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Soil Science Society of America Journal 67:806-816 (2003)
© 2003 Soil Science Society of America

DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Stabilization of 13C-Carbon and Immobilization of 15N-Nitrogen from Rice Straw in Humic Fractions

Jeffrey A. Bird*,a, Chris van Kesselb and William R. Horwatha

a Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA, 95616
b Dep. of Agron. and Range Sci., Univ. of California, Davis, CA 95616

* Corresponding author (jabird{at}socrates.berkeley.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The transition from open-field burning of straw residues to alternative residue management practices may affect soil C sequestration potential and the supply of nutrients to crops. A field study of dual-labeled (13C and 15N) rice (Oryza sativa L.) residues examined the effects of winter-fallow flooding (vs. nonflooded) and straw residue incorporation (vs. untilled, open-field burned residue) on straw C and N dynamics in soil organic matter (SOM) fractions. We examined the fate of C and N in the straw, crown, and root system in the incorporated treatments and the uncombusted stubble, crown, and roots in burned treatments during 1 yr. During the winter fallow, straw residue incorporation reduced residue 15N loss but increased residue 13C loss compared with burning. Straw 13C loss after 1 yr was unaffected by either winter flooding or straw management (77.1% of applied). Slightly more straw 15N was lost of that applied in burned (65.5 ± 3.5%) compared with incorporated (52.0 ± 3.8%) during 1 yr. A greater proportion of soil-recovered 13C remained as nonalkali extractable humics (humin) in burned (62.0%) compared with incorporated (40.8%). In contrast, incorporated treatments had a larger proportion of 15N remaining as mobile humic acid (MHA) than burned (42.4 vs. 37.7%). Straw incorporation increased the relative retention of straw 15N to 13C compared with burning, indicating that straw 15N additions with incorporation may increase soil organic N reserves at an even greater rate than the larger straw additions might predict. These results show that straw incorporation results in markedly different straw C and N sequestration pathways compared with untilled, open-field burned residues.

Abbreviations: GLM, general linear model • IRGA, infrared gas analyzer • IRMS, isotope ratio mass spectrometer • LF, light fraction • MFA, mobile fulvic acids • MHA, mobile humic acids • NF, nonwinter flooded • PVC, polyvinyl chloride • SMB, soil microbial biomass • SOM, soil organic matter • WF, winter flooded • WHC, water-holding capacity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
CROP RESIDUE MANAGEMENT affects the biological and chemical processes that govern the conversion of C and N to SOM and the residual availability of N to succeeding crops. Recent changes in rice straw management practices in California from open-field burning to straw incorporation with winter-fallow flooding to enhance straw decomposition have prompted a need to better understand the effects of straw management practices on the decomposition and humification of crop residue C and N. In California, rice is especially dependant on soil N for meeting crop demand (50–80% of N assimilated in fertilized fields) (Broadbent, 1979; Mikkelsen, 1987). Consequently, an enhanced knowledge of the effects of long-term soil incorporation of straw and winter flooding on crop residue C and N cycling would enable improved utilization of fertilizer and crop residue N.

Previously, we reported a sustained increase in soil microbial biomass (SMB) C, N, and labile SOM pools after four seasons of straw incorporation compared with straw burned (Bird et al., 2001; 2002). Recent work in tropical and temperate rice ecosystems has determined that the operationally-defined mobile humic fraction (Na4P2O7 or NaOH extractable) is influenced by agronomic practices and is more dynamic than humics associated with metal-oxides and mineral complexes (Olk et al., 1996; Devêvre and Horwath, 2001). A better understanding of the C and N humification rates and stability of C and N in humic and fulvic acid fractions may indicate the degree to which changes in crop residue management practices might alter the C sequestration potential and the availability of soil organic N reserves in rice systems. Although numerous field studies have reported the fate of added 14C-, 13C-, and/or 15N-labeled crop residues in situ, most investigations were limited to the measurement of N in the plant and total recovery of C and/or N in the soil (e.g., Jenkinson, 1971; Pal and Broadbent, 1975; Sørensen, 1987). Few field investigations have examined directly the pathways of crop residue C and N immobilization into the SMB, stabilization as SOM, or the fate of C and N concurrently (Stott et al., 1983; Voroney et al., 1989). Consequently, the regulation of C and N humification and turnover processes remain inadequately understood, especially in rice soils that are continuously flooded or use winter flooding.

The close relationship between crop residue C and N turnover in soil is influenced primarily by the labile C supply and the stability of the products formed by a growing microbial community (McGill et al., 1975). While it is well known that the turnover of crop residue C and N into mineral forms (CO2 and NH+4/NO-3) or stabilization into humic substances is determined in part by the interplay between climatic factors and substrate biochemistry (Stevenson, 1994), there has been limited research on the dynamics of C relative to that of N in sequestration processes that form stable SOM fractions (Ladd et al., 1981; Voroney et al., 1989).

The traditional practice of burning eliminates 70 to 80% of the C and N held in the straw, crown and roots (Hill et al., 1999). Most of the straw C and N left after burning remains in the form of uncombusted root, crown, and stubble. This investigation focused on the dynamics of uncombusted and biologically active straw remaining after burning compared with that of chopped, incorporated straw to further elucidate the processes important to the stabilization and turnover of SOM. A field study of dual-labeled (13C and 15N) crop residues was initiated in the sixth year of a long-term rice straw management experiment. Our main objective was to assess the effects of straw incorporation and winter flooding on crop residue decomposition, microbial C and N utilization, stabilization of added crop residue C and N in SOM, and subsequent plant-N availability. These biological properties are critical to the development of straw management practices that optimize C sequestration and the long-term N supply in flooded rice soils.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Field Site and Soil
In fall 1993, winter-flood and straw management treatments were established at a 28-ha field site located on a commercial rice farm in the northern Sacramento Valley, near Maxwell, CA, USA (39°20'00'' N, 122°08'00'' W). The soil is classified as a fine, smectitic, superactive, thermic Sodic Endoaquerts (Willows clay) (Soil Survey Staff, 1998). The field experiment was laid out as a split-plot design with four replications. Winter-flood management (flooded vs. nonflooded) was the main-plot treatment and straw management (incorporated vs. burned) was the split-plot treatment. The four main plot treatments were: (i) burned straw and winter flooded (WF); (ii) burned straw and nonwinter flooded (NF); (iii) incorporated straw and WF; and (iv) incorporated straw and NF. Each of the individual plots measured 0.75 ha. Before the initiation of the experiment in 1993, rice was cropped annually for {approx}50 yr and managed with open-field burning. In October 1993, straw management practices for the field plots were implemented following grain harvest as described in Bird et al. (2001). Winter-flooded plots were flooded annually to 100- to 150-mm water depth during the fallow period (November–March). Nonwinter-flooded plots received {approx}55 to 88 mm of precipitation per month during the fallow winter months (Fig. 1) . Rarely would this result in >10 mm standing water for more than 2 to 3 d at a time on nonflooded fallow fields.



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  		Fig. 1. Mean monthly precipitation and temperature (1961–1990) located in Colusa County, CA. Values summarized by the U.S. Western Regional Climate Center, Reno, NV.

 
Plant Residue Labeling
Rice was grown to maturity and labeled uniformly with 13C and 15N under controlled greenhouse conditions. A soil-free medium (washed fine sand) was used to provide support and to maximize 15N-uptake. Plants were labeled with 13CO2 and 15NH4+ and were grown in a half-strength Hoagland solution with two times the concentration of Fe and Zn (Hoagland and Arnon, 1950). Fertilizer 15N as NH4(SO4)2 was added periodically (every 2 to 3 wk) at 9.99% atom excess. M-103, a medium-grain, short-season rice cultivar adapted to the Sacramento Valley (California Cooperative Rice Research Foundation, Biggs, CA) was grown in 300-L basins at the University of California, Davis. Labeling was accomplished in a climate-controlled, plexiglass growth chamber (Fig. 2) modified from Horwath et al. (1994). Temperature was maintained using a water-cooled heat exchanger (Horwath et al., 1994). Carbon dioxide concentrations were monitored with a LI-6200 infrared gas analyzer (IRGA) (Li-Cor, Lincoln, NE). A 13CO2 generator was connected via a diaphragm pump (GAST, Benton Harbor, MI) to circulate 13CO2 into the chamber. The 13CO2 generator was a vessel containing concentrated H2SO4 attached to a reservoir containing 1 M NaHCO3 solution (9.99% atom excess 13C). A computer and data acquisition manager (21X Micrologger, Campbell Scientific, Inc., Logan, UT) controlled temperature and CO2 concentration level with solenoid actuated valves (Horwath et al., 1994). Rice was exposed to one photoperiod of enriched 13CO2 seven times during the course of its development (i.e., every 10–14 d after plant emergence from water). The labeling chamber remained over the basins for one photo period after labeling, and unlabeled 12CO2 was maintained at ambient levels to facilitate the reassimilation of night-respired 13C. Each plant received {approx}13 mg 13C during the labeling procedures for a final 13C enrichment of 1.75% atom excess per plant. The frequent labeling and reassimilation produced plants with relatively uniform 13C distribution. At maturity, the rice was dried at 25°C. Straw was uniformly enriched in 15N (9.9% atom excess). Rice roots and crowns were slightly depleted in 13C (443 0/00) compared with shoots (777 0/00).



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  		Fig. 2. Design of stable isotope 13C- and 15N-labeling growth chamber. Two square meters of rice (cultivar M-103) was produced in greenhouse facilities at the University of California, Davis.

 
13Carbon- and 15Nitrogen-Straw Field Study
Five successive rice crops were grown and five seasons of straw management treatments had been applied in the field study before the initiation of the labeled-straw experiment. Before the installation of the microcosms in September 1998, rice plants were removed from the soil in areas chosen for microplot installation. Microcosms [25-cm-diam. polyvinyl chloride (PVC), 20-cm height] were inserted into soil to 15 cm with a 5-cm rim above the soil surface in October 1998. In November 1998, rice straw residue, enriched in 13C and 15N, was added to the microcosms after field burning and straw incorporation and before winter flooding. Duplicate microcosms were installed in all treatments. One microcosm was excavated at the end of the winter-fallow period (161 d) in April 1999 and the second duplicate microcosm at crop maturity (312 d) in September 1999 immediately after soil was sampled.

The rice residue additions simulated the straw remaining after fall field treatments. For the incorporated treatments, the entire labeled plant residue (roots and straw without the panicle) was chopped to 2 to 3 cm and mixed into the soil to a 10-cm soil depth. In the burned treatment, only rice residue from roots, crowns, and the lower 5-cm of stubble (including the crown) of the rice plant was added after the main plots were burned. The crowns and roots were inserted into the soil to 8- to 10-cm soil depth with the crown remaining at the soil surface. In California, burning rice fields typically combusts 70 to 80% of the C and N held in the straw, crown, and roots (Hill et al., 1999). The burn combusts mainly straw from shoots cut during combining. The root, crown, and uncut stubble remain living and do not burn significantly. Unlabeled ash from equivalent soil areas was placed in the microcosm to simulate nutrient addition from the burned straw. The total amount of C, N, 13C, and 15N added to the burned and incorporated treatments are presented in Table 1. The tops of the microcosms were fit with screens (0.2- and 1-cm mesh) to contain the straw in the microcosm during the winter-fallow period. In addition, a 0.5-cm screened hole was placed in the side of the PVC microcosm 2.5 cm above the soil surface to facilitate water movement while limiting straw loss. In May 1999, each of the remaining microcosms was prepared for seeding by hand-tilling to a depth of 10 cm. Unlabeled ammonium sulfate-N fertilizer was applied to the microcosms at a rate of 157 kg N ha-1 just before planting in May 1999. Cultivar M-202 was aerial seeded on 13 May 1999 at a rate of 190 kg ha-1.


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  		Table 1. Straw treatment additions and practices applied to microcosms in November 1998. Incorporated treatments received straw, crown, and roots chopped to 2- to 3-cm lengths and incorporated to 10-cm soil depth. Unchopped crown (to 5-cm above soil surface) and roots were inserted into the soil in the burned treatment.

 
Soil Carbon and Nitrogen Analyses
Soil samples (0 to 15 cm) were collected from the microcosms using an intact core sampler (5-cm diam.) that allowed for bulk density determinations. During 1999, two soil cores per microcosm were collected on the two excavation dates (April and September) and separately fractionated into four SOM fractions. Large visible pieces of crop residue (>6 mm) were removed from the soil samples before analyses, and analyzed separately. Soil microbial biomass and inorganic N determinations were performed on field-moist soil for each of the duplicate soil cores. For SOM analyses, duplicate soil cores were combined, soil was air-dried until constant weight at 40°C, and ground using a ball-mill to pass a 250-µm sieve. Before the initiation of the field study in fall 1993, selected soil physical and chemical properties were determined (Bird et al., 2001; 2002). Total C and N in the SOM fractions were determined on a Carlo-Erba CHN analyzer (Costech Analytical Technologies, Inc., Valencia, CA) (Nelson and Sommers, 1996). Soil C and N determinations were expressed on a volumetric basis (dry-soil basis). Soil microbial biomass C, N, 13C, and 15N contents were determined on soil samples using the chloroform-fumigation incubation method (5-d chloroform period, Horwath and Paul, 1994; Bird et al., 2001). Carbon dioxide evolved during 10 d of incubation was measured using a Horiba PIR-2000 IGRA (Horiba Ltd., Kyoto, Japan). Microbial C was calculated without use of the control and adjusted using a KC constant of 0.41 (Voroney and Paul 1984; Horwath et al., 1996). Field moist soil subsamples and fumigated soil were extracted with 2 M KCl for exchangeable inorganic N (5:1 extractant:dry-soil ratio) and quantified colorimetrically on an automated N analyzer (Lachat Instruments, Milwaukee, WI) (Mulvaney, 1996). Soil microbial biomass N was calculated after subtracting the initial extractable soil NH4+; values were adjusted using a KN constant of 0.57 (Jenkinson, 1988).

Soil Organic Matter Fractionation
We fractionated soil samples into four SOM fractions with varying lability: (i) light fraction (LF), (ii) MHA, (iii) mobile fulvic acid (MFA), and (iv) nonalkali extractable humic acids (humin). Soil samples from the field and soil incubation studies were fractionated by physical separation of the LF using a density fractionation procedure described in Bird et al. (2002). The heavy fraction was fractionated into three SOM fractions: MHA, MFA, and humin described by McGill and Paul (1976) and Bird et al. (2002). Briefly, soil subsamples (20 g) were initially rinsed with 200 mL 0.1 M HCl to remove salts, carbonates, and particulate organic matter. Soil samples were subsequently extracted with 0.4 M NaOH, under N2, to yield MHA and MFA. Mobile fulvic acid solution subsamples were dialyzed to remove Na in deionized water using 1000 MW cellulose tubing (Spectrum Industries, Inc., Rancho Dominguez, CA). The remaining organic matter, humin, is bound in stable aggregates with metal polyvalent cations and clay minerals (Bird et al., 2002). The SOM fractions were analyzed for yield and isotopic enrichment (15N) after lyophilizing and homogenizing. Soil and SOM solutions were maintained at 4°C during and after fractionation.

Plant Analyses
At harvest in September 1999, rice plants were collected from microcosms by cutting just above the soil surface. Grain and straw dry matter were determined after drying samples until constant weight at 60°C and separating into grain and straw biomass. Dried straw samples were ground using a Wiley mill fit to pass a 2-mm screen; followed by ball milling to pass a 250-µm sieve. Total grain and straw C and N were determined as previously described. All plant determinations were expressed on a dry-matter basis (60°C).

Isotope Analyses
Natural abundance values (13C/15N) of plant, soil, SOM fractions, and inorganic N were determined from samples from a previous experiment (Bird et al., 2002). Unlabeled soil and plant samples were analyzed on a Europa Scientific Geo 20/20 isotope mass spectrometer (IRMS) (PDZ Europa Ltd., Crewe, UK). Labeled inorganic N extracts (NH+4 and NO-3) and KCl extracts from CFI were diffused onto ashed, Whatman GF/A filter paper using the teflon tape method described by Stark and Hart (1996). Isotopic enrichment of the 15N in diffused samples, and 13C and 15N in whole soil, plant, straw, and SOM fractions were determined on a Europa Scientific INTEGRA IRMS (PDZ Europa Ltd., Crewe UK). Respired C (13CO2) samples from CFI were analyzed on a Trace Gas Inlet System interfaced to the GEO 20/20 IRMS. The recovery of 13C and 15N in soil and plant pools analyzed by mass spectrometry was calculated with the following equations for C and N, initially in {delta}13C (0/00) and {delta}15N (0/00):

[1]

[2]
where R = 13C/12C or 15N/14N for C and N, respectively. Values were calculated relative to the international standard Vienna-Pee Dee Belemnite for 13C (R = 0.0112372) and for atmospheric N2 for 15N (R = 0.0036765). Atom % values presented for 15N were calculated for samples highly enriched in 15N from the fractional abundance of 15N.


[3]

The 13C and 15N enrichments were used to determine the fraction (f) of turnover of new C and N entering these pools (Balesdent and Mariotti, 1996):

[4]
where {delta}0 and {delta} are the {delta}13C values of a soil fraction at the beginning (i.e., natural abundance values) and end of the experiment (or sampling time point) and {delta}1 is the 13C signal of the substrate added. Similar calculations were done for 15N.

Statistical Analyses
Main effects of winter flooding and straw management were tested using a general linear model (GLM) designed for the split-plot design. All data were expressed as least squares means with standard errors of indicated treatments. Fisher (F) statistics and P values were indicated in text and tables for all GLM procedures. A significance level of P < 0.05 was set a priori as the {alpha}-level; and P values greater than 0.05 in Tables are listed as nonsignificant. Studentized t tests were performed among soil and plant C and N fraction data to compare 13C and 15N enrichment mean values on specific sample dates within incorporated and burned treatments and between sampling dates for the percentage recovery of straw 13C and 15N. Since proportional data are often not normally distributed, analyses on percentage data were performed after log-transformation. All statistical tests were performed using SYSTAT version 7.0 (SPSS Inc., Chicago, IL).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Soil Carbon and Nitrogen Pool Sizes
Total soil C and N were unaffected by straw or flooding management practices (Tables 2 and 3) at the end of 6 yr. The alkali-insoluble humin fraction represented the largest proportion of SOM recovered in all treatments, 52.4 ± 0.9% and 50.4 ± 0.8% of total C and N reserves, respectively (averaged across sample dates). The MHA fraction, the second largest soil C and N fraction examined (25.8 ± 0.6% and 27.3 ± 0.6% of total soil C and N), was significantly greater with annual straw incorporation than with burned (Tables 2 and 3). The labile LF-C, LF-N, and SMB-N pools, like the MHA fraction, were significantly greater with straw incorporation than open-field burning.


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  		Table 2. Quantities of various C fractions from soil sampled at a depth of 0 to 15 cm in April and September 1999 when straw was incorporated or burned and the fields were winter flooded (WF) or nonwinter flooded (NF). All treatments were flooded during the cropping season. Least-squares means, values in parentheses are standard errors (N = 4). P values greater than 0.05 are indicated as nonsignificant (NS).

 

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  		Table 3. Quantities of various N fractions from soil sampled at a depth of 0 to 15 cm in April and September 1999 when straw was incorporated or burned and the fields were winter-flooded (WF) or nonwinter flooded (NF). All treatments were flooded during the cropping season. Least-squares means, values in parentheses are standard errors (N = 4). P values greater than 0.05 are indicated as nonsignificant (NS).

 
While winter flooding did not affect C and N contents of the larger, more stable SOM fractions, LF-C (September) and inorganic N (April and September) were greater with annual winter flooding (Tables 2 and 3). A significant winter flood by straw interaction was present for inorganic N on both sampling dates in 1999. This interaction resulted in greater inorganic N with winter flooding and straw incorporation.

Straw 13Carbon and 15Nitrogen Dynamics during Winter Fallow
At the end of the winter-fallow period (November–April 1999; 161 d in situ), more straw 13C and 15N were recovered in the straw incorporated treatment compared with the burned (P = 0.001). The total percentage recovery of applied 13C (soil and >6 mm particulate straw), however, was almost two times greater for the burned straw treatment (82.5 ± 4.0%) than for straw incorporated (44.0 ± 2.3%) (P = 0.001; Fig. 3) . A large proportion of intact crown and root biomass (>6 mm) remained in the burned treatments while most of the visible straw in the incorporated treatments had decomposed. Treatments did not result in significant differences in the percentage of applied 13C recovered in the soil (<6 mm) (Fig. 3). A significant winter flood by straw interaction was present (F = 9.1; P = 0.024) for the total percentage recovery of applied 13C (soil and >6 mm). Winter flooding resulted in greater total recovery of applied 13C in burned treatments but had little effect in incorporated treatments (Fig. 3).



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  		Fig. 3. Recovery of 13C (top) and 15N (bottom)-crop residue as a percentage of that applied in November 1998 from soil sampled in April 1999 and September 1999. Soil recoveries were from soil sampled to the 0- to 15-cm depth when straw was incorporated (I) or burned (B) and the fields were winter flooded (WF) or nonwinter flooded (NF) (N = 4).

 
In contrast to 13C, total percentage recovery of applied 15N (soil and >6 mm) was greater in incorporated (78.5 ± 4.3%) compared with burned (62.3 ± 2.4%) treatments in April (P = 0.001; Fig. 3). The percentage of applied 15N recovered in the soil (<6 mm) was also greater in incorporated compared with burned treatments (Fig. 3). A small percentage of 15N was recovered as volunteer annual bluegrass (Poa annua L.) in the NF treatments.

At the end of the five-month winter-fallow period, the majority of 13C was recovered in the soil as humin and MHA (Table 4). The humin and MHA fractions were not significantly affected by straw management and averaged 51.8 ± 3.2% and 21.2 ± 0.9%, respectively, of soil-recovered 13C in April. The percentage of applied 13C recovered in soil as LF-C was affected by straw management and represented a greater proportion of soil-recovered 13C in the incorporated (16.2 ± 1.8%) compared with burned (9.0 ± 1.7%) treatments (Table 4).


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  		Table 4. Distribution of 13C recovered from soil (0- to 15-cm depth) among various fractions sampled in April and September 1999. Least-squares means, values in parentheses are standard errors (N = 4). P values greater than 0.05 are indicated as nonsignificant (NS).

 
Percentage recovery of applied 15N in soil as SOM was greatest in the MHA fraction for all treatments (Table 5). With soil incorporation of straw, recovery of applied 15N in soil as MHA and LF was greater than for burned treatments. Nonwinter-flooded conditions resulted in greater recovery of applied 15N in soil as LF than winter flooding (Table 5). In contrast to the LF, winter flooding increased the recovery of straw-15N as inorganic N in April 1999 compared with nonflooded fallow.


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  		Table 5. Distribution of 15N recovered from soil (0- to 15-cm depth) among various fractions sampled in April and September 1999. Least-squares means, values in parentheses are standard errors (N = 4). P values greater than 0.05 are indicated as nonsignificant (NS).

 
Straw 13Carbon and 15Nitrogen Dynamics during Crop Season
One year after application, more straw 13C and 15N were recovered in straw incorporated treatments compared with burned (September 1999; P = 0.001). Similar to April 1999, treatments did not result in significant differences in the percentage of applied 13C recovered in the soil at the end of 1 yr (Fig. 3). The percentage recovery of applied 15N in the soil for straw incorporated plots (41.7 ± 3.9%) exceeded straw burned (27.9 ± 2.8%) at the end of 1 yr, but this difference was not significant at a P < 0.05 (P = 0.061; Fig. 3).

The percentage recovery of applied 13C in soil as SOM was affected by straw incorporation and winter flooding (Table 4). In the humin fraction, the percentage of soil-recovered 13C was greater in the burned treatment compared with straw incorporated. In contrast, the percentage of 13C recovered as MHA- and SMB-C showed an opposite effect of straw management (Table 4). The partitioning of straw 13C recovered in soil as SOM fractions changed during the summer period (April–September 1999) and was affected by straw management treatments (Table 4). The proportion of soil-recovered 13C as MHA decreased from April to September in burned and increased in incorporated treatments (P < 0.05). The 13C enrichment of total soil and SOM fractions decreased from April to September in straw incorporated treatments (P < 0.05; Table 6). In contrast, 13C enrichments were similar on both sampling dates for total soil and SOM fractions in the straw burned treatments.


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  		Table 6. Enrichment of {delta}13C ({per thousand}) and 15N atom excess(%) in plant and soil (0- to 15-cm depth) pools sampled in April (preplant) and September (crop maturity) when straw was incorporated or burned (averages across winter-flooding treatments) the previous fall. Least-squares means, values in parentheses are standard errors (N = 8). Different letters following values correspond to differences among 15N or 13C enrichment values within straw management treatments and sampling date are significantly different (P < 0.05).

 
The recovery of applied 15N in soil as MHA and humin was affected by straw management treatments 1 yr after application. More 15N was partitioned into the humin fraction in burned treatments, whereas more 15N was partitioned into the MHA fraction in incorporated treatments (Table 5). The 15N enrichment of all soil N pools examined were lower in September than April, with the exception of the LF in the burned treatments (P < 0.05; Table 6).

At crop maturity, the recovery of 15N in rice, considered as a percentage of applied and as a percentage of total 15N recovered in April 1999, was not affected by treatments (Fig. 3 and Table 7). A significant winter flood x straw interaction was present for total rice-N uptake (soil plus residual straw N). This interaction resulted in greater N uptake in the WF soils compared with the NF soils in straw incorporated treatments and less in the WF soils in straw burned treatments (Table 7).


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  		Table 7. Total rice plant N uptake and 15N-crop residue uptake as a percentage of total 15N recovered (April 1999) as affected by incorporated or burned straw, winter flooding (WF), and nonwinter flooding (NF) fields at crop maturity (September 1999). Least-squares means, values in parentheses are standard errors (N = 4). P values greater than 0.05 are indicated as nonsignificant (NS).

 
13Carbon Loss:15Nitrogen Loss Ratios
The ratios of straw 13C loss:15N loss during each time period were calculated for each plot (N = 16; Table 8). During the winter-fallow period, 13C loss was lower than 15N loss for the burned treatments, while 13C loss was substantially greater than 15N loss in the incorporated treatments. The loss of 13C relative to that of 15N for the summer cropping period was reversed from that of the winter-fallow period. Overall, the net result of these opposing trends resulted in a straw 13C:15N loss ratio of 1.5 with straw incorporation and 1.1 with straw burning (Table 8). Winter flooding decreased the 13C:15N loss ratio compared with NF during the winter period and increased the 13C:15N loss ratio during the summer period. Consequently, no significant net effect of winter flooding was observed in the 13C:15N loss ratio during the course of 1 yr (Table 8).


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  		Table 8. Relative loss of 13C- and 15N-crop residue as a percentage of that applied in November 1998 during the initial winter-fallow period (November–April), summer crop period (May–September) and the entire 11 mo in situ study. Loss excludes plant assimilated 15N, and soil 13C and 15N below 15-cm depth. Least-squares means, values in parentheses are standard errors (N = 4). P values greater than 0.05 are indicated as nonsignificant (NS).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Straw Carbon and Nitrogen Decomposition
All straw management practices examined in this study facilitated the decomposition of enough residual straw residues for optimal seed bed preparation and produced similar grain yields under average fertilizer N rates (Eagle et al., 2000). As expected, more straw 13C and 15N were retained in the soil as SOM with straw incorporation (straw, stubble, crown, and roots) compared with straw (stubble, crown, and roots) in the burned treatment as more straw dry matter was applied in the straw incorporated treatment. The loss of applied 13C remaining after burning (74%) and incorporation (80%) are consistent with values reported for crop residue decomposition after 1 yr in temperate climates (Jenkinson and Rayner, 1977; Aita et al., 1997).

Loss of applied straw 13C during the winter fallow (November–April) was substantially reduced in burned (17%) compared with incorporated (56%) treatments. In burned treatments, slower labeled straw decomposition resulted in more straw recovered as intact crowns and root biomass (>6 mm) than recovered as total soil 13C. Similarly, field tillage (wet-rolling) increased straw decomposition during the winter fallow compared with untilled rice straw under WF conditions in California (Bird et al., 2000). Uncombusted straw crowns and stubble in burned treatments had limited soil contact compared with incorporation, thereby reducing microbial degradation. In contrast, percentage loss of applied 13C was greater during the summer in burned treatments compared with incorporated treatments. At this stage, remaining surface straw was incorporated into the soil following seedbed preparation in all treatments allowing for increased soil contact and exposure to decomposition processes, especially for surface residues in the burned treatment. As straw 13C was already converted into SMB- and SOM-C in incorporated treatments by early spring, decomposition of straw was consequently slower than in burned treatments during the growing season.

While the slower initial degradation of the residues remaining after burning may have been due to the effects of tillage, the differences in C quality may also have contributed to slower initial loss rates. As suggested by Balesdent and Balabane (1996), the relatively slow loss of root-derived C may be due to greater amounts of complex C compounds such as lignin and structural carbohydrates.

While winter flooding did not affect the loss of applied straw 13C at the end of 1 yr, decomposition of straw was slower in WF than NF soils in the burned treatments during the winter fallow. As anaerobic decomposition is generally considered less energetically efficient than aerobic (Broadbent, 1979), this result was expected. Winter flooding, however, did not affect the percentage loss of applied 13C in incorporated plots. Flooded soil conditions have either shown no effect on straw C loss rates compared with aerobic conditions (Neue and Scharpenseel, 1987; Devêvre and Horwath, 2000) or have shown increased straw C mineralization rates (Murayama, 1984). Flooded conditions during a laboratory incubation of straw added to soil from this study site depressed total CO2–C mineralization rates during the first 60 d compared with 50% water-holding capacity (WHC), but was similar thereafter (Devêvre and Horwath, 2000). Loss of C as methane from flooded soils, however, greatly exceeded those produced at 50% WHC and contributed {approx}9% of the total C emissions at 25°C under flooded conditions (Devêvre and Horwath, 2000). Thus, greater methane loss in WF treatments may partly explain the lack of observed differences in total C loss due to winter flooding with straw incorporation.

Our results indicate that, in the short term, winter flooding decreased decomposition of intact straw at or above the soil surface and had no net effect on soil-incorporated residues. The winter-flooding effect on 13C sequestration, however, did not persist at the end of 1 yr. In addition, winter flooding had no effect on total soil C, or on smaller more responsive SOM fractions such as MHA at the end of 6 yr of winter-flooding treatments. Consequently, winter flooding of fallow rice fields does not appear to be an effective method to accumulate C in rice soils, but may be worthwhile for waterbird habitat enhancement (Elphick and Oring, 1998).

Total percentage loss of applied straw 15N in all treatments was slightly higher, but consistent, with values reported at the end of 1 yr for medic (Medicago littoralis Rohde ex Loisel.) (35–45%, Ladd et al., 1981) and wheat (Triticum aestivum L.) residues (48–52%, Voroney et al., 1989). In contrast to straw 13C dynamics, loss of applied straw 15N was slightly higher in burned (65.5 ± 3.5%) compared with incorporated (52.0 ± 3.8%) treatments at the end of 1 yr. While this difference was not significant (P = 0.085), this trend was also apparent in recovery of applied 15N as soil N at the end of 1 yr. A high rep x straw error may have resulted in a Type II error for this comparison. The frequently saturated soils in temperate, flooded rice systems presumably contributed to higher N losses from the plant-soil system than unflooded soils. Loss pathways were most likely a combination of denitrification, ammonia volatilization, and leaching of 15N below the 15-cm soil depth. We suspect, however, that loss of straw-derived N below the 15-cm soil depth was small (<10% of applied) based on previous work that showed little movement of applied 15N fertilizer into the 15- to 90-cm depth 2 yr after application (Bird et al., 2001).

The higher loss of 15N relative to 13C for straw in burned treatments compared with incorporated at the end of 1 yr was the net result of markedly different 13C loss to 15N loss patterns during the two time periods. During the winter fallow, the burned treatments had low 13C:15N loss ratios (0.3 to 0.6). In contrast, 15N was more highly conserved in the soil than 13C with straw incorporation during the winter-fallow period. The low 13C loss relative to 15N in the burned treatments during the winter fallow emphasized the high proportion of intact straw residues and low microbial processing of applied straw C.

The straw x winter-flooding interaction observed in total rice N uptake was consistent with rice N uptake and yield observed at this site (Eagle et al., 2000). While the total amount of soil N recovered in the rice crop was greater in incorporated treatments compared with burned, the percentage of applied 15N recovered in the plant was similar among treatments. With an average uptake of 6.6% of applied straw N (averaged across treatments), straw N clearly plays a small but important role in supplying rice N under N-fertilized conditions. Ultimately, a slightly greater percentage of applied 15N remained in the soil in straw incorporated treatments compared with burned at the end of 1 yr. These results suggest that more straw 15N immobilized after straw incorporation was conserved in the soil as SOM than in burned treatments. Therefore, with time, more soil-immobilized straw N may be supplied to subsequent rice crops.

Soil-Microbial-Biomass-Carbon and Nitrogen
The increased SMB in incorporated compared with burned treatments was consistent with those reported previously during Years 4 through 6 of this field study (Bird et al., 2001). The sustained, larger SMB fraction in incorporated treatments was due, in part, to the larger labile MHA and LF fractions observed after 4 yr of straw incorporation compared with burned (Bird et al., 2002). While the SMB-C and N pool represented a small proportion (2 to 4%) of total C and N, the flux of 13C and 15N through the SMB was very high. This is illustrated well by the higher 13C and 15N enrichments in the SMB than all humic fractions on both sampling dates. The similar 15N enrichment of SMB to the inorganic N and LF-N indicate their close source-sink relationship. Furthermore, similarity of the relative 15N enrichments of the SMB to the LF at the end of 1 yr indicates that straw LF-15N was a major source of SMB-N in all treatments. In contrast, the 13C enrichment of the SMB in the burned treatment was significantly less than the LF at the end of 1 yr. The SMB in the burned treatment appeared to be more dependent on native soil C than LF-C.

Carbon and Nitrogen Stabilization and Turnover in Soil Organic Matter
Soil organic matter fractionation in combination with C and N stable isotope methodologies has provided a wealth of information on the important processes of humification in soils (McGill et al., 1975; Stott et al., 1983; Devêvre and Horwath, 2001). Our data show that at the end of 161 d in situ, 80 to 95% of soil-recovered straw 13C and 15N (<6 mm) was present in microbial and humic substances. The relative humification pathway of straw 13C was markedly different than straw 15N into SOM fractions. Furthermore, the pathways of 13C and 15N into and among SOM fractions varied significantly among straw and WF treatments.

More straw 13C entered the resistant humin fraction relative to straw 15N, while more straw 15N was stabilized as MHA than straw 13C. These trends in 13C and 15N partitioning, which were present in all treatments, suggest divergent humification pathways for C and N from straw byproducts. Further research in this area, however, is needed to consider whether these trends are maintained with time, if these observations are apparent in the humification pathways of other ecosystems, and if other SOM fractionation procedures reflect similar results.

Our results indicate that crop residue management practices not only can affect soil C and N stabilization rates, but can also affect the relative contributions of C and N to different SOM fractions. At the end of 1 yr, the proportions of 13C and 15N recovered in the soil as MHA were greater in straw incorporated compared with burned treatments. Similarly, the proportion of 13C recovered in the soil as humin was greater in burned treatments compared with incorporated. The labeled straw addition results were consistent with the longer-term effects (i.e., 6 yr) of straw management practices observed in greater total MHA-C, MHA-N, and slightly lower total humin-C with straw incorporation compared with straw burning. The implications of these findings suggest that the MHA-C, MHA-N, and humin-C fractions contain sizable portions of recent straw additions and reflect, most significantly, recent agronomic practices.

Mobile humic acid N is an important source of crop N needs in temperate rice (Bird et al., 2002). Rice straw residue incorporation during six seasons has contributed substantially to this source of plant-available N and may warrant a reduction in fertilizer N needs compared with annual straw burning. While total soil C and N have not increased at the end of 6 yr of straw incorporation of residues, the trends apparent in greater MHA-, SMB-, and LF-C pools suggest that, in the long term, soil incorporation of residues may have a significant impact on soil C accumulation.


   ACKNOWLEDGMENTS
 
This research was supported by grants from the California Rice Research Board, Ducks Unlimited, and the California Energy Commission Farm Energy Assistance Program. We would like to acknowledge the generous contributions made by Canal Farms, D. Harris, M. Hair, J.E. Hill, W. Komaroff, K. Epps, J. Slagter, and E. Humphreys. The University of California Davis Stable Isotope Facility provided isotope analysis.

Received for publication March 25, 2002.


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