7. The efficacy and loss of fertilizer N in lowland rice

Nitrogen fertilization is a key input in increasing rice production in East, South, and Southeast Asia. The introduction of high-yielding varieties has greatly increased the prospect of increasing yields, but this goal will not be reached without great increases in the use and efficiency of N on rice. Nitrogen enters a unique environment in flooded soils, in which losses of fertilizer N and mechanisms of losses vary greatly from those in upland situations. Whereas upland crops frequently use 40–60% of the applied N, flooded rice crops typically use only 20–40%. There is a great potential for increasing the efficiency of N uptake on this very responsive crop to help alleviate food deficits in the developing world.This article reviews current use of N fertilizers (particularly urea) on rice, the problems associated with rice fertilization, and recent research results that aid understanding of problems associated with N fertilization of rice and possible avenues to increase the efficiency of N use by rice.     

Nitrogen behavior in tropical wetland rice soils. 2. The efficiency of fertilizer nitrogen,priming effect and A-values

Results of tracer pot experiments show that in tropical wetland rice soils, rice plants recovered 50–69% of applied fertilizer N in the first cropping, 7–12% in the second cropping and 1–4% in the third cropping. Recovery of fertilizer N in the presence of incorporated rice straw was decreased to 45–53% (first cropping), 9–12% (second cropping), and 3–5% (third cropping), respectively. Application of fertilizer N resulted in the increase in plant uptake of native soil nitrogen due to priming effect which valued 3–29% of total N uptake by the rice plants. A-values calculated show overestimated amounts of available soil N in relation to plant uptake of native soil N. Perhaps their use in assessing fertilizer requirement in tropical wetland rice soils would be of limited meaning.     

Dryland wheat yields in relation to soil organic carbon, applied nitrogen, stored water and rainfall distribution

Correlations of long-term yields with soil, climatic and management variables would increase our understanding about their contribution to yield and help balance them for optimising the same. Therefore, grain yields of dryland wheat which received variable N ranging from 0–100 kg ha^–1 collected over 13 years, were analysed statistically. Wheat responded significantly to applied N in all but two years when the yields were the lowest. Grain yields across years did not relate with applied N alone (r ² = 0.00), but inclusion of seasonal water supply and organic carbon content of soil (OC) in the regression accounted for 64 per cent variation in yield. When water supply was split into stored water and growing season rain the relationship improved further (R ⁴ = 0.68) and the regression exhibited a significant interaction between seasonal rain and applied N.Three splits of seasonal water supply,viz. rain during 45 days after sowing (DAS) plus available water storage at seeding, rain during 46-120 DAS and rain during 121 DAS till a week before physiological maturity; soil OC and applied N explained 80 per cent variation in grain yield. Response to increase in post seeding water supplies was larger than that to the preseeding storage. Validation of the best regression against independent published data gave excellent agreement.