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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Immobilization of Fertilizer Nitrogen in Rice

Effects of Straw Management Practices

^a Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616
^b Univ. of California Cooperative Extension, Kearney Agricultural Center, Parlier, CA, 93648
^c Dep. of Agronomy and Range Science, University of California, Davis, CA 95616

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

A recent transition in rice straw management, from open-field burning to soil incorporation in combination with winter-fallow flooding, has led to uncertainty in evaluating long-term N fertility. A 2-yr field study of ¹⁵N-labeled fertilizer and crop residue was initiated in the fourth year of a rice straw management trial to examine the impacts of winter flooding and straw management on N fertilizer immobilization and crop uptake. After six seasons of residue incorporation and winter flooding, no effect on total soil C or N was observed. During the fifth and sixth year of the field study, microbial biomass C and N were greater for straw incorporation than for straw burned. Microbial biomass contained a sizable portion of soil-recovered ¹⁵N fertilizer after the first (23%) and second (10%) crop season of the ¹⁵N study. The half-life of the ¹⁵N in the biomass ranged from 0.55 to 0.87 yr. One year after ¹⁵N-fertilizer application, greater recovery of ¹⁵N in the soil from straw incorporation versus burning (22.2 versus 18.7%) resulted in a slight increase in residual fertilizer N recovery in grain in the second growing season of the ¹⁵N study. Increased soil ¹⁵N recovery 1 yr after fertilizer application in the straw incorporation treatment, however, was offset by higher grain recovery of ¹⁵N in the burned treatment during the first growing season. Hence, the net result of these competing soil and plant sinks for fertilizer N led to similar ¹⁵N losses after 2 yr (50.3 ± 2.2%) under burned and incorporated straw. The cumulative effects of straw incorporation resulted in greater net N mineralization, an increase in microbial biomass N, and greater recovery of ¹⁵N in soil one year after application. Clearly, an active, labile N pool was formed when straw was incorporated that led to a reduction in fertilizer N dependency for rice.

Abbreviations: SMB-N, soil microbial biomass N • SMB-C, soil microbial biomass C • SOM, soil organic matter • WF, winter flooded • NF nonwinter flooded

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