Incorporation of S into soil organic matter in the field as determined by the natural abundance of stable S isotopes
1996
Eriksen, J.
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 cat 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 cat 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.
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