Riparian plant isotopes reflect anthropogenic nitrogen perturbations: robust patterns across land use gradients

Riparian plants incorporate nitrogen (N) from aquatic, terrestrial, and atmospheric sources, and their stable isotope compositions (δ 15 N) may reflect land use impacts on N sources and transformations over scales of sites to watersheds. We surveyed leaf δ 15 N values of 11 common riparian tree, shrub, and herbaceous species from 20 streams and rivers spanning three fifth‐order watersheds in northern Utah, USA ( n = 255 sites and 819 leaf samples). Streams spanned undeveloped montane forests to suburban, urban, and agricultural lands. Mean species‐specific differences in leaf δ 15 N values were relatively small within sites (1.2 ± 2.2‰), although emergent aquatic macrophytes had higher within‐site δ 15 N values than other growth forms. Leaf δ 15 N values varied significantly across land‐use categories, and were lowest in undeveloped montane reaches (0.5 ± 1.9‰; mean and standard deviation), intermediate in suburban and urban reaches (2.3 ± 2.6 and 3.2 ± 3.4‰), and greatest in agricultural reaches (4.1 ± 3.1‰). The substantial variation in leaf δ 15 N values within a land use category often corresponded with local management differences. In an undeveloped montane canyon permitting off‐leash dogs, leaf δ 15 N values (1.5 ± 1.3‰) exceeded similar canyons that strictly prohibited dogs (δ 15 N = −0.7 ± 1.1‰). Canyons with cattle grazing had leaf δ 15 N values enriched by 1.4 and 2.8‰ relative to similar, but un‐grazed canyons. Variation in traffic between 0 and 5000 vehicles per day did not significantly affect leaf δ 15 N values, although a canyon with 50,000 vehicles per day showed a 5.7‰ increase relative to low‐trafficked canyons. Urban leaf δ 15 N values were consistently enriched by 2.5 ± 0.6‰ relative to leaves in un‐grazed montane reaches, and leaves in a septic‐impacted suburban reach were enriched by 4.6‰ relative to upstream samples. Samples from a sewage‐impacted urban river averaged 8.0 ± 4.1‰ and reached 22‰ adjacent to publicly owned treatment works (POTW). Another urban river displayed similar values in the absence of POTWs, implicating leaky sewers. Our results demonstrate the capacity of N isotopes from a diverse riparian plant community to inform our spatial understanding of watershed N‐cycling perturbations, and illustrate the impact of human activities on N cycling even within protected watersheds.