Natural and predicted time course behaviour of ammonium nitrogen release in lowland rice soils

Abstract

An experiment was conducted on Typic Haplustall" soil at the Tamil Nadu Agricultural University, Coimbatore to study the effects of green manuring and N timings on soil ammonium-N release pattern under lowland flooded rice soils. The main-plot treatments consisted of three green manures viz., sesbania (Sesbania rostrata), cowpea (Vigna imguiciilnta) and parthenium (Parthenium hysterophoms L.) to supply 54 kg N ha"1 and one non-green-manure treatment. Application of 150 kg N ha"' as urea at varying quantities, at different growth stages of the crop along with one control formed the sub-plot treatments. Incorporation of green manures increased soil ammonium-N content. The effect was more pronounced during the peak period of soil ammonium-N release, which occurred between the second and fourth week after transplanting rice. Application of N fertilizer resulted in an immediate spurt in soil ammonium-N, but it declined within a few days. Due to faster N uptake by the crop, the decline was rapid during late growth stages. Continued splits (150 kg N ha'1 in six equal splits) ensured a steady ammonium-N supply throughout the growth period. Similarly, skipping of basal N and application of 150 kg N in three equal splits starting from early tillering stage onwards also adequately met the crop N demand for high yield. Mathematical models were developed on the ammonium-N release pattern from green manures and they predicted similar ammonium-N release pattern as observed in the experiments.