Connections and Gaps Between Mineral Weathering and Biology in Forested Watersheds of the Northeastern USA.

Assessments of nutrient sustainability and critical loads are commonly formulated to consider the rooting zone as the domain where mineral weathering occurs and recharges nutrient supplies. However, common limitations of weathering studies are inadequate determination of mineralogic composition and insufficient sampling depth to determine location and advancement of weathering reactions. We determined mineral stocks through EPMA mapping of Al, Ca, Fe, P, and Si content of soil samples and an image analysis routine that assigned mineral composition based on the content of these elements. Portions of classified maps were confirmed by optical petrography or full elemental analysis by SEM-EDS.  We measured element depletion relative to Ti as an immobile index and carbonate content by pressure calcimetry. Samples were analyzed for soil profiles up to 2.5m depth, well past the upper boundary of the C horizon at ~0.7m. Sites spanned a composition range found in glaciated northeastern USA including Winnisook, NY (sandstone parent material, 100 mg/kg Ca), Hubbard Brook, NH (granite, 0.9% Ca), and Sleepers River, VT (calcareous granulite, 3.5% Ca). Physical evidence of mycorrhizal weathering of silicates in the rooting zone was found at Hubbard Brook and Sleepers River. All profiles exhibited a weathering front, or threshold above which the most reactive minerals (calcite, apatite) have been depleted, which was generally well below the C horizon interface. Catchment scale Ca exports reflect this deeper weathering source while rooting zone exchangeable Ca was highly variable, reflecting spatial variation in hydrologic flowpaths, which bring deeper weathering products to the surface only in certain landscape positions. Results suggest that nutrient cycling and critical loads models need to be refined to account for deeper weathering and spatial patterns of lateral and upward hydrologic fluxes. Recovery from cultural acidification may be limited where hydrologic connections do not provide a vehicle for weathering products to recharge the solum.