Sulphur cycling and its implications on sulphur fertilizer requirements of grazed grassland ecosystems

The need for assessing sulphur (S) requirements in intensively-managed grassland ecosystems is becoming increasingly important because of the worldwide emphasis on sustaining non-renewable fertilizer resources and reducing atmospheric S emission. This paper reviews major factors affecting S cycling and efficient utilization of S fertilizers and soil S reserves in grazed grasslands. Models used for quantitatively describing S cycling in these ecosystems are also discussed. The review shows that most (95%) of the total soil S in intensively managed grazed grasslands is present as organic S and the remainder is in readily soluble and adsorbed S. Soil organic S that has been accumulated with time in grazed grasslands receiving regular S fertilizer applications can provide a significant amount of S for plant S nutrition. Carbon-bonded S is the major (50–85%) form of soil organic S, owing to the regular return of animal excreta and plant litter. Grazing animals have a major effect on the amount and rate of S cycling as most (87–90%) of S ingested by grazing animals is returned to soils as excreta. Excretal S is either recycled or subjected to leaching losses and transferred to stock camps. Leaching losses are expected to be higher in camp than in non-camp areas as camp soils contain higher soil organic and inorganic S fractions. Likewise more soil organic S is mineralized in camp than non-camp soils. This may provide S exceeding that taken up by plants and hence could lead to substantial S leaching losses in camp areas. This review identifies the lack of quantitative data on: (1) effects of soil microbial activity and plant S uptake on the fate of applied S fertilizers and excretal S in grazed grassland; (2) rates of soil S transformations as influenced by management and seasonal variables; (3) the contribution of soil organic S to plant S uptake and leaching losses in camp and non-camp areas of grazed grasslands. This scarcity of quantitative data is a major constraint in the development of dynamic S cycling models for predicting rates of S cycling and for estimating plant S requirements in grassland ecosystems where S cycling has not attained a steady state. When a steady state exists, mass-balance S models are more reliable than the traditional practice of using only soil or plant S tests for assessing long-term annual plant S rerequirements.