Investigating plantation-induced near-surface soil hydrophobicity and its impact on groundwater recharge in the Nebraska Sand Hills, USA

Abstract:

Although numerous studies in diverse environmental settings have demonstrated that plantations tend to reduce soil moisture and recharge rates, research on physical mechanisms affecting these linkages tend to focus mainly on the effects of evapotranspiration and interception. This study investigates the extent of soil hydrophobicity resulting from land use changes and its impact on groundwater recharge in a century-old experimental forest surrounded by grassland in the Northern High Plains (Nebraska National Forest). Water Drop Penetration Tests (WDPT) and Nuclear Magnetic Resonance (NMR) spectroscopy were used to investigate soil hydrophobicity on 50 cm soil cores collected from experimental plots beneath 5 land cover types. WDPT analysis indicated that most near-surface soils (0–12.5 and 12.5–25 cm) beneath pine plots were moderately to strongly hydrophobic. NMR spectroscopy analysis comparing ratios of hydrophobic (3.2–0.5 and 8.5–6.5 ppm) to hydrophilic (6.5–3.2 ppm) regions suggests that surface soils beneath the plantations were uniformly more hydrophobic than grasslands (by ~30 to 260%). Unsaturated zone soil cores were collected from beneath each experimental plot for comparison of hydrophobicity with recharge rates based on chloride and sulfate mass balance. Recharge estimates beneath the plantations (4–10 mm yr^-1) represent reductions of 86–94% relative to the surrounding native grassland, suggesting a link between soil hydrophobicity and reduced infiltration beneath the plantations.