Topographic controls on soil nitrogen availability in a lowland tropical forest

Geomorphic position often correlates with nutrient cycling across landscapes. In tropical forests, topography is known to influence phosphorus (P) availability, but its effect on nitrogen (N) cycling has received less exploration, especially in lowland forests where widespread N richness is frequently assumed. Here, we report significant effects of topographic slope and landscape position on multiple aspects of the N cycle across a highly dissected lowland tropical forest on the Osa Peninsula, Costa Rica. A suite of N cycle metrics measured along a topographic sequence revealed a distinct gradient in N availability. Values of soil delta N-15, inorganic N pools, net nitrification rates, and nitrification potentials were all substantially lower on a flanking steep hillslope (similar to 28 degrees) compared to a relatively flat ridge top (similar to 6 degrees), indicating lower N availability and a less open N cycle in steep parts of the landscape. Slope soils also hosted smaller total carbon and nitrogen stocks and notably less weathered soil minerals than did ridge soils. These latter findings suggest that elevated N loss resulting from high rates of soil and particulate organic matter erosion could underpin the spatial variation in N cycling and availability. Expanding our analysis to the larger study landscape, a strong negative linear relationship between soil delta N-15 values and surface slope angles was observed. N isotope mass balance models suggest that this pattern is most plausibly explained by an increase in N loss via erosive, non-fractioning pathways from steep zones, as most other variables commonly assumed to affect soil delta N-15 values (such as temperature, precipitation, and vegetation type) did not vary across the sampled region. Together, these results reveal notable hillslope-scale variation in N richness and suggest an important role for non-fractionating N loss in the maintenance of this pattern. Such findings highlight the importance of geomorphology and the significant capacity of erosion to influence N availability in steepland ecosystems.