Seasonal variations in forms of extractable sulfur in some New Zealand soils

Abstract

Seasonal variations in different forms of extractable sulfur (S) were examined in three New Zealand soils under permanent pasture. The concentrations of total extractable S, extractable inorganic sulfate-S (SO2-4-S), and extractable organic HI-reducible sulfur (HI-S) and carbon-bonded sulfur (C-S) were determined in field moist samples using four different extractants. During the course of the year, extractable SO2-4-S levels varied substantially. Concentrations of SO42--S in the soils peaked in the early spring and, for two of the soils, remained at relatively low levels throughout the rest of the year. The third soil, however, following a decrease in concentration in late spring, showed a continuous increase in SO2-4-S during the summer and autumn period. Concentrations of extractable organic C-S in soils were generally highest in the winter and declined significantly during the spring. Extractable organic HI-S concentrations fluctuated throughout the year with no clear trends. The proportions of the three forms of extractable S in soils also varied seasonally. The proportions of extractable organic HI-S in soils were more constant compared with the other two forms of S and generally accounted for less than 20% of the total extractable S. The seasonal changes in the proportions of SO2-4-S appeared to be inversely related to the changes in extractable C-S. This suggests that under the field conditions, the extractable organic C-S fraction is readily mineralized to sulfate when conditions are favourable for mineralization to take place.