Nutrient Dynamics in an Agricultural Watershed: Observations on the Role of A Riparian Forest

Abstract

Nutrient (C, N, and P) concentration changes were measured in surface runoff and shallow groundwater as they moved through a small agricultural (cropland) watershed located in Maryland. During the study period (March 1981 to March 1982), dramatic changes in water—borne nutrient loads occurred in the riparian forest of the watershed. From surface runoff waters that had transited °50 m of riparian forest, an estimated 4.1 Mg of particulates, 11 kg of particulate organic—N, 0.83 kg of ammonium—N, 2.7 kg of nitrate—N and 3.0 kg of total particulate—P per ha of riparian forest were removed during the study year. In addition, an estimated removal of 45 kg°ha—1°yr—1 of nitrate N occurred in subsurface flow as it moved through the riparian zone. Nutrient uptake rates for the cropland are riparian forest were estimated. These systems were then compared with respect to their pathways of nutrient flow and ability to retain nutrients. The cropland appeared to retain fewer nutrients than the riparian forest and is thought to incur the majority of its nutrient losses in harvested crop. The dominant pathway of total—N loss from the riparian forest seemed to be subsurface flux. Total phosphorus loss from the riparian forest appeared almost evenly divided between surface and subsurface losses. Nutrient removals in the riparian forest and thought to be of ecological significance to receiving waters and indicate that coupling natural systems and managed habitats within a watershed may reduce diffuse—source pollution.