Biological Cycling of Mineral Nutrients in a Temperate Forested Shale Catchment

Abstract

Biological pumping of mineral elements (root uptake from the soil and concentration at the surface via litterfall) may be an important mechanism influencing their loss from terrestrial ecosystems by accelerating transport in runoff, though few estimates exist to assess this. In the Susquehanna Shale Hills Critical Zone Observatory (a temperate forested watershed in central Pennsylvania), we compared two independent methods (litterfall-based and transpiration flow-based) for estimating the total uptake of elements (Ca, K, Mg, Mn, Si, Sr, and Al) by canopy trees. Elemental concentrations were measured monthly in leaf tissue and xylem sap of dominant species Quercus rubra (chestnut oak), Q. prinus (red oak), and Acer saccharum (sugar maple) for two growing seasons. Species-specific litterfall and transpiration (estimated by eddy covariance) were used to scale concentrations to annual fluxes. For most elements, both methods generated comparable gross fluxes in the range of 53–85 (Ca), 9–19 (Mg), 14–28 (Mn), 0.2–0.6 (Al), and 0.1–0.3 (Sr) kg·ha−1·year−1. For K, litterfall-based methods generated substantially lower estimates than transpiration, though neither accounted comprehensively for leaching from live foliage, resorption, or potential recycling within the growing season. For most elements, uptake rates were similar in magnitude to stream losses, implying a low degree of recycling. For K and Mn however, biological uptake exceeded losses by 1–2 orders of magnitude, suggesting a biological role in ecosystem retention. We conclude that litter- and transpiration-based methods can be combined to broadly estimate the magnitude of biological pumping, with additional measures of wood-/root-based turnover necessary for more accurate budgets.