Landscape and Buffer Effects on Phosphorus Transport in Groundwater Under Rotational Grazing.

Surface runoff is considered the primary transport mechanism of phosphorus (P) from upland areas to surface water, while groundwater transport is assumed negligible. The objective of this research was to determine the landscape and buffer effects on groundwater transport of P under livestock management. The study design consisted of rotationally grazed grass buffer (GB) and agroforestry buffer (AB) watersheds in loess soils with tall fescue (Festuca arundinacea Schreb.), red clover (Trifolium pretense L.), and lespedeza (Kummerowia stipulacea Maxim.). The AB had four rows of poplar trees (Populus tremula L.) at the footslope. Two transects of six groundwater monitoring wells were installed in a catena sequence and weekly groundwater samples were analyzed for orthophosphate, total P, nitrate-N, total N and ammonium-N (PO₄-P, TP, NO₃-N and NH₄⁺-N) concentrations for two years. Median 2015 AB (0.11 mg L^-1) and GB (0.17 mg L^-1) footslope PO₄-P were greater than concentrations in 2016 (AB 0.039 mg L^-1 and GB 0.074 mg L^-1). Median 2015 AB (0.14 mg L^-1) and GB (0.2 mg L^-1) footslope TP were also greater than the concentrations in 2016 (AB 0.04 mg L^-1 and GB 0.08 mg L^-1). Median PO₄-P and TP at the footslope were significantly lower (p<0.001) than the concentrations of <0.1 mg L^-1 PO₄-P and <0.1 mg L^-1 TP at the summit and backslope in 2016 with low precipitation. Median PO₄-P and TP concentrations at footslope of AB and GB were not significantly different (p=0.20). Orthophosphate showed a significant positive correlation with NH₄-N, suggesting that reduced conditions promoted dissolution of Fe and Mn phosphates which increased the release of P to groundwater. The results demonstrated that significant P transport to groundwater occurred in these loess soils following long-term cattle grazing, and P concentrations in groundwater increased with greater precipitation and low redox conditions.

Key words: Agroforestry, Loess soils, Midwest USA, Precipitation, Redox