Short- and long-term effects of rice straw application on nitrogen uptake by crops and nitrogen mineralization under flooded and upland conditions

We investigated short- and long-term effects of rice straw application to paddy soils on crop growth, mineralization of straw N, and soil N supply under flooded and upland conditions. For investigating the short-term effects, pot and laboratory incubation experiments with 15N-labeled rice straw were conducted. In the incubation experiment over 90 days at 25 °C, 23– 24% of N in the 15N-labeled rice straw was mineralized under upland and flooded conditions. In the pot experiment, 13– 14% of N in the 15N-labeled rice straw was taken up by maize (Zea mays L.) and paddy rice (Oryza sativa L.). These values were greater than those with the 15N-labeled rice straw compost, however, N derived from the rice straw contributed to soil inorganic N and N uptake by crops only a small extent under flooded and upland conditions. Continuous application of rice straw (5 Mg ha^–1 year^–1 for 12 years) increased soil total N and total C. Nitrogen uptake by paddy rice, upland rice and maize in the field experiment, and inorganic N in the incubation experiment over 91 days at 25°C increased in the soil with the continuous rice straw application compared with the soil without rice straw application. Gross rates of N mineralization under flooded conditions also increased by the continuous rice straw application. The degrees of increase in N uptake by paddy rice and upland crops and increase in gross rates of N mineralization by continuous rice straw application were higher than the degree of increase in total N in the soil. Continuous application of rice straw contributed to the improvement of soil fertility and the promotion of growth and N uptake by paddy rice and upland crops, while short-term effects on dry matter production and N uptake by crops were little. The appropriate timing of rice straw incorporation and combination use of rice straw and mineral fertilizer would improve the short-term effectiveness.