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

straw burned (Bird et al., 2001; 2002). Recent work in tropical and temperate rice ecosystems has determined The transition from open-field burning of straw residues to alternathat the operationally-defined mobile humic fraction tive residue management practices may affect soil C sequestration potential and the supply of nutrients to crops. A field study of dual(Na4P2O7 or NaOH extractable) is influenced by agrolabeled (13C and 15N) rice (Oryza sativa L.) residues examined the nomic practices and is more dynamic than humics associeffects of winter-fallow flooding (vs. nonflooded) and straw residue ated with metal-oxides and mineral complexes (Olk et incorporation (vs. untilled, open-field burned residue) on straw C and al., 1996; Devêvre and Horwath, 2001). A better underN dynamics in soil organic matter (SOM) fractions. We examined the standing of the C and N humification rates and stability fate of C and N in the straw, crown, and root system in the incorporated of C and N in humic and fulvic acid fractions may inditreatments and the uncombusted stubble, crown, and roots in burned cate the degree to which changes in crop residue mantreatments during 1 yr. During the winter fallow, straw residue incoragement practices might alter the C sequestration poporation reduced residue 15N loss but increased residue 13C loss comtential and the availability of soil organic N reserves pared with burning. Straw 13C loss after 1 yr was unaffected by either in rice systems. Although numerous field studies have winter flooding or straw management (77.1% of applied). Slightly more straw 15N was lost of that applied in burned (65.5 3.5%) reported the fate of added 14C-, 13C-, and/or 15N-labeled compared with incorporated (52.0 3.8%) during 1 yr. A greater crop residues in situ, most investigations were limited proportion of soil-recovered 13C remained as nonalkali extractable to the measurement of N in the plant and total recovery humics (humin) in burned (62.0%) compared with incorporated of C and/or N in the soil (e.g., Jenkinson, 1971; Pal and (40.8%). In contrast, incorporated treatments had a larger proportion Broadbent, 1975; Sørensen, 1987). Few field investigaof 15N remaining as mobile humic acid (MHA) than burned (42.4 vs. tions have examined directly the pathways of crop resi37.7%). Straw incorporation increased the relative retention of straw due C and N immobilization into the SMB, stabilization 15N to 13C compared with burning, indicating that straw 15N additions as SOM, or the fate of C and N concurrently (Stott with incorporation may increase soil organic N reserves at an even et al., 1983; Voroney et al., 1989). Consequently, the greater rate than the larger straw additions might predict. These results regulation of C and N humification and turnover proshow that straw incorporation results in markedly different straw C and N sequestration pathways compared with untilled, open-field cesses remain inadequately understood, especially in burned residues. 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 residue management affects the biological and C supply and the stability of the products formed by a chemical processes that govern the conversion of growing microbial community (McGill et al., 1975). C and N to SOM and the residual availability of N to While it is well known that the turnover of crop residue succeeding crops. Recent changes in rice straw manageC and N into mineral forms (CO2 and NH 4 /NO 3 ) or ment practices in California from open-field burning stabilization into humic substances is determined in part to straw incorporation with winter-fallow flooding to by the interplay between climatic factors and substrate enhance straw decomposition have prompted a need biochemistry (Stevenson, 1994), there has been limited to better understand the effects of straw management research on the dynamics of C relative to that of N in practices on the decomposition and humification of crop sequestration processes that form stable SOM fractions residue C and N. In California, rice is especially de(Ladd et al., 1981; Voroney et al., 1989). pendant on soil N for meeting crop demand (50–80% The traditional practice of burning eliminates 70 to of N assimilated in fertilized fields) (Broadbent, 1979; 80% of the C and N held in the straw, crown and roots Mikkelsen, 1987). Consequently, an enhanced knowl(Hill et al., 1999). Most of the straw C and N left after edge of the effects of long-term soil incorporation of burning remains in the form of uncombusted root, straw and winter flooding on crop residue C and N crown, and stubble. This investigation focused on the cycling would enable improved utilization of fertilizer dynamics of uncombusted and biologically active straw and crop residue N. remaining after burning compared with that of chopped, Previously, we reported a sustained increase in soil incorporated straw to further elucidate the processes microbial biomass (SMB) C, N, and labile SOM pools important to the stabilization and turnover of SOM. A after four seasons of straw incorporation compared with field study of dual-labeled (13C and 15N) crop residues J.A. Bird and W.R. Horwath, Dep. of Land, Air, and Water Resources, Abbreviations: GLM, general linear model; IRGA, infrared gas anaUniv. of California, Davis, CA, 95616; and C. van Kessel, Dep. of lyzer; IRMS, isotope ratio mass spectrometer; LF, light fraction; MFA, Agron. and Range Sci., Univ. of California, Davis, CA 95616. Received mobile fulvic acids; MHA, mobile humic acids; NF, nonwinter flooded; 25 Mar. 2002. *Corresponding author ([email protected]). PVC, polyvinyl chloride; SMB, soil microbial biomass; SOM, soil organic matter; WF, winter flooded; WHC, water-holding capacity. Published in Soil Sci. Soc. Am. J. 67:806–816 (2003).