Controls on Soil and Stream Nitrogen Cycling in a Mountain-to-Urban Watershed

Abstract:

Human activities in cities contribute large quantities of nitrogen (N) to adjacent ecosystems, but it is unclear how various sources of anthropogenic N contribute to and move through local watersheds. We analyzed myriad soil and water samples from across the Jordan River Valley, Salt Lake City, UT in order to assess N dynamics in terrestrial systems, at the riparian-stream interface, and in streams in this coupled human-natural system. We used data from two terrestrial headwater sites to demonstrate that forests tend to be more N-rich in topographic lows compared to hillslopes. Regardless of landscape position, soils beneath herbaceous vegetation had high nitrate concentrations and enriched δ¹⁵N values, suggesting overall N richness compared to forests. Isotope data showed that nitrate from all soils and headwater streams was of microbial, rather than direct anthropogenic, origin. In addition, nitrate from nearby streams was isotopically distinct from upland soils, suggesting low hydrologic connectivity between the two. Using data from the headwaters as well as eight additional downstream sites, we found that riparian soil N pools were increasingly decoupled from stream N dynamics lower in the watershed. This was related to where the stream transitioned from gaining to losing water from the groundwater system. Stream N contents were low in undisturbed mountain waters, but increased ten-fold at sites contaminated with urban groundwater. Across five watersheds spanning the Jordan Valley, we found anthropogenic N increasingly impacted streams as watershed size and land use intensity increased. Wastewater treatment plants imparted a further order-of-magnitude increase in stream nitrate concentrations and isotope values. Our work demonstrates that controls on N dynamics shift from topography and vegetation in upper watersheds to groundwater-surface water interactions and human activities in lower, more developed reaches. While the adjacent wildland ecosystem appears to have relatviely high capacity to buffer anthropogenic N inputs, it is important to consider the fate of N in lower, urbanized zones.