Climate, Topography, and Canopy Chemistry Exert Hierarchical Control Over Soil N Cycling in a Neotropical Lowland Forest
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Nutrient availability varies substantially across lowland tropical forests and constrains their responses to global change. However, interactions among regional, landscape, and local controls of nutrient availability are poorly understood. In that context, we explored the effects of rainfall, topography, and canopy chemistry on nitrogen (N) cycling across the Osa Peninsula (Costa Rica). We sampled soils from catenas in regions receiving 3000 versus 5000 mm y−1 rainfall. In both regions, we sampled catenas starting on narrow, knife-edged ridges, and in the less humid region we compared catenas starting on rapidly eroding knife-edged ridges to catenas with ridges consisting of slowly eroding terraces. On the stable terraces, we sampled soils from 0.25 ha plots with either high or low mean canopy N. In all sites, we measured metrics of long- (soil δ15N) and short-term (net nitrification, net N mineralization, and KCl-extractable N) N availability. Mean soil δ15N was elevated in the less humid region (3.8 ± 0.16 vs. 3.1 ± 0.14‰; P = 0.003). Within that region, mean δ15N was enriched by approximately 1‰ on stable terraces (5.3 ± 0.14‰) relative to nearby knife-edged ridges (4.0 ± 0.24‰; P < 0.001). Short-term N metrics did not vary with rainfall or topography (P > 0.05). By contrast, short-term soil N metrics differed under canopies with high versus low canopy N, but soil δ15N did not. These results illustrate the role of climate and topography as dominant drivers of long-term N cycling in the region, as well as the potential for canopy characteristics, which are likely determined by species composition in this system, to impose small-scale heterogeneity within those broader constraints. Overall, our work suggests the utility of a hierarchical framework for understanding how diverse drivers of nutrient status interact across space and time in tropical forests.
Keywords15N canopy chemistry Carnegie Airborne Observatory climate imaging spectroscopy LiDAR nitrogen soil topography
This work was supported through a collaborative Grant awarded by the National Science Foundation (DEB-0918387 to SP), and an IGERT (DGE-0966060 to D. Rand). Carnegie Airborne Observatory (CAO) data collection and processing was funded privately by the Carnegie Institution for Science. The CAO has been made possible by grants and donations to G.P. Asner from the Avatar Alliance Foundation, Margaret A. Cargill Foundation, David and Lucile Packard Foundation, Gordon and Betty Moore Foundation, Grantham Foundation for the Protection of the Environment, W. M. Keck Foundation, John D. and Catherine T. MacArthur Foundation, Andrew W. Mellon Foundation, Mary Anne Nyburg Baker and G. Leonard Baker Jr, and William R. Hearst III. From the Carnegie Airborne Observatory, we thank R. Martin, C. Anderson, D. Knapp and N. Vaughn for assistance with data collection and processing. We thank Osa Conservation and the Ministeria de Ambiente y Energía for assistance with research permits and site access. The authors appreciate field and laboratory assistance from M. Lopez as well as B. Cannon, A. Ginter, B. Munyer, A. Swanson and R. Ho. Funding was provided by Blue Moon Foundation.
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