Tropical soil nutrient distributions determined by biotic and hillslope processes

Abstract

Understanding the drivers of nutrient distributions across heterogeneous tropical landscapes, and the implication of those distributions for ecosystem processes, remains a challenge in biogeochemistry. Guided by a high-resolution digital elevation model, we determined soil chemical characteristics, including rock-derived nutrient availability and total element concentrations, along 16 hillslopes with two different morphologies on a terra firme landscape in the lowland Peruvian Amazon. We assessed the role of geomorphic processes in rejuvenating soils, and the role of biotic processes in retaining nutrients in surface soils along toposequences. Our results show that the chemical weathering state of subsurface soils decreases down convex slopes, suggesting a role for erosion-driven rejuvenation. Total phosphorus (P) and calcium (Ca) are less depleted in surface soils than subsurface soils, in contrast to the vertical patterns observed for total magnesium (Mg) and potassium (K). Exchangeable Ca and Mg are highly concentrated in surface soils relative to the total available in the top meter of soil, which remains true as the total amount of these nutrients available in the top meter of soil increases along convex hillslopes. The combined results suggest that Ca is especially tightly cycled by this tropical forest ecosystem, and that steep, eroding hillslopes play a major role in the distribution and rejuvenation of rock-derived nutrients in regions far from marine aerosol and dust inputs. Geomorphically-mediated distributions of these nutrients, in turn, may underpin spatial variation in tropical forest carbon dynamics.