Factors influencing the mineralization of organic nitrogen, phosphorus and sulphur in soils
Part I of this thesis is a critical review of the literature on the contents, availability and factors that individually and/or severally influence mineralization of organic N, P and S in soils. The review also pinpointed the importance of mineralized portions of these plant nutrients in tropical rainforest farming systems. After establishing the efficiency of the useof bicarbonate anion exchange resin as an extractant for available P and S in soils, the method was employed throughout the remainder of this investigation to assay plant available P and S in ten Nigerian and three Scottish soils either before, during, or after the incubation and greenhouse studies. In order to identify those factors that most strongly influence mineralization of organic N, P and S in soils, the following facts were revealed through several incubation experiments with both Scottish and Nigerian soils. (i) There was higher microbial activity and hence higher production of mineral N, extractable P and S at the soil moisture content of 60 than 40% WHC. (ii) Drying and rewetting cycles positively affected NH4-N, P and S extractability but had no effect on NO3-N production. (iii) Increased soil C:N ratios had a complex effect on microbial activities and nutrient availabilities. At a ratio of 25, (a) there was complete immobilization of soil N with subsequent slow remineralization even after 96 days, (b) there was immobilization of P or decrease in resin extractable P, (c) there was increase in resin extractable S probably due to (d) lower pH levels that resulted from glucose additions, (e) there were always substantial increases in exchangeable Al, Fe and Mn following the addition of glucose, and this might have contributed to the decreases in resin extractable P observed. (iv) Nitrification was increased following the addition of NH4-N to a soil with high initial N content but had a depressing effect where the initial N content was low. (v) Added P stimulated the production of both NH4-N and NO3-N. (vi) Liming decreased accumulation of NH4-N, extractable Al, Fe and Mn but resulted in higher rates of nitrification, and increased resin extractable P and S. (vii) Partial sterilization of soil with toluene resulted in the accumulation of mineral-N entirely as NH4-N, an indication that toluene was effective as a nitrification inhibitor. Greenhouse studies using the Stanford and DeMent seedling short-term nutrient absorption technique showed positive effects of soil incubation, liming, N and P additions on the availability and offtake of N and S, the magnitude of which depended very much on soil type. However, P offtake was decreased by soil incubation prior to cropping. Similarly, the increase in resin extractable P due to liming observed during incubation studies did not manifest during the nutrient absorption studies. In general, there are significant interactions between factors affecting mineralization of N, P and S and therefore their effects cannot be evaluated in isolation of one another. The nitrification process is much more sensitive to adverse soil conditions than ammonification, and NO3-N production can be supressed by increased soil acidity, soil active S and the heavy metallic cations. Al which complexes rapidly with P in soil, follows the trends in pH changes that occur upon liming, application of acid nutrient fertilizers, and increased soil C:N ratio through glucose additions. The flush in active Al as pH falls may account for decrease in available P observed upon increase C:N ratio rather than microbial P-immobilization. It appears that soil pH and C:N ratio are the two factors most strongly influencing availability of N, P and S in soils. The effects of other factors on mineralization depend on the extent to which they themselves directly or indirectly affect soil pH and C:N ratio. In acid tropical rainforest soils which usually receive heavy annual precipitation, the accumulation of mineral-N as NH4-N. may be beneficial in preventing N losses through leaching. On the other hand, leaching losses of SO4-S in these soils may result in S-deficiencies. Hence, there should be well-researched liming and fertilizer programmes in order to enhance the availability of both added and mineralized N, P and S.