Influence of redox conditions and rice straw incorporation on nitrogen availability in fertilized paddy soils

Temporal nitrogen (N) availability in fertilized rice paddies is the result of a balance of processes, mainly the gross rates of N mineralization, microbial and abiotic immobilization, and N losses. Water and crop residue management practices often confound these established relationships making N the most difficult nutrient to manage in rice cropping systems. To investigate and quantify the interactive effects of soil redox conditions and straw incorporation on temporal fertilizer-N availability, we treated a paddy soil with enriched ammonium-N-15 and incubated for 160 days under flooded or non-flooded conditions, with or without the addition of rice straw. Changes in total available N as well as available and immobilized fertilizer-derived N (FDN) during incubation were evaluated. Under both oxic and waterlogged soils, about 45-53 % of applied N was rapidly immobilized. Whereas in the former most of this FDN was released contributing to the available N pool, flooded soils experienced significant losses from the soil/water system (a parts per thousand 67 % of applied N). Addition of rice straw enhanced N immobilization, particularly under flooded conditions, that also contributed to limiting losses. Moreover, turnover of this labile organic matter pool supplied significant amounts of available N towards the later stages of the incubation, partly compensating for the immobilization of fertilizer-N.