Incorporation of S into soil organic matter in the field as determined by the natural abundance of stable S isotopes

In agricultural systems with low S inputs, soil organic matter is a major source of S and the transformations between organic and inorganic S pools are important for the supply of S to plants. This study was conducted to determine the effect of S fertilizer on the size and activity of organic S pools. For 5 years S fertilizer with a known composition of stable S isotopes was applied to a rotation on a loamy soil and a coarse sandy soil at rates higher than the plant demand. Total organic S in soil organic matter was not affected by sulphur application, but a small increase occurred in the sulphate ester fractions (P<0.05). Inorganic sulphate concentrations in the soil reflected the S application in the year of sampling, whereas S applied in earlier years was not recognized. Organic matter below the plough layer in both soils was enriched with S, possibly as a result or organic matter leaching or an increased clay content in the subsoils. At 0–20 cm, the C:S ratio in organic matter was ca. 100 for both soils, decreasing to 73 and 46 at 60–80 cm for the coarse sandy soil and the loamy soils, respectively. In both soils, isotope data showed that ca. 30% of organic-bonded S at 0–20 cm originated from fertilizer S applied during the last 5 years, irrespective of the S application rate. At 20–40 cm the rate of incorporations was lower and at 40–60 cm no incorporation of fertilizer S into organic matter was recognized. The fertilizer application did not induce net changes in the total organic S fraction, but isotope data indicated that a considerable part of the organic S pool was involved in S cycling in the field.