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Mineralization of soil sulfur and its relation to soil carbon : nitrogen and phosphorus Kowalenko, Charles Grant


The mineralization of soil sulfur was studied particularly in relation to soil carbon, nitrogen and phosphorus. The investigation involved two aspects; an examination of pertinent methods and a consideration of the processes involved in soil sulfur mineralization using an incubation procedure. The examination of pertinent methods involved both analytical and incubation techniques. The bismuth sulfide colorimetric finish was shown to be a suitable alternative to the methylene blue finish in the hydriodic acid reduction method of quantifying sulfate. A chromotropic acid nitrate analysis method was shown to provide several advantages over the phenoldisulfonic acid method. The aerobic status of a particular closed incubation system was evaluated by comparing mineralization results with an open incubation system. The nature of the reagent, the soil to solution ratio and air drying were evaluated for their influence on using extractable sulfate as a measure of soil sulfur mineralization during an incubation study. Utilization of data not considering these influences were critically discussed and several recommendations were made. The established techniques were used to examine the mineralization of sulfur particularly in relation to other soil parameters. Relatively close correlations were found among calcium chloride extractable sulfate values and carbon dioxide evolved, nitrogen mineralized, and soil pH through an incubation over 14 weeks at 30°C and 100 cm water tension. Poorer correlations of calcium chloride extract-able sulfate with both sodium bicarbonate extractable phosphate and arylsulfatase activity were found. Relationships between total phospholipid phosphorus and sulfur mineralized were relatively close, however, the reason for this remained unclear. It appeared that phospholipids may be an important phosphorus or carbon source for microorganisms. Relationships among carbon, nitrogen and sulfur mineralization were discussed. The form and distribution of sulfur compounds, whether Hl-reducible S or C-bonded S was indicated as being an important consideration in mineralization studies. Attempts at separating organically bound sulfate from inorganic sulfate in phosphate buffer and bicarbonate extracts were largely unsatisfactory, however, significant amounts of the organic form were shown to be present. It was suggested that the inclusion of organic sulfate in phosphate buffer, bicarbonate and probably acetate extract analyses contributed to poor correlations with microbial activity. An incubation experiment using two soil samples, treated with five nitrogen sources at four rates was conducted to study the influence of nitrogen additions on sulfur mineralization. The analyses were done after eight weeks incubation using the previously established methods. Simple, partial and multiple correlations showed that the sulfate mineralized was influenced by nitrogen applications through the latter's effect on microbial activity and the resulting soil pH. An analysis of variance using a factorial design showed the following very highly significant interactions: nitrogen source with rate; nitrogen source with soil; rate with soil; and soil, source and rate all together. Hence, the nitrogen source had an influence on microbial activity and microbial environment which in turn influenced the soil sulfur mineralized. This effect varied with the soil sample, and nitrogen source and rate. The possible implications that nitrogen additions may have on plant-available soil sulfur and areas for further research were briefly discussed.

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