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