Sulphur cycling in Danish agricultural soils: Turnover in organic S fractions

Abstract

The turnover of S in the microbial biomass and its incorporation into organic matter with different degrees of physical protection was investigated. In a field study, three soils were mixed with carrier-free 35S and combinations of S and cellulose in PVC cylinders cropped with perennial ryegrass. In the absence of cellulose, the recovery of 35S in soil organic matter was 5–15 and 10–20% for the +S and −S treatments, respectively. The presence of a C source increased the total 35S incorporation considerably, and the recovery in the +S and −S treatments was 20–30 and 40–50%, respectively. Incorporation of 35S into the fraction not protected by soil structure reached a maximum between 8 and 14 d after amendment and then slowly decreased over the next months. In the fraction physically-protected by soil structure, the incorporation of 35S was slow in the first 2 wk, but then increased until 12–16 wk after the start of the experiment. The results suggest that, initially, 35S was incorporated into organic matter not protected by soil structure and then gradually redistributed into physically protected organic matter. Organic S was not accumulated in the unprotected fraction, whereas the protected fraction appeared to be a storage for organic S, at least in the short term. The treatments did not significantly affect microbial biomass S, which may be partly due to variations caused by the field conditions employed. In contrast, the incorporation of 35S into microbial biomass was significantly higher in the C treatments (19% recovery in the −S + C treatment) than without C amendment (4% recovery). The recovery of 35S in the microbial biomass remained relatively constant, although immobilization increased, indicating that 35S immobilized by the microbial biomass was transformed directly into soil organic S.