Divergent patterns of Carbon, Nitrogen and Sulfur storage in grassland soils

Sulfur (S) in soils mainly occurs in organic forms, and its cycling should be primarily controlled by the same factors as those for carbon (C) and nitrogen (N), two other main constituents of soil organic matter. Here, we aim to test this assumption based on a global meta-analysis of soil organic C, N and S contents in grassland soils. We reviewed existing literature with a focus on grassland soils as one of the major global ecosystems, including both native grasslands and managed grasslands with additional fertilization. Element concentrations and supplementary parameters (mean annual temperature and precipitation, texture, pH, soil group, management) were retrieved from the studies, while C:S and N:S element ratios were either directly obtained from the studies or calculated. Additionally, we analyze isotope ratios of the respective elements (δ¹³C, δ¹⁵N and δ³⁴S) in soil samples collected from native and managed grassland sites along climatic transects across Europe and North America.

In literature data concentrations of OC and N, but not S, were significantly higher in pastures compared to native grassland. As a consequence, C:S and N:S ratios were significantly lower in cultivated grassland than in native sites. Further, climatic conditions and soil group significantly affected C:S and N:S ratios, with significantly lower ratios in arid climate and in soil groups typically occurring there (e.g. Kastanozems) compared to more humid conditions and respective soil groups (e.g. Luvisols). However, the variation of C:S and N:S ratios was considerably higher than for C:N ratios. This was also evident in the isotope data obtained from the soil samples along the continental transects. Here, compared to δ¹³C and δ¹⁵N, δ³⁴S values showed strong variation that was only partially explained by climate and land use, and was additionally affected by the specific parent material at each site. We therefore suggest that the availability and turnover of S in organic matter of grassland soils is not strictly analogous to C and N cycling.