Links between soil hydrophobicity and groundwater recharge under plantations in a sandy grassland setting, Nebraska Sand Hills, USA

This study addresses relationships between soil hydrophobicity and groundwater recharge through the unsaturated zone in semiarid sandy grasslands containing mature tree plantations. Field and laboratory investigations of soil properties and recharge rates were undertaken at 10 experimental plots within the Nebraska Sand Hills. Plots included a range of plantation species and planting densities in addition to grasslands. Hydrophobicity was characterized using a combination of methods, including water and ethanol drop penetration tests, nuclear magnetic resonance spectral analysis, and field infiltration tests. Water and ethanol drop tests indicate that surface soils beneath pine plots were moderately to strongly hydrophobic and that plantations are 3‐13 times more hydrophobic than the grasslands. The spectral analysis suggested that the surface soil organic carbon beneath pine plantations contains up to 3 times the ratio of hydrophobic components compared with that for the grasslands. Mini-infiltrometer tests demonstrate that changing grassland into tree plantations leads to declining soil infiltration capacity and lowering of sorptivity and hydraulic conductivity (at −2 cm pressure head) by an order of magnitude. Previously published chloride-based annual recharge estimates beneath these plantations represent reductions of up to 90% relative to the grassland and showed a strong relationship (R 2 = 0.94) with unsaturated hydraulic conductivity retrieved from the field infiltration experiments. HYDRUS-1D synthetic numerical modeling was also performed to corroborate decreasing recharge rates (25‐31.4%) under land-use change dynamics.