Nitrogen fixation of soybean in rice-based cropping systems

Soybean dry matter and crop nitrogen uptake increased with increasing rate of nitrogen fertilizer applied to either rice soybean. However, there was no additional improvements in soybean dry matter and nitrogen accumulation when the starter nitrogen was supplied following applications of 100 kg N/ha and 300 kg N/ha to rice. Nodulation was inhibited by nitrogen fertilizer applications to either rice or soybean during early growth. This reflected in the depression of the relative abundance of ureides nitrogen in xylem sap. These effects, however, disappeared later. Nitrogen fixation in soybean was increased by the use of nitrogen fertilizer with either rice or soybean. Without nitrogen to either crop, soybean fixed 122 kg N/ha. Supply of either starter nitrogen at 25 kg N/ha and 50 kg N/ha, or the residual effect of 100-300 kg N/ha to rice increased the amount of nitrogen fixed to between 132 and 140 kg N/ha. Starter nitrogen had no effect on nitrogen fixation when 100 kg N/ha was supplied on rice, but starter nitrogen at 50 kg N/ha significantly decreased nitrogen fixation following the application of 300 kg N/ha on rice. There was small improvement in soybean seed yield with the use of fertilizer nitrogen, but nitrogen fertilizer may affect long term nitrogen balance in the soil. After the seed harvest and straw removal, which was the common practice by farmers, it was estimated that the removal of nitrogen exceeded the input of nitrogen from fixation, ranging from -6 kg N/ha to -35 kg N/ha. However, when straw was returned to the soil, nitrogen balance was somewhat improved. There was a slight positive balance up to 9 kg N/ha in treatments with some starter nitrogen or some residual nitrogen from rice; the net negative balance of nitrogen, at -10 kg N/ha, occurred in the treatment with highest of nitrogen at 300 kg N/ha to rice plus 50 kg N/ha starter nitrogen to soybean.