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 (13C and 15N) rice (Oryza sativa L.) residues examined the effects of winter-fallow flooding (vs. nonflooded) and straw residue incorporation (vs. untilled, openfield 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.