Nutrient Cycling in Semi-Arid Grasslands and Savannas: Its Influence on Pattern, Productivity and Stability

The grazing animal carrying capacity of tropical semi-arid rangelands is largely controlled by rainfall, but above about 500 mm per annum, soil nutrient status has an overriding role. On low-nutrient soils, which cover vast areas of the ancient land surfaces of Africa, South America and Australia, it is the amount of grass eaten and digested that controls animal production, not the amount produced. On nutrient­poor soils (particularly those deficient in nitrogen and phosphorus) the primary production continues to increase with increasing rainfall, but the acceptability of the forage to ruminants declines, On the nutrient-rich soils of younger land surfaces, both primary production and animal carrying capacity continue to increase with increasing rainfall. Much of the spatial pattern of plant and animal communities in dry and mesic tropical rangelands is linked to variations in soil nutrient supply. The clay content and type is a key factor related to parent material. Clay is not only a source of nutrients itself, but also determines the degree to which organically bound nitrogen and phosphorus can accumulate in the soil. There are several abiotic and biotic mechanisms that redistribute clay and nutrients in the landscape, resulting in nutrient-rich and nutrient-poor patches which are differentially exploited by herbivores. It is likely that the creation and maintenance of such patches is necessary for the survival of grazers in nutrient-poor environments.