A Simplified Model for Estimating Nitrogen Mineralization in Paddy Soil.

Abstract

The estimation of available nitrogen in paddy soil is important in order to evaluate rice productivity and to improve fertilizer management. In this study, a simplified model is proposed for estimating nitrogen mineralization in paddy soil based on incubation experiments. The main factors considered are soil moisture percentage on dry weight basis before submergence (θ) and temperature (T). Soil samples were collected from an experimental paddy field (Alluvial, SL) at Kyoto University. Incubation experiments were conducted for wet and air dried soil under four different temperatures of 15°, 20°, 25° and 30°C. To investigate the effect of preflooding soil moisture on N mineralization, we set eight levels of soil moisture before submergence, after which samples were subjected to a similar incubation experiment. Mineralized N in wet soil (NW) increased linearly, while the drying effect (ND), defined as a difference in N mineralized between air-dried soil and wet soil, was found to have an equal ceiling value over the incubation temperature conditions, though the rate toward the asymptote varied with temperature. We employed the following equations to express the N mineralization rate :dN/dt=dNW/dt+dND/dt dNW/dt=KW dND/dt=KDNOexp(-KDt) where, KW and KD are temperature dependent rate constants expressed by the Arrhenius' equation, of which the apparent activation energy was 24, 803 and 14, 058 cal·mol-1, respectively. No is the maximum effect of soil drying expressed as a function of θ before submergence as follows : NO=7.171-0.66θ+0.015θ2, θl19.56% =0 θ>19.56% The present model was able to estimate satisfactorily the amount of NH4-N mineralized under specific conditions. Increasing the model generality is a subject for further study.