Stabilization of 13C-carbon and immobilization of 15N-nitrogen from rice straw in humic fractions

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 (C-13 and N-15) 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 N-15 loss but increased residue C-13 loss compared with burning. Straw C-13 loss after 1 yr was unaffected by either winter flooding or straw management (77.1% of applied). Slightly more straw N-15 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 C-13 remained as nonalkali extractable humics (humin) in burned (62.0%) compared with incorporated (40.8%). In contrast, incorporated treatments had a larger proportion of N-15 remaining as mobile humic acid (MHA) than burned (42.4 vs. 37.7%). Straw incorporation increased the relative retention of straw N-15 to C-13 compared with burning, indicating that straw N-15 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.