Land Cover Influences on Stream Nitrogen Dynamics During Storms

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Virginia Tech

Previous studies on the effects of land cover influence on stream nitrogen have focused on base flow conditions or were conducted specifically within urbanized or primarily agricultural watersheds. While these studies have shown relationships between land cover and nitrogen, this relationship and the scale of influence could change during storms. The purpose of my study was to understand how land cover influences nitrogen in streams during storms. This was address using nine watersheds within the Little Tennessee Basin in North Carolina. While this basin is primarily forested, the nine watersheds have mixed agricultural, built, and forest land cover. Land cover influences were addressed through nitrogen concentration/discharge patterns, nitrogen concentration relationship to land cover, and comparison of storm and base flow nitrogen concentrations over time. Weekly base flow samples and samples from six storm were collected in 2010-2011. Total dissolved nitrogen (TDN), nitrate (NO??), dissolved organic nitrogen (DON), and ammonium (NH?⁺) concentrations were compared among sites. During most storms, DON peaked before the peak of the discharge while NO?? peaked after the peak of the storm. This suggest that DON could be coming from a near stream source or surface runoff while NO?? could be from longer pathways such as subsurface flow or from sources further away on the watershed. NO?? concentration varied among sites, while DON concentration varied more between base flow and storm samples. Examining the different landscape scales from 200-m local corridor, 200-m stream corridor, and entire watershed, watershed land cover was the best predictor for all the nitrogen concentrations. Agricultural and built combined best predicted TDN and NO??, while agricultural land cover was a better predictor of DON. For storms, nitrogen concentrations did not show seasonal patterns but was more related to discharge. Nitrogen concentration increased with discharge during storms and the more intense and longer storms had higher TDN and NO?? concentrations. However, conflicting seasonal trends were seen in monthly base flow. The more forested watersheds had high NO?? during the summer and low NO?? in the winter. For sites with higher NO??, the seasonality was reversed, with higher winter NO?? concentration. The least forested site had relatively constant nitrogen through the year at base flow and concentration decreased for most storms. Further studies on storms and nitrogen transport are needed to understand better the seasonal patterns of nitrogen input during storms.

dissolve organic nitrogen, nitrate, land cover, high flow event