al., 1994; Johnson, 1995). Because most detrital C in the terrestrial biosphere is found in mineral soil (Jobbágy and Jackson, 2000), soil C and N mineralization rates also are predicted to control how terrestrial C storage responds to changes in climate or vegetation (Vegetation/Ecosystem Modeling an Analysis Project, 1995). Accurate modeling of soil C and N mineralization rates and their response to a changing environment therefore requires a detailed understanding of the factors that control these biogeochemical processes. The quality of detrital inputs and soil texture are hypothesized to control soil C and N mineralization rates (Wedin and Tilman, 1990; Bonde et al., 1992; Schimel et al., 1994), but relative to the decomposition of fresh litter, little is known about how these variables alter C and N mineralization rates in mineral soil (Bauhus et al., 1998). Few such studies have been conducted in forests where most of the Earth's soil C is stored (Jobbágy and Jackson, 2000).

Three competing hypotheses have been proposed for how initial litter quality influences litter decomposition rates and N release from decomposing litter. The first hypothesis suggests that litter decomposition and N release are positively related to initial litter quality (Fogel and Cromack, 1977; Berg, 1986). In early stages of litter decomposition, C/N may be the best predictor of mass loss and N release (Taylor et al., 1989), while lignin indexes become increasingly important in later stages of decay (Berg, 1986; McClaugherty and Berg, 1987). In the decay filter hypothesis (Melillo et al., 1989), differences in initial litter quality (lignin/N and lignin/cellulose) alter litter decomposition and N release rates in the early stages of litter decomposition. However, as substrate quality converges to some low value during decomposition, initial litter quality has a decreasing influence on late-stage decomposition rates, which instead are controlled by climate, soil texture, and exogenous sources of labile C and nutrients. A third hypothesis suggests that litter decomposition rates and N release may relate inversely to lignin and N-based estimates of initial litter quality. For example, high N content may actually retard litter decomposition rates later in the decomposition process, particularly if lignin levels are also high (Berg, 1986; Fog, 1988; Berg and Matzner, 1997). From these three hypotheses, litter quality can be expected to positively alter, negatively alter, or have no influence on C and N mineralization rates in mineral soil.

In addition to litter quality, clay content is hypothesized to alter soil C and N mineralization rates by binding with organic matter to form soil aggregates that protect soil C and N from heterotrophic soil organisms (Oades, 1988; Six et al., 1999). In situ, ¹³C field studies show that within a site where climate, litter quality, and biota vary minimally, soil C loss occurs more slowly in clay-sized than in sand-sized soil fractions (Bonde et al., 1992; Desjardins et al., 1994). Despite a near universal pattern of declining decomposition rates with decreasing particle-size, an inverse relationship between C mineralization rates and soil clay content across ¹³C field studies was not observed (Giardina and Ryan, 2000). In laboratory studies that control temperature and moisture, the effect of clay content on soil C mineralization is weak (Motavalli et al., 1994) or nonexistent (Srenson, 1981; Hassink et al., 1993; Scott et al., 1996). Evidence for a relationship between soil clay content and net N mineralization is also mixed. At a grassland site, net N transformation rates were lowest in high clay soils (Hassink et al., 1993), but at other sites, clay content did not influence transformation rates (Srenson, 1981; Motavalli et al., 1995).

The purpose of this study was to examine the effects of litter quality and soil clay content on C and net N mineralization rates in mineral soils sampled from two subalpine forest types of the central Rocky Mountains. We used 16-mo laboratory incubations of soil sampled from sites with contrasting litter quality inputs (pine or aspen) and a range of soil clay content (70–390 g kg^-1 soil) to test the hypotheses that C and N mineralization rates are higher in soils receiving inputs of high quality litter, but decrease as clay content increases. Because microbes mediate the effects of species type and clay content on soil C and N mineralization rates, we also examined bacterial and fungal biomass in soil.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Study Sites
We used differences in species litter quality and site soil texture (Table 1) as natural experiments to test our hypotheses. We selected six closed-canopy lodgepole pine and six closed-canopy aspen sites from among sites in northern Colorado, USA, previously examined by Stump and Binkley (1993). Based on traditional indexes of litter quality, aspen trees produce higher quality leaf litter (less lignin and more N) than do lodgepole pine trees. Aspen roots contain similar concentrations of lignin, but 50% more N than pine roots (Table 2). Further, aspen leaves and roots decomposed more quickly than pine material during 6-mo laboratory incubations (Stump and Binkley, 1993).

View this table: [in this window] [in a new window]   Table 2. Litter quality for aspen and lodgepole pine foliage and fine roots.

Soil Sampling, Analysis, and Incubation
Approximately 2 kg of mineral soil (0–15 cm) were collected from three randomly selected points in stands at each of the 12 sites originally studied by Stump and Binkley (1993). Prior to sampling, forest floor material was removed. The soils were composited by site, and stored for 60 d at 2°C without sieving before chemical analysis and incubation. Immediately prior to analysis and incubation, soils were sieved to 2 mm to remove rocks and roots. Moisture capacity of each soil was then determined by saturating 200 g of soil with water, letting soil drain freely for 24 h on cheesecloth, and weighing a subsample of wetted soil before and after oven drying at 104°C for 24 h. Twelve 200-g samples per soil type (144 in all) were then adjusted to moisture capacity. To improve drainage during leaching, 50-g (wet weight) subsamples of each soil were mixed with 40 g (dry weight) of acid washed quartz sand. The 90-g mixture was placed in Falcon Filter micro-lysimeter cups (Becton Dickinson, Model 7102, Franklin Lakes, NJ) to permit dilute nutrient leaching during the incubation without disturbing the soil, as described by Nadelhoffer et al. (1991). The soil-sand mixture was maintained at field capacity (90 g total weight) throughout the incubation by periodic additions of deionized water. Twelve laboratory replicates of each of the 12 soil types (144 in all) were incubated for 12 mo. From Months 13 to 16, only three of the 12 laboratory replicates per site were used because the other 9 laboratory replicates were used for another experiment.

Soil C mineralization rates were measured by placing microlysimeter cups plus soil into sealed 1-L glass jars with 2 M NaOH traps (Anderson, 1982). For the 16-mo incubation period, NaOH traps were sampled and replaced at 30-d intervals. After addition of 1.0 mL of 0.75 M BaCl₂, 1.0-mL subsamples of the NaOH were titrated with 1.0 M HCl to determine the quantity of NaOH neutralized by absorbed CO₂. Empty microlysimeter cups (blanks) were incubated concurrently throughout the 16-mo incubation period. Changes in base concentration caused by water absorption were estimated by weighing base traps before and after each incubation period and assuming that water absorption caused any increases in weight. Based on CO₂ production, the highest O₂ consumption rate observed was 0.006 mol 31 d^-1. Because a 1-L mason jar contains 0.015 mol O₂, the incubations should have been aerobic for the entire course of study. Incubation temperature was selected to match mean annual air temperature, and was held at 5 ± 1°C throughout the incubation.

Soil NH⁺₄ and NO^-₃ were initially extracted from a subsample of each soil by shaking 10 g of sieved, field-moist soil with 100 mL of 2 M KCl on a reciprocating shaker for 60 min. Extracts were settled for 30 min, and then passed through preleached (2 M KCl) #42 Whatman filters (Whatman International, Kent, England); extracts were stored frozen until analysis (Keeney and Nelson, 1982). To estimate net N mineralization rates and to limit the accumulation of NO^-₃ in soils, each of the soil incubations and blanks were leached at the end of Months 4, 8, 12, and 16 with 100 mL of dilute, N-free nutrient solution (Nadelhoffer et al., 1991), and samples were frozen until analysis. At the end of Month 16, a final 2 M KCl extraction was performed to remove any mineral N that had not been previously removed by the final dilute nutrient leaching. Sample extracts were analyzed for NH⁺₄and NO^-₃ on a Lachat Instruments AE Flow Injection Autoanalyzer (Lachat Instruments, Milwaukee, WI) according to Lachat Instruments QuikChem Method 12-107-06-2-A (1990) for NH⁺₄ and QuikChem Method 12-107-04-1-B (1992) for NO^-₃ and nitrite (NO^-₂). To estimate net N mineralization rates for the 16-mo incubations, we made the following calculations: net N mineralization rate equals the NH⁺₄ and NO^-₃ removed in the final KCl extract plus the NH⁺₄ and NO^-₃ removed during the four leaching events minus NH⁺₄ + NO^-₃ removed in the initial, time-zero KCl extract. We assumed no N was lost to dentrification.

Concentrations of C and N in soil were determined on a Leco 1000 CHN analyzer (Leco, St. Joseph, MI) by dry combustion. The Soil Testing Lab at Colorado State University determined soil clay content by hydrometer method (Gee and Bauder, 1986), pH by glass electrode in a 1:2 mixture with deionized water, and total soil P by NHClO₄ digestion followed by inductively coupled plasma-spectrometry (Kuo, 1996). At the end of the incubations, a direct count method was used to analyze the three remaining laboratory replicates of each of the 12 soil types for total and active fungal and bacterial biomass (Hendricks et al., 1998). All C, N, and P data are presented on an oven dried soil basis (24 h at 104°C). We express soil C and N mineralization rates as g C or N released per kg soil C or N. Monthly estimates of soil C mineralization are based on soil C content for that month rather than on initial soil C content; the quantity of soil C released during previous months was subtracted from initial soil C. Cumulative release rates are based on soil C content at the beginning of the measurement period in question.

Statistical Analyses
Twelve laboratory replicates for Months 1 to 12 and three laboratory replicates for Months 13 to 16 were used to estimate measurement means for the 12 sites. We used analysis of covariance with species type (aspen or pine) as a fixed factor (n = 6) and clay content as the covariate to examine the effects of initial litter quality and clay content on soil C and N mineralization rates, extracted N pools, and microbial biomass (SPSS, version 8.0, Chicago, IL). Patterns of N release during the leaching events were analyzed by repeated measures ANOVA with species and clay content as fixed factors; soils with <18% clay were classified as low clay, while soils with >18% clay were classified as high clay. Relationships among soil clay content, total C and N content, C and N mineralization rates, and microbial biomass were analyzed by means of simple and multiple linear regression. Data were log transformed when the assumption of homogeneity of variance was not met. In all comparisons, = 0.05 was used to protect against Type I errors.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Soil Carbon Mineralization Rates and Soil Carbon Content
Throughout the incubation, C mineralization rates (g C kg^-1 soil C) were significantly higher in pine than in aspen soils (Fig. 1). At the end of the first 152 d of incubation, cumulative soil C release for pine was 136 g C kg^-1 soil C compared with 51 g C kg^-1 soil C for aspen (P < 0.01; Fig. 2). Differences between species persisted throughout the incubation; from Day 377 to 483, cumulative soil C release for pine was 22 g C kg^-1 soil C compared with 12 g C kg^-1 soil C for aspen (P < 0.01; Fig. 2). By the end of 16 mo, total soil C release for pine was 238 g C kg^-1 soil C compared with 103 g C kg^-1 soil C for aspen (P < 0.01). In contrast, C loss kg^-1 soil over 16 mo did not differ between species (pine: 4.58 g C kg^-1 soil; aspen: 6.14 g C kg^-1 soil; P = 0.74).

View larger version (21K): [in this window] [in a new window]   Fig. 1. Means with standard errors for soil C mineralization rates (g C kg-1 soil C 31 d-1), which is total soil C release normalized for initial differences in soil C content and adjusted each month for losses of soil C in previous months (n = 6).

View larger version (21K): [in this window] [in a new window]   Fig. 2. Means with standard errors for cumulative C release (g C kg-1 soil C) from pine and aspen soils (n = 6). (A) shows cumulative soil C release during the first 152 d of incubation. (B) shows cumulative soil C release during the last 130 d of incubation.

View larger version (29K): [in this window] [in a new window]   Fig. 3. (A) Relationship between total C release (g C kg-1 soil C) over 16 mo and soil clay content (g kg-1 soil) for pine and aspen soils (n = 6); (B) Relationship between soil C content (g C kg-1 soil) and soil clay content (g kg-1 soil) for pine and aspen soils (n = 6). Regression equations are from linear regression analyses.

View larger version (24K): [in this window] [in a new window]   Fig. 4. (A) Means with standard errors for 16 mo cumulative release of NH+4 (g N kg-1 soil N) from mineral soil sampled from pine and aspen stands during the four leaching events (n = 6); (B) Cumulative release of NO-3 (g N kg-1 soil N); (C) Cumulative release of total mineral N (g N kg-1 soil N).

View larger version (23K): [in this window] [in a new window]   Fig. 5. Means with standard errors for total quantities of active bacterial, total bacterial, active fungal, and total fungal biomass (mg kg-1 soil C) in pine and aspen soils (n = 6). Total fungal biomass was divided by 100 to fit the data on the graph.

View larger version (22K): [in this window] [in a new window]   Fig. 6. Relationships between the ratio of active fungi/active bacteria (kg kg-1) and soil clay content (g kg-1 soil) for pine and aspen soils (n = 6), across gradients of soil clay content. Regression equations are from linear regression analyses.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Our results are consistent with long-term litter decomposition studies showing that rapid decomposition rates for high quality litter are typically not sustained. In later stages of litter decay, decomposition rates for high quality litter may slow down more rapidly than do rates for low quality litter (Berg, 1986; Prescott et al., 2000). For example, Berg (1986) found that low N content Scot's pine (Pinus sylvestris L. Dict. Gard) litter decomposed more slowly in the first two years than high N content Scot's pine litter, but after two years, decomposition rates for the high N content litter declined while decomposition rates for low N content litter continued unchanged. By Year 3, total mass loss and lignin mass loss for low N content litter had surpassed losses for high N content litter (Berg, 1986). Similarly, Prescott et al. (2000) found that first year mass loss for lodgepole pine litter was lower than for aspen litter (30% and 40%, respectively), but by Year 2, differences had disappeared (50%), and by Year 4, mass loss for pine had surpassed aspen (70% and 60%, respectively). Overall, high quality litter from broad leaved trees decomposed more quickly in the first two years compared with conifer litter, but in subsequent years, mass loss for conifer litter often surpassed broad leaved litter (Prescott et al., 2000). In line with these results, Kaye et al. (2000) reported that soil C of grass origin decomposed more slowly when sites were planted to Albizia falcataria (L.) Fosberg, a N-fixing tree that produces high quality litter, than when planted to Eucalyptus saligna Sm., a similarly fast-growing tree that produces relatively low quality litter. Berg (1986) hypothesized that inverse relationships between litter quality and decomposition were caused by: high N availability inhibiting lignase production or activity; low quality lignin occurring in overall high quality litter; or, decomposition of high N litter leading to the formation of organic matter that is more resistant to decay than organic matter formed from low N content litter.

At the end of our incubations, pine soil C supported microbial communities with a higher proportion of active fungi to active bacteria (Fig. 6). Because certain groups of fungi decompose low quality substrates more effectively than do bacteria (Paul and Clark, 1996), the fungi dominated communities in pine soils may decompose the low quality remains and byproducts of litter decomposition more effectively than the bacteria dominated communities in aspen soils. Conversely, high quality aspen litter may select for organisms that are adapted to higher quality litter, but would do poorly with the low quality remains and byproducts of aspen litter decomposition. Hence, the apparent negative effect of high litter quality on soil C mineralization rates may reflect the influence of tree species on the composition of soil microbial communities.

The C/N of soil did not explain species differences in soil C mineralization rates. Soil C/N did not differ between species (pine: 20.4; aspen: 17.4, P = 0.81), and was unrelated to both C/N of initial litter (P = 0.75; Tables 1 and 2) and soil C mineralization rates (pine: R² = 0.33, P = 0.23; aspen: R² = 0.07, P = 0.60). While soil C/N has been used to index soil C quality (Parnas, 1976; Schimel et al., 1994), soil C/N was a poor predictor of C mineralization rates across our sites.

Soil Clay Content, Soil Carbon Mineralization Rates, and Soil Carbon Content
In line with previous studies (Srenson, 1981; Hassink et al., 1993; Motavalli et al., 1994; Scott et al., 1996), soil C availability was unrelated (aspen) or weakly related (pine) to soil clay content. Variation in climate is unlikely to have masked a relationship between soil C mineralization rates and clay content because climate was similar across sites (Stump and Binkley, 1993). Secondary indexes of soil C quality (microbial biomass and composition) showed mixed results. After 16 mo of incubation, most measures of microbial biomass were unrelated to soil clay content, suggesting that C quality did not vary with clay content (Bauhus et al., 1998). In contrast, analysis of covariance showed that the ratio of active fungi/active bacteria increased with clay content (P = 0.03; Fig. 6), suggesting lower soil C quality in high clay soils.

Despite a weak relationship with soil C mineralization rates, soil clay content was a strong predictor of soil C content across our sites (Fig. 3B). These findings appear contradictory, but soil clay content can influence plant productivity (Pastor et al., 1984), and higher soil C content in high clay soils may represent an effect of clay on detritus production rather than an effect on soil C mineralization rates. For example, soil texture can influence water holding capacity and nutrient availability, both of which can influence plant productivity. Similarly, Srenson (1981) found that while high clay soils retained more ¹⁴C labeled cellulose than low clay soils, with differences established in the first 10 d of incubation, soils that retained more cellulose also mineralized proportionally more cellulose over the 4 yr incubation.

Net Nitrogen Mineralization Rates
Large differences between pine and aspen litter quality did not lead to species differences in soil C/N or net N mineralization rates (P = 0.81 and P = 0.68, respectively). These results are consistent with the decay filter hypothesis (Melillo et al., 1989): early in the decomposition process, low quality litter will release less N than high quality litter because available nutrients are immobilized more rapidly by microbes decomposing low quality, nutrient poor litter. As the quality of diverse litter types converges during decomposition, the influence of initial litter quality on N release should also decline. In contrast to soil N mineralization rates, NH⁺₄ was leached at higher rates from pine than aspen soils (Fig. 4A), possibly because abiotic fixation or microbial competition for NH⁺₄ were greater in aspen soils. Overall, differences in leached NH⁺₄ were small relative to mineralization rates. Cumulative N release increased more rapidly in pine than in aspen soils, suggesting that species differences in N mineralization rates may have emerged had the incubations continued.

Previous studies have shown mixed effects of initial litter quality on net N mineralization rates. In a common garden experiment with perennial grasses, increasing litter quality led to increased net N mineralization rates kg^-1 soil N (Wedin and Tilman, 1990). In a common garden study with trees, Scott (1998) found that soils under European larch (Larix deciduo P. Mill.), which had the highest leaf litter and root quality of five trees examined, had the highest net N mineralization rates kg^-1 soil N. In contrast, red oak soils had moderate N mineralization rates, but red oak roots and leaf litter were lowest and second lowest in quality.

The decay filter hypothesis predicts that soil texture, climate, and new sources of labile C will control N release in later stages of litter decomposition (Melillo et al., 1989). Across our sites, soil N mineralization rates did not relate to soil clay content, despite constant temperature and moisture during the incubation, and no inputs of labile C. Clay content did not influence quantities of N leached from soil, and analysis of repeated measures showed that clay content did not alter patterns of N release (P = 0.23). Our findings are consistent with two long-term studies reporting no relationship between N mineralization rates and soil clay content (Srenson, 1981; Motavalli et al., 1995), but conflict with a grassland study reporting lower N mineralization rates in high clay soils (Hassink et al., 1993).

Most N in soil is covalently bound to soil C, and the fate of soil N is assumed to follow that of soil C (Schimel et al., 1994; Paul and Clark, 1996). In our study, N mineralization rates were unrelated to soil C mineralization rates. That our estimates of N mineralization represent net rates (i.e., the balance of gross mineralization and immobilization processes in soil) while estimates of soil C mineralization represent gross rates, could explain the poor relationship between N and C mineralization rates. Hart et al. (1994) showed that gross N transformation rates can vary closely with CO₂ release during long-term incubations (r² = 0.97), but that net N mineralization rates can correlate poorly with gross N transformation rates. If soil C mineralization rate is a good predictor of gross N transformation rates across our soils, then gross N transformations rates would be higher in pine than in aspen soils, but would not vary with soil clay content.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Based on short-term litter decomposition studies (1 to 2 yr), ecosystem models identify C/N and lignin/N as important predictors of soil C formation and mineralization. Our results suggest, however, that these relationships may be relevant only over the time scales from which they were developed. For the pine and aspen soils compared here, low quality litter resulted in high quality soil C, and faster mineralization of pine C appears to be the explanation for the lower C content of pine soils. Similarly, while large-scale relationships between soil C and clay content are used to make inferences about how soil C mineralization rates are altered by soil clay content, we found that despite a strong relationship between soil C and clay content, clay was a weak predictor of soil C and N mineralization rates. While in agreement with previous incubation and cross-site comparison studies examining the effects of soil clay content on C mineralization rates, these results conflict with ¹³C-based particle-size fractionation studies at individual sites reporting a strong influence of soil texture on C turnover. Overall, our results suggest that current assumptions about litter quality and soil textural controls on soil C formation and turnover need to be reevaluated.

	ACKNOWLEDGMENTS

 
We thank Rudy King for helpful advice in design and analysis of the study, Neal Scott for help with modification of the microlysimeters, Ingrid Dckersmith for review of the manuscript, and Chuck Troendle for obtaining financial support for C.P. Giardina. The work was supported by the USDA-Forest Service.

Received for publication September 12, 2000.

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