ACCUMULATION OF SOIL ORGANIC-MATTER AND THE FORMS, MINERALIZATION POTENTIAL AND PLANT-AVAILABILITY OF ACCUMULATED ORGANIC SULFUR - EFFECTS OF PASTURE IMPROVEMENT AND INTENSIVE CULTIVATION

The amount and distribution of organic C, N and S in the soil from a 37 yr old grazed pasture supplied with superphosphate at rates of 0, 188 and 376 kg ha-1 annually was investigated. Results were compared with a wilderness site which had not been used for agriculture and an arable site that had been intensively cultivated for 11 consecutive years. Levels of organic C, S and total N in the 0-4 cm soil layer followed the order 376 = 188 > control greater-than-or-equal-to wilderness >> arable. Compared with the control, accumulation of total N and organic S in the 188 and 376 treatments was proportionately greater than that of C reflecting the inputs of fertilizer sulphate-S and increased clover growth and N2 fixation in superphosphate-treated plots. Under wilderness and grazed pasture there was a rapid decline in soil organic matter content, biomass C and N and arylsulphatase activity with increasing soil depth whilst in the arable soil these properties were uniformly distributed within the surface 20 cm. Mineralization of S from the various soils (0-4 cm layer) as measured in an open incubation followed the order 376 > 188 > wilderness > arable > control. When measured by plant uptake in a pot experiment the same order of mineralization was followed except S uptake from control exceeded that from arable. This was attributed to poor soil physical properties limiting growth and S uptake from the arable soil. Differences in total organic S content, hydriodic acid-reducible and C-bonded S and arylsulphatase activity between treatments did not explain the magnitude of S mineralization for the various treatments. Bicarbonate-extractable and heat-soluble organic S appeared to be better indicators of potentially mineralizable organic S than other indices since they showed the trend 376 > 188 > control and the amount extracted from wilderness was greater than that from control and arable.