Abstract

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.
2009 Portland GSA Annual Meeting (18-21 October 2009)

Citation

Jin, L., Andrews, D., Kaiser, N., Kaye, J., Lin, H., and Brantley, S. L. (2009): Interaction between nutrient dynamics and chemical weathering at Shale Hills catchment, a Critical Zone Observatory in Pennsylvania, USA. GSA Annual Fall Meeting.

This Paper/Book acknowledges NSF CZO grant support.