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Abstract

While topographic variability and land-­‐use change have been shown to alter nutrient availability in diverse tropical landscapes, the combined effects of these two factors on nitrogen (N) cycling are poorly understood. In all ecosystems, however, it is vital to understand controls on N availability, as N is an element that is commonly associated with nutrient limitation in ecosystems. This study investigated the effects of topographic position and land-­‐cover type on multiple pools and fluxes associated with the N cycle in a heterogeneous wet tropical forest in southwestern Costa Rica. A suite of N cycle metrics were measured across four land-­‐cover types (primary forest, pasture, 10-­‐15 year successional forest, and 25-­‐30 year successional forest), and at two topographic positions (gentle slope ~10° and steep slope ~43°). Six of seven N metrics assessed differed significantly for both topography and forest type, with the exception being δ15N for topographic position and ammonium (NH4+) for land-­‐cover type. Concentrations of N were higher on gentle versus steep slopes across land-­‐cover types, and pastures exhibited the highest N concentration, with N concentrations declining through secondary forest succession. These findings suggest that differences in slope angle, likely associated with erosion rates, and anthropogenic land-­‐cover changes that alter forest dynamics and species composition may cause spatial variation in N cycling and availability. This may have important repercussions for our understanding of nutrient availability across heterogeneous tropical landscapes that are diverse in both topography and land-­‐cover.