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Soil Science Society of America Journal 63:1385-1396 (1999)
© 1999 Soil Science Society of America

DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Maize Residue Decomposition Measurement Using Soil Surface Carbon Dioxide Fluxes and Natural Abundance of Carbon-13

Philippe Rochettea, Denis A. Angersa and Lawrence B. Flanagana

a Dep. of Biological Sciences, Univ. of Lethbridge, 4401 University Dr., Lethbridge, AB, Canada, T1K 3M4

rochettep{at}em.agr.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
The decomposition rate of crop residues in soils directly impacts organic matter content and nutrient cycling. We hypothesized that natural abundance 13C analyses could be used with soil CO2 flux measurements to quantify the short-term decomposition rates of maize (Zea mays L.) residues under undisturbed field conditions. For this purpose, maize was grown in a sandy loam (Umbric Dystrochrept) that developed under C3 vegetation. Residues were returned to the field at the end of the growing season. During the following snowfree period (May to November), the maize residue decomposition rate was calculated for plots that were either under no-till or moldboard plowed, using the C isotope ratio (13C/12C) of the soil CO2, the C isotope ratio of the plant and soil substrates, and the soil respiration rate. The incorporation of residue-derived C into the soil microbial biomass was also evaluated. Maize residue decomposition increased the C isotope ratio of the soil CO2 by 2 to 7{per thousand} relative to unamended control plots. Decomposition rates peaked in June (2–3 g C m-2 d-1) and were low at both the beginning and end of the growing season (<0.5 g C m-2 d-1). For a given soil temperature, the decomposition was more active early than late in the season because of decreased substrate availability as decomposition proceeded. The decomposition rate of maize-derived C correlated with the fraction of the microbial biomass derived from maize residues. This active pool represented 9% of microbial biomass and showed a high level of specific activity. The total maize residue-C losses during the study corresponded with 35% of the added residue C under no-till plots and 40% with moldboard plowing. Natural abundance 13C analyses may be successfully used with respiration measurements to quantify crop residue decomposition rates under undisturbed field conditions.

Abbreviations: DM, dry matter • MBC, microbial biomass C


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
SOIL ORGANIC MATTER is central to soil functioning and the maintenance of soil quality. Its role in determining soil structural processes, erosion control and nutrient supply is well established. Soil organic matter is also a significant pool of biospheric C, and changes in soil C content directly affects the atmospheric CO2 (Buyanovsky and Wagner, 1998). Crop residues are the major and often the only input of C in agricultural soils. It is therefore essential to understand the dynamics of their decomposition in soils in order to predict the effects of soil and crop management on soil C content.

The decomposition rate of 14C- or 13C-labeled crop residues has been studied under laboratory and field conditions (e.g., Jenkinson, 1977; Broadbent and Nakashima, 1974; Voroney et al., 1989; Aita et al., 1997). Essentially, the methodology consisted of incorporating previously labeled residues into the soil and determining the amount of residue-derived C remaining in the soil after a given period. This technique provides precise estimates of decomposition rates. However, the information obtained is mostly relevant to aboveground crop residues. The natural abundance of 13C has also been used to study C transformation in soils where the C isotope ratio of the crop was contrasting with that of the soil C (Cerri et al., 1985). This technique was successfully used to determine the medium-term (5–30 yr) dynamics of soil organic matter and its various fractions under different soil and management conditions (Balesdent et al., 1990; Angers and Giroux, 1996; Gregorich et al., 1996). One of the major advantages of the use of the natural abundance of 13C compared with the incorporation of prelabeled residues, is that the above- and belowground crop residues are incorporated into the soil via natural crop growth and typical agricultural management practices.

An alternative to the monitoring of the disappearance of the residues in soil is the measurement of the rate at which the CO2 is produced during the decomposition of the crop residues. Respiration rates provide quantitative information on the short-term dynamics of the decomposition processes. Decomposition rates of residues have been determined in the laboratory, where soil samples amended with residues were incubated under controlled conditions. The CO2–C originating from the residues was then measured using either isotope techniques (Broadbent and Nakashima, 1974; Wu et al., 1993) or by comparison with unamended control plots (Reinertsen et al., 1984; Recous et al., 1995). There are few reports of the use of respiration-based techniques for measuring the decomposition of specific C sources under field conditions. The rate of decomposition of manure (Gregorich et al., 1998; Rochette and Gregorich, 1998) and crop residues (Jensen et al., 1997) has been measured using the difference in total soil respiration rates between amended and control plots, while Aita (1996) used an isotope technique to quantify the decomposition rate of artificially 13C-labeled wheat (Triticum aestivum L.) residues. Rochette and Flanagan (1997) and Rochette et al. (1999) used the difference in C isotope ratio between C3 and C4 plants to separate the total soil respiration into root and soil components under undisturbed field conditions. They estimated the instantaneous decomposition rates of native soil organic C at regular intervals during the growing season.

In this study, we hypothesized that natural abundance 13C analyses could be used with soil CO2 flux measurements to quantify the short-term decomposition rates of above- and belowground maize crop residues under undisturbed field conditions.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Study Site and Description of the Experiment
The study site was located at the Chapais Experimental Farm, Lévis, Québec in a 0.4-ha field with 5% slope facing northwest. The soil was a St-Pacôme sandy loam (Umbric Dystrochrept) with 0.70 g g-1 sand, 0.11 g g-1 silt, and 0.19 g g-1 clay, a pHH2O of 6.5 ± 0.1 and average C and N contents of 19.9 ± 2.0 g kg-1 and 1.78 ± 0.19 g kg-1, respectively. The average bulk density prior to tillage was 1.31 ± 0.09 g cm-3. Barley (Hordeum vulgare L.) was grown in this field from 1993 to 1995 under conventional tillage practices (fall moldboard plowing). To our knowledge, no C4 plants have ever been grown in the field.

The experimental design was a randomized complete block with four replicates. The treatments consisted of (i) maize followed by barley under fall moldboard plowing (moldboard), (ii) maize followed by barley under no-till (no-till), and (iii) bare soil control (control). The study was carried out across 2 yr on 7 by 11 m plots. In 1996, maize was grown on the total surface of the field except for the four control plots, which were kept free of vegetation by manual weeding. On Day 278 in 1996, the maize crop was cut and the aboveground residues (10.83 Mg dry matter [DM] ha-1; 431.0 g C kg-1 DM; 9.4 g N kg-1 DM) were left on the soil surface. Moldboard plowing to a depth of 0.20 m was performed on Day 299 in 1996, while the no-till plots were left untouched. The four control plots were also kept bare in 1997 by regularly removing vegetation by hand and by one rototillage on Day 167. In 1997, barley was seeded at a rate of 156 kg ha-1 on Day 132 on the plots in which maize was grown in 1996. Fertilizers were applied prior to planting according to soil tests. Following harvest (Day 217), barley crop residues were left undisturbed on the soil surface for the rest of the season.

Total Soil Respiration
Four rectangular acrylic collars (100 cm long by 15 cm wide by 14 cm high with a wall thickness of 6.35 mm) were inserted to a depth of 10 cm on each of the 12 plots on Day 122 in 1997. The collars were left in the soil for the duration of the experiment except for a brief period in May to allow for planting. Total soil respiration (Rt) was measured with a dynamic closed-chamber system described by Rochette et al. (1997). The system consisted of a rectangular acrylic chamber (100 cm long by 15 cm wide by 15 cm high with a wall thickness of 6.35 mm) connected to a portable CO2 analyzer (LI-6200, Li-Cor, Lincoln, NE). At the time of measurement, the chamber was attached to the acrylic collars by clamps for a period of {approx}2 min during which the rate of increase of CO2 concentration was monitored in four successive periods of 20 s. The respiration rate was calculated for each 20-s period and the four values were averaged for the flux estimate (Rochette and Angers, 1999). On each sampling day, Rt was measured successively on all 48 collars between 1000 and 1400 h. Measurements of Rt began on Day 125 and were repeated weekly (21 times) until Day 307.

Soil temperature at 10 cm was measured inside each soil respiration collar at the time of respiration measurement using copper-constantan thermocouples. The thermocouples were installed at the same time as the respiration collars and were left in place for the duration of the experiment. They were read with digital thermometers (model HH23, Omega Inc., Stamford, CT and model 600-2820, Barnant Co., Barrington, IL). Soil moisture in the 5- to 25-cm layer of plots in Repetition 2 was measured using time domain reflectometry probes (Topp et al., 1984) on 17 d from Day 172 to the end of the season. Instrument malfunction prevented the measurement of soil moisture in Repetitions 1, 3 and 4, and prior to Day 172.

Carbon Isotope Ratio of Soil Carbon Dioxide and Organic Substrates
Soil air samples were taken at the soil surface inside enclosures. One enclosure, consisting of a collar identical to those used for measuring respiration, was installed in each plot. This collar was inserted 14 cm deep so that the top of the collar was even with the soil surface. Twenty four hours before sampling, an acrylic lid (thickness 6.35 mm) was sealed to the collar with vinyl tape (thickness 0.013 cm, width 7.5 cm). When tested under lab conditions, the vinyl tape kept leaks below 2.5% for periods of 6 d. The absence of a headspace kept contamination of samples with atmospheric CO2 to a minimum. Sampling of air in soil surface enclosures was chosen as an alternative to direct sampling of soil air to account for the CO2 produced by the decomposition of the residues at the surface of the no-till soil. At the time of sampling, air was removed from the enclosures via a plastic tube (Bev-A-Line IV, Labcor, Anjou, Québec) connected through the lid to an evacuated 250-mL glass gas sampling bulb (Kontes, Vineland, NJ). Before collecting the soil air sample, 10 mL of air was removed from the tube and discarded to account for the dead volume of the tube. After collecting the air sample, the bulbs were returned to a lab for cryogenic extraction of the CO2 according to the technique described by Rochette and Flanagan (1997). The purified CO2 was analyzed on a gas isotope ratio mass spectrometer (Sira 12, VG Isotech, Middlewich, Cheshire, UK) at the Ottawa-Carleton Stable Isotope Facility.

Maize and barley plants were collected from the field. The plants were washed and subsamples of leaves and roots were taken from a range of positions on the plant. The subsamples were combined for an individual plant, and the material was dried at 65°C, and ground to a fine powder with a tissue grinder. Prior to the experiment, soil samples were taken from the 20-cm surface layer and dried in an oven at 60°C for 24 h. Roots from the previous vegetation were removed from the samples, and the soil was ground to a fine powder with a mortar and pestle. Plant and soil samples were analyzed on an isotope ratio mass spectrometer interfaced with a Robo-Prep Sample Converter (Europa Scientific, Crewe, UK) at the Stable Isotope Facility of the Department of Soil Science at the University of Saskatchewan.

Isotopic compositions are expressed using delta notation:

(1)

The international standard for CO2 samples is CO2 from Pee Dee Belemnite (PDB) limestone (Ehleringer and Osmond, 1989). The values are presented in parts per thousand ({per thousand}). Isotope ratios in soil CO2 samples were corrected for the presence of N2O by the procedure described by Friedli and Siegenthaler (1988).

Maize Residue Decomposition Rates
A mass balance approach was used to calculate the contribution of maize residue decomposition to the total soil surface CO2 flux (Rresid,frac). This approach is similar to that proposed by Mary et al. (1992) after simplification by assuming that there was no contamination of soil air by atmospheric CO2:

(2)
where Rresid is the emission rate of the CO2 produced by the decomposition of the maize residues, {delta}13CO2 cn and {delta}13CO2 ctl are the {delta}13C of the soil CO2 in the maize and control plots, respectively, and {delta}13Ccn and {delta}13Csoil are the {delta}13C of the maize C and soil C, respectively. This approach assumes that the {delta}13C of the soil C and of the barley C are the same and that there is no isotopic fractionation during respiration processes. Mary et al. (1992) reported a fractionation of several per thousand when various substrates were incubated in the laboratory. On the other hand, Lin and Ehleringer (1997) showed that there is little fractionation during dark respiration processes in plants. It was also observed that the CO2 respired by the roots of young wheat plants had the same {delta}13C as the root tissues (Cheng, 1996), and that the {delta}13C of soil CO2, once corrected for the slower diffusion of the heavier 13C isotope, was identical to the {delta}13C of soil C (Cerling et al., 1991). We feel justified, therefore, in making the assumption that no fractionation occurred during respiration processes.

The decomposition rates of maize residues were calculated as the product of the total soil respiration by the contribution of residues to the total respiration:

(3)

Cumulative estimates of C loss from maize decomposition were made by linearly interpolating between sampling dates. Soil respiration measurements made between 1000 and 1400 h were assumed to be representative of average daily values. In Eastern Canada, soil respiration is largely controlled by temperature (Rochette and Gregorich, 1998), and for this assumption to be valid, soil temperature between 1000 and 1400 h has to be similar to the daily average. Measurements of soil temperature at 10 cm made in the field between Days 185 and 195 in 1998, showed that mean values between 1000 and 1400 h varied from 0.9°C cooler to 0.7°C warmer than the daily average.

Microbial Biomass Carbon and its {delta}13C
Soil samples were collected from the 15-cm surface soil layer 19 times during the experiment. Samples were sieved in the field at 6 mm and stored immediately at 3°C. Soil microbial biomass C measurements were carried out within 24 h of sampling using the chloroform-fumigation extraction technique (Wu et al., 1990). Two 50-g subsamples of field-moist soils were placed in 100-mL beakers. One subsample was fumigated for 24 h in a vacuum desiccator containing 25 mL of CHCl3. The other subsample was kept in the dark at 3°C for 24 h. Both fumigated and nonfumigated soils were extracted with 100 mL of 0.25 M K2SO4. After shaking for 1 h on an oscillating shaker, the suspensions were centrifuged at 1000 g and filtered. The organic C content of the extracts was determined by UV-persulfate oxidation on a DC-180 Carbon Analyzer (Dohrmann Co., Santa Clara, CA). An extraction efficiency (Kec factor) of 0.45 was used to calculate the microbial biomass C (Wu et al., 1990). Microbial biomass C was expressed in g C m-2 using a 15-cm sampling depth and bulk densities. For that purpose, soil bulk density was measured in each plot once in August 1997 using cylindrical soil cores (3 cm high x 6 cm i.d.). Average values (± standard deviation) were 1.30 (0.07), 1.39 (0.03), and 1.30 (0.04) g cm-3 for the moldboard, no-till, and control treatments, respectively. The approach to determine the {delta}13C of the K2SO4 extracts was adapted from Wanniarachni (1997). The K2SO4 extracts of the samples from four sampling dates (Day 141,168, 210, and 253) were dialyzed using 11.5-mm-diameter dialysis tubing with a molecular weight cutoff of 3500 (Spectra/Por 3500, Spectrum Laboratories, Laguna Hills, CA) in distilled water first for 16 h followed by an additional 4 h. The dialyzed extracts were freeze-dried and analyzed on an isotope ratio mass spectrometer interfaced with a Robo-Prep Sample Converter (Europa Scientific, Crewe, UK) at the Stable Isotope Facility of the Soil Science Department at the University of Saskatchewan.

The {delta}13C of the microbial biomass C ({delta}13CMBC) was calculated using the formula:

(4)
where CFUM and CNF are the amounts of C extracted from the fumigated and nonfumigated samples, respectively, and {delta}13CFUM and {delta}13CNF are the corresponding isotope ratios.

Statistical Analysis
A separate analysis of variance using a randomized complete block design was performed at each sampling date for the parameters related to CO2 emissions as these were measured at the same location (fixed chamber) throughout the experiment. For parameters related to microbial biomass, pooled samples were obtained from various locations in each plot at each sampling date and a split-plot analysis was used with treatments as the main-plot treatment and sampling time as the subplot treatment. Contrast analysis was used to compare the means (Snedecor and Cochran, 1980). All statistical analyses were performed using the SAS GLM procedure (SAS Inst., 1989).


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Soil Temperature and Soil Water Content
The first soil air samples and CO2 flux measurements were taken 5 d after snowmelt when soil temperatures at 10 cm were between 6 and 8°C (Day 125) (Fig. 1a) . Maximum temperatures (20–24°C) were reached around Day 180 and were followed by a gradual decrease to reach 8°C on Day 307. Temperature in the cropped plots was lower (P <= 0.05) than in control plot between Days 147 and 211, as a result of the shading of the soil by the barley crop. The insulating effect of the maize residues on the surface of the no-till soil resulted in lower (P <= 0.05) temperature in the no-till than in the plowed soil early in the season (Day 147–162). Maize residues had no effect (P > 0.43) on soil temperature after canopy closure (Day 180) and there were no significant (P > 0.35) differences in temperature among treatments after harvest (following Day 217).



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  		Fig. 1 (a) Soil temperature at 10 cm and (b) soil water content in the 5- to 25-cm layer during the 1997 growing season in control, no-till, and moldboard-plowed plots. Error bars indicate standard deviations (n = 4)

 
At the time of maximum crop vegetative development (Day 170–180), soil water content fluctuated between 0.2 and 0.3 m3 m-3 (Fig. 1b). Evapotranspiration coupled with low rainfall (29 mm between Day 185 and 224) decreased soil water content in cropped soils to levels below 0.15 m3 m-3 between Day 200 and 225. Abundant rain between Day 225 and 234 (135 mm) raised soil moisture to levels >0.25 m3 m-3. Soil moisture remained high after harvest due to frequent rainfall and the absence of crop transpiration. No-till soil had a tendency to be wetter than plowed soil during most of the season, while soil moisture in the control was less affected by the mid-season dry period, thereby highlighting the critical role of crop transpiration on soil moisture dynamics.

Total Soil Respiration
Total soil respiration rates in cropped plots varied between 1 and 7 g C m-2 d-1 during the experiment (Fig. 2) . These respiration rates are similar to those reported for other soils under continuous cropping in eastern Canada (Rochette et al., 1991, 1992). Respiration in cropped treatments followed a similar pattern early and late in the season (Fig. 2). Significant differences (P <= 0.05) were observed only on Day 155 and 162, resulting from the relatively high variability of Rt. The absence of both root respiration and decomposition of previous-year crop residues resulted in significantly lower respiration (P <= 0.05) in the control plots than in cropped plots on all days except Days 155, 162, 170, and 178.



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  		Fig. 2 Total soil respiration during the 1997 growing season in control, no-till, and moldboard-plowed plots. Error bars indicate standard deviations (n = 4)

 
Soil respiration in cool and moist climates is mostly controlled by soil temperature. Variations in soil temperature have been shown to explain as much as 95% of the variation in soil respiration when other factors such as moisture and substrates were not limiting (Rochette and Gregorich, 1998). In our study, only 19 to 46% of the variation in respiration was explained by temperature (Fig. 3) . However, the respiration–temperature relationships can be used to identify periods when factors other that temperature affected soil respiration. In the control plots, two events increased respiration rates well above the levels predicted by the respiration–temperature curve (Fig. 2 and 3). Rototillage of the plots on Day 167 had a strong stimulating effect on decomposition and resulted in high respiration rates in the following two sampling dates. Respiration on Day 237was also higher than expected. This probably reflected a period of high but short-lived respiration observed following rewetting of the soil after a dry period in August (Rochette et al., 1991). In the plowed soil, the only period during which a factor other than temperature appeared to have a strong influence on respiration was the midsummer drought (Fig. 3b). During that period (Day 200–230), respiration rates were well below the respiration–temperature curve, indicating that soil moisture was limiting respiration. In the no-till soil, the midsummer drought had a lower effect on respiration than in plowed soil (Fig. 3c). Respiration in no-till plots was unusually high on Day 279 when measurements were made during a warm spell in the fall (Fig. 1a and 2).



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  		Fig. 3 Total soil respiration as a function of soil temperature at 10 cm in (a) control, (b) no-till, and (c) moldboard-plowed plots. Open squares indicate the measurements made during the 2 wk following rototillage of the control plots on Day 167. Open triangles indicate measurements made during the period when soil water content was below 0.15 m3 m-3 (Day 200–225)

 
Carbon Isotope Ratio of Soil Carbon Dioxide
The C isotope ratio of soil CO2 in cropped plots was significantly (P <= 0.01) enriched in 13C on all sampling dates but Day 304. The {delta}13C values were maximum (-15{per thousand}) at the beginning of the season and decreased linearly to about -20{per thousand} on Day 260 (Fig. 4) . The isotope ratio of soil CO2–C in no-till was significantly different (P <= 0.05) than in plowed plots only on Day 189. The {delta}13C values of the CO2 in the control plots fluctuated between -22 and -24{per thousand} during most of the experiment (Fig. 4). These soil CO2 isotope ratios are 4 to 6{per thousand} higher than the {delta}13C value of the native soil C (-28.2{per thousand}), which is the presumed source of respired soil CO2–C. This difference between the {delta}13C value of the soil CO2 and the source respired CO2 was consistent with the 4.4{per thousand} value predicted by models based on the slower diffusion of heavier 13CO2 molecules than that of the 12CO2 molecules (Dorr and Munnich, 1980; Cerling et al., 1991).



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  		Fig. 4 Carbon isotope ratio of soil CO2. Error bars indicate standard deviations (n = 4)

 
The {delta}13C value of soil CO2 in the control plots was higher than -22{per thousand} for the first and the last sampling dates. Similar increases in {delta}13C of the soil CO2 in control plots (Rochette and Flanagan, 1997) and under grass (Hesterberg and Siegenthaler, 1991) late in the season have been previously reported. They are probably the result of the changes in the concentration of CO2 within the soil air space that are caused by low respiration rates following frost, and potential contamination of the soil air by atmospheric CO2 by convective transfer associated with the air being cooler than soil. In the absence of a reliable measurement of the {delta}13C of the soil CO2 on Day 304, the {delta}13C measured on Day 284 was used in the decomposition calculations for Day 304.

Decomposition Rates of the Maize Residues
The C isotope ratio of the substrates for respiration was –12.0{per thousand} ± 0.3 for the maize tissues, –28.2{per thousand} ± 0.1 for the native soil C and –28.3{per thousand} ± 0.1 for the barley tissues. These values were used in Eq. [2] to calculate the contribution of the decomposition of maize residues to total soil respiration. The contribution of residues increased in May, peaked at almost 50% in early June and decreased gradually to <20% at the end of the season (Fig. 5) . The contribution of residues was similar in no-till and plowed plots except for the period from Day 160 to 189 when values were lower in no-till. However, the difference was significant (P <= 0.05) only for Day 189.



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  		Fig. 5 Contribution of the decomposition of maize crop residues to the total soil respiration during the 1997 growing season in no-till and moldboard-plowed plots. Error bars indicate standard deviations (n = 4)

 
Estimates of instantaneous decomposition rates of the maize residues were made by multiplying the total respiration rates by the relative contributions of maize residues to soil CO2 (Eq. [3]). An estimate was produced for each day when respiration was measured by using the nearest estimate of residue contribution to respiration. Decomposition rates were significantly greater than zero (P <= 0.05) at all sampling dates except Days 125, 253, and 307, and rates were larger (P <= 0.05) in moldboard than in no-till plots on Days 162 and 189 (Fig. 6) . Decomposition rates were low in May, peaked in June at 2.2 g C m-2 d-1 for no-till and 3.0 for moldboard, and gradually decreased from early July to the end of the season. These values are in agreement with decomposition rates (1–2 g C m-2 d-1) of wheat straw artificially labeled with 13C measured from the emitted 13C–CO2 under field conditions (Aita, 1996).



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  		Fig. 6 Decomposition rates of the maize residues during the 1997 growing season in no-till and moldboard-plowed plots. Error bars indicate standard deviations (n = 4)

 
Examination of the decomposition–temperature graphs provides information on the decomposition during the experiment. Measurements made early in the season produced greater decomposition rates at a given temperature than measurements made later in the season, as indicated by different (P < 0.05) linear relationships before and after Day 156 (Fig. 7) . As the season progressed, decomposition slowed, in part due to proportionally more resistant maize-C compounds undergoing decomposition with time. The larger slope in no-till (0.18 g C m-2 d-1 C-1) than in the plowed plots (0.13 g C m-2 d-1 C-1) early in the season (Fig. 7a and 7b) may have been due to less decomposition in these plots prior to Day 125. This observation has practical importance since it suggests that decomposition in no-till soils can be as high as in plowed soils early in the year despite cooler soil temperature due to the insulating effect of the residues at the soil surface of the no-till soil. Also, during the mid-season drought, the decomposition rates in the plowed plots were well below the decomposition–temperature curve, while the decomposition in no-till was apparently not limited by low soil moisture (Fig. 7a and 7b). This lower sensitivity of decomposition to dry conditions in no-till than in plowed soils may be explained by the buffering of the extreme environmental conditions (temperature and moisture) by the residue layer on the soil surface. Finally, when sampling dates during spring and drought are removed, the response of decomposition of maize residues to temperature was nearly identical in both treatments (moldboard: decomposition = 0.113Ts – 0.75, r2 = 0.87, n = 10; no-till: decomposition = 0.115Ts – 0.76, r2 = 0.69, n = 12), where Ts is soil temperature (Fig. 7a and 7b). This similar response of decomposition to changes in temperature in both treatments in summer and fall when soil moisture was not limiting suggests that similar maize-C compounds were being decomposed in both treatments during that part of the season.



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  		Fig. 7 Decomposition rate of the maize residues as a function of soil temperature at 10 cm in (a) no-till and (b) moldboard-plowed plots. Open triangles indicate measurements made during the period when soil water content was below 0.15 m3 m-3 (Day 200–225)

 
Cumulative estimates of C loss from maize decomposition were made by linearly interpolating between sampling dates. Cumulated residue-C losses for the season were 208 g C m-2 in the plowed soil and 183 g C m-2 in the no-till soil (Fig. 8) . These values represent 40 and 35% of the amount of maize residues returned to the soil, respectively. Field studies of the decomposition rate of 14C-labeled crop residues reported disappearance of 45 to 70% of the residues during the growing season following incorporation (Oberlander and Roth, 1968; Shields and Paul, 1973; Jenkinson, 1977; Amato et al., 1987; Wu et al., 1993). The lower estimates of decomposition in our study may be partly explained by the exclusion of the losses that occurred in late fall of 1996 and during the 1996-1997 winter when a large fraction of the labile C may have been respired. Indeed, incubations in the laboratory have shown that the decomposition rates of residues are highest shortly after the incorporation of the residues because of the decomposition of readily decomposable compounds (Broadbent and Nakashima, 1974; Reinertsen et al., 1984; Recous et al., 1995). We estimated that between 350 and 500 soil degree days (>0°C) accumulated between harvest of the maize crop and the initiation of the CO2 flux measurements. Gilmour et al. (1998) observed that {approx}20% of the maize residues was decomposed when incubated under equivalent heat accumulation.



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  		Fig. 8 Cumulative decomposition of maize residues during the 1997 growing season in no-till and moldboard-plowed plots. A total of 520 g m-2 of residue C was returned to the field in the fall of 1996. Error bars indicate standard deviations (n = 4)

 
Microbial Biomass Carbon
Microbial biomass C contents varied from 17 to 53 g C m-2 during this study (Fig. 9) which is in the range of values reported for Eastern Canadian soils (e.g., Bolinder et al., 1999). The analysis of variance showed that both the effects of cropping and tillage were statistically significant (P < 0.05) with average values of 22.5 g C m-2 for the control, 34.1 g C m-2 for the plowed soil, and 38.9 g C m-2 for the no-till soil. The higher values for the no-till and plowed soils relative to the control are due to the input of maize and barley residues (above- and belowground residues) during the 2 yr of the experiment. Similar rapid and persistent effects of crop residue incorporation on MBC have been reported previously (Jensen et al., 1997). The higher values in the no-till soil than in the plowed soil is in line with previous studies in Eastern Canada (e.g., Carter, 1992; Angers et al., 1993) and further illustrate that MBC responds very rapidly to changes in tillage practices. Only small temporal variations were observed with, for example, a high at Day 158 to 160 in the cropped plots, which coincides with the highest values of crop residue decomposition (Fig. 6).



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  		Fig. 9 Microbial biomass C in the 15-cm surface soil layer during the 1997 growing season in control, no-till, and moldboard-plowed plots. Error bars indicate standard deviations (n = 4)

 
The {delta}13C value of the microbial biomass was greater in the cropped soil than in the control, which clearly reflects the contribution of the maize-derived C to the microbial biomass (Table 1) . On average, 8 and 10% of MBC was of maize origin for the plowed and no-till soils, respectively. Using the same stable isotope technique, Ryan and Aravena (1994) found that 30 to 55% of the MBC was derived from C4 plants after 5 yr of continuous grain maize, while Angers et al. (1995) reported a contribution of 35% after 11 yr of silage maize. The much higher values in these studies than in our study are explained by the fact that maize-derived C was supplied for many years prior to soil sampling in addition to the supply of root-derived C during the current season. The contribution of the growing plant can be significant. In a field study, Bruulsema and Duxbury (1996) found that 23% of MBC was from C4 after a single year of maize, whereas values ranging from 1 to 11% were found after 42 d of maize growth in a pot study (Qian and Doran, 1996).


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  		Table 1 Carbon isotope ratio ({delta}13C value), quantity, and origin of microbial biomass carbon (MBC) as affected by the treatments on four sampling dates in 1997

 
The amount of MBC derived from C4 material varied from 2 to 7 g C m-2, which represents from 0.4 to 1.4% of the estimated initial C4 inputs. Using natural abundance of 13C under controlled conditions, Qian and Doran (1996) estimated this incorporation to be 1.5 to 3.5% after 42 d of maize growth. Studies on the decomposition of 14C-labeled plant residues have shown that right after the incorporation of the residues, and for a period of a few weeks, the proportion of the residue-C input recovered in the microbial biomass can range from 5 to 20% (Ladd et al., 1981; Bremer and van Kessel, 1992; Wu et al., 1993; Witter and Dahlin, 1995). However, Ladd et al. (1981) observed that this proportion decreased rapidly after 32 wk and stabilized to values approaching 1%. (Ladd et al., 1981). The values observed in our study and their relative stability with time confirm that, at or near equilibrium, {approx}1% of the C input is recovered in the microbial biomass. However, comparisons of the rate of incorporation of residue-derived C in the microbial biomass between studies should be made with caution. Differences can be due to the methods used to measure MBC — mainly fumigation-extraction vs. fumigation-incubation — but also to variations in the extraction efficiency factor (Kec) used in the various experiments (Wu et al., 1993). Significant uncertainty is also associated with the estimation of the total C inputs in field studies, especially the belowground portion. Under field conditions, the depth or amount of soil considered in measuring microbial biomass should also be kept in mind; in this study, the top 0- to 15-cm layer was considered.

Attempts to relate soil respiration measured under field conditions to soil microbial biomass have been relatively unsuccessful (Jensen et al., 1997; Gregorich et al., 1998; Rochette and Gregorich, 1998). In these three studies, total soil respiration was mainly a function of soil temperature and moisture. The relationship between MBC and either soil respiration or maize residue decomposition was not significant in our study either (data not shown). It has been hypothesized that the soil microbial biomass is not uniform and that it is composed of various fractions, each having different levels of activity and turnover times (e.g., Ladd et al., 1995). One of these fractions would be formed from the decomposition of fresh residues and have a rapid turnover (Ladd et al., 1995). This hypothesis is difficult to test experimentally, especially under field conditions. Nealy et al. (1991) found a close relationship between substrate-induced respiration — a measure of the active microbial biomass — and residue decomposition rate as measured using litter bag loss. In our study, the fraction of the microbial biomass involved in the decomposition of relatively fresh maize-derived C has been quantified and can be considered as an active fraction. We tested the hypothesis that the quantity of MBC derived from maize was related to the decomposition of maize residues. Except for one data point (no-till at Day 253), the relationship was linear and highly significant (Fig. 10) . The slope of the relationship, which represents an estimate of the specific activity of the biomass fraction decomposing the maize-derived C, was constant at 0.45 g CO2–C g-1 MBC d-1. The specific activity was much lower (0.10 g CO2–C g-1 MBC d-1) for the no-till treatment at Day 253 due to a very low total MBC value at that particular date. Our values, which were obtained from field soil respiration measurements, are in the high range of those reported in the literature for laboratory respiration experiments (Santruckova and Straskraba, 1991). However, in those studies, the specific activities were determined from total MBC, whereas in our study, we were concerned with a much smaller fraction representing from 6 to 14% of the total MBC. The 13C approach was therefore successful at relating an active soil biological fraction to residue decomposition rates measured directly in the field.



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  		Fig. 10 Decomposition rate of the maize residues as a function of the microbial biomass C derived from maize. The regression equation does not include the data point for no-till Day 253

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
The purpose of this study was to use the natural abundance of 13C and the soil surface CO2 fluxes to measure the decomposition rate of maize residues under undisturbed field conditions. The decomposition of maize residues during the year following their return to the soil significantly increased the C isotope ratio of the soil CO2 relative to the unamended control plots. Decomposition rates peaked in June (2–3 g C m-2 d-1) and were lowest at both ends of the season (<0.5 g C m-2 d-1). For a given soil temperature, the decomposition was more active earlier than later in the season, possibly as a result of differences in substrate availability as the decomposition proceeded. The decomposition of the residues was also more affected by low soil water content in the moldboard plow than in the no-till plots. The total maize residue–C losses during the study corresponded with 35% of the added residue–C in the no-till plots and 40% in the moldboard-plowed plots. Except for one data point, the decomposition rate of maize-derived C was proportional to the fraction of the microbial biomass derived from maize residues but not to the total biomass. This fraction, although representing only 6 to 14% of the total MBC, showed a high level of specific activity, thereby indicating that this fraction was very active compared with the rest of the microbial biomass.

The results obtained in this study suggest that measurements of the natural abundance of 13C in soil CO2 can be successfully used to quantify the decomposition rates of crop residues. The techniques based on the disappearance of prelabeled incorporated residues are robust and less labor-intensive than the technique proposed in this study. However, the combined use of natural abundance of 13C and CO2 emissions allowed for (i) study of the decomposition of residues that were incorporated into the soil via natural root growth and typical agricultural practices and (ii) measurement of instantaneous decomposition rates, which provided information on the same time scale as changes of the variables that modulate decomposition of residues in soils (e.g., soil temperature and soil moisture). Moreover, the approach allowed us to relate an active microbial biomass C fraction to residue decomposition rates.Lin Ehlringer 1997; SAS Institute 1989


   ACKNOWLEDGMENTS
 
This work was supported by the AAFC PERD program. We thank Nicole Bissonnette for her assistance in the statistical analysis, and François Gagné, Patrice Jolicoeur, Philippe Drouin, and René Baillargeon for assistance in soil respiration measurements, sample collection, laboratory analyses, and plot preparation and maintenance.

Received for publication August 21, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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