SULFUR MINERALIZATION AND TRANSFORMATIONS IN SOILS AS INFLUENCED BY ADDITIONS OF CARBON, NITROGEN AND SULFUR

The mineralisation of soil organic sulphur was examined for two soils using an open incubation system. Combinations of glucose-carbon, nitrate-nitrogen and sulphate-S were added to the incubated soils at regular intervals to examine the effects of these nutrients on S mineralisation. Soil microbial activity was monitored by CO2 evolution from the incubated samples. At the end of the 14-week incubation, the incubated soils were analysed to determine the sources of S mineralised during the incubation. In both soils, the addition of C, N and S had a considerable effect on S mineralisation. Additions of C or S decreased S mineralisation whereas N additions slightly enhanced mineralisation. The mineralised S appeared to be derived almost exclusively from C-bonded forms of soil organic S. In both soils, there were substantial increases in hydriodic acid (HI)-reducible forms of S during the incubation. Additions of C, N and S influenced the amounts of S mineralised or transformed from or to C-bonded and HI-reducible forms of soil organic S. The results from this study are discussed in relation to the dichotomous model of S cycling in soil as proposed by McGill and Cole (Geoderma, 26, 267-286, 1981). In some respects, the S mineralisation data appeared to support the model. However, in relation to the source of mineralised S, there were large differences between what the model would predict for certain treatments and what was actually observed. This was particularly true with regard to the transformations of HI-reducible forms of organic S.