The Fate of Fertilizer Nitrogen Applied to the Paddy Field and its Absorption by Rice Plant : VIII. Comparative studies of the fate of basal nitrogen and the absorption of nitrogen by rice plant in different paddy fields

Abstract

The behavior or basal nitrogen and absorption of nitrogen by the rice plants in two different paddy fields, Odawara and Takadate, were compared. The results obtained are as follows; 1) The soils of both fields belonging to Gray-Brown Soil were poor ill soil oraganic matter, and were poorly drained. However, the clay contents and CEC values of the plowed layers were much greater in Takadate than in Odawara. Furthermore, the clay fraction of Takadate soil was dominated by montmorillonites. On the other hand, the amounts of nitrogen mineralized by incubation at 30°C were treater in Odawara soil than in Takadate soil. 2) The amount of soil ammonium nitrogen in both fields decreased exponentially after transplanting, and almost all of it was disappeared at the accumulated effective thermal index (AETI) of about 400. The decrease of soil ammonium nitrogen in both fields was shown as an exponential equation of AETI. Soil ammonium nitrogen derived from basal nitrogen was disappeared about the same time. 3) The amounts of nitrogen per unit area absorbed by the rice plants (y) in both fields were indicated to be closely related to the AETI (x). An exponential equation (y=cdx) was obtained for the early growth stage, and linear one (y=ax+b), for the middle and late growth stages. The crossing point of these equations was the AETI of about 400. However, there were some differences in the parametcrs between Odawara and Takadate. It was assumed that the limiting factors were the ability of nitrogen absorption of the rice plants, for the exponential part, and the rate of mineralization of soil organic nitrogen, for the linear part. 4) The average recoveries of basal nitrogen (60-70 kg-N/ha applied) in the rice plants were 43% in Takada, and 27% in Odawara, reflecting the ammonium absorption of the plowed layers as mentioned in 1). On the other hand, the amounts of soil nitrogen absorbed by the rice plants at the ear mergence were 71 kg-N/ha in Odawara, and 44 kg-N/ha in Takadate, reflecting the amounts of nitroten mineralization by incubation. 5) The amounts of residual basal nitrogen in plowed layers were 16% in Takadate, and 19% in Odawara. Therefore, the total recoveries (the recovery of basal nitrogen in the rice plants and residual basal nitrogen in the plowed layers) were 68% in Takadate and 46% in Odawara. Since the water permeability of both fields was extremely small, it was assumed that almost all of unrecovered basal nitrogen was lost by denitrification.