Agroforestry buffers on nitrogen reduction in groundwater on a grazed hillslope

Agricultural practices often contribute to the transport of solutes into groundwater; thus, low-cost strategies that extract nutrients from groundwater are essential to address water pollution. This study evaluated the effects of agroforestry (tree + grass; AB [cottonwood {Populus deltoides Bortr. ex Marsh.}]) and grass buffers (GB; [tall fescue Schedonorus phoenix (Scop.) Holub, Red clover {Trifolium pretense L.}, and Lespedeza {Lespedeza Michx}]) on groundwater nitrogen (N) concentrations. The experiment consisted of two grazing watersheds, one with an AB and another with a GB treatment. Buffers were not grazed since 2001. Three wells representing summit, backslope, and foot-slope positions were installed at each watershed. Water samples were collected biweekly from November 2019 to January 2022 and analyzed for total nitrogen (TN) and dissolved N (DN). Dissolved nitrogen (DN) and TN concentrations after the AB in the foot-slope well were 99% (5.36-0.06 mg L-1) and 85% (9.04-1.37 mg L-1) lower than the mean concentration of the summit and backslope wells. Similarly, DN and TN concentrations after the GB in the foot-slope well were 94% (1.95-0.11 mg L-1) and 62% (2.86-1.07 mg L-1), lower than the mean concentration of the summit and backslope wells. Dissolved N concentrations were lower during warm periods probably due to plant uptake and denitrification in the buffer zone. Results showed that buffers, especially with deep-rooted trees in the proximity of the water table, decreased TN and DN concentrations in groundwater and can be used as a conservation practice to address water pollution.