Cropping and Sulfur Fertilization Influence on Sulfur Transformations in Soil

Abstract

AbstractThe dynamics of S cycling in soils are not well understood. A 2‐yr field study was conducted to determine the effects of two S fertilizers (CaSO4·2H2O [CS] or elemental S [ES]) at two fertilization rates (0 or 50 kg S ha−1 yr−1) on the temporal distribution of soil S fractions (inorganic SO4, ester sulfate, and C‐bonded and residual S), arylsulfatase activity, and microbial‐biomass C and S in soil. The soil studied (a fine‐silty, mixed, mesic Aquultic Argixeroll) was cropped with winter rape, Brassica napus L. var. napus or left uncropped. Sulfate levels (0–15 cm depth) were significantly (P < 0.01) affected by S‐fertilizer treatment. During the rainy winter season, SO4 levels ranged from 7 to 13 mg kg−1 in CS plots, compared with 2 to 7 mg kg−1 in control plots. In the months from March to May, biomass S increased and SO4 levels decreased (<6 mg SO4‐S kg−1 soil), indicating S immobilization was occurring in the spring. Ester sulfate and residual S varied seasonally, whereas C‐bonded S showed minimal seasonal fluctuations. Cropping significantly increased biological activity (arylsulfatase activities and biomass C) over uncropped treatments. Subsoil SO4 appeared to be both a source (upward movement during high evapotranspiration) and a sink for S in surface soil, with extractable SO4 in the 60‐ to 90‐cm depth being significantly affected by cropping and S fertilization. The limitations of using extractable SO4 as a soil test were shown by wide variations in SO4 over time (2–14 mg SO4 kg−1 soil) in control plots, even though there was a seed yield response to S.