12:15 PM - 1:00 PM
[P2-31] Nitrogen Dynamics in Paddy Fields under Different Rice Bran Levels
*Nominated for Presentation Awards
The application of organic material (OM) is essential for sustainable rice production, and understanding decomposability is important for the effective use of OM. We hypothesize that easily-decomposable OM such as rice bran (RB) sharply increases inorganic nitrogen (IN) as plant growth begins, and the mineralization ceases in advance of plant maturity. To test this hypothesis, the mineralization, leaching, and plant uptake of nitrogen were investigated from winter to summer in rice fields where early-maturing cultivar Natsuhikari was applied with 0, 40, 80, and 160kgNha-1 of RB. NH4- and NO3-nitrogens were quantified in soils at 0-2, 2-10, 10-20, and 20-25cm depths for estimation of mineralization, and soil water pressure was measured at 15 and 30cm depths for assessment of leaching. Aboveground biomass was collected at five growth stages for estimation of nitrogen uptake. Mineralization started long before irrigation at low temperatures and prolonged until plant maturity. Soil nitrogen was the primary source for plants throughout the growth period; it contributed 69.4% of total nitrogen uptake. RB contribution was estimated at 3.9%, 31.5%, and 37.3% in 40, 80, and 160kgNha-1, respectively. A sharp decline of mineralized nitrogen was observed, implying the occurrence of immobilization despite the low C/N ratio (19) of RB. The concentration of IN was three times higher at harvest than at the start of the experiment. These results indicate that a gradual increase in temperature linearly increases nitrogen mineralization, and larger amounts of RB application allow continuous mineralization of nitrogen throughout the growth period with potential involvement of immobilization.