Nitrogen dynamics and fertilizer use efficiency in rice following straw incorporation and winter flooding

Incorporation of rice (Oryza sativa L.) straw, when compared with burning, affects soil N supply by increasing N and C inputs. This study determined the effects of long-term alternative rice straw management and winter flooding on seasonal crop uptake of N, fertilizer N use efficiency (FNUE), and crop N uptake from N-15-labeled straw. Microplots were established on two sites in California, Maxwell and Biggs, by applying N-15-labeled fertilizer during Year 4 of a long-term rice straw management study. At the end of the year, N-15-labeled straw was applied to assess crop uptake of straw N in the following season. Fertilizer use efficiency by N-15 dilution (FUE-N-15) over the growing season at Maxwell and at final harvest at Biggs was significantly higher when straw was burned rather than incorporated. Fertilizer use efficiency by N difference was significantly greater than FUE-N-15, suggesting an apparent added N interaction (ANI). Straw management did not significantly affect the uptake of residual fertilizer N-15 or of straw N-15 in the subsequent year. Winter flooding had no significant effect on measured parameters. Data indicate a decrease in FNUE with a concomitant and more significant increase in the plant available soil N following the change in straw management from burning to incorporation. Belowground N-15 pools appear to be a larger source of N for the following crop than aboveground residue.