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Potential net soil N mineralization and decomposition of glycine-13C in forest soils along an elevation gradient
Soil Biology & Biochemistry 36 (2004) 1491–1496 www.elsevier.com/locate/soilbio
Potential net soil N mineralization and decomposition of glycine-13C in forest soils along an elevation gradient Charles T. Garten Jr.* Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Mail Stop 6038, Oak Ridge, TN 37831-6038, USA Received 18 March 2003; received in revised form 9 April 2004; accepted 23 April 2004
Abstract The objective of this research was to better understand patterns of soil nitrogen (N) availability and soil organic matter (SOM) decomposition in forest soils across an elevation gradient (235 – 1670 m) in the southern Appalachian Mountains. Laboratory studies were used to determine the potential rate of net soil N mineralization and in situ studies of 13C-labelled glycine were used to infer differences in decomposition rates. Nitrogen stocks, surface soil (0– 5 cm) N concentrations, and the pool of potentially mineralizable surface soil N tended to increase from low to high elevations. Rates of potential net soil N mineralization were not significantly correlated with elevation. Increasing soil N availability with elevation is primarily due to greater soil N stocks and lower substrate C-to-N ratios, rather than differences in potential net soil N mineralization rates. The loss rate of 13C from labelled soils (0– 20 cm) was inversely related to study site elevation (r ¼ 20:85; P , 0:05) and directly related to mean annual temperature (þ0.86; P , 0:05). The results indicated different patterns of potential net soil N mineralization and 13C loss along the elevation gradient. The different patterns can be explained within a framework of climate, substrate chemistry, and coupled soil C and N stocks. Although less SOM decomposition is indicated at cool, high-elevation sites, low substrate C-to-N ratios in these N-rich systems result in more N release (N mineralization) for each unit of C converted to CO2 by soil microorganisms. q 2004 Elsevier Ltd. All rights reserved. Keywords: Soil organic matter; N availability; Climate change; C-to-N ratios; Litter chemistry; Glycine
1. Introduction Many studies indicate that both soil respiration (Raich and Schlesinger, 1992; Kirschbaum, 1995; Raich and Potter, 1995) and the decomposition of labile soil organic matter (SOM) (Trumbore et al., 1996; Garten et al., 1999; MacDonald et al., 1999; Kirschbaum, 2000) increase with temperature. Meta-analysis also shows that warmer temperatures increase annual fluxes of net soil N mineralization (Rustad et al., 2001). Because mean annual temperature (MAT) declines with increasing elevation (Shanks, 1954), one might expect both decomposition of SOM and net soil N mineralization to decrease with increasing altitude. However, studies in the southern Appalachian Mountains indicate increasing forest soil N availability with increasing elevation (Van Miegroet et al., 1992; Garten and Van Miegroet, 1994). These observations are contrary to an * Tel.: þ1-865-574-7355; fax: þ 1-865-576-8646. E-mail address: [email protected] 0038-0717/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2004.04.019
expected inverse association between temperature and net soil N mineralization that has been reported along other elevation gradients (e.g. Joshi et al., 2003). Some studies in mountainous terrain also indicate annual fluxes (kg N ha21) and rates (year21) of soil N mineralization can exhibit different patterns with a change in elevation (Powers, 1990; Bohlen et al., 2001). In earlier studies, it was hypothesized that changes in natural forest soil C isotope ratios, C concentrations, and C-to-N ratios in the southern Appalachian Mountains are interrelated because of temperature and litter chemistry controls on SOM decomposition (Garten et al., 2000). The objective of this research was to better understand patterns of soil N availability and SOM decomposition across an elevation gradient in the southern Appalachian Mountains in west Tennessee, USA. For the purpose of the research, laboratory studies were undertaken to determine the potential rate of net forest soil N mineralization and in situ studies of 13C-labelled glycine were used to infer site differences in decomposition rates. The working hypothesis
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was that rates of both net soil N mineralization and glycine2- 13C decomposition would decline with increasing elevation or decreasing mean annual temperatures.
Table 2 Nitrogen concentrations, soil column N content, potentially mineralizable soil N ðN0 Þ; and potential net N mineralization rate (k) in soils from the six study sites Depth (cm) Parameter
LP1
WB2
SB3
MH4
BB5
SP6
2. Materials and methods 0 –5
mg N g21 soil Soil N (mg) N0 (mg N) k (d21)
1.2 6.0 1188 0.025
1.3 6.5 799 0.025
5.3 26.5 2655 0.024
3.9 19.5 3413 0.019
4.2 21.0 3924 0.029
4.7 23.5 3797 0.029
5 –20
mg N g21 soil Soil N (mg) N0 (mg N) k (d21)
0.6 2.9 603 0.024
0.3 1.8 402 0.022
2.4 12.0 1569 0.034
1.3 6.5 1878 0.023
1.5 7.5 1747 0.031
1.0 5.0 458 0.022
2.1. Study sites During 1995, six study sites (Table 1) were located in 50 þ year-old forest ecosystems along an elevation gradient in the southern Appalachian Mountains. Climates, soil properties, and vegetation at the six study sites were described by Garten et al. (1999, 2000). The dominant overstory tree species at each site were as follows: LP1, loblolly pine (Pinus taeda); WB2, yellow poplar (Liriodendron tulipifera) and red maple (Acer rubrum); SB3, Carolina silverbell (Halesia carolina) and eastern hemlock (Tsuga canadensis); MH4, mixed oaks (Quercus spp.), and mixed maples (Acer spp.); BB5, red spruce (Picea rubens) and beech (Fagus grandifolia); and SP6, mature red spruce. Soil samples were collected from the six study sites on 10 different occasions from 1995 to 1997 (Garten et al., 1999). Soil samples from the 0 to 5 and 5 to 20 cm depth were airdried, crushed, and weighed to estimate soil density (g cm23). Soil density was based on the known volume of the sample (cm3) and the mass of dry soil that passed a 2 mm sieve (g). Soil , 2 mm was ground and homogenized in a sample mill and analyzed for total C and N (elemental analysis by combustion methods). Soil N stocks (g N m22) at each study site were calculated as the product of N concentration (g N g21 soil), soil density (g soil m23), and increment depth (m). Stocks in the 0– 5 and 5 –20 cm increments were summed to calculate total N stocks over the surface 20 cm of mineral soil. 2.2. Nitrogen mineralization studies Potential net N mineralization in soils from the six study sites was measured using an aerobic soil column leaching experiment. Detailed methods associated with the technique are described in Campbell et al. (1993) and are briefly summarized here. The method permits calculation of both Table 1 Elevation, mean annual temperature (MAT), mean (^ SE) mineral soil N stocks to a 20 cm soil depth ðn ¼ 10Þ; sand, and silt content at the six forest sites 22
Study site
Elevation (m)
MAT (8C)
gNm
LP1 WB2 SB3 MH4 BB5 SP6
235 335 940 1000 1650 1670
12.6 13.1 10.3 10.5 6.2 6.4
186 ^ 9 136 ^ 10 451 ^ 20 220 ^ 16 328 ^ 30 280 ^ 17
%Sand
%Silt
13 33 33 21 40 52
73 57 55 63 45 36
All data except soil N stocks are from Garten et al. (1999).
potentially mineralizable soil N and the potential rate of net soil N mineralization. Mineral soil, collected in early 1996, over 0– 5 and 5 – 20 cm depths at each site was used in the soil column leaching experiments. Replicate columns (13 cm long £ 2.6 cm dia.) were prepared using 5 g of air-dry soil (, 2 mm) mixed with 25 g of clean, washed beach sand [illustrated in Campbell et al. (1993)]. The initial total soil N in each column (mg N) was calculated based on N concentrations (elemental analysis using combustion methods) in the 0– 5 and 5 –20 cm deep soil samples from each study site (Table 2). The soil columns were leached using 100 ml of 10 mM CaCl2, followed by 25 ml of a N-free nutrient solution (Campbell et al., 1993), at the beginning of the experiment and after 15, 29, 48, 64, 112, 137, and 193 d of laboratory incubation at room temperature (21 8C). Excess leachate was removed under vacuum (60 mm Hg). Clear leachates from the soil columns were analyzed for NH4 – N and NO3 – N concentrations (mg N ml21) colorimetrically using a Bran þ Luebbe autoanalyzer (TRAACS Model 800 or AA3). Total N leached (mg NH4 – N þ mg NO3 –N) from each soil column was calculated as the product of leachate volume (ml) and concentration (mg N ml21). According to the principles of this method (Campbell et al., 1993), some part of the initial total soil N in each column is potentially mineralizable and the remainder is resistant soil organic N. The relationship between net soil N mineralized (Nmin ; mg N), as inferred from cumulative N leached, and time (t; days) is described by a nonlinear equation Nmin ¼ N0 ð1 2 expð2ktÞÞ
ð1Þ
where N0 is the potentially mineralizable soil N (mg N) and k (d21) is the rate of net soil N mineralization. Parameters N0 and k were derived from nonlinear regressions of cumulative inorganic N leached versus time. Relatively large amounts of N may be leached on the first day of the incubation due to a release of N from microorganisms that died when soil samples were air-dried. To correct for this potential artifact, the mass of N leached on the first day from
C.T. Garten Jr. / Soil Biology & Biochemistry 36 (2004) 1491–1496
the soil columns was subtracted from amounts leached on all subsequent days. The data transformation set the initial amount of N leached from the soil columns to zero at each study site. 2.3. Decomposition of
13
C-glycine
Following removal of the O-horizon, mineral soil (0 – 20 cm) was sampled from the six forest sites using a bucket auger in late 1996. The soil was air-dried, sieved (2 mm), and thoroughly homogenized. The homogenized, air-dry soil from each site was packed into 24 butyrate, plastic tubes (20 cm long £ 2.4 cm dia.) to a mass density of 0.9 –1.2 g cm23. In the field, glycine-2-13C (99 at.%) was added to half the tubes using enough water to sufficiently wet the soil without drainage. The amount of 13C added to each tube varied between 4 and 21 mg depending on the soil C concentration. Tubes containing glycine-2- 13C and paired controls (unlabelled) were buried flush with the surface of the ground at each study site. The tubes were covered with a fine mesh nylon screen to exclude invertebrates and debris. A labelled soil tube and its paired control were removed from each site at the beginning of the experiment and at approximately 1, 2, 4, 10, 13, and 18 months following addition of the isotope. In the laboratory, soils were extruded from the recovered plastic tubes and air-dried to a constant weight at room temperature (21 8C). A subsample of the air-dried soil was ground and homogenized using a mortar and pestle. The finely ground subsample was used for analysis of soil C and N concentrations (elemental analysis by combustion methods) and analysis of 13C-to-12C ratios. Methods used for the analysis of stable C isotopes were described by Garten et al. (2000). Soils were combusted in sealed glass tubes containing copper oxide wire and reduced Cu. Carbon dioxide was separated from other combustion products by cryogenic distillation on a vacuum line following sample combustion. Carbon isotope ratios were measured using a dual inlet stable isotope ratio mass spectrometer along with reference materials from the National Institute of Standards and Technology (Gaithersburg, MD). The fraction of 13C remaining (Y) in the six forest soils as a function of time was described as a one-phase exponential decay Y ¼ A expð2k0 tÞ þ B
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3. Results 3.1. Nitrogen mineralization studies High-elevation sites (BB5, SP6) had greater soil N stocks (0 –20 cm) than low-elevation study sites (LP1, WB2). However, the correlation between N stocks and elevation was not statistically significant because of a large variation in soil N between two mid-elevation study sites (SB3, MH4) (Table 1). Surface soil (0 – 5 cm) N concentrations for samples used in the laboratory incubations (Table 2) tended to increase with study site elevation (r ¼ þ0:81; P , 0:10). Soil from the 0 –5 cm depth increment released more N over the course of the incubation than soil from the 5 to 20 cm depth increment. Potentially mineralizable N ðN0 Þ and the potential rate of net N mineralization (k) (Table 2) were calculated for soils from the different study sites using Eq. (1) and data on cumulative N leached (Fig. 1). The pool of potentially mineralizable N ðN0 Þ in surface soils (0 – 5 cm) increased with study site elevation (r ¼ þ0:95; P , 0:01). The potential rate of net N mineralization varied from 0.019 to 0.029 d21 for samples from the 0 –5 cm soil depth and from 0.022 to 0.034 d21 for samples from the 5– 20 cm soil depth (Table 2). Rates of potential net soil N mineralization in both depth increments were not correlated with study site elevation. 3.2. Decomposition of
13
C-glycine
Table 3 summarizes data from studies of glycine-2-13C decomposition during in situ exposure of soils at the six
ð2Þ
where A þ B is the initial amount of 13C, B is the remaining amount of 13C for large values of t (days), and k0 is the decay rate (d21). The data from each site were normalized using initial atom% excess 13C. The mean residence time (MRT), in days, for the disappearance of glycine-2-13C was calculated as 1=k 0 :
Fig. 1. Cumulative N leached from surface (0–5 cm) mineral soils collected at six mature forest sites along an elevation gradient in the southern Appalachian Mountains. Site codes (LP1, WB2, SB3, MH4, BB5, SP6) are explained in the text. Paired data points at each time step show leaching results from replicate soil columns using methods described by Campbell et al. (1993).
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Table 3 Coefficients for a one-phase exponential decay of 13C following the addition of glycine-2-13C to forest soils along an elevation gradient Study site
A
k0
B
MRT
r2
LP1 WB2 SB3 MH4 BB5 SP6
0.846 0.743 0.775 0.774 0.727 0.670
0.035 0.040 0.027 0.030 0.022 0.028
0.144 0.253 0.214 0.225 0.256 0.311
28.8 25.1 36.8 33.4 45.1 35.6
0.97 0.96 0.98 0.96 0.96 0.95 13
The data were normalized to the initial atom% excess C, start at A þ B; and decay to an asymptote (B) at rate k0 : The mean residence time (MRT) for 13C was calculated as 1=k0 : The coefficient of determination ðr2 Þ indicates the goodness of fit for the data to the nonlinear regression against elapsed time (days).
study sites. Immediately after labelling, the atom% excess 13 C in soils from the six sites ranged from 0.0789 to 0.1054. The former values were calculated based on the natural abundance of 13C in the control soils at each site that received no 13C addition. Soils analyzed during the first few weeks after isotope addition indicated a rapid disappearance of 13C and consequently, a rapid approach to B; the asymptotic amount of 13C. At each study site, a one-phase exponential decay was a good fit for describing the loss of 13C over time (coefficients of determination were all $ 0.95; see Table 3). Across the six sites, calculated loss rates (k0 ) were not related to the fraction of soil 13C remaining at the end of the in situ exposure (B). The lost rate of 13C (k0 ) from labelled soils was inversely related to study site elevation (r ¼ 20:85; P , 0:05), thus MRT increased with elevation (Fig. 2). The correlation
Fig. 2. Mean residence time (MRT) of 13C during in situ exposures of labelled soils and fraction of 13C remaining at the end of decomposition of glycine-2-13C (B) at six forest sites along an elevation gradient in the southern Appalachian Mountains. Linear regressions against elevation are shown for both MRT (y ¼ 24:9 þ 9:5 £ 1023 x) and B (y ¼ 0:17þ 6:5 £ 1025 x).
between the MRT for 13C (from Table 3) and MAT (from Table 1) across the six sites was þ 0.86 ðP , 0:05Þ: There was also a tendency for the fraction of 13C remaining at the end of the in situ exposure (B) to increase with study site elevation (r ¼ þ0:72; P ¼ 0:10) (Fig. 2).
4. Discussion Nitrogen stocks (Table 1), surface soil (0 – 5 cm) N concentrations, and the pool of potentially mineralizable surface soil N (Table 2) all tended to increase from low to high-elevation sites. However, the laboratory leaching experiments indicated no relationship between potential rates of net soil N mineralization (k) and elevation. Actual field rates may, of course, be different from those measured in the laboratory at a constant temperature, but at least one study indicates results from laboratory measurements of net soil N mineralization correlate with field rates (Zak et al., 1989). Soil microbial communities along the elevation gradient have undoubtedly adapted to long-standing differences between sites in both MAT and litter chemistry. For this reason, potential net N mineralization rates measured in the laboratory probably reflect actual rates. In any case, greater soil N stocks can produce a greater net soil N mineralization flux (g N m22 year21) even if the rate (fraction mineralized per unit time) is not correlated with elevation. Experiments on the decomposition of glycine-2-13C indicate a direct relationship between its decomposition rate (k0 ) and MAT. This result agrees with studies on the effect of temperature on SOM decomposition (Simmons et al., 1996; Kirschbaum, 2000) and studies indicating declines in forest soil respiration with increasing altitude in the southern Appalachian Mountains (L.S. Chambers1, 1998). Temperature effects on SOM decomposition are almost certainly a major factor increasing soil C stocks with elevation (Garten et al., 1999) because other studies in the region indicate a decline in aboveground net primary production with increasing altitude (Bolstad et al., 2001). Glycine is weakly adsorbed by soil, because of its neutral charge, and remains primarily in soil solution where it is readily assimilated by soil microorganisms (Barak et al., 1990; Vinolas et al., 2001). Due to the solubility of glycine, different amounts of precipitation along the elevation gradient may have affected 13C loss from in situ soil exposure. The effect of increased precipitation at higher elevations (beyond that attributable to respiration) appeared to be negligible because the fraction of the 13C remaining at the end of the in situ exposures tended to increase with altitude. In addition to temperature, differences in substrate chemistry may contribute to the increase in 13C MRT with 1
Unpublished MS Thesis, University of Tennessee, Knoxville.
C.T. Garten Jr. / Soil Biology & Biochemistry 36 (2004) 1491–1496
elevation. A 3-year study had shown that aboveground litterfall C-to-N ratios at the six study sites (see Table 3; Garten et al., 1999) decline with increasing elevation (r ¼ 20:8; P , 0:05). This change in litter C-to-N ratios is probably caused by increasing soil N availability with altitude. The change in litter chemistry with elevation could modify both SOM decomposition (Berg et al., 1996) and net soil N mineralization (Janssen, 1996) independent of temperature. Decomposition of SOM is reduced by high soil N ˚ gren et al., 2001), and lower availability (Fog, 1988; A substrate C-to-N ratios limit SOM decomposition (Berg, 2000). Thus, potential effects of MAT and N availability on 13C loss rates are confounded and, in all likelihood, may act together to reduce SOM decomposition with increasing elevation. Although less SOM decomposition is indicated at cool, high-elevation study sites, low substrate C-to-N ratios in these N-rich systems result in more N release (N mineralization) for each unit of C converted to CO2 soil microorganisms. Thus, lower substrate C-to-N ratios (in addition to greater soil N stocks) potentially contribute to increasing soil N availability with elevation. Theoretically, N mineralization and the decomposition of glycine-2-13C are related (i.e. the deamination of simple amino acids by soil microorganisms yields NH4 –N) and, according to the hypothesis for this study, decomposition rates and rates of net soil N mineralization might covary. However, the results indicated unrelated patterns of potential net soil N mineralization and glycine-2-13C decomposition along the elevation gradient. Greater amounts of N released at a slow rate from decomposition of substrates with a low C-to-N ratio in soils from cool, high-elevation forests and lesser amounts of N released at a fast rate from decomposition of substrates with a high C-toN ratio in soils from warm, low-elevation forests can result in apparent comparable rates of net soil N mineralization. Thus, different patterns in net soil N mineralization and glycine decomposition with elevation can be explained within a framework of climate, substrate chemistry, and coupled soil C and N stocks.
Acknowledgements I wish to thank the following persons, currently or formerly affiliated with Oak Ridge National Laboratory, for their valuable technical contributions to the work: Bonnie Lu, Deanne Brice, Ronda Webster, and Ramie Wilkerson. This research was sponsored by the US Department of Energy, Office Science, Biological and Environmental Research, Terrestrial Carbon Processes Program under contract with Oak Ridge National Laboratory, managed by UT-Battelle, LLC.
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References ˚ gren, G.I., Bosatta, E., Magill, A.H., 2001. Combining theory and A experiment to understand effects of inorganic nitrogen on litter decomposition. Oecologia 128, 94– 98. Barak, P., Molina, J.A.E., Hadas, A., Clapp, C.E., 1990. Mineralization of amino acids and evidence of direct assimilation of organic nitrogen. Soil Science Society America Journal 54, 769–774. Berg, B., 2000. Litter decomposition and organic matter turnover in northern forest soils. Forest Ecology and Management 133, 13 –22. Berg, B., Ekbohm, G., Johansson, M.B., McClaugherty, C., Rutigliano, F., DeSanto, A.V., 1996. Maximum decomposition limits of forest litter types: a synthesis. Canadian Journal of Botany 74, 659– 672. Bohlen, P.J., Groffman, P.M., Driscoll, C.T., Fahey, T.J., Siccama, T.G., 2001. Plant–soil– microbial interactions in a northern hardwood forest. Ecology 82, 965 –978. Bolstad, P.V., Vose, J.M., McNulty, S.G., 2001. Forest productivity, leaf area, and terrain in southern Appalachian deciduous forests. Forest Science 47, 419–427. Campbell, C.A., Ellert, B.H., Jame, Y.W., 1993. Nitrogen mineralization potential in soils. In: Carter, M.R. (Ed.), Soil Sampling and Methods of Analysis. Lewis Publishers, Boca Raton, pp. 341 –349. Fog, K., 1988. The effect of added nitrogen on the rate of decomposition of organic-matter. Biological Reviews 63, 433–462. Garten, C.T. Jr., Van Miegroet, H., 1994. Relationships between soil nitrogen dynamics and natural 15N abundance in plant foliage from Great Smoky Mountains National Park. Canadian Journal of Forest Research 24, 1636–1645. Garten, C.T. Jr., Post, W.M., Hanson, P.J., Cooper, L.W., 1999. Forest soil carbon inventories and dynamics along an elevation gradient in the southern Appalachian Mountains. Biogeochemistry 45, 115– 145. Garten, C.T. Jr., Cooper, L.W., Post, W.M., Hanson, P.J., 2000. Climate controls on forest soil C isotope ratios in the southern Appalachian Mountains. Ecology 81, 1108–1119. Janssen, B.H., 1996. Nitrogen mineralization in relation to C:N ratio and decomposability of organic materials. Plant and Soil 181, 39– 45. Joshi, A.B., Vann, D.R., Johnson, A.H., Miller, E.K., 2003. Nitrogen availability and forest productivity along a climosequence on Whiteface Mountain, New York. Canadian Journal of Forest Research 33, 1880–1891. Kirschbaum, M.U.F., 1995. The temperature dependence of soil organic matter decomposition, and the effect of global warming on soil organic C storage. Soil Biology and Biochemistry 27, 753–760. Kirschbaum, M.U.F., 2000. Will changes in soil organic carbon act as a positive or negative feedback on global warming? Biogeochemistry 48, 21– 51. MacDonald, N.W., Randlett, D.L., Zak, D.R., 1999. Soil warming and carbon loss from a Lake States spodosol. Soil Science Society America Journal 63, 211–218. Powers, R.F., 1990. Nitrogen mineralization along an altitudinal gradient— interactions of soil-temperature, moisture, and substrate quality. Forest Ecology and Management 30, 19– 29. Raich, J.W., Potter, C.S., 1995. Global patterns of carbon-dioxide emissions from soils. Global Biogeochemical Cycles 9, 23–36. Raich, J.W., Schlesinger, W.H., 1992. The global carbon-dioxide flux in soil respiration and its relationship to vegetation and climate. Tellus Series B—Chemical and Physical Meteorology 44, 81– 99. Rustad, L.E., Campbell, J.L., Marion, G.M., Norby, R.J., Mitchell, M.J., Hartley, A.E., Cornelissen, J.H.C., Gurevitch, J., 2001. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologica 126, 543–562. Shanks, R.E., 1954. Climates of the Great Smoky Mountains. Ecology 35, 354– 361.
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Simmons, J.A., Fernandez, I.J., Briggs, R.D., Delaney, M.T., 1996. Forest floor carbon pools and fluxes along a regional climate gradient in Maine, USA. Forest Ecology and Management 84, 81 –95. Trumbore, S.E., Chadwick, O.A., Amundson, R., 1996. Rapid exchange between soil carbon and atmospheric carbon dioxide driven by temperature change. Science 272, 393–396. Van Miegroet, H., Cole, D.W., Foster, N.W., 1992. Nitrogen distribution and cycling. In: Johnson, D.W., Lindberg, S.E. (Eds.), Atmospheric
Deposition and Forest Nutrient Cycling: A Synthesis of the Integrated Forest Study. Springer, New York, NY, pp. 178 –196. Vinolas, L.C., Vallejo, V.R., Jones, D.L., 2001. Control of amino acid mineralization and microbial metabolism by temperature. Soil Biology and Biochemistry 33, 1137–1140. Zak, D.R., Host, G.E., Pregitzer, K.S., 1989. Regional variability in nitrogen mineralization, nitrification, and overstory biomass in northern Lower Michigan. Canadian Journal of Forest Research 19, 1521– 1526.
Potential net soil N mineralization and decomposition of glycine-13C in forest soils along an elevation gradient Charles T. Garten Jr.* Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Mail Stop 6038, Oak Ridge, TN 37831-6038, USA Received 18 March 2003; received in revised form 9 April 2004; accepted 23 April 2004
Abstract The objective of this research was to better understand patterns of soil nitrogen (N) availability and soil organic matter (SOM) decomposition in forest soils across an elevation gradient (235 – 1670 m) in the southern Appalachian Mountains. Laboratory studies were used to determine the potential rate of net soil N mineralization and in situ studies of 13C-labelled glycine were used to infer differences in decomposition rates. Nitrogen stocks, surface soil (0– 5 cm) N concentrations, and the pool of potentially mineralizable surface soil N tended to increase from low to high elevations. Rates of potential net soil N mineralization were not significantly correlated with elevation. Increasing soil N availability with elevation is primarily due to greater soil N stocks and lower substrate C-to-N ratios, rather than differences in potential net soil N mineralization rates. The loss rate of 13C from labelled soils (0– 20 cm) was inversely related to study site elevation (r ¼ 20:85; P , 0:05) and directly related to mean annual temperature (þ0.86; P , 0:05). The results indicated different patterns of potential net soil N mineralization and 13C loss along the elevation gradient. The different patterns can be explained within a framework of climate, substrate chemistry, and coupled soil C and N stocks. Although less SOM decomposition is indicated at cool, high-elevation sites, low substrate C-to-N ratios in these N-rich systems result in more N release (N mineralization) for each unit of C converted to CO2 by soil microorganisms. q 2004 Elsevier Ltd. All rights reserved. Keywords: Soil organic matter; N availability; Climate change; C-to-N ratios; Litter chemistry; Glycine
1. Introduction Many studies indicate that both soil respiration (Raich and Schlesinger, 1992; Kirschbaum, 1995; Raich and Potter, 1995) and the decomposition of labile soil organic matter (SOM) (Trumbore et al., 1996; Garten et al., 1999; MacDonald et al., 1999; Kirschbaum, 2000) increase with temperature. Meta-analysis also shows that warmer temperatures increase annual fluxes of net soil N mineralization (Rustad et al., 2001). Because mean annual temperature (MAT) declines with increasing elevation (Shanks, 1954), one might expect both decomposition of SOM and net soil N mineralization to decrease with increasing altitude. However, studies in the southern Appalachian Mountains indicate increasing forest soil N availability with increasing elevation (Van Miegroet et al., 1992; Garten and Van Miegroet, 1994). These observations are contrary to an * Tel.: þ1-865-574-7355; fax: þ 1-865-576-8646. E-mail address: [email protected] 0038-0717/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2004.04.019
expected inverse association between temperature and net soil N mineralization that has been reported along other elevation gradients (e.g. Joshi et al., 2003). Some studies in mountainous terrain also indicate annual fluxes (kg N ha21) and rates (year21) of soil N mineralization can exhibit different patterns with a change in elevation (Powers, 1990; Bohlen et al., 2001). In earlier studies, it was hypothesized that changes in natural forest soil C isotope ratios, C concentrations, and C-to-N ratios in the southern Appalachian Mountains are interrelated because of temperature and litter chemistry controls on SOM decomposition (Garten et al., 2000). The objective of this research was to better understand patterns of soil N availability and SOM decomposition across an elevation gradient in the southern Appalachian Mountains in west Tennessee, USA. For the purpose of the research, laboratory studies were undertaken to determine the potential rate of net forest soil N mineralization and in situ studies of 13C-labelled glycine were used to infer site differences in decomposition rates. The working hypothesis
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was that rates of both net soil N mineralization and glycine2- 13C decomposition would decline with increasing elevation or decreasing mean annual temperatures.
Table 2 Nitrogen concentrations, soil column N content, potentially mineralizable soil N ðN0 Þ; and potential net N mineralization rate (k) in soils from the six study sites Depth (cm) Parameter
LP1
WB2
SB3
MH4
BB5
SP6
2. Materials and methods 0 –5
mg N g21 soil Soil N (mg) N0 (mg N) k (d21)
1.2 6.0 1188 0.025
1.3 6.5 799 0.025
5.3 26.5 2655 0.024
3.9 19.5 3413 0.019
4.2 21.0 3924 0.029
4.7 23.5 3797 0.029
5 –20
mg N g21 soil Soil N (mg) N0 (mg N) k (d21)
0.6 2.9 603 0.024
0.3 1.8 402 0.022
2.4 12.0 1569 0.034
1.3 6.5 1878 0.023
1.5 7.5 1747 0.031
1.0 5.0 458 0.022
2.1. Study sites During 1995, six study sites (Table 1) were located in 50 þ year-old forest ecosystems along an elevation gradient in the southern Appalachian Mountains. Climates, soil properties, and vegetation at the six study sites were described by Garten et al. (1999, 2000). The dominant overstory tree species at each site were as follows: LP1, loblolly pine (Pinus taeda); WB2, yellow poplar (Liriodendron tulipifera) and red maple (Acer rubrum); SB3, Carolina silverbell (Halesia carolina) and eastern hemlock (Tsuga canadensis); MH4, mixed oaks (Quercus spp.), and mixed maples (Acer spp.); BB5, red spruce (Picea rubens) and beech (Fagus grandifolia); and SP6, mature red spruce. Soil samples were collected from the six study sites on 10 different occasions from 1995 to 1997 (Garten et al., 1999). Soil samples from the 0 to 5 and 5 to 20 cm depth were airdried, crushed, and weighed to estimate soil density (g cm23). Soil density was based on the known volume of the sample (cm3) and the mass of dry soil that passed a 2 mm sieve (g). Soil , 2 mm was ground and homogenized in a sample mill and analyzed for total C and N (elemental analysis by combustion methods). Soil N stocks (g N m22) at each study site were calculated as the product of N concentration (g N g21 soil), soil density (g soil m23), and increment depth (m). Stocks in the 0– 5 and 5 –20 cm increments were summed to calculate total N stocks over the surface 20 cm of mineral soil. 2.2. Nitrogen mineralization studies Potential net N mineralization in soils from the six study sites was measured using an aerobic soil column leaching experiment. Detailed methods associated with the technique are described in Campbell et al. (1993) and are briefly summarized here. The method permits calculation of both Table 1 Elevation, mean annual temperature (MAT), mean (^ SE) mineral soil N stocks to a 20 cm soil depth ðn ¼ 10Þ; sand, and silt content at the six forest sites 22
Study site
Elevation (m)
MAT (8C)
gNm
LP1 WB2 SB3 MH4 BB5 SP6
235 335 940 1000 1650 1670
12.6 13.1 10.3 10.5 6.2 6.4
186 ^ 9 136 ^ 10 451 ^ 20 220 ^ 16 328 ^ 30 280 ^ 17
%Sand
%Silt
13 33 33 21 40 52
73 57 55 63 45 36
All data except soil N stocks are from Garten et al. (1999).
potentially mineralizable soil N and the potential rate of net soil N mineralization. Mineral soil, collected in early 1996, over 0– 5 and 5 – 20 cm depths at each site was used in the soil column leaching experiments. Replicate columns (13 cm long £ 2.6 cm dia.) were prepared using 5 g of air-dry soil (, 2 mm) mixed with 25 g of clean, washed beach sand [illustrated in Campbell et al. (1993)]. The initial total soil N in each column (mg N) was calculated based on N concentrations (elemental analysis using combustion methods) in the 0– 5 and 5 –20 cm deep soil samples from each study site (Table 2). The soil columns were leached using 100 ml of 10 mM CaCl2, followed by 25 ml of a N-free nutrient solution (Campbell et al., 1993), at the beginning of the experiment and after 15, 29, 48, 64, 112, 137, and 193 d of laboratory incubation at room temperature (21 8C). Excess leachate was removed under vacuum (60 mm Hg). Clear leachates from the soil columns were analyzed for NH4 – N and NO3 – N concentrations (mg N ml21) colorimetrically using a Bran þ Luebbe autoanalyzer (TRAACS Model 800 or AA3). Total N leached (mg NH4 – N þ mg NO3 –N) from each soil column was calculated as the product of leachate volume (ml) and concentration (mg N ml21). According to the principles of this method (Campbell et al., 1993), some part of the initial total soil N in each column is potentially mineralizable and the remainder is resistant soil organic N. The relationship between net soil N mineralized (Nmin ; mg N), as inferred from cumulative N leached, and time (t; days) is described by a nonlinear equation Nmin ¼ N0 ð1 2 expð2ktÞÞ
ð1Þ
where N0 is the potentially mineralizable soil N (mg N) and k (d21) is the rate of net soil N mineralization. Parameters N0 and k were derived from nonlinear regressions of cumulative inorganic N leached versus time. Relatively large amounts of N may be leached on the first day of the incubation due to a release of N from microorganisms that died when soil samples were air-dried. To correct for this potential artifact, the mass of N leached on the first day from
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the soil columns was subtracted from amounts leached on all subsequent days. The data transformation set the initial amount of N leached from the soil columns to zero at each study site. 2.3. Decomposition of
13
C-glycine
Following removal of the O-horizon, mineral soil (0 – 20 cm) was sampled from the six forest sites using a bucket auger in late 1996. The soil was air-dried, sieved (2 mm), and thoroughly homogenized. The homogenized, air-dry soil from each site was packed into 24 butyrate, plastic tubes (20 cm long £ 2.4 cm dia.) to a mass density of 0.9 –1.2 g cm23. In the field, glycine-2-13C (99 at.%) was added to half the tubes using enough water to sufficiently wet the soil without drainage. The amount of 13C added to each tube varied between 4 and 21 mg depending on the soil C concentration. Tubes containing glycine-2- 13C and paired controls (unlabelled) were buried flush with the surface of the ground at each study site. The tubes were covered with a fine mesh nylon screen to exclude invertebrates and debris. A labelled soil tube and its paired control were removed from each site at the beginning of the experiment and at approximately 1, 2, 4, 10, 13, and 18 months following addition of the isotope. In the laboratory, soils were extruded from the recovered plastic tubes and air-dried to a constant weight at room temperature (21 8C). A subsample of the air-dried soil was ground and homogenized using a mortar and pestle. The finely ground subsample was used for analysis of soil C and N concentrations (elemental analysis by combustion methods) and analysis of 13C-to-12C ratios. Methods used for the analysis of stable C isotopes were described by Garten et al. (2000). Soils were combusted in sealed glass tubes containing copper oxide wire and reduced Cu. Carbon dioxide was separated from other combustion products by cryogenic distillation on a vacuum line following sample combustion. Carbon isotope ratios were measured using a dual inlet stable isotope ratio mass spectrometer along with reference materials from the National Institute of Standards and Technology (Gaithersburg, MD). The fraction of 13C remaining (Y) in the six forest soils as a function of time was described as a one-phase exponential decay Y ¼ A expð2k0 tÞ þ B
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3. Results 3.1. Nitrogen mineralization studies High-elevation sites (BB5, SP6) had greater soil N stocks (0 –20 cm) than low-elevation study sites (LP1, WB2). However, the correlation between N stocks and elevation was not statistically significant because of a large variation in soil N between two mid-elevation study sites (SB3, MH4) (Table 1). Surface soil (0 – 5 cm) N concentrations for samples used in the laboratory incubations (Table 2) tended to increase with study site elevation (r ¼ þ0:81; P , 0:10). Soil from the 0 –5 cm depth increment released more N over the course of the incubation than soil from the 5 to 20 cm depth increment. Potentially mineralizable N ðN0 Þ and the potential rate of net N mineralization (k) (Table 2) were calculated for soils from the different study sites using Eq. (1) and data on cumulative N leached (Fig. 1). The pool of potentially mineralizable N ðN0 Þ in surface soils (0 – 5 cm) increased with study site elevation (r ¼ þ0:95; P , 0:01). The potential rate of net N mineralization varied from 0.019 to 0.029 d21 for samples from the 0 –5 cm soil depth and from 0.022 to 0.034 d21 for samples from the 5– 20 cm soil depth (Table 2). Rates of potential net soil N mineralization in both depth increments were not correlated with study site elevation. 3.2. Decomposition of
13
C-glycine
Table 3 summarizes data from studies of glycine-2-13C decomposition during in situ exposure of soils at the six
ð2Þ
where A þ B is the initial amount of 13C, B is the remaining amount of 13C for large values of t (days), and k0 is the decay rate (d21). The data from each site were normalized using initial atom% excess 13C. The mean residence time (MRT), in days, for the disappearance of glycine-2-13C was calculated as 1=k 0 :
Fig. 1. Cumulative N leached from surface (0–5 cm) mineral soils collected at six mature forest sites along an elevation gradient in the southern Appalachian Mountains. Site codes (LP1, WB2, SB3, MH4, BB5, SP6) are explained in the text. Paired data points at each time step show leaching results from replicate soil columns using methods described by Campbell et al. (1993).
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Table 3 Coefficients for a one-phase exponential decay of 13C following the addition of glycine-2-13C to forest soils along an elevation gradient Study site
A
k0
B
MRT
r2
LP1 WB2 SB3 MH4 BB5 SP6
0.846 0.743 0.775 0.774 0.727 0.670
0.035 0.040 0.027 0.030 0.022 0.028
0.144 0.253 0.214 0.225 0.256 0.311
28.8 25.1 36.8 33.4 45.1 35.6
0.97 0.96 0.98 0.96 0.96 0.95 13
The data were normalized to the initial atom% excess C, start at A þ B; and decay to an asymptote (B) at rate k0 : The mean residence time (MRT) for 13C was calculated as 1=k0 : The coefficient of determination ðr2 Þ indicates the goodness of fit for the data to the nonlinear regression against elapsed time (days).
study sites. Immediately after labelling, the atom% excess 13 C in soils from the six sites ranged from 0.0789 to 0.1054. The former values were calculated based on the natural abundance of 13C in the control soils at each site that received no 13C addition. Soils analyzed during the first few weeks after isotope addition indicated a rapid disappearance of 13C and consequently, a rapid approach to B; the asymptotic amount of 13C. At each study site, a one-phase exponential decay was a good fit for describing the loss of 13C over time (coefficients of determination were all $ 0.95; see Table 3). Across the six sites, calculated loss rates (k0 ) were not related to the fraction of soil 13C remaining at the end of the in situ exposure (B). The lost rate of 13C (k0 ) from labelled soils was inversely related to study site elevation (r ¼ 20:85; P , 0:05), thus MRT increased with elevation (Fig. 2). The correlation
Fig. 2. Mean residence time (MRT) of 13C during in situ exposures of labelled soils and fraction of 13C remaining at the end of decomposition of glycine-2-13C (B) at six forest sites along an elevation gradient in the southern Appalachian Mountains. Linear regressions against elevation are shown for both MRT (y ¼ 24:9 þ 9:5 £ 1023 x) and B (y ¼ 0:17þ 6:5 £ 1025 x).
between the MRT for 13C (from Table 3) and MAT (from Table 1) across the six sites was þ 0.86 ðP , 0:05Þ: There was also a tendency for the fraction of 13C remaining at the end of the in situ exposure (B) to increase with study site elevation (r ¼ þ0:72; P ¼ 0:10) (Fig. 2).
4. Discussion Nitrogen stocks (Table 1), surface soil (0 – 5 cm) N concentrations, and the pool of potentially mineralizable surface soil N (Table 2) all tended to increase from low to high-elevation sites. However, the laboratory leaching experiments indicated no relationship between potential rates of net soil N mineralization (k) and elevation. Actual field rates may, of course, be different from those measured in the laboratory at a constant temperature, but at least one study indicates results from laboratory measurements of net soil N mineralization correlate with field rates (Zak et al., 1989). Soil microbial communities along the elevation gradient have undoubtedly adapted to long-standing differences between sites in both MAT and litter chemistry. For this reason, potential net N mineralization rates measured in the laboratory probably reflect actual rates. In any case, greater soil N stocks can produce a greater net soil N mineralization flux (g N m22 year21) even if the rate (fraction mineralized per unit time) is not correlated with elevation. Experiments on the decomposition of glycine-2-13C indicate a direct relationship between its decomposition rate (k0 ) and MAT. This result agrees with studies on the effect of temperature on SOM decomposition (Simmons et al., 1996; Kirschbaum, 2000) and studies indicating declines in forest soil respiration with increasing altitude in the southern Appalachian Mountains (L.S. Chambers1, 1998). Temperature effects on SOM decomposition are almost certainly a major factor increasing soil C stocks with elevation (Garten et al., 1999) because other studies in the region indicate a decline in aboveground net primary production with increasing altitude (Bolstad et al., 2001). Glycine is weakly adsorbed by soil, because of its neutral charge, and remains primarily in soil solution where it is readily assimilated by soil microorganisms (Barak et al., 1990; Vinolas et al., 2001). Due to the solubility of glycine, different amounts of precipitation along the elevation gradient may have affected 13C loss from in situ soil exposure. The effect of increased precipitation at higher elevations (beyond that attributable to respiration) appeared to be negligible because the fraction of the 13C remaining at the end of the in situ exposures tended to increase with altitude. In addition to temperature, differences in substrate chemistry may contribute to the increase in 13C MRT with 1
Unpublished MS Thesis, University of Tennessee, Knoxville.
C.T. Garten Jr. / Soil Biology & Biochemistry 36 (2004) 1491–1496
elevation. A 3-year study had shown that aboveground litterfall C-to-N ratios at the six study sites (see Table 3; Garten et al., 1999) decline with increasing elevation (r ¼ 20:8; P , 0:05). This change in litter C-to-N ratios is probably caused by increasing soil N availability with altitude. The change in litter chemistry with elevation could modify both SOM decomposition (Berg et al., 1996) and net soil N mineralization (Janssen, 1996) independent of temperature. Decomposition of SOM is reduced by high soil N ˚ gren et al., 2001), and lower availability (Fog, 1988; A substrate C-to-N ratios limit SOM decomposition (Berg, 2000). Thus, potential effects of MAT and N availability on 13C loss rates are confounded and, in all likelihood, may act together to reduce SOM decomposition with increasing elevation. Although less SOM decomposition is indicated at cool, high-elevation study sites, low substrate C-to-N ratios in these N-rich systems result in more N release (N mineralization) for each unit of C converted to CO2 soil microorganisms. Thus, lower substrate C-to-N ratios (in addition to greater soil N stocks) potentially contribute to increasing soil N availability with elevation. Theoretically, N mineralization and the decomposition of glycine-2-13C are related (i.e. the deamination of simple amino acids by soil microorganisms yields NH4 –N) and, according to the hypothesis for this study, decomposition rates and rates of net soil N mineralization might covary. However, the results indicated unrelated patterns of potential net soil N mineralization and glycine-2-13C decomposition along the elevation gradient. Greater amounts of N released at a slow rate from decomposition of substrates with a low C-to-N ratio in soils from cool, high-elevation forests and lesser amounts of N released at a fast rate from decomposition of substrates with a high C-toN ratio in soils from warm, low-elevation forests can result in apparent comparable rates of net soil N mineralization. Thus, different patterns in net soil N mineralization and glycine decomposition with elevation can be explained within a framework of climate, substrate chemistry, and coupled soil C and N stocks.
Acknowledgements I wish to thank the following persons, currently or formerly affiliated with Oak Ridge National Laboratory, for their valuable technical contributions to the work: Bonnie Lu, Deanne Brice, Ronda Webster, and Ramie Wilkerson. This research was sponsored by the US Department of Energy, Office Science, Biological and Environmental Research, Terrestrial Carbon Processes Program under contract with Oak Ridge National Laboratory, managed by UT-Battelle, LLC.
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