SEASONAL FLUCTUATIONS OF NATIVE AVAILABLE N AND SOIL-MANAGEMENT IMPLICATIONS

The concentration of native available N in tropical soils fluctuates c onsiderably in response to seasonal changes in soil water potential. S uch fluctuation reflects the net effect of inputs of N from mineraliza tion, fertilizers and the atmosphere, and removal by plant uptake, imm obilization, leaching and gaseous losses. The greatest concentrations normally occur during the transition between the dry and wet seasons. In East-Africa, up to 184 kg mineral N ha(-1) has been measured in the 0-40 cm soil layer and in Trinidad, 143 kg ha(-1) was found in the 0- 10 cm layer. Release and accumulation of mineral N occur as a result o f the influence of soil water potential on microbial activity. This is due to changes in microbial motility, solute diffusion, microbial sur vival and the release of protected organic matter. A quantitative unde rstanding of these processes should increase the efficiency of use of this valuable N resource by crops. Current methods of forecasting mine ralization under field conditions include measurement of the soil mine ralization potential, the release of N from seasonal inputs of litter and model predictions. Litter quality is important. Its composition, i n particular its nitrogen, lignin and soluble polyphenol content has a major impact on its N mineralization rate. Crop uptake, gaseous and l eaching losses decrease the concentration of soil mineral N during the wet season. These losses are important under moist tropical condition s. For example, at Port Harcourt and Ibadan in Nigeria, leaching losse s were large in spite of NO3- adsorption which decreased the depth of NO3- leaching relative to through-flow. To minimise these losses, it i s essential to synchronise plant nutrient demand with supply by minera lisation. This is particularly important at the start of the tropical rainy season when high rates of mineralisation often in excess of the relatively low levels of crop demand, are observed. Fertilizer recomme ndation, the time table for cropping and the farming system used there fore need to take into account the seasonal availability of N. The CER ES model simulates crop growth and development and the N-cycle. As dev elopment and validation continue, such models should provide a strong basis for better soil, crop and fertilizer management practices. A bet ter understanding of the processes should provide a strong basis for f uther development of such models.