Effects of topography and forest change on soil nutrient dynamics in wet tropical forests

A widely accepted paradigm for lowland tropical forests is that phosphorus (P) commonly limits plant growth while nitrogen (N) cycles in relative excess. And yet, increasing evidence suggests substantial heterogeneity in the availability of these essential elements from local to regional scales, driven by high variability in both biotic and abiotic factors. In this thesis, I used a series of field experiments in Costa Rica alongside detailed laboratory analyses to explore how changes in organic matter inputs, tree species and topography can all affect tropical forest nutrient cycling. First, I showed that changes in organic matter inputs, such as those that could result from altered land-use or forest productivity under elevated CO2, altered the availability of soil N and P and the production of enzymes that acquire them. Reduced leaf litter quantities exacerbated soil P limitation, yet leaf litter doubling shifted soils toward N constraints, similar to observations in temperate biomes exposed to elevated CO2. Next, using a series of mono-dominant native tree plantations, I demonstrated that species promote up to eight-fold differences in soil emissions of nitrous oxide (N2O), a potent greenhouse gas. Species differences in N2O efflux were strongly and inversely related to rates of fine-root growth. This link between belowground C allocation and emission of N2O may be important to forestry-related climate mitigation programs such as REDD+. Finally, I documented the importance of topography to N cycling in a diverse primary forest, where flat ridge-tops display much greater N-richness than do steep hillslopes. My data suggested that surface erosion might drive this landscape level pattern, thus for my last experiment I monitored fluxes of nitrogen in eroding soil, leaf litter, and runoff. I found that erosion of soil and organic N in steep regions is roughly equal to atmospheric N inputs in the short and long term, implying that erosion may maintain N limitation across substantial portions of topographically complex tropical forests. Taken as a whole, my work underscores several multi-faceted aspects of heterogeneous tropical forest nutrient cycling. This complexity is necessary to unravel in order to tackle societally-relevant issues ranging from climate prediction to forest restoration