The fate of fertilizer nitrogen applied to the paddy field and its absorption by rice plant part 8 comparative studies of the fate of basal nitrogen and the absorption of nitrogen by rice plant in different paddy fields
Shoji, S.; Nogi, T.; Takahashi, J.; Wada, G.
Japanese Journal of Crop Science 45(2): 226-231
Two paddy fields, Odawara and Takadate, Japan, were compared. In both fields gray-brown soils were poor in soil organic matter, and were poorly drained. However, the clay contents and CEC [cation exchange capacity] values of the plowed layers were much greater in Takadate than in Odawara. The clay fraction of Takadate soil was dominated by montmorillonites. The amounts of N mineralized by incubation at 30.degree. C were greater in Odawara soil than in Takadate soil. The amount of soil ammonium N in both fields decreased exponentially after transplanting, and most of it disappeared at the accumulated effective thermal index (AETI) of about 400. The decrease of soil ammonium N in both fields was shown as an exponential equation of AETI. Soil ammonium N derived from basal N disappeared about the same time. The amounts of N per unit area absorbed by the rice plants (y) in both fields were closely related to the AETI (x). An exponential equation (y = cdx) was obtained for the early growth stage, and a 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 parameters between Odawara and Takadate. It was assumed that the limiting factors were the ability of N absorption of the rice plants, for the exponential part, and the rate of mineralization of soil organic N, for the linear part. The average recoveries of basal N (60-70 kg N/ha applied) in the rice plants were 43% in Takadate and 27% in Odawara, reflecting the ammonium absorption of the plowed layers. The amounts of soil N absorbed by the rice plants at ear mergence were 71 kg N/ha in Odawara and 44 kg N/ha in Takadate, reflecting the amounts of N mineralization by incubation. The amounts of residual basal N in plowed layers were 16% in Takadate and 19% in Odawara. The total recoveries (the recovery of basal N in the rice plants and residual basal N 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 most of the unrecovered basal N was lost by denitrification.