Long-term impacts of land cover change on nitrate export in mixed-use exurban and suburban watersheds

Understanding long-term drivers of nitrate export is critical for managing nutrient pollution in rapidly developing watersheds. This study examines how land use/land cover (LULC) change and nitrogen (N) inputs influence streamwater nitrate concentrations across six mixed-use watersheds (<1000 km²) in southeastern Pennsylvania over a 22-year period (1998-2020). Flow-normalized NO₃-N concentrations were calculated using the USGS Weighted Regressions on Time, Discharge, and Season (WRTDS). Trends were then parsed into management (MTC) and discharge (QTC) components to assess relative drivers. Flow-normalized NO₃-N concentrations increased in four watersheds and decreased in two with elevated seasonal trends during fall and winter months. MTC was the dominant contributor to overall water quality trends (WQT), suggesting human management practices primarily drive NO₃-N variability. Long-term analysis of N-inputs suggests the importance of commercial fertilizer and human waste followed by agricultural inputs. A cross-correlation analysis revealed a 1- to 8-year lag between N input and nitrate export in five watersheds indicating variable nitrate retention and legacy effects linked to past land use practices. Despite consistent increases in developed land and declines in agricultural and forested land across all watersheds, land cover composition alone did not explain NO₃-N patterns. However, landscape metrics of developed land within riparian buffer zones, specifically patch cohesion and shape complexity, were positively correlated with elevated nitrate concentrations when values were averaged over 5-year intervals. These findings highlight the need for integrated watershed management that considers not only land use extent, but also spatial configuration within riparian zones, temporal lags, and seasonal nitrate dynamics.