Soil sulphate changes in the presence and absence of growing plants

Abstract

To determine whether sulphate addition to sulphur-deficient soils affects mineralization of organic sulphur, an experiment was conducted in undrained pots with five soils of differing properties. Initial and final extractable soil sulphate (water-soluble and adsorbed), and sulphur taken up by Phalaris tuberosa L., were measured. Where plants were grown, mobilization of the organic sulphur occurred at the nil, 4 p.p.m., 12 p.p.m., and 36 p.p.m. levels of sulphate sulphur addition in four out of the five soils. In the fifth soil, a lateritic krasnozem, only at the nil and 4 p.p.m. levels was there any net release of sulphate. All five soils immobilized added sulphate at the 108 p.p.m. level. In the absence of plants, with one exception, no net mineralization of organic sulphur occurred following additions of sulphate. In the pots without added sulphate there was slight mineralization (less than under plants) in all soils but the lateritic krasnozem. The modifying effect of growing plants is obviously of great significance in the cycling of sulphur in soil. The relative intensity of immobilization and mineralization was affected by the presence of growing plants. This effect was probably due to the activities of rhizosphere microorganisms.