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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

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

^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)

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

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