The influences of organic residues on utilization of urea N by rice

Abstract

The organic materials of vetch straw, isotopically labeled with¹⁵N and unlabeled, rice straw and¹⁵N-enriched urea were applied to rice in a greenhouse experiment to evaluate the release of available N during the decomposition of vetch material and its uptake by rice, and to measure the effect of organic materials on the efficiency of urea-N utilization by rice. Measurements were made at three sampling stages during the growth period. As expected, vetch material decomposed readily and furnished a continuous supply of N for the growth of rice, although only 18.1% of vetch-N was utilized by the rice crop. However, this was not sufficient to support the survival of all tillers until harvest. After harvest, 70% of vetch-N still remained in paddy soil. The influence of organic materials on urea-N absorption by rice became apparent at about the stage of panicle initiation. The highest urea-N uptake by rice was 42.2% in vetch straw-mixed soil. Otherwise, rice straw retarded urea-N uptake by rice. Nitrogen distribution data indicated that the vetch material would stimulate urea-N uptake by rice plants.

The residual effect of vetch material was evaluated by planting Sudan grass immediately after rice was harvested. Only 4.4% of residual vetch-N was utilized in 20 weeks. This low percentage of N uptake and its low availability ratio demonstrated the poor residual effects of this leguminous material.