Tropical forest ecosystem processes play an important role in global biogeochemical cycles, including the uptake and storage of carbon. However, these processes can be constrained by the supply of rock derived nutrients, including the base cations calcium, magnesium, and potassium. Available forms of these nutrients are held on cation exchange sites within soils, but can be lost from ecosystems through leaching and the chemical and physical weathering of soil minerals. However, they can also be replenished through erosion and the subsequent incorporation of less weathered material into the soil profile. The influence of these countervailing processes can vary across regions and within landscapes to dictate the availability of rock derived nutrients within the soil column. In this study we consider how the rock derived nutrient availability and the capacity of soils to retain them on the exchange column shifts across a basin-to-treeline elevation gradient in the Andes-Amazon region, and how hillslope processes interact to moderate these shifts at the scale of local landscapes. To address this question we sampled soils to one meter, along six hillslopes at each of three locations along an elevation gradient on the eastern slope of the Andes in southern Peru to examine how elevation, hillslope position, and soil depth influence rock derived nutrient availability across a landscape.
We found a general decrease in surface-soil cation exchange capacity (ECEC) with decreasing elevation, while base saturation is highest at mid-elevations and lowest in low elevations across all soil depths. This pattern in base saturation is driven by high levels of available Ca at mid-elevation sites in all soil depths. This combination of high base saturation, mid range ECEC, and a high proportion of Ca at the mid-elevation site could be the combined effect of parent material and high erosion rates at this site. Hillslope variation in nutrient availability occurs primarily in subsurface soils. The CEC remains relatively constant along hillslopes at all elevations in surface soils, while it increases along low and mid elevation slopes in subsurface soils. In addition, the base saturation of subsurface soils increases along slopes at all elevations. These results indicate that hillslope processes vary based on local conditions at different elevations to control distributions of rock derived nutrients across landscapes, with implications for the distributions of ecosystem processes across the region.