THE EFFECT OF PLANTS ON GROUNDWATER CARBONATE CHEMISTRY IN EXPERIMENTAL SAND ECOSYSTEMS

Chemical weathering reactions are a primary source of alkalinity generation in groundwater systems involving the transfer of carbon from the atmosphere (as CO₂) to the hydrosphere (asH₂CO₃, HCO₃^- and CO₃^2-). In spite of the fact that the hydrosphere represents a larger pool of C than soils, the detailed mechanisms of CO2 sequestration in shallow terrestrial groundwater systems have been largely unexamined. How do vascular plants affect the rates of alkalinity generation and what particular processes are involved? We have studied pH and carbonate chemistry in shallow groundwater from experimental sand ecosystems (“sandboxes”) at the Hubbard Brook Experimental Forest in north-central NH. The sandboxes (7.5 m x 7.5 m x 1.5 m) consist of granitic sand from a local glacial outwash deposit and are fully lined to collect drainage water and solute exports. We have monitored the sandboxes for the last 13 years to examine how plants control mineral weathering and denudation, nutrient cycling, and carbonate-system chemistry. Here we report results from three sandboxes with different established plant covers (moss/lichen or nonvascular, grass, red pine). A clear seasonal cycle of groundwater pH occurs in the moss/lichen box that is inversely related to soil temperature. Groundwater pH values in nonvascular discharge range from 7.6-5.4, with maximum and minimum values occurring in the winter and summer, respectively. Groundwater pH in the grass and red pine discharge has a more narrow range (6.0-7.0) and shows no relationship to soil temperature. Circum-neutral pH values occur in the boxes with rooted plant systems, even when subsurface CO₂ values are very large (1-6 x 10³ ppm_v). Conventional gas-liquid carbonate equilibrium may not apply in these systems due to short groundwater residence times (days to weeks) or a lack of biochemical mechanisms to increase C exchange between phases. Bicarbonate fluxes ranged over the experiment from 100-700 mol C/ha/yr; for rooted plant systems, fluxes have decreased as plants matured and increased following plant disturbance via harvest and/or mortality. In the absence of rooted vegetation, C transfers to the hydrosphere still occur; fluxes are controlled by microbial activity and amount of soil organic matter available in the system.