Unsaturated flow and hydrogeochemistry beneath different plant covers: Field observations from the Hubbard Brook sandbox experiment

Experimental ecosystems (“sandboxes”) within the Hubbard Brook Experimental Forest (New Hampshire) were used to investigate water budgets, porewater residence times and porewater chemical evolution beneath different plant communities established on a sand substrate of granitic origin. The sandboxes (7.5 m x 7.5 m x 1.5 m deep) are fully lined with plastic fabric and instrumented to collect all water/solute exports. Three sandboxes, each planted with a different cover (red pine, bunch grass and a novascular cover of moss/lichen), were used for this study. Daily sandbox water inputs and outputs were measured for two years. Depth profiles of vadose-zone porewater were collected on a monthly basis and analyzed for major inorganic cations and anions. Water export from these sandboxes is characterized by a steady, small discharge (∼2 mm/d) during both winter and growing seasons. Beneath all plant covers, drainage discharge increased by 1–2 orders of magnitude for periods of days to weeks in response to snowmelt and/or large rainstorms. Evapotranspiration is largest and discharge is smallest beneath red pine > grass > moss/lichen cover, while water contents and pressure heads are largest beneath moss/lichen > grass > red pine. Mean porewater residence times (MRTs) determined from hydrologic techniques ranged from 1–5 months beneath each sandbox. A stable isotope tracer test conducted on the moss/lichen sandbox yielded an estimate that was within the range of MRTs based on hydrologic techniques. Acidic precipitation inputs (pH 4.2) are being neutralized within the upper 15 cm of the subsurface (porewater pH ∼6) beneath all plant covers. Decreases in exchangeable base cation concentrations on the solid phase were measured beneath all plant covers over a 5 year interval. A simple, annual mass balance of hydrogen ion suggests that cation exchange is a sink for 34–60% of the hydrogen ion sources in these ecosystems, with the majority of the remainder attributed to consumption in primary mineral weathering reactions.