Immobilization of Fertilizer Nitrogen in Rice : Effects of Straw Management Practices

rope have been required to adopt alternative straw management practices, because burning has raised air polluA recent transition in rice straw management, from open-field tion concerns (Ocio et al., 1991; Eagle et al., 2000). In burning to soil incorporation in combination with winter-fallow flooding, has led to uncertainty in evaluating long-term N fertility. A 2-yr California, soil incorporation of crop residues during field study of 15N-labeled fertilizer and crop residue was initiated in the fall and shallow flooding of fields during the winterthe fourth year of a rice straw management trial to examine the fallow period has replaced open-field burning. These impacts of winter flooding and straw management on N fertilizer changes in straw management have prompted a reexamimmobilization and crop uptake. After six seasons of residue incorpoination of N immobization–mineralization dynamics ration and winter flooding, no effect on total soil C or N was observed. and their effect on N fertility in rice soils. In California, During the fifth and sixth year of the field study, microbial biomass rice straw production is 8 to 10 Mg ha 1, and it contains C and N were greater for straw incorporation than for straw burned. 50 to 70 kg N ha 1 (Brandon et al., 1995). The traditional Microbial biomass contained a sizable portion of soil-recovered 15N practice of burning eliminates 70 to 80% of the C and fertilizer after the first (23%) and second (10%) crop season of the N held in the straw, crown, and roots (Hill et al., 1999). 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 Most of the straw C and N left after burning remains 15N in the soil from straw incorporation versus burning (22.2 versus in the form of noncombusted root, crown, and stubble. 18.7%) resulted in a slight increase in residual fertilizer N recovery Consequently, the change from burning to straw incorin grain in the second growing season of the 15N study. Increased soil poration and winter flooding will likely alter the cycling 15N recovery 1 yr after fertilizer application in the straw incorporation of C and N in soil. treatment, however, was offset by higher grain recovery of 15N in the Although many studies have reported the fate of burned treatment during the first growing season. Hence, the net added 15N-labeled fertilizer N in lowland rice agroecoresult of these competing soil and plant sinks for fertilizer N led to systems, most investigations were limited to the measimilar 15N losses after 2 yr (50.3 2.2%) under burned and incorposurement of total fertilizer N in the plant and recovery rated straw. The cumulative effects of straw incorporation resulted in the soil (Patnaik and Broadbent, 1967; Patrick and in greater net N mineralization, an increase in microbial biomass N, and greater recovery of 15N in soil one year after application. Clearly, Reddy, 1976; Clement et al., 1995). Few investigations an active, labile N pool was formed when straw was incorporated that have directly examined the pathways of N immobilizaled to a reduction in fertilizer N dependency for rice. tion into the microbial biomass (Paul and Juma, 1981). We determined the effects of alternative rice straw management practices on the dynamics of N immobilization I of fertilizer and crop residue N in soil of applied fertilizer and crop residue N. 15N stable isois one of the most critical aspects affecting long-term tope methodology was used to follow the turnover of fertility in rice (Oryza sativa L.). The most important added fertilizer and crop residue N in the inorganic, misource of plant-available N for rice is soil organic N, crobial, and total soil and plant N pools. A 2-yr study of representing 50 to 80% of total N assimilated by the 15N fertilizer and crop residues was initiated in a 4-yrcrop (Broadbent, 1979; Mikkelsen, 1987). The relatively old rice straw management experiment. Our main objeclow fertilizer N use-efficiency in lowland rice systems tive was to assess the effects of straw incorporation and compared with upland crops (40–60% recovery of apwinter flooding on crop N availability, microbial N useplied N) has been attributed to higher losses due to efficiency, and the stabilization of added fertilizer-N and denitrification and volatilization, and a greater degree crop residue N in the soil. These parameters are critical of immobilization in soil (Broadbent and Nakashima, to develop straw management practices that lead to an 1970; Craswell et al., 1985; Vlek and Byrnes, 1986). The optimization of long-term N supply in flooded soils. mechanisms controlling immobilization of fertilizer and soil N in crop residue and their impact on long-term MATERIALS AND METHODS soil fertility are not well understood, especially in flooded rice soils. A better knowledge of the biological Field Site controls on N immobilization would enable improved In fall 1993, winter-flood and straw management treatments utilization of N from fertilizers and crop residues. were established at a 28-ha field site located on a commercial Recently, grain producers in North America and Eurice farm in the northern Sacramento Valley, near Maxwell, CA (USA). The soil is classified as a fine, smectitic, superacJeffrey A. Bird and William R. Horwath, Dep. of Land, Air and tive, thermic, Sodic, Endoaquert (Willows clay). The field Water Resources, Univ. of California, Davis, CA 95616; Alison J. experiment is a split-plot design with four replications. Winter Eagle, Univ. of California Cooperative Extension, Kearney Agriculflood management (flooded vs. unflooded) is the main-plot tural Center, Parlier, CA, 93648; Chris Van Kessel, Dep. of Agronomy and Range Science, University of California, Davis, CA 95616. Received 22 June 2000. *Corresponding author (wrhorwath@ucdavis. Abbreviations: SMB-N, soil microbial biomass N; SMB-C, soil microedu). bial biomass C; SOM, soil organic matter; WF, winter flooded; NF nonwinter flooded. Published in Soil Sci. Soc. Am. J. 65:1143–1152 (2001).