Nitrogen behavior in tropical wetland rice soils

Laboratory and greenhouse pot experiments show that tropical wetland rice soils are very diverse in the inherent fertility. The mineralization of native soil nitrogen differed among the soils studied. Air-drying and oven-drying of soils resulted in a large increase in nitrogen mineralized. The main source of mineralized N was the amino acid and amino sugar fractions.N uptake by rice was well correlated with N mineralized in incubation of air-dried soil which should reflect accurately the nitrogen-supplying capacities of tropical wetland rice soils during the growing season. However, plant uptake of available soil nitrogen was also correlated with total nitrogen content of soils, and it would appear that total soil nitrogen which is simple to determine is a satisfactory index of the N supplying capacities of tropical wetland rice soils. Exclusion of problem soils like acid sulfate soils improved the correlation.     

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.