Nutrient Dynamics in Riparian Zones of Agricultural Watersheds

Agricultural practices can lead to an increased export of nitrogen (N) and phosphorus (P) and impaired water quality in downstream ecosystems. Eutrophication of lakes and coastal zones leads to anoxia in bottom waters, loss of biodiversity and degraded water quality for human consumption. Stream restoration is increasingly being used as a strategy to improve local water quality and mitigate the effects of nutrient enrichment in downstream ecosystems. While natural headwater streams and adjacent riparian zones have been identified as hot spots for nutrient removal, the potential for restored systems to achieve similar functions remains unclear. The goal of my research is to analyze the effects of integrated stream and riparian restoration on the removal of dissolved nutrients. To do so, I compared nutrient retention in the riparian zone of four different stream types: a restored agricultural stream/riparian zone, an incised agricultural ditch, an agricultural stream with a narrow riparian buffer and a stream in a forested section of a primarily agricultural watershed. I measured changes in ammonium (NH4+), nitrate (NO3 -), soluble reactive phosphorus (SRP), total dissolved organic carbon (TOC) and total dissolved nitrogen (TDN) in groundwater from the edge of field to near stream seasonally from spring 2013 to fall 2015. Results show a decreasing but variable trend in dissolved nutrients from edge of field to near stream in sites with adjacent agriculture. Results also varied seasonally with highest concentrations observed at the edge of field in spring for all sites. This is likely attributed to fertilization of adjacent and upstream agricultural fields. I also developed a statistically based model of nutrient concentrations in riparian groundwater as a function of potential explanatory variables, including riparian width, adjacent land use, vegetation cover, crop rotation and precipitation records. Results of the regression model confirm the control of inputs from adjacent agricultural practices on riparian function. Additionally, they suggest that type of vegetation may play a secondary role in removal efficiency with grasses and other herbaceous vegetation more effective at removing nutrients from shallow groundwater flow paths. These results help to improve understanding of stream-riparian interactions in agricultural systems, particularly the role of riparian restoration in improving water quality