The incorporation and transformation of 35S in soil: effects of soil conditioning and glucose or sulphate additions

Sulphur transformations in soil maintained in a closed incubation system were monitored by means of labelling with radioactive 35S-SO4. The distribution of 35S 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 35S-SO4 was added to soil, the amount and rate of 35S 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 35S, there was a comparatively slow rate of incorporation. An average of 8-19% 35S 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% 35S 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 35S into soil organic fractions within 5 days of incubation. A maximum of 94% incorporation was reached within 16 days. The incorporation of 35S 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 35S incorporation into C-bonded S. In the longer term, in the air-dried and glucose-treated samples, some of the 35S 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 35S incorporation into the biomass within 3 days, and could account for the higher level of 35S in C-bonded forms in these treatments. In the glucose-treated soil, in the short-term, up to 90% of the incorporated 35S Was found in the biomass-S pool. Addition of carrier sulphate together with 35S-SO4 appeared to have a retarding effect on the incorporation of the 35S into the soil. In the presence of added sulphate, most of the incorporated 35S was present as HI-reducible S and there was no significant labelling of the soil biomass-S during 32 days of incubation.