Sulphur fractionation in calcareous soils and bioavailability to plants

Sulphur fractionation and availability to plants are poorly understood in calcareous soils. Sixty-four calcareous soils containing varying amounts of CaCO3 were collected from ten provinces in China and their S fractions determined. Organic S was the predominant fraction of S, accounting for on average 77% of the soil total S. The amounts of adsorbed sulphate were found to be negligible. 1 M HCl extracted substantially more sulphate than either 0.01 M CaCl2 or 0.016 M KH2PO4, indicating the existence of water-insoluble but acid-soluble sulphate, probably in the form of sulphate co-precipitated with CaCO3. The concentrations of water-insoluble sulphate correlated positively with the contents of CaCO3 and accounted for 0.03–40.3% (mean 11.7%) of soil total S. To test the bioavailability of water-insoluble sulphate, a sulphate-CaCO3 co-precipitate labelled with 35S was prepared and added to a calcareous soil in a pot experiment with either $\mathrm{N}{\mathrm{H}}_{4}^{+}$ or $\mathrm{N}{\mathrm{O}}_{3}^{-}$ as the N source. In 29 days, wheat plants took up 10.6% and 3.0% of the 35S added to the soil in the $\mathrm{N}{\mathrm{H}}_{4}^{+}$ and $\mathrm{N}{\mathrm{O}}_{3}^{-}$ treatments, respectively. At the end of the pot experiment, the decrease of water-insoluble, acid-soluble, sulphate was more apparent in the $\mathrm{N}{\mathrm{H}}_{4}^{+}$ than in the $\mathrm{N}{\mathrm{O}}_{3}^{-}$ treatment. The results indicate that sulphate co-precipitated with CaCO3 in calcareous soils may become partly available for plant uptake, depending on rhizosphere pH, if the field precipitate is similar to the laboratory prepared sample studied.