Effects of co-situs application of controlled-availability fertilizer on fertilizer and soil nitrogen uptake by rice (Oryza sativa L.) in paddy soils with different available nitrogen

Improved recovery of fertilizer nitrogen (N) would permit a reduction in the rate of application of fertilizer, which would save resources and decrease N outflow from paddy fields into adjacent bodies of water. Contact application of controlled-availability fertilizer (CAF) to plant roots (i.e. "co-situs application") is a possible method to increase the recovery of fertilizer N by plants. We investigated the effects of co-situs CAF application on the uptake of fertilizer and soil N and the yield of paddy rice (Oryza sativa L.) in seven soils with different levels of available N. The experiment was conducted in a paddy field in which the plow layer soil had been replaced with the sample soils. The uptake of fertilizer N increased to 4.5-5.3 g m(-2) (65-75% recovery) with co-situs application (7 g N m(-2)) from 1.9-2.6 g m(-2) (27-37% recovery) with conventional readily available fertilizer (RAF). The increased uptake of fertilizer N was not significantly correlated with available soil N. However, the uptake of soil N decreased by 0.2-1.6 g m(-2) with co-situs CAF application, and the decrease in soil N uptake was greatest in soils with the highest available N. The net (total) N uptake after co-situs application was 0.9-2.4 g m(-2). Brown rice yield was strongly related to N uptake in the RAF and CAF plots. Nitrogen application rates could be reduced by 19-53% with co-situs application, while still providing a N uptake equal to that with conventional RAF application. However, the potential reduction in the application rate of N was smaller in soils with more available N.