Immobilization of Fertilizer Nitrogen in Rice

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