Riparian Plant Isotopes Relflect 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 (δ15 N) may reflect land use impacts on N sources and transformations over scales of sites to watersheds. We surveyed leaf δ15 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 δ15N values were relatively small within sites (1.2 ± 2.2‰), although emergent aquatic macrophytes had higher within-site δ15 N values than other growth forms. Leaf δ15 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 δ15N values within a land use category often corresponded with local management differences. In an undeveloped montane canyon permitting off-leash dogs, leaf δ15N values (1.5 ± 1.3‰) exceeded similar canyons that strictly prohibited dogs (δ15 N = - 0.7 ± 1.1‰). Canyons with cattle grazing had leaf δ15 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 δ15N values, although a canyon with 50,000 vehicles per day showed a 5.7‰ increase relative to low-trafficked canyons. Urban leaf δ15N 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. © 2015 Hall et al.