Resilience of phosphorus transformations in tropical forest and derived ecosystems

Abstract

The soils associated with most tropical forests are exposed to significant leaching. This leaching has weathered the soils’ mineral suite and reduced their capacity to bind nutrients, exacerbating the potential for nutrient loss from the forest or any derived ecosystem such as slash-burn agriculture. Phosphorus is often the most limiting nutrient in ecosystems on weathered tropical soils. Examples are given for P budgets of South American forests and for an estimate of P loss from a highly dystrophic rain forest. Despite a biological turnover of 6% of the forests P every year, only about 0.001% of the total P stocks are lost annually, indicating that biogeochemical P recycling is highly efficient. The mechanisms for such efficient cycling are internal retranslocation of P, and a litter and root mat with abundant mycotrophism which result in an effective separation of the biological P cycle from a potentially P-fixing soil. In the examples of less dystrophic forests from NE Brazil, P retention in the litter mat is less efficient and P uptake from the soil is more important. Therefore, the potential for non-forest land use is greater in NE Brazil, although P tranformations to less available forms limit the length of cultivation cycles. Few data exist on alternative land uses, such as tree crops or agroforestry, and the complex methodology of studying P has limited the number of experiments aiming at an understanding of the complete set of P transformations which determine an ecosystem’s resilience to disturbance and limit net P losses.