THE INCORPORATION AND TRANSFORMATIONS OF S-35 IN SOIL - EFFECTS OF SOIL CONDITIONING AND GLUCOSE OR SULFATE ADDITIONS

Sulphur transformations in soil maintained in a closed incubation system were monitored by means of labelling with radioactive (S-SO4)-S-35. The distribution of S-35 between phosphate-extractable S, biomass-S, hydriodic acid (HI)-reducible S and carbon-bonded S was determined at intervals of between 1 and 120 days of incubation. In studies were carrier free (S-SO4)-S-35 was added to soil, the amount and rate of S-35 incorporation into the soil depended on the treatment of the soil prior to incubation. When soil was conditioned (maintained at 20-degrees-C and 75% field moisture capacity for 2 weeks) prior to the addition of S-35, there was a comparatively slow rate of incorporation. An average of 8-19% S-35 incorporation occurred within 1-5 days and a maximum of 48% incorporation between 90 and 120 days. Air-dried soil incubated without conditioning, showed a faster incorporation in the short to medium term, 14-28% S-35 incorporated within 1-5 days, and a maximum of 43% incorporation after c 60 days. A third treatment, using air-dried soil to which 1% glucose C had been added showed a very high and rapid rate of incorporation of S-35 into soil organic fractions within 5 days of incubation. A maximum of 84% incorporation was reached within 16 days. The incorporation of S-35 into HI-reducible and C-bonded forms of S showed considerable differences between treatments. In the short-term, most of the incorporation in the conditioned soil was into HI-reducible forms. The two treatments which received no conditioning showed a much higher proportion of S-35 incorporation into C-bonded S. In the longer term, in the air-dried and glucose-treated samples, some of the S-35 originally incorporated into C-bonded forms of S appeared to be transformed to HI-reducible S or remobilized to extractable forms of S. Microbial biomass in the conditioned soil showed no significant labelling until the 24th day of incubation. The other two treatments showed significant amounts of S-35 incorporation into the biomass within 3 days, and could account for the higher level of S-35 in C-bonded forms in these treatments. In the glucose-treated soil, in the short-term, up to 90% of the incorporated S-35 was found in the biomass-S pool. Addition of carrier sulphate together with (S-SO4)-S-35 appeared to have a retarding effect on the incorporation of the S-35 into the soil. In the presence of added sulphate, most of the incorporated S-35 was present as HI-reducible S and there was no significant labelling of the soil biomass-S during 32 days of incubation.