Plant-cover effects on hydrology and pedogenesis in a sandy vadose zone

Abstract

The hydrology of three field lysimeters was monitored after 15 years of plant growth to examine pedogenic impacts of plant-induced changes to water budgets, flow, and chemical denudation. Between 70% and 87% of annual water discharge from all lysimeters (“sandboxes”) was generated from short (usually ≤5 days) precipitation events and longer (usually ≤25 days) snowmelt events. These infiltration events occur only 10–23% of the time throughout the year. Average annual evapotranspiration (ET) from the red pine sandbox was 73% of precipitation and 1.4–2.6 times larger than ET from the grass and moss/lichen boxes, respectively. In situ unsaturated hydraulic conductivity estimates, based on Darcy's law, are on the order of 10 −8 m/s and did not appear to vary significantly with plant cover. The upper zone of each sandbox is characterized by rapid, large changes in the magnitude of pressure head (−700 to −6 cm), water content (1 to 9%), and magnitude and direction of the hydraulic gradient. The vertical extent of this upper zone varies with plant cover; it extends to a depth between 30 and 70 cm beneath moss/lichen and grass whereas the upper zone beneath red pine extends to at least 120 cm during the growing season. Synoptic field measurements of water content and pressure head were very different from laboratory measurements of hanging-column moisture-retention curves. The red pine sandbox consistently exhibited the smallest water contents and the most negative pressure heads. Upper-zone soil water samples, collected at small and large tension during events, had different silica concentrations, which supports the existence of a dynamic flow system consisting of water residing in large and small pore spaces, respectively. However, large-tension silica concentrations increased systematically with depth in all sandboxes, indicating chemical reaction progresses in bulk soil water along flow paths. A conservative estimate of the shortest mean soil water residence time in the sandboxes is 9 days, which is sufficient time for exchange and weathering reactions to occur. Silica concentrations at 95 cm beneath red pine were twice as large as those beneath the moss/lichen cover. However, larger water fluxes from the moss/lichen sandbox produced an annual silica denudation flux of 910 mol Si/ha/year, whereas silica denudation from the pine sandbox was only 470 mol Si/ha/year. These results suggest that water and solute uptake by rooting vegetation may actually decrease chemical denudation from young, soil-building ecosystems.