INTERACTIONS BETWEEN NUTRIENT DYNAMICS AND CHEMICAL WEATHERING AT SHALE HILLS CATCHMENT, A CRITICAL ZONE OBSERVATORY IN PENNSYLVANIA, USA

Earth's surface comprises a weathering engine that solubilizes and disaggregates rock to form regolith/soil. These processes provide nutrients to sustain terrestrial ecosystems, which in turn modify the soil environments by, for example, secreting organic acids and increasing soil CO2 concentrations. Here, we investigated the influence of biology on water-rock interaction along a steep, planar hillslope in the Shale Hills catchment, a Critical Zone Observatory recently established in Pennsylvania, USA. Measurements of carbon and nitrogen content were carried out in three soil profiles along the hillslope (ridge top, mid-slope, and valley floor). Weekly soil water samples collected from nested lysimeters were analyzed for dissolved organic carbon (DOC). Soil CO2 concentration profiles and the emitted CO2 fluxes were monitored weekly for about two months during summer 2009. Soil carbon and nitrogen, as well as C/N ratios, decrease with depth. Pore water DOC also generally shows decreases with depth. Additionally, a positive correlation is observed between DOC and dissolved Al and Fe in the pore waters, suggesting that dissolution of Al- and Fe-bearing minerals from the underlying shale is mediated by organic ligands. Soil pCO2 shows both temporal and spatial variation. Carbon dioxide shows a diffusion profile, with concentrations up to 50 times as high as atmospheric CO2. The CO2 concentrations are higher in the summer than those in the winter and spring. Frequent sampling enables us to conclude that CO2 emission flux from soils is controlled at least in summer by diffusion coefficient, thus the physical properties of soils (e.g., porosity, texture, soil moisture) while the CO2 gradient is almost constant. Due to the presence of such high soil gas CO2 and dissolved organic acids, soil pore water pH values were as low as 3.5, a condition that is favorable to clay mineral dissolution. In summary, soil, soil gas, and soil water phases were characterized in a watershed, developed entirely on shale and we presented several examples how biosphere, supported by soils, could influence the chemical weathering reactions and soil formation.