Groundwater Nitrogen and Phosphorus Dynamics Under Grazed Pasture: Landscape and Management Effects.

Abstract

Surface runoff is considered the primary transport mechanism of nutrients and while groundwater transport is assumed negligible. The objective of this research was to determine the landscape and buffer effects on groundwater transport of nitrogen (N) and phosphorus (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 nitrate-N (NO₃-N), total N (TN), ammonium-N (NH₄-N), orthophosphate (PO₄-P), and total (TP) for two years. Landscape position and denitrification were the major factors affected significantly low (p<0.001) NO₃-N and TN at the footslope. Trees in the AB treatment further reduced NO₃-N at the footslope. Seasonal differences in TN and NO₃-N were most apparent in the footslope wells, and the seasonal pattern in concentrations was different between the two watershed treatments. 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 PO₄-P and TP at the footslope were significantly lower (p<0.001) than that 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. Orthophosphate and TP were not significantly different (p=0.20) between AB and GB footslope. Positive correlation of PO₄-P with NH₄-N suggested that reduced conditions promoted dissolution of Fe and Mn phosphates which increased the P release 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, Cattle manure, Loess soils, Midwest USA, Precipitation