Temporal and Spatial Patterns of Soil Nutrient Availability in a Wet Tropical Forest, Costa Rica

Tropical wet forests now cover approximately 13.70% the earth's land surface (Melillo et al. 1993). Despite their relatively limited distribution, they support a disproportionately large percentage of the earth's biodiversity, and play a significant role in the global carbon budget (Brown & Lugo 1982, Melillo et al. 1993, Field et al. 1998, Clark et al. 2001). At present, human disturbance in the form of land use alteration is the primary driver of environmental change in tropical ecosystems, but changes in global atmospheric composition and subsequent climate changes may become ecologically significant in the future (Scholes and van Breeman 1997). Altered climatic conditions could affect the functioning of tropical ecosystems through the disruption of biogeochemical cycles. The limited temporal and spatial resolution of nutrient cycling studies in tropical ecosystems constrains our understanding of controls on nutrient availability in highly weathered soils.

This work presents a comprehensive investigation of temporal and spatial patterns of nutrient (in particular P and N) availability in a mature tropical rain forest on timescales of hours to years and spatial scales of meters to kilometers and explores the potential environmental and biological mechanisms driving these patterns. Chapter one presents the results from a field study to determine the biotic and abiotic controls on diurnal fluctuations in soil P availability of a wet tropical forest. Chapter two includes results from a field study in which anion exchange resin membranes were used to quantify variability in labile P over days to weeks. The influence of environmental conditions and vegetation characteristics on patterns of soil P availability were compared in two sites of contrasting P fertility. Chapter three presents the results of a three year field study quantifying monthly soil nutrient (P and N) availability across a natural three-fold gradient in soil total P content. The influence of environmental conditions and vegetation characteristics on seasonal and interannual patterns of soil nutrient availability were assessed. Finally, Chapter four presents an investigation of potential links between P and N cycles as well as other soil chemical properties across a three-fold gradient in tropical forest soil P content.

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