Relationship between sulfur speciation in soils and plant availability

Both pot experiments and field survey were conducted to investigate the relationship between sulfur speciation in soils and sulfur concentration in plants. Soil sulfur was fractionated into water-soluble sulfate (S1), adsorbed sulfate (S2), carbonate-occluded sulfate (S3), ester sulfate (S4) and carbon-bonded sulfur (S5). The correlation was well described by multiple linear regression equations with plant sulfur concentration being the dependent variable and soil sulfur species being the independent variable. All the correlations were found to be highly significant (P < 0.01). Moreover, the regression equations show consistent forms with respect to the relative magnitude of the factors before each of the variables, by the order S1 > S2 > S4 > S5 > S3. The average values of the factors before S1, S2, S4, S5 and S3 were 28.6, 11.0, 3.2, 0.47 and −0.14, respectively, indicating that water-soluble sulfate was the most easily available form of soil sulfur, followed by adsorbed sulfate, ester sulfate, carbon-bonded sulfur and carbonate-occluded sulfate. Changes in concentrations of the different forms of soil sulfur after pot experiments gave direct evidence to the order of availability. S1 and S2 decreased significantly, the content of S4 also reduced, whereas S5 and S3 remained almost unchanged. These results suggested that water-soluble and adsorbed sulfate are directly available to plants, while the mineralization of sulfate from organic sulfur, especially from ester sulfate, also made an important contribution to plant uptake of sulfur. It appears that the sum of water-soluble sulfate, adsorbed sulfate and ester sulfate covers most of the available sulfur in soils and can be used as an indicator of the status of soil sulfur. Based on the above consideration a simplified speciation method was recommended to determine the available forms of soil sulfur.