Spatial and temporal scaling of nitrogen cycling and export: resolving three paradoxes for a forested Piedmont watershed

Nitrogen is a limiting nutrient in many forest and aquatic ecosystems. Reducing nutrient pollution to downstream receiving waters is an important scientific and management goal. Disproportionate amounts of nitrogen cycling and export occur in small portions of the landscape and over brief periods of time. These hot spots and hot moments are highly heterogeneous in space and time, making scaling of nitrogen cycling and export a major scientific challenge. This dissertation addresses how linkages between the physical structure of a watershed and related hydrological, ecological, and biogeochemical processes control the spatial and temporal patterns of nitrogen cycling and export. This work was conducted at Pond Branch, a small, forested watershed in the Piedmont physiographic province in Maryland (USA). This dissertation has three related multi-scale questions that are addressed by employing three different views of watershed ecohydrology (plan view, cross sectional, and longitudinal). Detailed observations and analysis at Pond Branch suggest that the ecohydrological processes in the riparian zone (which accounts for 4% of the catchment area) control nitrogen cycling and export at the watershed scale. In particular: 1) Denitrification accounts for approximately 25% of atmospheric N deposition. 99% of watershed scale denitrification occurs in riparian hollows, which comprise less than 1% of the watershed. 2) Water table declines and diurnal variations during the summer likely decrease the retention efficiency of riparian hollows, driving seasonal stream export patterns. Annual stream export is approximately 20% of denitrification, so moderate decreases in the retention in such close proximity to the stream could easily account for stream nitrogen patterns. 3) The ecohydrology of the riparian zone exerts a strong control on the timing and magnitude of nitrogen export as shown in evolving concentration-discharge relationships. Overall, this dissertation increases understanding of how the geomorphic template is an important control for nitrogen cycling and export at the watershed scale. It advances our understanding of hot spots and hot moments from a geomorphic perspective that helps link biogeochemical reactions and hydrologic transport mechanisms to explain seasonal patterns in nitrogen export.Doctor of Philosoph