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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Nitrogen Dynamics in Humic Fractions under Alternative Straw Management in Temperate Rice

^a Dep. of Land, Air and Water Resources, One Shields Avenue, University of California, Davis, CA 95616
^b Dep. of Agronomy and Range Science, University of California, Davis, CA 95616

* Corresponding author (wrhorwath{at}ucdavis.edu)

Crop residue management practices can affect N immobilization and stabilization processes important to efficient utilization of N from fertilizers, crop residues, and soil organic matter (SOM). A 2-yr, ¹⁵N-labeling field study was conducted to examine the effects of winter-fallow flooding (vs. unflooded) and straw residue incorporation (vs. burning) on the rates of sequestration and stability of specific SOM pools critical in sustaining N fertility in rice (Oryza sativa L.). Five SOM fractions were examined from soil samples obtained over Years 4 to 6 of a field trial: light fraction (LF), mobile humic acid (MHA), mobile fulvic acid (MFA), metal-associated humic acid (MAHA), and alkali-insoluble humics (HUM). After 4 yr of straw management treatments, soil incorporation of straw increased MHA and LF C and N compared with burned straw. Immobilization of N fertilizer peaked in all SOM fractions after one growing season (120 d) and was greatest in the MHA fraction over the 2-yr ¹⁵N study. Nitrogen fertilizer sequestration in MHA and LF was greater with straw incorporation compared with burned. Turnover of immobilized ¹⁵N-fertilizer in the stable organic components was fastest in the labile MHA and MFA fractions (7- to 9-yr half-life) compared with the half-lives of the moderately resistant MAHA fraction (53 yr) and most stable HUM fraction (153 yr). While the MAHA and HUM fractions played a significant role in N fertilizer immobilization and turnover, the MHA and LF fractions represented the primary active sink and source of sequestered N affecting both short- and long-term soil fertility.

Abbreviations: GLM, general linear model • HUM, alkali-insoluble humics • IRMS, isotope ratio mass spectrometer • LF, light fraction • MAHA, metal-associated humic acids • MHA, mobile humic acids • MFA, mobile fulvic acids • NF, nonwinter flooded • SMB, soil microbial biomass • SOM, soil organic matter • WF, winter flooded

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