PAVEMENT, PASTURE, PONDS, AND POOP: NITROGEN BIOGEOCHEMISTRY IN HUMAN TRANSFORMED LANDSCAPES OF THE SOUTHEASTERN PIEDMONT

Humans have dramatically increased the amount of reactive nitrogen (Nr) in the global system over the last century. The increase in reactive nitrogen is especially noticeable in urban environments with larger sources and diminished sinks of Nr. In the Piedmont of South Carolina, rapid urban expansion into a rural landscape of predominantly pasture and forest has been occurring for over three decades. The topography of the region results in a concentration of impervious surfaces being located in the headwaters of watersheds. High vehicle traffic is a major source of ammonium and nitrate dry deposition on impervious surfaces that enters the system during rain events. Leaky sewer pipes along roads act as point sources to headwater streams. Downstream, urbanization has converted second growth forest to large extents of impervious surface, turfgrass, and ornamental urban tree cover. In some cases, abundance of woody debris in urban headwater streams is reduced compared to rural headwaters. These land and stream transformations from natural forest cover diminish the landscape’s capacity for nitrogen assimilation and denitrification, as evidenced by high dissolved nitrogen concentrations in urban streams. In contrast, ponds in the urban landscape act as nitrogen sinks. The result is high nitrate concentrations at the headwaters that decrease downstream, but typically remain above 1.0 mg/L nitrate. Human influence on nitrogen biogeochemistry also is evident in rural landscapes. For example, nitrate concentrations in pasture-dominated watersheds, though lower than in urban streams, are higher than in completely forested watersheds and positively correlate with percent of watershed area covered by pasture. In some rural watersheds, row crop agriculture and poultry production also may contribute to elevate stream nitrate concentrations. In larger stream and rivers, wastewater treatment plants add effluent to streams and rivers and superimpose spikes of nitrogen well above the background concentrations. Initial analysis of human appropriation of net primary production (HANPP) indicates that transformation of forest to urban and agricultural landscapes increases HANPP, suggesting that HANPP is a predictor of changes in the biogeochemistry of nitrogen in the landscape.