Seasonal fluctuations of sulphate and soil microbial biomass-S in the surface of a Wakanui soil

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Abstract
A field consisting of pasture, cultivated, and fallow plots was conducted on a Wakanui silt loam soil at Lincoln, Canterbury to examine the seasonal variations in sulphate and microbial biomass-S concentrations in the surface soil. In general, soil sulphate levels during the summer and spring months remained higher than in the winter months. Sulphate concentrations appeared to be decreased by substantial rainfall events in the week before sampling. However, when rainfall high enough to cause sulphate leaching occurred more than 1 week before sampling, and providing temperatures were relatively warm, subsequent sulphur (S) mineralisation appeared to return sulphate-S to pre-rainfall levels. Sulphate levels generally increased with temperature, were reduced in the presence of actively growing plants, and were clearly inter-related with microbial biomass-S concentrations. The observed fluctuations in sulphate concentrations may have implications for the interpretation of current soil S tests based on extracted of sulphate-S. Soil biomass-S concentrations appeared to be strongly influenced by a combination of soil moisture and temperature. Although biomass-S concentrations tended generally to follow increases and decreases in soil temperature, there was also evidence that biomass-S was reduced at low soil moisture contents. During the course of the year, biomass-S fluctuated from 0.4 to 1.8% of the total soil S. Pasture plots had substantially higher biomass-S concentrations than either the cultivated or fallow plots.

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