Soil Nitrogen Mineralization as Affected By Drying, Liming and Sewage Sludge Addition

Ahmad, Nazir
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This investigation was undertaken to determine the effect of drying and liming on soil nitrogen mineralization, to measure in N-supplying power of soils and to assess the effect of soil type on mineralization of nitrogen from sewage sludge. Laboratory, greenhouse and field experiments were conducted on a number of soils from the Hawaiian Islands. Air and oven-drying for 12 weeks increased the amount of mineral-N in five soils studied. Oven-drying released about 10-40 times more N than air-drying. Most of the mineral-N was released as NH^-N. Cultivation of undisturbed soil in the field resulted in more N being mineralized than from soil which was left undisturbed. Dry matter yield of corn was also affected when grown on air and oven-dried soils. Air-drying was more beneficial to corn yield in the Kaiwiki (Typic Hydrandepts). However, oven-drying had harmful effects on the growth of corn in this soil. Air-drying had little effect on the growth of corn in the Maile 7 (Hydric Dystrendept) soil. But in the oven-dried soil plants were much more healthy and more dry matter yield of corn was produced, compared to the control. The effect of lime application on N mineralization was studied on the Paaloa (Humoxic Tropohumult) and Wahiawa (Typic Eutrustox) soils in the field. The Paaloa soil had never previously been cultivated or limed. The Wahiawa soil had been limed twice before. Liming at 2 and 4 tons/ ha resulted in greater N mineralization than the control in the Paaloa soil. However, liming from a pH of 4.7 to 7.1 had little effect on the amount of N mineralized in the Wahiawa soil. An evaluation of the N supplying power of seven soils was made. Two chemical extraction methods as well as aerobic incubation procedures at 25°C and 35°C were used to obtain an index of N availability. Nitrogen mineralized was correlated with N-uptake by corn in the greenhouse. Aerobic incubation at 25°C was significantly correlated (r=0.96) with the N-uptake by corn. Incubation at 35°C was also highly correlated (r=0.92). The chemical methods employed were, by comparison, unsuitable in evaluating the N status of these soils. The amount of N mineralized from anaerobically-dried sewage sludge when mixed with two soils was very small, and depended on the soil type and the sludge rate. In the Waimea soil (Typic Eutrandept), 3-4% more N was mineralized than in the Wahiawa soil (Humoxic Tropohumult) when the sludge was applied at 22.4 and 44.8 tons/ha rates. N minrealization potential (Nq ) did not adequately predict the actual N mineralized from the soil: sludge mixtures. A two-function equation with two values of N q may be necessary to adequately describe N mineralization in sludge amended soil, because of two different N mineralization rates. One function describes the release of N from a readily-mineralizeable fraction in early incubation (0-4 weeks), while the other function describes the later r e lease of N from the more stable fraction in sewage sludge.
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