CROPPING AND SULFUR FERTILIZATION INFLUENCE ON SULFUR TRANSFORMATIONS IN SOIL

The 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.