Immobilization of fertilizer nitrogen in rice: Effects of straw management practice

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 N-15-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 N-15 fertilizer after the first (23%) and second (10%) crop season of the N-15 study. The half-life of the N-15 in the biomass ranged from 0.55 to 0.87 yr. One year after N-15-fertilizer application, greater recovery of N-15 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 N-15 study. Increased soil N-15 recovery 1 yr after fertilizer application in the straw incorporation treatment, however, was offset by higher grain recovery of N-15 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 N-15 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 N-15 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.