FOREST CANOPY CONTRIBUTIONS TO THE WEATHERING PROFILE

Soil profiles are commonly used to evaluate the geomorphic stability and age of landforms, and to characterize changes in regional climate. While the general factors that govern the development of weathering profiles are well established, we currently lack a means of quantifying the contributions of a particular factor to pedogenesis. Here we use fallout radionuclides and chemical extractions to trace the contributions of litterfall inputs to the formation of spodic horizons in forested soils of the northeastern United States and Norway. While the uptake of iron and aluminum by forest vegetation is relatively low on an annual basis, and usually on the order of 250 to 750 g ha^-1 y^-1, the decomposition of litter on the soil surface provides a top-down flux of these elements to the soil profile that is significant on timescales of millennia. We show that 30 to 50% of the secondary iron and aluminum in the upper mineral soil (Bs horizon) can be attributed to forest cycling since the last glacial retreat approximately 12,000 years ago. The techniques that we develop here give us a unique perspective on how soil horizons develop, and allow us to quantify the effect that the biosphere can have on weathering profiles.