Long-term effects of land use and fertilizer treatments on sulfur cycling

The unique archived samples from the Rothamsted Broadbalk Experiment, England, were used to evaluate long-term effects of changing S inputs from atmospheric deposition and fertilization on soil S pools and soil S isotope ratio since 1843. The effects of changing land uses were also investigated. Large S inputs from atmospheric deposition and from sulfate fertilizers did not result in any significant accumulation of soil organic or inorganic S in the arable plots where organic C remained stable. Inputs of sulfate in excess of crop uptake were lost mainly through leaching. Organic S accumulated markedly in the arable plot receiving farmyard manure (FYM) or where arable land was allowed to revert to woodland or grassland. In the latter two systems soil organic C accumulated faster than organic S. In all soils investigated the S isotope ratio (delta S-34) decreased substantially during the last 150 yr. The decrease in delta S-34 was greater in the woodland, grassland, and the arable FYM plot than in other arable plots receiving either inorganic fertilizers only or no fertilizers. The results indicate that atmospheric S was more depleted in S-34 than the soil native S at the experimental site, and that atmospheric S was incorporated into the organic pool to varying degrees depending on the C pool. In conclusion,land use had a large effect on the S cycling in soils, which is driven mainly by soil organic C cycling. Without accumulating soil organic C, there appears to be little scope for S retention in temperate soils with neutral pH.