Nutrient Cycling in an Agricultural Watershed: I. Phreatic Movement

Much of the runoff from agricultural fields in the southeastern Coastal Plain is carried to a stream channel system in a shallow phreatic aquifer. This subsurface runoff often passes through a band of riparian forest before becoming streamflow. It is hypothesized that the riparian ecosystem acts as a nutrient sink and reduces the concentrations and loads of nutrients in the shallow aquifer before the nutrients reach the stream channel. Concentrations and loads (kg/m 2 ) of NO 3 ‐N, NH 4 ‐N, organic N, dissolved molybdate‐reactive P 1 total P, Ca, Mg, K, Cl, and SO 4 ‐S were measured in shallow phreatic wells at 37 locations on an agricultural watershed near Tifton, Ga. Total water volume moving off the watershed in subsurface flow was < 1% of streamflow with corresponding small amounts of nutrients. Nitrate‐N, Ca, and Mg had significantly higher concentrations in field wells than in forest or streamside wells. Concentrations of Cl were not reduced as water moved from field to forest. Processes within the riparian zone apparently converted primarily inorganic N from fields (76% NO 3 ‐N, 6% NH 4 ‐N, 18% organic N) into primarily organic N in streamside wells (10% NO 3 ‐N, 14% NH 4 ‐N, and 76% organic N). Concentration differences between field and forest wells indicated the riparian forest's ability to act as a sink for NO 3 ‐N, Ca, Mg, K, and SO 4 ‐S. Due to their role as nutrient sinks, riparian forests are important in maintaining stream water quality on agricultural watersheds.