Sulfur, besides phosphorus, is crucial for the nutrition of plants on tropical
soils. Its availability is closely related to the turnover of soil organic
matter. To get a better insight into transformation of soil S forms during the
decomposition of organic matter, we studied inorganic and organic S pools in
bulk samples and alkaline extracts of soils under different land uses
representative of the tropical highlands of northern Thailand. Samples were
taken from a cabbage cultivation, a Pinus reforestation,
a secondary forest, and a primary forest. Total S ranged from 483 549
mg/kg in the subsoil to 1909 376 mg/kg in the organic layers, which is
relatively high for tropical soils. The major S component in soil was organic
S, comprising 75–99% of total S. Organic S was significantly
correlated with total S, organic C, and total N, indicating that there is a
close relationship between C, N, and S cycling in soil. C-bonded S was the
predominant form in the topsoils (35–99% of total S) but its
presence decreased with soil depth. The maximum concentrations of ester
SO4-S were found in the A horizons (128 49 mg/kg),
whereas the concentrations of inorganic SO4-S were small
in all horizons. Compared with the forest site, the cabbage cultivation site
was strongly depleted in S. C-bonded S was more depleted than ester
SO4-S.
A comparison of the S forms in NaOH extracts with S forms in bulk soil and C
forms as indicated by 13C-NMR spectroscopy showed
(i) that the extracts were very representative of soil
organic S fractions and (ii) that ester
SO4-S was mainly associated with O-substituted aliphatic
C. In contrast, C-bonded S seemed to be connected to more-or-less all C
binding types.
transformation of soil organic matter, sulfate.
Short-Term Changes in Fertility Attributes and Soil Organic Matter Caused by the Addition of EM Bokashis in Two Tropical Soils
