From Sewers to Salix and Tailpipes to Typha: Riparian Plants Reflect Anthropogenic Nitrogen Sources Across Montane to Urban Gradients

Abstract:

Urban and suburban streams typically receive anthropogenic nitrogen (N) from multiple sources, and their identification and partitioning is a prerequisite for effective water quality management. However, stream N fluxes and sources are often highly variable, limiting the utility of water samples for source identification. Nitrate in perennial streams can provide an important N source for riparian vegetation in semi-arid environments. Thus, riparian plant tissue may integrate the stable isotope composition (δ¹⁵N) of stream nitrate over longer timescales and assist in source identification. Here, we tested whether δ¹⁵N of riparian plant leaves could provide an effective indicator of spatial variation in N sources across land use gradients spanning wildland to urban ecosystems in Salt Lake City, Utah, and the surrounding Wasatch Range Megapolitan Area. We found that leaf δ¹⁵N varied systematically within and among eight streams and rivers (n = 378 leaf samples) consistent with spatial land use variations. Plants from a suburban stream adjacent to homes with septic systems (δ¹⁵N = 5.1‰) were highly enriched relative to similar species from an adjacent undeveloped stream (δ¹⁵N = -0.7 ‰), suggesting an important contribution of enriched human fecal N to the suburban stream. Plants from a montane stream in a largely undeveloped recreational canyon that permitted off-leash dogs (δ¹⁵N = 1.8 ‰) were enriched relative to an adjacent canyon with similar land use that strictly prohibited dogs but had comparable vehicle traffic (δ¹⁵N = -0.7 ‰), suggesting the contribution of dog waste to stream N. Plants from urban stream reaches were enriched by 1.3 – 2.8 ‰ relative to upstream wildland reaches, and δ¹⁵N increased by 0.2 ‰ per km in the urban streams. Mechanisms leading to this urban enrichment could include leaky municipal sewers, atmospheric N deposition, and/or increased rates of N cycling and gaseous losses. Overall, our results demonstrate the potential utility of riparian plant N isotopes as a simple diagnostic of N source inputs to inform watershed management.