Soil organic matter dynamics under alternative rice straw management practices

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 recent transition in straw management, from open-field burning to soil incorporation in combination with winter-fallow flooding, has prompted a re-examination of N immobilization-mineralization dynamics and their effect on N fertility in temperate rice (Oryza sativa L.) soils.;This research addressed two main questions: (1) What are the effects of four to six years of soil incorporation of rice straw and winter-fallow flooding compared to straw burned on labile and resistant SOM C and N? (2) Are C and N immobilization and humification processes similar in soil after four to six years of alternative straw management practices?;Carbon and N cycling were examined with two stable isotope tracer studies (labeled 15N fertilizer and 15N and 13C straw) conducted during the fourth through sixth years of a long-term field study examining alternative straw management practices. The rates, pathways and stability of C and N additions were determined using SOM fractionation (five labile and resistant fractions) and microbial (phospholipid fatty acid (PLFA) and chloroform fumigation-incubation (CFI)) methodologies. The effects of waterfowl foraging on rice straw decomposition was investigated in a separate, one-year field study examining the effects of waterfowl in tilled and untilled winter-flooded fields.;Total soil C and N were unaffected by six years of soil incorporation of straw and winter-fallow flooding compared with burned straw. However, four seasons of straw incorporation increased C and N in labile SOM fractions (mobile humic acids (MHA) and light fraction, (LF)), and soil microbial biomass (SMB) compared with straw burned. Similarly, straw incorporation increased the immobilization of fertilizer and straw N into the SMB, LF, and MHA pools. Humification pathways were dissimilar for straw C and N; both were affected by treatments. Waterfowl foraging substantially increased straw decomposition in flooded, fallow, rice fields. These results provide a process-based understanding of the changes in C and N dynamics due to alternative straw management practices; they justify reduced N fertilizer application rates after four seasons of straw incorporation.