Abstract
Humans have altered Earth's nitrogen cycle so dramatically that reactive nitrogen (Nr) has doubled. This has increased Nr in aquatic ecosystems, which can lead to reduced water quality and ecosystem health. Apportioning sources of Nr to specific ecosystems, however, continues to be challenging, despite this knowledge being critical for mitigation and protection of water resources. Here we use Δ17O, δ18O and δ15N from Uinta Mountain (Utah, USA) snow, inflow and lake nitrate in combination with a Bayesian-based stable isotope mixing model, to show that at least 70% of nitrates in aquatic systems are anthropogenic and arrive via the atmosphere. Moreover, agricultural activities, specifically nitrate- and ammonium-based fertilizer use, are contributing most (∼60%) Nr, and data from other North American alpine lakes suggest this is a widespread phenomenon. Our findings offer a pathway towards more effective mitigation, but point to challenges in balancing food production with protection of important water resources.
Highlights
Humans have altered Earth’s nitrogen cycle so dramatically that reactive nitrogen (Nr) has doubled
Our results show that atmospheric deposition of Nr from anthropogenic sources contributes at least 70% of the total nitrate to Uinta Mountain inflows and lakes; the majority of this is from atmospherically delivered fertilizers
Of 41 samples analysed here, all but one have D17O–NO3À that are elevated above the terrestrial fractional line (D17O 1⁄4 0±1%) (Figs 3 and 4)
Summary
Humans have altered Earth’s nitrogen cycle so dramatically that reactive nitrogen (Nr) has doubled. This has increased Nr in aquatic ecosystems, which can lead to reduced water quality and ecosystem health. Apportioning sources of Nr to specific ecosystems, continues to be challenging, despite this knowledge being critical for mitigation and protection of water resources. Agricultural activities, nitrate- and ammonium-based fertilizer use, are contributing most (B60%) Nr, and data from other North American alpine lakes suggest this is a widespread phenomenon. Reactive nitrogen (Nr), or biologically available nitrogen, which includes the inorganic forms ammonia nitric acid (NH3), ammonium (NH4þ ), nitrogen (HNO3), nitrous oxide (N2O), nitrate o(xNidOe3À(N) Oanx)d, others (for example, peroxyacetyl nitrates, as well as organic compounds (for example, urea, amines, proteins and nucleic acids)), has been doubled by human activities[3]. Alpine lakes can be limited by nitrogen[8,9,10,11] and even modest increases in nitrogen deposition can have significant effects including eutrophication, acidification and reduction of biodiversity[12]
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