14
C distribution in soil organisms and respiration after the
decomposition of crop residue in conventional tillage
and no-till agroecosystems at Georgia Piedimont
Shenglei Fu
*, David C. Coleman, Rita Schartz, Robert Potter,
Paul F. Hendrix, D.A. Crossley Jr.
Institute of Ecology, University of Georgia, Athens, GA 30602-2202 USA
Received 8 September 1999; received in revised form 21 March 2000; accepted 6 June 2000
Abstract
This study illustrated how crop residue-derived carbon interacts with non-residue carbon (e.g., native soil carbon) in agroecosystems and how carbon is allocated to soil organisms and respiration under different tillage regimes. The carbon dynamics in crop residue, soil microorganisms, nematodes and respiration were monitored using14C-labeled corn residue. In addition, the carbon budget was estimated for both conventional tillage (CT) and no-till (NT) agricultural ecosystems during the short period after residue application. A laboratory and a ®eld study were conducted separately to assess the above objectives. The results illustrated that the general patterns of carbon allocation were similar in both laboratory and ®eld studies but at a lower magnitude in the ®eld. Most14C input to soil was released into air through soil respiration (93±98%) under both CT and NT regimes, with only a small portion bound in microbial (1.8±6.5%) and nematode biomass (0.01±0.12%). However, more 14C was retained in microbial and nematode biomass under CT than under NT, while the 14C distributed in soil respiration was similar under both tillage regimes. The 14C speci®c activities of soil microorganisms, nematodes and respiration were signi®cantly higher under CT than under NT. The higher14C speci®c activities of soil microorganisms and
nematodes, and more 14C retained in the biomass of soil microorganisms and nematodes under CT, suggested that soil organisms might use C more ef®ciently under CT than under NT. During the short-term experiments, cumulative soil respiration was signi®cantly higher but residue-derived carbon contributed less to soil respiration under NT than under CT. Consequently, more non-residue carbon (e.g., native soil carbon) was decomposed and respired by soil organisms under NT than under CT after 40 days of the residue application. It is suggested that residue application might cause a net loss of soil carbon in agroecosystems possibly because of the priming effect of crop residue, particularly under NT regime under the short term.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Carbon budget; Conventional tillage; Georgia Piedmont; No-till; Soil organisms; Soil respiration
1. Introduction
Since 1960s, there has been a signi®cant increase in interest in no-till (NT) agriculture, and many studies have focused on the NT practice. Results have shown that NT agricultural practice has many advantages *Corresponding author. Present address: Department of
Envir-onmental Studies, University of California, Santa Cruz, CA 95064, USA. Tel.:1-831-459-3791.
E-mail address: [email protected] (S. Fu).
over conventional tillage (CT). These include erosion control, improved water retention, reduced labor requirements, and reduced fuel usage (Phillips and Phillips, 1984). A comparison of 22 agroecosystem components and processes between NT and CT in Georgia also showed that NT was a better agricultural practice than CT (House et al., 1984). Some studies showed rapid losses of soil organic matter following cultivation of soils converted from forest or sod of the southern Appalachian Piedmont (Giddens, 1957; Jones et al., 1966). Tillage increases the rate of organic matter decomposition by burying residue into soil and breaking soil aggregates, thereby increasing the sur-face area of soil organic matter to microbial coloniza-tion (Phillips and Phillips, 1984; Burke et al., 1995; Hendrix et al., 1998). The decomposition rate of CT buried litter is much faster than NT surface litter (Phillips and Phillips, 1984; Parmelee and Alston, 1986). A ®eld study using 14C-labeled wheat straw showed that a less proportion of added 14C was retained in the incorporated-straw treatment than in the surface-straw treatment (Holland and Coleman, 1987).
Some studies showed that soils managed by NT for extended periods of time have higher organic matter contents than if they are plowed (Lal, 1976; Blevins et al., 1977). Some other studies agreed that conserva-tion management practice (e.g., NT) can increase carbon concentrations in degraded soils (Kern and Johnson, 1993; Dixon et al., 1994; Burke et al., 1995). However, Angers et al. (1997) found that there was no signi®cant difference between tillage treat-ments in the total C storage down to 60 cm depth, however, the depth distribution of soil C varied with tillage. In the surface 0±10 cm, C content was higher under NT than under MP (conventional moldboard plowing), whereas at deeper levels (20±40 cm) the reverse trend was observed. Hendrix et al. (1998) found that intensive cultivation resulted in no obser-vable change in total C content at the end of 3 years, but there were carbon losses at the end of 16 years under both CT (40% loss in soil C) and NT (18% loss in soil C) regimes. In general, most studies have focused on the overall change of soil organic matter under different tillage regimes; unfortunately, the contribution of crop residue and non-residue carbon (e.g., native soil carbon) to soil system cannot be estimated separately.
Glucose and plant materials labeled with14C have been used in laboratory and in ®eld to estimate the carbon partitioning in different soils. Soil types, pH conditions, soil organisms and substrate nature have been proven to be important factors to affect carbon allocation in soil system (Marchant and Nicholas, 1974; Ladd et al., 1981, 1995, 1996; Kretzschmar and Ladd, 1993; Nelson et al., 1996; Sorenson et al., 1996; Ferris et al., 1997; Chotte et al., 1998). However, little infor-mation is available to address how residue-derived carbon interacts with non-residue carbon in agroeco-systems and how carbon is allocated to soil organisms and respiration under different tillage regimes.
The objectives of this study were to monitor the carbon dynamics of soil organisms and respiration and to estimate the carbon budget and carbon partitioning for both CT and NT during the short period after the application of crop residue. The14C speci®c activities of soil organisms and respiration were measured and
14
C distribution in soil system was also estimated to evaluate the C use ef®ciency of soil organisms under both CT and NT. The ultimate goals were to give a better understanding of the dynamic of soil carbon under different tillage regimes and to provide scien-ti®c information for the conservation of soil carbon in different agroecosystems.
2. Materials and methods
2.1. Site description
2.2. Experimental design
2.2.1. Laboratory study
On March 16, 1997, intact soil cores were taken from CT and NT ®eld plots, respectively. Each soil core from CT ®eld plot was crushed gently and visible residues were picked out by hand, and 1 gm of14 C-labeled corn leaf litter was then mixed with soil (equivalent to 509 g mÿ2). The corn leaf litter was cut into 11 cm2pieces. Soils were placed back in the metal core (5 cm in diameter and 5 cm in height) of the soil corer, representing CT treatment. Soil cores from NT ®eld plots were kept intact in the metal core but surface residues were carefully removed. One gram of corn leaf litter was then applied on the surface of each soil core, representing the NT treatment. Each CT or NT soil core was placed in a 1 l Mason jar with some wet ®lter paper to keep the moisture relatively con-stant. All Mason jars were placed in a growth chamber maintained temperature at a range of 22±258C. The Mason jars were opened periodically to moisten the ®lter paper and to allow for aeration, but were kept air-tight when soil respiration measurements were in progress. Three CT and three NT soil cores were destructively sampled at 2, 3, 8, 17, 32 and 40 days, respectively.
2.2.2. Field study
On April 6, 1998, CT plots were plowed by hand using a shovel and visible residues were removed. PVC tubes (10 cm in diameter, 10 cm in height) were placed into soil 5 cm deep. Four grams of14C-labeled corn leaf litter were then incorporated into soil in each core (equivalent to 509 g mÿ2
). Surface residue was removed from NT plots and PVC tubes were set 5 cm deep and 4 gm of corn leaf litter was applied on the soil surface inside each tube. Three CT and three NT soil cores were sampled at 3, 13, 32 and 40 days, respectively.
2.3. Measurements
2.3.1. Soil respiration and14C activity
Soil respiration was measured using static base traps (Anderson, 1982). Total CO2absorbed in alkali
(20 ml, 1 M) for 24 h was determined by titration of aliquots (5 ml) with standard HCl (0.25 M). The14C activity of soil respiration was measured by adding
0.5 ml of NaOH to Ecolite scintillation ¯uor (ICN Biomedicals, Costa Mesa, CA) and counting for 20 min in a model of LS-3801 liquid scintillation counter (Beckman Instruments, Fullerton, CA).
2.3.2. Soil nematodes and14C activity
Soil nematodes were extracted using the Baermann funnel method (McSorley, 1987). After ®xation in 4% formaldehyde solution nematodes were counted and measured using an inverted microscope. 1000 indivi-dual nematodes were randomly measured for width and length. Nematode biomass was then calculated according to Andrassy's (1956) formula and converted to dry weight assuming a dry mass content of 25% (Yeates, 1979). Nematode samples were ®ltered with glass micro®ber ®lters (25mmf, Whatman Cat No. 1820±025, Whatman International). Each ®lter with nematodes was put into a 20 ml vial. Nematodes were then digested with 1 ml of Scintigest (Fisher Scien-ti®c, Fair Lawn, NJ) for at least 48 h. The samples were diluted with 1 ml deionized water and were neutralized with 1 ml 0.6 M acetic acid. Twenty milli-liters of Scintiverse (Fisher Scienti®c, Fair Lawn, NJ) was added to each vial and14C activity was measured on a liquid scintillation counter. All values were corrected by quench curve and background counts were subtracted.
2.3.3. Soil microbial biomass and14C activity Soil microbial biomass and its 14C activity were measured in the ®eld study by the following proce-dure. Soil samples were stored at 48C for 2 days and then analyzed for microbial biomass using the chloro-form fumigation direct extraction method (Vance et al., 1987). Gravimetric moisture was determined for each soil sample. Roots were removed from soil samples by hand. Two subsamples, a fumigation (F) and non-fumigation (NF), of 20 g dry weight equiva-lents were taken from each ®eld soil sample. F sub-samples were fumigated with distilled chloroform for 48 h. F and NF subsamples were extracted with 60 ml of 0.5 M K2SO4. Extracts were analyzed for total
organic carbon (TOC) using a Total Organic Carbon Analyzer (TOC-500, Shimadzu, Kyoto, Japan). Microbial biomass carbon (MBC) was calculated as: MBC(FTOCÿNFTOC)/Kc. The Kc used was
K2SO4 extract to 15 ml of Ecolite scintillation ¯uor
(ICN Biomedicals, Costa Mesa, CA) and counting for 20 min with a model LS-3801 liquid scintillation counter. In the laboratory, soil microbial biomass was estimated from daily soil respiration by using the ratio of soil microbial biomass to daily soil respiration in the ®eld study. The14C speci®c activity of soil microbial biomass in laboratory study was estimated from the14C speci®c activity of soil respira-tion by using the ratio of 14C speci®c activity of microbial biomass to 14C speci®c activity of soil respiration in ®eld study.
2.3.4. Residue decomposition and14C lost
The corn residues were dried at 708C. Dry weights were recorded before the experiment started and at each sampling time, and subsamples were oxidized in an OX-500 Biological Oxidizer (R.J. Harvey Instrument Company, Hillsdale, NJ). Ash free dry weight (AFDW) was used to determine the decom-position rate of corn residues. The 14C activity of corn residue was measured on a liquid scintillation counter at the beginning and end of the experiment period.
2.4. Statistical analysis
Statistical analyses for all data were performed using SAS software (SAS Institute, 1985) GLM pro-cedure. Comparison among means was carried out using the LSD test for equal sample sizes and Scheffe's test for unequal sample sizes. Signi®cance levels were set atP<0.05.
Unless otherwise stated, all measurements were conducted at 0±5 cm depth and all data were expressed on dry mass basis. To calculate a carbon budget, all data were converted to a per square meter basis.
3. Results
3.1. Decomposition of corn residue and14C input
After 40 days, corn leaf residue decomposed 37.8 and 32.7% under CT and NT treatments, and 14C amounts lost to soil through decomposition were 7.2106and 6.5106Bq mÿ2under CT and NT for the laboratory study (Table 1). Decomposition rates of
corn residues in the ®eld were slightly lower than in the laboratory study, accounting for 36.4 and 23.7% under CT and NT treatments, respectively. The 14C amounts lost through decomposition were 5.1106 and 3.3106Bq mÿ2
under CT and NT for the ®eld study. In conclusion,14C lost through residue decom-position was at a signi®cantly lower rate under NT than under CT in both laboratory and ®eld, though residue decomposition rates were not signi®cantly different between the two tillage regimes in laboratory (Table 1).
3.2. Soil respiration
In the laboratory study, soil respiration was signi®-cantly higher under NT treatment than that under CT treatment except for a ¯ush under CT during the ®rst few days after the experiment started (Fig. 1A). Cumu-lative CO2-C under NT was almost twice that of CT
during the 40-day experimental period. They were 2.83105 and 1.46105mg mÿ2 for NT and CT, respectively. However, 14C speci®c activity of soil respiration was signi®cantly higher under CT than under NT, accounting for 63.13 and 29.23 Bq mgÿ1 CO2-C (Fig. 2A, Table 2). Taking both soil respiration
and its14C speci®c activity into consideration, cumu-lative CO2
-14
C for CT and NT was very similar during the 40-day experimental period, accounting for 6.2106and 6.3106Bq mÿ2
, respectively.
The trends of soil respiration and its 14C activity under both tillage regimes in the ®eld study were similar to those in the laboratory. More carbon (cumulative CO2-C) was released through respiration
under NT (3.5105mg mÿ2
) than under CT Table 1
Carbon and14C lost in the 40 days decomposition of corn residue in Horseshoe Bend soil, Athens, GA
Tillage Placement C lost (%)a 14C lost (Bq mÿ2)a
(1.6105mg mÿ2) throughout the entire period, how-ever, a ¯ush of soil respiration was not observed under CT at the beginning of the experiment (Fig. 1B). The
14
C speci®c activity of CO2was signi®cantly higher
under CT (31.6 Bq mgÿ1
CO2-C) than under NT
(10.67 Bq mgÿ1
CO2-C) (Fig. 2B, Table 2).
Never-theless, the14C speci®c activity of soil respiration was lower compared with that in the laboratory study. Cumulative CO2-14C was also less in the ®eld than
in the laboratory.
3.3. Soil microbial biomass and14C activity
Soil microbial biomass was signi®cantly higher under NT than under CT in the laboratory study. Their mean values were 639 and 455 mg C kgÿ1
soil, respec-tively (Fig. 3A). The 14C speci®c activity of soil microbial biomass was signi®cantly higher under CT than under NT throughout the entire experimental period, averaging 12.70 and 2.26 Bq mgÿ1
C (Fig. 4A, Table 2). For the ®eld study, soil microbial biomass and14C speci®c activity follow the same pattern as in
laboratory but their values were lower compared with those in laboratory. Soil microbial biomass was sig-ni®cantly higher under NT (720 mg C kgÿ1
soil) than under CT (355 mg C kgÿ1
soil) treatment (Fig. 3B). However, the14C speci®c activity of soil microorgan-Fig. 1. Daily soil respiration under different tillage regimes after
residue addition: (A) in the laboratory; (B) in the ®eld. Each data point represents the mean and the standard error of three replicates.
Fig. 2. The14C speci®c activity of soil respiration under different tillage regimes after residue addition: (A) in the laboratory; (B) in the ®eld. Each data point represents the mean and the standard error of three replicates.
Table 2
The 14C speci®c activity (Bq mgÿ1 C) of soil respiration, microorganisms and nematodes in Horseshoe Bend soil, Athens, GAa
Tillage Respiration Microorganisms Nematodes
Laboratory
CT 63.1311.43 12.692.90 68.5710.18 NT 29.236.34 2.260.23 18.486.12
Field
CT 31.6012.68 3.951.10 210.8518.04 NT 10.673.71 0.730.31 77.827.89
isms was much higher under CT than under NT at all times, averaging 3.95 and 0.73 Bq mgÿ1
C, respec-tively (Fig. 4B, Table 2).
3.4. Soil nematodes and14C activity
In the laboratory study, soil nematode biomass increased rapidly under CT, peaking around 1 month after the experiment began. Under NT, soil nematode biomass also increased but at a lower rate compared to CT (Fig. 5A). At the beginning of the experiment, soil nematode biomass was higher under NT than under CT, however, the trend was reversed 2 weeks later. The
14
C speci®c activity of soil nematodes was the highest on day 8 of the experiment and declined thereafter under CT and it increased gradually but did not peak until 1 month later under NT (Fig. 6A).
In general, there was signi®cantly higher nematode biomass under NT than under CT in the ®eld study. However, total nematode biomass did not change much in both CT and NT throughout the experimental
period and was generally much lower compared with the laboratory data (Fig. 5B). In the ®eld,14C speci®c activity of soil nematodes was signi®cantly higher under CT than under NT. The 14C speci®c activity peaked on day 13 of the experiment and declined thereafter in CT and it was hardly detectable until 1 month after residue application in NT (Fig. 6B).
In all cases, nematode biomass was much lower than microbial biomass (Figs. 3A and B, 5A and B). However, the14C speci®c activities of soil nematodes were much higher compared with those of soil micro-organisms (Table 2).
3.5. 14C and C budget
After 40 days of the experiment, most14C assimi-lated by soil organisms was released cumulatively into air via respiration (93.3±98.2%), with only a small portion bound in the soil microbial biomass (1.8± 6.5%) and nematodes (0.01±0.12%) under both tillage regimes in either laboratory or ®eld study (Table 3). Fig. 3. Dynamics of soil microbial biomass under different tillage
regimes after residue addition: (A) in the laboratory; (B) in the ®eld. Each data point represents the mean and the standard error of three replicates.
Total14C recovered in soil organisms and respiration was in the range of 88±99%. However, there was a tillage effect on 14C distribution and 14C recovery. More14C were bound in soil microbial and nematode biomass under CT than under NT while14C
distribu-tion in soil respiradistribu-tion were similar under both tillage regimes (Table 3). In general, total 14C recovered in soil organisms and respiration was slightly less under CT than under NT. The same patterns were observed in both the laboratory and ®eld studies.
Fig. 5. Soil nematode biomass under different tillage regimes after residue addition: (A) in the laboratory; (B) in the ®eld. Each data point represents the mean and the standard error of three replicates.
Fig. 6. The14C speci®c activity of soil nematodes under different tillage regimes after residue addition: (A) in the laboratory; (B) in the ®eld. Each data point represents the mean and the standard error of three replicates.
Table 3
The distribution (%) of the recovered14C in soil respiration, microorganisms and nematodes after residue decomposition, Horseshoe Bend, Athens, GA
Tillage 14C recovery (%)a The distribution (%) of recovered14C
Cumulative CO2b Microorganismsc Nematodesc
Laboratory
CT 92.4 93.35.8 6.51.4 0.120.02
NT 99.0 97.36.5 2.60.6 0.040.01
Field
CT 88.0 94.81.9 5.21.2 0.020.01
NT 97.9 98.26.4 1.80.3 0.010.00
a 14C recovery (%) is the percentage of total14C recovered in soil microorganisms, nematodes and respiration to total14C lost through the
decomposition of corn residue.
bMeans and standard errors of three replicates across 40 days.
3.6. Carbon contribution of residue to soil organisms and respiration
Similar trends of the contribution of residue-derived carbon to soil organisms and respiration were found in both the laboratory and ®eld studies and a signi®cant tillage effect was observed in both studies (Table 4). Under CT, most carbon in soil respiration (74.7± 84.1%) and nematodes (53.2±78.0%) were derived from residue through decomposition, but only a small portion of carbon in soil microorganisms (12.5± 17.8%) was derived from residue. Under NT, the fractions of residue-derived carbon in soil respiration and nematodes were also higher than that in microbial biomass though they were all lower than those under CT.
4. Discussion
4.1. Tillage effect
Soil respiration was signi®cantly higher under NT than under CT except that a ¯ush occurred under CT immediately after plowing. Soil microbial biomass was much higher under NT than under CT. Residue decomposed faster under CT than under NT, particu-larly in the ®eld. The 14C speci®c activities of soil microorganisms and nematodes and the percentages of
14
C distributed to soil microorganisms and nematodes were higher under CT than under NT, indicating that the carbon utilization ef®ciency of soil organisms was higher under CT than under NT. There was a lag under NT in terms of response of soil organisms to residue application. The placement of crop residue would be responsible for this tillage effect. The incorporation of crop residue into soil under CT resulted in an intimate contact between residue and soil matrix and a ready availability of carbon source to soil biota. Thus, the susceptibility of residue to soil microbial decomposi-tion was increased. In contrast, residues were strati®ed on the soil surface under NT and the contact of residues with soil organisms, particularly with deep soil-dwelling organisms, was restricted (Parmelee and Alston, 1986; Holland and Coleman, 1987).
A ¯ush of soil respiration was observed under CT immediately after residue incorporation. This may be attributed to the ¯ush of microbial activity responding to the availability of soluble carbon from residue. Swift et al. (1979) reported that plant residues could contain up to 25% soluble, readily decomposable compounds and Chotte et al. (1998) observed that the mineralization rate of substrate-derived 14C was highest during the ®rst 3 days of incubation. Another possible explanation is that soil manipulation under CT affects carbon availability by disturbing soil structure and exposing protected organic materials (Hendrix et al., 1988).
The pattern for the 14C budget was intriguing. In both tillage treatments, most 14C assimilated by soil organisms was lost through respiration, with only a small portion bound in biomass of soil organisms. However, more14C was bound in the biomass of soil microorganisms and nematodes under CT than under NT though14C distributed in soil respiration under CT was similar to that under NT. It is hypothesized that this might be an alternative mechanism to retain carbon in soil by sequestering more carbon in the biomass of soil organisms under CT.
The overall14C recovered under CT (88±92%) was less than under NT (98±99%). This raises the question, where was the rest of the14C under CT? Kretzschmar and Ladd (1993) suggested that there could be some gaseous loss in a form other than CO2in soils of high
compaction. Anaerobic products of decomposition such as methane (CH4) were not measured when plant
material was buried at depth. In the present study, soil Table 4
Percentages of residue-derived carbon to total carbon in soil respiration, microorganisms and nematodes, Horseshoe Bend, Athens, GA
Tillage Fraction of residue-derived carbon (%)a
CO2 Microorganisms Nematodes
aValues with different letters in the same column or in the same
row were signi®cantly different atP<0.05 level.
bMeans and standard errors from three replicates across ®ve
sampling dates.
cMeans and standard errors from three replicates across four
cores taken from NT ®eld plots were kept intact, while soil cores taken from CT ®eld plots were mixed with residue and then returned to the metal core by hand, consequently, soil was more compacted under CT treatment. Under ®eld situations, repeated tractor passage over CT plots might also cause soil compac-tion (Aritajat et al., 1977). The bulk densities of CT and NT were 1.25 and 1.18 g cmÿ3in the laboratory study and 1.30 and 1.10 g cmÿ3in the ®eld, re¯ected that soil was more compacted under CT treatment. In an incubation study, Chotte et al. (1998) also found a
14
C de®cit. They hypothesized that the 14C de®cit might result from the failure to trap all 14CO2
pro-duced during the ®rst few days of rapid microbial decomposition of added substrate. In the present study,
14CO
2for the ®rst 3 days was extrapolated because no
measurements were made. This might cause a greater error under CT than under NT due to the faster decomposition rate of residue under CT than under NT during the ®rst few days.
4.2. The effect of substrate quality
The14C speci®c activities of soil microbial biomass and nematodes in the present study were lower com-pared with the Yeates et al. (1998) study. This might be due to the different quality of substrate in the two studies. Crop residue (particulate carbon) was used as substrate in the present study while root exudates (soluble substrate) were the carbon source in the Yeates et al. (1998) study. Sorenson et al. (1996) and Chotte et al. (1998) illustrated that14C incorpora-tion into microbial biomass was higher when using soluble substrate rather than particulate carbon. Another cause of the difference between the present and Yeates et al. (1998) on the result of14C speci®c activity of soil nematodes, is that only one sedentary species (Heterodera trifolii) was selected in the Yeates et al. (1998) study whereas in the present study the entire nematode population was considered.
4.3. Environmental effect
In the laboratory, the biomass of soil organisms, particularly soil nematodes, increased tremendously soon after residue application. However, in the ®eld, there was generally a lag in the response of soil organisms to residue application. The biomass of soil
organisms in the ®eld study was little changed. The peak of 14C speci®c activities of soil respiration, microbial biomass and nematodes also occurred later in the ®eld compared with the laboratory. In addition, the 14C speci®c activities of soil respiration and soil microbial biomass in the ®eld were signi®cantly lower, only half of that of the laboratory study. How-ever,14C speci®c activity of soil nematodes was higher in the ®eld than laboratory study. The causes remain unclear. In general, the patterns of C and14C budget for both studies were similar.
The different moisture and temperature conditions might be responsible for the differences between laboratory and ®eld studies. Quemada and Cabrera (1997) found that total CO2 evolved from residue
decomposition increased exponentially with water potential (c) when c increased from ÿ5.0 to ÿ0.003 (saturation) MPa, but increasingchad little effect on total CO2evolution as saturation approached.
Quemada and Cabrera (1997) also stated that the effect of con CO2evolution was enhanced as
tem-perature (T) increased. A linear relationship between CO2 evolved and temperature was found when soil
water potential was held at a speci®c level (Stott et al., 1986). In the present study, soil water potentials were close to saturation in both laboratory (ÿ0.004 to ÿ0.008 MPa) and in ®eld (ÿ0.002 to ÿ0.007 MPa); therefore, they might not cause signi®cant difference of residue decomposition between laboratory and ®eld studies. However, different temperature in laboratory (22±258C) and in ®eld (13±178C) might have caused the signi®cant difference between the laboratory and ®eld studies. Soil organisms obtain bene®t from favor-able soil temperature in the laboratory, and conse-quently, utilize carbon more ef®ciently and their biomass increased more rapidly than in the ®eld.
4.4. Contribution of residue-derived carbon and non-residue carbon
Under CT, residue-derived carbon contributed to soil respiration and soil nematodes much more than non-residue carbon; nevertheless, a greater proportion of non-residue carbon was bound in microbial bio-mass. The carbon in soil organisms and respiration are mainly from non-residue carbon under NT.
lost through soil respiration, which illustrated that a certain amount of non-residue carbon (e.g., native soil carbon) was decomposed by soil organisms possibly resulting from the priming effect of crop residue (Shen and Bartha, 1997; Pascual et al., 1998; Kuzyakov et al., 2000). In other words, residue application might have accelerated the decomposition of native soil carbon and caused a net loss of total soil carbon in agroeco-systems, particularly under NT regime under a short term.
5. Conclusion
The results illustrated that the general patterns of carbon allocation in different tillage systems were similar in both laboratory and ®eld studies but at a lower magnitude in the ®eld. The higher14C speci®c activities of soil microorganisms and nematodes, and more14C retained in the biomass of soil microorgan-isms and nematodes under CT suggested that soil organisms might use C more ef®ciently under CT than under NT. Cumulative soil respiration was signi®cantly higher but residue-derived carbon contributed less to soil respiration under NT than under CT, therefore, more non-residue carbon (e.g., native soil carbon) was decomposed and respired by soil organisms under NT than under CT after 40 days of the residue application. It is suggested that residue application might cause a net loss of soil carbon in agroecosystems, particularly under NT regime for a short term.
Acknowledgements
This work was supported by a grant from the National Science Foundation to the Institute of Ecology, University of Georgia. We thank Keith W. Kisselle, Carol J. Garrett, Betty Weise, Paula Marci-nek, Patricia Huback, Kathy Sasser, Brent Andrews and Sherry Farly for their ®eld and laboratory assis-tance.
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