Thesis Abstract of AGS Students

Nitrogen fixation of soybean in rice based cropping systems

Ying Jifeng (1990)

A field experiment was conducted to measure the amount of nitrogen fixed in soybean grown after rice. There were 9 nitrogen treatments which were factorial combinations of three levels of nitrogen supplied to rice (0, 100, 300 kgN/ha) and three levels of starter nitrogen (0, 25, 50 kg N/ha) to the subsequent soybean crop. The experiment was carried out on San Sai soil, low in total nitrogen (0.05-0.06%), at the Multiple Cropping Center Research Station of Chiang Mai University, Thailand. Estimates of soybean nitrogen fixation in soybean were made by the xylem sap analysis method.

Soybean dry matter and crop nitrogen uptake increased with increasing rate of nitrogen fer-tilizer applied to either rice or soybean. However, there was no addittional improvements in soybean dry matter and nitrogen accumulation when the starter nitrogen was supplied following applications of 100 kgN/ha and 300 kgN/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 kgN/ha. Supply of either starter nitrogen at 25 kgN/ha and 50 kgN/ha, or the residual effect of 100-300 kgN/ha to rice increased the amount of nitrogen fixed to between 132 and 140 kgN/ha. Starter nitrogen had no effect on nitrogen fixation when 100 kg N/ha was supplied to rice, but starter nitrogen at 50 kgN/ha significantly decreased nitrogen fixation following the application of 300 kgN/ha to rice.

There was small improvement in soybean seed yield with the use of fertilizer nitrogen, but nitrogen fertilizer may effect 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 exceed the input of nitrogen from fixation, ranging from -6 kgN/ha to -35 kgN/ha. However, if straw was returned to the soil, nitrogen balance was somewhat improved. There was a slight positive balance upto 9 kgN/ha in treatments with some starter nitrogen or some residual nitrogen from rice; the net negative balance of nitrogen, at -10 kgN/ha, occurred in the treatment with highest of nitrogen at 300 kg N/ha to rice plus 50 kgN/ha starter nitrogen to soybean.

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