IBL Tropical Forest Research Program Land Use Change Ecosystems worldwide are undergoing unprecedented rates of land conversion and land use change. Land-management practices such as fire, grazing, tillage, and fertilizer application, among others, affect ecosystem composition, the distribution of organic matter, and a variety of nutrient cycling processes, including losses of limiting and essential elements to air and water The most dramatic recent land use changes have been centered in tropical and sub-tropical forested ecosystems, where deforestation now removes roughly 2% of the remaining forest cover per year, most of which becomes cattle pasture. The biogeochemical consequences of land use change in the tropics are potentially quite different from those in temperate regions, in large part because tropical ecosystems frequently occur on extremely old soils. The combination of greater soil age along with a warm, often wet climate, leads to highly weathered soils that are typically depleted in phosphorus (P) and base cations (Ca, Mg, K), rich in iron and aluminum oxides, and of variable charge; these are the Oxisols and Ultisols that dominate many tropical regions. Carbon uptake and storage in ecosystems on such soils are often limited by phosphorus and/or base cation supply Thus, any land use driven changes in P and/or base cation availability may be central to predicting the sustainability of cleared land, as well as land-atmosphere exchanges of carbon and trace gases following forest conversion. Our lab has therefore studied the biogeochemical effects of tropical deforestation to cattle pasture in both Brazil and Costa Rica, with an emphasis on changes in P and base cation cycling, as well as the effects of forest-pasture conversion on microbial function. Additional details on can be found in the publications list.