Abstract

Plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems.  Hydrologic and other losses are offset by inputs from atmospheric deposition and weathering. This multi-tracer study investigates these effects using ⁸⁷Sr/⁸⁶Sr, Ca/Sr, and Ge/Si ratios from solid (soil profiles and bedrock), biological (leaves and sap waters), and water (pore-, ground-, stream-waters) samples to study Ca, Sr, Ge, and Si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central Pennsylvania.  Leaves and sap waters were found to have similar Ge/Si ratios <1 μmol/mol which is consistent with biological fractionation occurring at the root–soil water interface. Ge/Si ratios in soil porewaters were higher near the surface and increased over the growing season suggesting plant preferential uptake of Si over Ge. Ca/Sr ratios in oak leaves (1360±40 mol/mol) were significantly higher than those in maple leaves (650±20mol/mol). Ca/Sr ratios were also generally higher in leaves than in sap waters which are consistent with preferential Sr adsorption or Sr uptake during transpiration.  ⁸⁷Sr/⁸⁶Sr ratios in both leaves and sap waters were similar for a given site implying that trees access similar pools of Sr and Ca, although there are site-to-site differences. Additionally, ⁸⁷Sr/⁸⁶Sr ratios in plant tissues were most similar to shallow pore waters (0–50 cm) suggesting that mineral nutrients are obtained by trees from the near sub-surface. ⁸⁷Sr/⁸⁶Sr ratios in soil solutions from ridgetop and swale sites are best explained by mixing Sr derived from shale and atmospheric deposition. Valley bottom soil reservoirs and stream and groundwater samples include Sr and Ca derived from dissolution of two isotopically distinct generations of carbonates. A preliminary estimate of the Sr and Ca stream fluxes and isotopic mass balances imply propagation of a carbonate weathering front of ca. 300 m Myr−1, a value larger than previously reported for regolith weathering advance rates calculated based on cosmogenic nuclides and U-series isotopes. The data for Ca, Sr, Si, and Ge in soil, soil solutions, and streamwaters reflect the interaction of slower weathering processes with rapid, biologically driven cycling between soils and biomass.

Citation

Meek, Katherine, Derry, Louis, Sparks, Jed, and Cathles, Lawrence (2016): 87Sr/86Sr, Ca/Sr, and Ge/Si ratios as tracers of solute sources and biogeochemical cycling at a temperate forested shale catchment, central Pennsylvania, USA. Chemical Geology, vol. 445, 84–102. DOI: 10.1016/j.chemgeo.2016.04.026