Origin and Regulation of Nutrient Supply to Plants in Humid Tropical Grasslands

Stocks and fluxes of carbon and nutrients ln tropical grasslands are determined by a suite of hierarchically organised factors.· Rain foll patterns and edaphic factors first determine the nature of the vegetation, the quantity and quality of organic inputs, and the relative allocation to the production of shoots or roots. Grazing by cattle or native herbivores, and fires, are other processes that significantly affect nutcient cycling. The nature and intensity of constraints exerted by these factors separate different types of tropical grasslands, i.e., savannas induced by the seasonality and limited abundance of rainfall, savannas induced by the poverty of soils and artificial pastures developed in rain forest areas. Tropical grasslands have in common general features like, e.g., the importance of roots as major organic sources for the soil, the impact of fires and/or grazing on the above­ground biomass, and the role of soil fauna, especially earthworms and termites, in the regulation of soil organic matter and nutrient dynamics at different scales of time and space. Detail of these regulations and specific interactions in the rhizosphere (e.g., allelopathy and general effects of the quality of the root material) differs among different types of grasslands. Descriptions of the above­mentioned structures and processes are given in this paper, with specific emphasis on soil organic matter turnover and nitrogen cycling.