Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue5.2000:

Temperature e€ects on the diversity of soil heterotrophs and the

d

13

C of soil-respired CO

₂

Je€rey A. Andrews

a,

*, Roser Matamala

a

, Kristi M. Westover

a

, William H. Schlesinger

a, b

a

Department of Botany, Duke University, Durham, NC 27708, USA

b

Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708, USA

Received 5 May 1999; received in revised form 15 September 1999; accepted 28 October 1999

Abstract

We measured the respiration rates, d13C of respired CO2, and microbial community composition in root-free bulk soils

incubated at 4, 22 and 408C. The soils were obtained from the Duke Forest Free-Air CO2 Enrichment (FACE) experiment

where organic carbon in soils sampled from the elevated CO2plots contained a unique 13C label that was derived from FACE

fumigation. The CO2produced by soil heterotrophs at 48C was 2.2 to 3.5- enriched in 13

C relative to CO2respired at 22 and

408C and was similarly enriched relative to bulk soil carbon. There was no isotopic di€erence between CO2produced at 22 and

408C. Respiration rates increased exponentially with temperature from 0.25 mg CO2g soil

ÿ1

dÿ1at 48C to 0.73 mg CO2g soil

ÿ1

dÿ1 at 408C. Microbial community composition, as measured by the di€erences in populations of morphology types, di€ered across the temperature range. Only eight of 67 microbial morphology types were common to all three incubation temperatures, while six types were unique to 48C soil, 17 to 228C soil and 18 to 408C soil. Species richness, approximated from morphology type, was signi®cantly lower at 48C than at 22 and 408C. This change in microbial community structure from 4 to 22 and 408C caused a shift in mineralizable carbon pools, resulting in a shift in the isotopic composition of CO2 respired at the low

temperature.72000 Elsevier Science Ltd. All rights reserved.

Keywords:Isotope fractionation; Soil respiration; Soil carbon; Soil microbes

1. Introduction

Studies of carbon isotopes provide a powerful tool to elucidate carbon dynamics in soils and are widely used to understand ecosystem carbon budgets, gas ¯ux from the soil and decomposition dynamics. The isoto-pic composition of soil CO2is determined by the inter-play of a variety of biological and physical processes, as well as chemical reactions, in the soil environment (Amundson et al., 1997). The carbon isotope ratios in

soil CO2are determined by the mixture of CO2derived from three sources: inward di€usion from the over-lying atmosphere, respiration of live roots and the

oxi-dation of soil organic matter (SOM) by soil

heterotrophs.

Carbon isotope ratios in soil organic matter change during decomposition. This change is mediated by both the isotopic composition of the chemical constitu-ents of SOM as well as discrimination during de-composition by soil heterotrophs (AÊgren et al., 1996). The metabolic fractionation of carbon in plants during photosynthesis and during the synthesis of various bio-chemical compounds (O'Leary, 1981) results in low contents of 13C in slowly decomposing substances, such as lignin (Benner et al., 1987). However, 12C may be used preferentially by decomposers (Blair et al.,

0038-0717/00/$ - see front matter72000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 ( 9 9 ) 0 0 2 0 6 - 0

www.elsevier.com/locate/soilbio

* Corresponding author. Present address: Department of Ecology and Evolutionary Biology, MS 170, Rice University, P.O. Box 1892, Houston, TX 77251-1892, USA. Tel.: 713-527-8750; fax: +1-713-285-5232.

1985), resulting in a tendency for 13C enrichment in the remaining SOM (Nadelho€er and Fry, 1988). Therefore, variation in the composition of SOM and the activity of soil heterotrophs will result in changes in the carbon isotope composition of soil CO2.

It is widely known that temperature has an e€ect on microbial activity (MacDonald et al., 1995) and that certain groups of microbes are adapted to particular temperature regimes (Allen and Brock, 1968; Zogg et al., 1997). Zogg et al. (1997) have postulated that tem-perature-induced changes in the soil microbial commu-nity could result in a shift in the use of available substrates in the soil. Because of the di€erences in the carbon isotopic ratios found among components of SOM, it is likely that such a microbial community shift would impact the isotopic composition of mi-crobe-produced soil CO2and soil respired CO2. Recog-nizing this e€ect on the isotopic composition of soil-derived CO2 is an important factor for the interpret-ation of an increasing number of biogeochemical

stu-dies of soil carbon dynamics. In addition, an

understanding of how soil microbes a€ect the isotopic composition of soil CO2 will increase our understand-ing of SOM formation.

Although there have been many studies on the isoto-pic composition of SOM, relatively few have examined the factors controlling the carbon isotope ratios of soil-derived CO2 (Amundson et al., 1997). In this study, we examined the e€ect of temperature on the isotopic composition of CO2 produced by soil hetero-trophs and the concurrent relationship between tem-perature and soil microbial community composition. Soils for our experiments were sampled from the Duke Forest Free Air CO2Enrichment (FACE) site. FACE technology was developed to study the e€ects of high CO2on intact ecosystems without the use of enclosures (Hendrey et al., 1999). The CO2 used to fumigate this experiment is derived from natural gas and, because it is strongly depleted in 13C, it serves as an isotopic label of new plant tissues and soil organic matter. We utilized this unique isotopic label to better characterize soil heterotroph activity at various temperatures.

2. Material and methods

2.1. Study site

The study site was the Duke Forest Free-Air CO2 Enrichment (FACE) experiment, located in the central Piedmont region of North Carolina, USA (35897'N 79809'W). The site was cleared of a mixed forest in 1981, drum chopped and burned in 1982 and estab-lished as a loblolly pine (Pinus taeda L.) plantation in 1983. Soils are of the Enon series, a low-fertility Ultic Al®sol that is typical of many upland areas in the

southeastern US. The soil is derived from igneous rock, yielding a well-developed, acidic…pHˆ5:75† pro-®le with mixed clay mineralogy.

The site contains six 30-m dia circular plots in a 16-y-old loblolly pine forest. Each of the plots are sur-rounded by an array of vertical vent pipes that extend to the top of the forest canopy. Three `elevated' plots are fumigated with CO2 through the vent pipes to maintain an atmospheric CO2concentration inside the plots that is 200 ml lÿ1above ambient (575ml lÿ1 aver-age, May±October 1997). The remaining three `con-trol' plots are fumigated with ambient air only. To account for site variability each elevated plot is paired with a control plot of similar topography and veg-etation density. Continuous fumigation began 27 August 1996.

The CO2used for fumigation is derived from natural gas and is, therefore, strongly depleted in 13C …d13Cˆ ÿ43:120:6- (2S.E.) versus PDB). The standard ex-pression of stable carbon isotope ratios is in di€eren-tial notation (Craig, 1953), where

d13_Cˆ

and is expressed in parts per thousand (-). The accepted standard is the PDB carbonate (Craig, 1957). Using this CO2 to elevate the atmospheric concen-tration by 200 ml lÿ1 changes the d13C of CO2 in the FACE plot from ÿ8 to ÿ20-. The photosynthetic fractionation (Farquhar et al., 1989) by the loblolly

pine results in new photosynthate with

d13Cˆ ÿ39:321:4-, as measured in young needles sampled from September 1997 (Ellsworth, 1999). Fine roots (<1 mm dia) grown under FACE conditions between August 1997 and August 1998 had a d13_Cˆ

ÿ39:520:1- (R. Matamala, unpublished data). This unique isotopic signature in organic carbon enters the soil as above- and below-ground litter, shifting the

d13C of soil organic matter to more negative values.

2.2. Sampling methods

removed by hand from the soils in a temperature-con-trolled room set to the temperature of the soils as measured in situ at the time of sampling. All soil samples from each ring were composited and hom-ogenized and a sub-sample was removed for mass loss determination of water content after drying at 1108C to constant mass. The composited samples were stored overnight at 48C.

2.3. Sample analysis

Field capacity for these soils had been determined previously to be 300 g kgÿ1. For each composited sample, approximately 250 g of root-free soil was placed in a 1-l Mason2 jar and adjusted to and main-tained at 240 g water kgÿ1 content by mass with de-ionized water. The jars were capped with a perforated lid to allow exchange of air with ambient room air. For series A, sample jars were placed in water baths maintained at 4 and 258C. After 24 d of incubation, the samples maintained at 48C were warmed to 258C. For series B, samples were maintained at 48C (cold room), 228C (room temperature) and 408C (water bath) for 48 d. Throughout the incubation, samples were kept dark except while measurements were in progress.

2.3.1. CO2¯ux analysis

We measured the respiration rate of the soils (rate of CO2production) and the d13C of the respired CO2 at intervals during a period of 25 d (series A) and 48 d (series B). The jars were capped with a lid with two ports where 0.5-cm dia Bev-a-line2 IV tubing was used to form a closed-loop through an IRGA (PPSys-tems, EGM-1). We estimated the total volume of the chamber system using an estimated soil particle density of 2.65 g cmÿ3and the measured water content of the sample. Respiration rates were calculated based on the rate of accumulation of CO2in the chamber system as measured at 1-min intervals over 30 min. Following the respiration measurement, gas samples were taken from the incubation jars in 75-m Whitey2 _stainless

steel gas cylinders that were sealed with Nupro2

bel-lows valves equipped with Kel-F2stem tips. The cylin-ders were pre-evacuated to 10ÿ5 Pa. The sample jar was ¯ushed with CO2-free air for 2 min and the sample cylinder attached directly to the access tubes in the jar lid with a stainless steel Swage-loc2 tube con-nector and CO2 was allowed to accumulate in the sample jar for 1 h before the sample was drawn. Car-bon dioxide was concentrated in the gas sample via cryogenic puri®cation and vacuum distillation (Bout-ton, 1991) and d13C was determined by stable isotope ratio mass spectrometry (VG ISOGAS series 2) at the Duke University Phytotron. The soil samples were

maintained at the appropriate incubation temperatures throughout both measurements.

2.3.2. Microbial community analysis

On d 14 of series B incubation, approximately 2 g of soil from each of the 18 homogenized samples was col-lected and serially diluted using 0.1% CaSO4. Small (100 ml) aliquots from dilutions (10ÿ3, 10ÿ4, 10ÿ5, 10ÿ6 and 10ÿ7) of each sample were spread-plated onto 10% tryptic soy agar (TSA) (Wollum, 1982). This reduced concentration of TSA was used to provide a less nutrient rich environment. Plates were incubated at the temperature of the soil samples from which they were removed (4, 22 or 408C). Colonies from each di-lution series were counted for 10 consecutive days. In this way, fast- and slow-growing organisms were included in the analysis. All colonies were described according to four criteria: color, sheen, con®guration and margin. The number of colony forming units (CFU) per gram of soil for each morphology type was estimated.

2.3.3. Bulk soil carbon analysis

At the end of the incubation, sub-samples of the soils were dried at 908C and ground to a ®ne powder. Bulk soil d13C was determined by stable isotope ratio mass spectrometry (VG ISOGAS series 2) following whole sample combustion in an elemental analyzer (Carlo Erba Instrumentazione, NA1500 series 1).

2.4. Statistical analysis

Statistical analysis of the isotope and respiration data collected on a speci®c date was performed using a paired t-test (JMP, SAS Institute), where in series A,

nˆ3 and in series B, nˆ6: Because these measure-ments were performed over a time series, analysis to compare results over the entire incubation were per-formed pair-wise using repeated measures analysis of variance (ANOVA). For analysis of the microbial community data, a multivariate analysis of variance (MANOVA) of the ranked estimated numbers of each morphology type was conducted on those types iso-lated from multiple samples from each of the

treat-ments (SAS Institute, 1990). A repeated pro®le

3. Results

3.1. CO2respiration rate and isotopic composition

Initial respiration rates in both elevated and control plot soil in series B increased exponentially with tem-perature, from an average of 0.02520.003 mg CO2 g soilÿ1dÿ1at 48C to 0.27220.021 mg CO2g soilÿ1dÿ1 at 228C and to 0.72920.079 mg CO2 g soilÿ1 dÿ1 at 408C (Fig. 1A). Respiration rates at all temperatures were signi®cantly di€erent on each measurement date through day 12 (P< 0.01), when rates at 22 and 408C began to converge. Nevertheless, temperature had a signi®cant e€ect on respiration rate between all tem-peratures (repeated measures ANOVA, P < 0.001). Soils collected from plots under FACE CO2 fumi-gation had signi®cantly higher initial (day 1) respir-ation rates at 4 and 408C (paired t-test, PR0.05) but not at 228C…Pˆ0:24).

Soils incubated at 48C showed higher values ofd13C in soil heterotroph-respired CO2 than those incubated at higher temperatures; however, this temperature-dependent isotopic fractionation of carbon in CO2 respired from series B soils was not exhibited between

22 and 408C (Fig. 1B). On d 1, the d13C of CO2 respired from soils at 48C was signi®cantly enriched in 13

C by 3.0-relative to the CO2respired from 22 and 408C (paired t-test, PR0.001), but there was no sig-ni®cant di€erence in the carbon isotope ratios of CO2 respired from 22 and 408C soils (paired t-test,

Pˆ0:36). The di€erence of d13C in CO2 respired at 48C and CO2 respired at 22 or 408C was exhibited throughout the 48-d incubation (repeated measures ANOVA, Pˆ0:02† while there was never a di€erence between 22 and 408C (repeated measures ANOVA,

Pˆ0:85).

In series A soils, soil heterotroph respiration rates were signi®cantly higher at 258C than at 48C over the

period measured (repeated measures ANOVA, Pˆ

0:01†(Fig. 2). In addition, the d13C of CO2respired at 48C was 2.2- enriched in 13C relative to the CO2 respired at 258C on d 2 of the incubation (Fig. 2). The di€erence in the isotopic composition of the CO2 increased during the course of the incubation so that by d 10, the d13C of CO2 respired at 48C was 3.5 -enriched in 13C compared to CO2 respired at 258C. For samples incubated at 48C, a temperature shift to 258C on d 23 of the incubation induced a change in the d13C of soil heterotroph-respired CO2from ÿ23.0 20.3 toÿ28.520.2-48 h later.

In series B, which included soils from FACE elev-ated and control plots, the d13C of CO2 from FACE elevated plot soils was depleted relative to the CO2 from control plot soils throughout the entire incu-bation (Fig. 1B). At 48C, the depletion ranged from 3.9 to 2.4-. At the higher incubation temperatures, the magnitude of 13C depletion decreased with time, shifting from 4.5- on d 1 to 1.7- on d 48 at 228C and from 4.3 to 0.4- at 408C. However, the FACE treatment had no e€ect on the initial (d 1) magnitude of the temperature-dependent fractionation of carbon

Fig. 2. Mean respiration rates and meand13_{C of respired CO} 2from

series A incubation, where nˆ3 and error bars show 1 standard error of the mean.

Fig. 1. (A) Mean respiration rate and (B) meand13_{C of respired CO} 2

in CO2between 4 and 228C (pairedt-test, Pˆ0:44†or between 4 and 408C (pairedt-test,Pˆ0:59).

At the end of the incubations, the d13C of bulk soil samples was similar to thed13C of CO2respired at the higher temperatures. In series B soils, there was no sig-ni®cant di€erence between the d13C of bulk soil and the d13C of CO2respired at the end of the incubation at 228C (paired t-test, Pˆ0:64† or 408C …Pˆ0:21). However, there was a large enrichment measured in the 13C of CO2 respired at 48C relative to bulk soil, where the d13C was 2.9420.60- less negative in elev-ated and control plot soils (paired t-test, Pˆ0:004). Similar results were found in series A; the last measured d13C of respired CO2 at 258C was similar (paired t-test, Pˆ0:81† to the mean bulk soil d13C. However, the CO2respired at 48C was enriched in13C by 3.7820.20-…Pˆ0:03).

3.2. Microbial community structure

Incubation temperature signi®cantly a€ected com-munities of soil heterotrophs, as measured by

di€er-ences in estimated population sizes of various

morphology types (Table 1). A total of 67 di€erent morphology types were characterized based on the four criteria: color, sheen, con®guration and margin. Of these 67 types, only eight were common to all three incubation temperatures. Six morphology types were unique to 48C soil, 17 to 228C soil and 18 to 408C soil (Fig. 3). Using morphology types to approximate mi-crobial species, both 228C and 408C soil temperatures had signi®cantly greater species richness than 48C

…Fˆ15:72,d:f:ˆ2,P< 0.001, ANOVA, Tukey HSD-test). Absence of a particular morphology type in a di-lution series does not indicate that an organism was not present in the soil because plating techniques are selective not only for culturable organisms, but also

for numerically dominant organisms. However,

absence of a morphology type does indicate lack of

ac-tivity. When the eight common types were included in the MANOVA analysis, there were di€erences in com-munity composition associated with soils incubated at di€erent temperatures, as indicated by a signi®cant pro®le by incubation temperature interaction (Wilke's LambdaFˆ6:32,d:f:ˆ14, 8,P< 0.01) (Table 1).

4. Discussion

We found that in soils collected from a loblolly pine

Table 1

Multivariate analysis of variance of e€ects of CO2and temperature on estimated population sizes of morphology types a

E€ect Num d.f. Den d.f. Wilke's lambda F P

Overall di€erences

Block 16 6 4.6410ÿ2 1.36 NS

CO2 8 3 1.4810ÿ1 2.15 NS

Temperature 16 6 2.6610ÿ3 _{6.88 0.05}

CO2temperature 16 6 4.2310ÿ2 1.44 NS

Interaction with pro®le

Block 14 8 4.9010ÿ2 _{2.01 NS}

CO2 7 4 1.6310ÿ1 2.94 NS

Temperature 14 8 6.8810ÿ3 _{6.32 0.006}

CO2temperature 14 8 2.5310ÿ1 0.56 NS

a_{The heading `overall di€erences' presents tests for the e€ects of treatment on estimated population sizes of morphology types. The heading}

`interaction with pro®le' presents tests for the speci®c hypothesis that estimated population sizes of morphology types (i.e. bacterial community composition) depend upon the designated treatment e€ect.

ecosystem, there is a temperature-dependent C isotope fractionation in soil heterotroph-respired CO2 that is associated with a shift in microbial community struc-ture. Our results clearly demonstrate the relationship between soil temperature and the stable carbon isotope ratio of CO2 produced by soil heterotrophs. This re-lationship, however, is not linear across the entire ex-perimental temperature range of 4±408C. Samples incubated at 22 and 408C show no di€erence in the

d13C of respired CO2 and the d13C of respired CO2 was similar to thed13C of bulk soil. However, samples incubated at 48C are enriched in 13C relative to both CO2 respired at higher temperatures and to bulk soil C. These results suggest that at low temperatures, either there is a change in the kinetics of carbon miner-alization or there is a shift in the carbon pool being mineralized.

We found a consistent exponential increase in respir-ation rates across the entire 4±408C temperature range, as measured by the rate of CO2production by soil het-erotrophs. This relationship between temperature and microbial respiration has been found in numerous stu-dies (e.g. Wiant, 1967; de Jong et al., 1974; Kirsch-baum, 1995). Microbial activity is strongly controlled by temperature because of e€ects on both microbial

biomass (Insam, 1990) and enzyme activity

(McClaugherty and Linkins, 1990). We calculated the

Q10 of this soil to be 1.9 by ®tting an exponential curve to our data following Boone et al. (1998) and Davidson et al. (1998). This value compares well with the Q10 range of 1.7±1.9 reported by Winkler et al. (1996) for similar soils sampled from the Duke Forest and with otherQ10s reported for other soils containing no roots (Holland et al., 1995; Boone et al., 1998; KaÈt-terer et al., 1998).

The increase in respiration rate across the entire temperature range and the enrichment in 13C only at 48C rule out a strictly kinetic explanation for the observed carbon isotope fractionation. In addition, there is no theory that suggests a very di€erent ratio of reaction rates of13C compared to12C in slow versus fast reactions (AÊgren et al., 1996). We suggest that the shift in carbon isotopic ratios in respired CO2 is the result of a shift in the use of carbon substrates in the soil. In laboratory incubations, MacDonald et al. (1995) found an apparent increase in the pool of labile C with an increase in soil temperature. This change in available C with temperature could be explained by changes in the structure and size of the active soil mi-crobial community (Richards et al., 1985; Ellert and Bettany, 1992).

Zogg et al. (1997) found that an apparent increase in the pool size of labile C with increasing temperature was associated with changes in the composition of the microbial community as determined through molecular analysis. Indeed, it has long been recognized that

cer-tain groups of soil microbes are adapted to particular temperature regimes (Allen and Brock, 1968). Our data suggest a shift in the soil microbe community structure and size with temperature (Table 1, Fig. 3). We can ®nd no evidence in the literature that increased microbial activity as a result of only population size could account for a shift in substrate C pools. How-ever, because soil microbes vary substantially in their anity for various substrates, a change in the diversity of the microbial may render additional compounds to the labile C pool.

Biochemical constituents of SOM vary in carbon isotope composition as a result of two processes. Metabolic fractionation in plants during tissues syn-thesis (Park and Epstein, 1961; O'Leary, 1981) results in structural compounds that are depleted in 13C (Ben-ner et al., 1987). Lignin, for instance, is ge(Ben-nerally depleted in 13C by ca. 4±7- relative to cellulose. In the soil 12C tends to be used preferentially, to some degree, by microbes during decomposition (Blair et al., 1985) so that the remaining SOM is enriched in 13C. AÊgren et al. (1996) used the continuous quality theory (Bosatta and AÊgren, 1991) to demonstrate that isotope discrimination during decomposition is determined by both initial substrate properties and microbial proper-ties. The CO2 that results from decomposition will vary in isotopic composition dependent on which mi-crobe are active. We suggest that the less negative iso-topic ratio of CO2 produced at 48C indicates that a smaller fraction of lignin or lignin-like compounds are used as substrates at that temperature.

There were di€erences in microbial community com-position between all temperatures as measured by mor-phology types. However, the relative lack of microbial diversity at 48C is striking. Soils at 48C contained only one-third as many unique morphology types as did soils incubated at either of the higher temperatures; additionally, morphology type richness was signi®-cantly lower in 48C soils. The lack of microbial diver-sity coupled with the enrichment in 13C in CO2relative to bulk soil carbon suggests that only speci®c carbon compounds (those enriched in 13C) can be mineralized by microbes that are active at low temperatures.

occurred at all incubation temperatures demonstrating that some new organic material is part of the labile carbon pool at all temperatures. However, the much smaller enrichment in 13C of respired CO2 from elev-ated relative to control plot soils at 48C shows that proportionately less of the new, isotopically-labeled carbon is being mineralized by the microbes. This trend away from labile carbon at 48C is present despite the indication that there is more labile carbon in the soils under elevated CO2 as suggested by higher initial rates of soil respiration and by a recent study by Hun-gate et al. (1997).

The hypothesis that part of the SOM pool is pro-tected from mineralization at low temperatures is ad-ditionally supported by the data from the series A incubation. After 24 d of incubation at 48C, the tem-perature was raised to 258C. Thed13C of CO2respired at the new temperature was isotopically similar to the CO2 produced early in the 258C incubation, as if a C substrate readily mineralized by the microbial commu-nity at 258C was unavailable to the community at 48C. The enrichment of 13C in microbially-respired CO2 at 48C may have important implications for the in-terpretation of isotopic data in studies of soil carbon processes. Several studies have found that thed13C of soil and soil-respired CO2 becomes less negative (enriched in13C) during cold winter months (Reardon et al., 1979; Parada et al., 1983). During these months, CO2production in the soil is diminished so the shift in

d13C is certainly due, at least in part, to di€usion of CO2 into the soil from the overlying atmosphere (Amundson et al., 1997). However, our data suggest that such a shift could also be the result of the tem-perature-dependent shift in the active microbial com-munity causing in a shift in the carbon pool being mineralized. Accounting for this interaction between temperature and d13C of respired CO2 will be import-ant to experiments that rely on laboratory incubations of soils to examine soil carbon dynamics (e.g. Nadel-ho€er and Fry, 1988; Hsieh, 1996). Given the potential for a signi®cant isotope fractionation, care must be exhibited to match the incubation temperature to the ®eld temperature of the soil at the time the sample was taken.

Acknowledgements

We thank Larry Giles for technical assistance with isotopic measurements and Miquel GonzaÁlez-Meler for valuable advice. This research was conducted with sup-port from the US Department of Energy, the National Science Foundation, and the Electric Power Research Institute as a contribution to the Duke Forest FACE Project. Additional support was granted to J.A.A. by a NASA Earth System Science Fellowship and an

NSF Doctoral Dissertation Improvement Award, to R.M. by a fellowship from the Ministerio de Educa-cioÂn y Ciencia (Spain) and to K.M.W. by a grant from the US Department of Agriculture.

References

AÊgren, G.I., Bosatta, E., Balesdent, J., 1996. Isotope discrimination during decomposition of organic matter: a theoretical analysis. Soil Science Society of America Journal 60, 1121±1126.

Allen, S.D., Brock, T.D., 1968. The adaptation of heterotrophic microcosms to di€erent temperatures. Ecology 49, 343±346. Amundson, R., Stern, L., Baisden, T., Wang, Y., 1997. The isotopic

composition of soil and soil-respired CO2. Geoderma 82, 83±114.

Benner, R., Fogel, M.L., Sprague, E.K., Hodson, R.E., 1987. Depletion of 13_{C in lignin and its implications for stable carbon}

isotope studies. Nature 329, 708±710.

Blair, N., Leu, A., MunÄoz, E., Olsen, J., Kwong, E., Des Marais, D., 1985. Carbon isotopic fractionation in heterotrophic mi-crobial metabolism. Applied Environmental Microbiology 50, 996±1001.

Boone, R.D., Nadelho€er, K.J., Canary, J.D., Kaye, J.P., 1998. Roots exert a strong in¯uence on temperature sensitivity of soil respiration. Nature 396, 570±572.

Bosatta, E., AÊgren, G.I., 1991. Dynamics of carbon and nitrogen in the organic matter of soil: a generic theory. American Naturalist 138, 227±245.

Boutton, T.W. 1991. Stable carbon isotope ratios of natural ma-terials: I. Sample preparation and mass spectrometric analysis. In: Coleman, D.C., Fry, B. (Eds.), Carbon Isotope Techniques. Academic Press, San Diego, pp. 189±213.

Craig, H., 1953. The geochemistry of stable carbon isotopes. Geochimica et Cosmochimica Acta 3, 53±92.

Craig, H., 1957. Isotopic standards for carbon and oxygen and cor-rection factors for mass-spectrometric analysis of carbon dioxide. Geochimica et Cosmochimica Acta 12, 133±149.

Davidson, E.A., Belk, E., Boone, R.D., 1998. Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperature mixed hardwood forest. Global Change Biology 4, 217±227.

de Jong, E., Schappert, J.V., MacDonald, K.B., 1974. Carbon diox-ide evolution from virgin and cultivated soil as a€ected by man-agement practices and climate. Canadian Journal of Soil Science 54, 299±307.

Ellert, B.H., Bettany, J.R., 1992. Temperature dependence of net nitrogen and sulfur mineralization. Soil Science Society of America Journal 56, 1133±1141.

Ellsworth, D.S., 1999. CO2enrichment in a maturing pine forest: are

CO2exchange and water status in the canopy a€ected? Plant Cell

& Environment 22, 461±472.

Farquhar, G.D., Ehleringer, J., Hubick, K.T., 1989. Carbon isotope discrimination during photosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 9, 121±137.

Hendrey, G.R., Ellsworth, D.S., Lewin, K.F., Nagy, J., 1999. A free-air enrichment system for exposing tall forest vegetation to elev-ated atmospheric CO2. Global Change Biology 5, 293±309.

Holland, E.A., Townsend, A.R., Vitousek, P.M., 1995. Variability in temperature regulation of CO2¯uxes and N mineralization from

®ve Hawaiian soils: implications for a changing climate. Global Change Biology 1, 115±123.

Hsieh, Y.-H., 1996. Soil organic carbon pool of two tropical soils inferred by carbon signatures. Soil Science Society of America Journal 60, 1117±1121.

Mooney, H.A., Field, C.B., 1997. The fate of carbon in grass-lands under carbon dioxide enrichment. Nature 388, 576±579. Insam, H., 1990. Are the soil microbial biomass and basal

respir-ation governed by the climatic regime? Soil Biology & Biochemistry 22, 525±532.

KaÈtterer, T., Reichstein, M., Andren, O., Lomander, A., 1998. Temperature dependence of organic matter decomposition: a critical review using literature data analyzed with di€erent models. Biology & Fertility of Soils 27, 258±262.

Kirschbaum, M.U.F., 1995. The temperature dependence of soil or-ganic mater decomposition, and the e€ect of global warming on soil organic C storage. Soil Biology & Biochemistry 27, 753±760. MacDonald, N.W., Zak, D.R., Pregitzer, K.S., 1995. Temperature

e€ects on kinetics of microbial respiration and net nitrogen and sulfur mineralization. Soil Science Society of America Journal 59, 233±240.

McClaugherty, C.A., Linkins, A.E., 1990. Temperature responses of enzymes in two forest soils. Soil Biology & Biochemistry 22, 29± 33.

Nadelho€er, K.J., Fry, B., 1988. Controls on natural nitrogen-15 and carbon-13 abundances in forest soil organic matter. Soil Science Society of America Journal 52, 1633±1640.

O'Leary, M.H., 1981. Carbon isotope fractionation in plants. Phytochemistry 20, 553±567.

Parada, C.B., Long, A., Davis, S.N., 1983. Stable-isotopic compo-sition of soil carbon dioxide in the Tucson Basin, Arizona, USA. Isotope Geochemistry 1, 219±236.

Park, R., Epstein, S., 1961. Metabolic fractionation of13_{C and}12_C

in plants. Plant Physiology 36, 133±138.

Parton, W.J., Schimel, D.S., Cole, C.V., Ojima, D.S., 1987. Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Science Society of America Journal 51, 1173± 1179.

Reardon, E.J., Allison, G.B., Fritz, P., 1979. Seasonal chemical and isotopic variations of soil CO2 at Trout Creek, Ontario. Journal

of Hydrology 43, 355±371.

Richards, B.N., Smith, J.E.N., White, G.J., Charley, J.L., 1985. Mineralization of soil nitrogen in three forest communities from the New England region of New South Wales. Australian Journal of Ecology 10, 429±441.

SAS Institute, 1990. SAS/STAT User's Guide, Release 6.03. SAS Institute, Cary, NC.

Trumbore, S.E., 1993. Comparison of carbon dynamics in two soils using measurements of radiocarbon in pre- and post-bomb soils. Global Biogeochemical Cycles 7, 275±290.

Wiant, H.V., 1967. In¯uence of temperature on the rate of soil res-piration. Journal of Forestry 65, 489±490.

Winkler, J.P., Cherry, R.S., Schlesinger, W.H., 1996. The Q10

re-lationship of microbial respiration in a temperate forest soil. Soil Biology & Biochemistry 28, 1067±1072.

Wollum II, A.G. 1982. Cultural methods for soil micro-organisms. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis Part 2: Chemical and Microbiological Properties. American Society of Agronomy, Madison, WI, pp. 781±802. Zogg, G.P., Zak, D.R., Ringelberg, D.B., MacDonald, N.W.,