δ15N systematics in two minerotrophic peatlands in the eastern U.S.: Insights into nitrogen cycling under moderate pollution

Abstract Availability of reactive nitrogen (N r ; mainly ammonium, NH 4 + , and nitrate, NO 3 − ) via precipitation and upwelling groundwater affects carbon (C) accumulation in peat deposits and biological greenhouse gas emissions. Isotope composition of N r was studied at Wolf Swamp (WS) and The Glades (GL), two peatlands located in western Maryland (U.S.), to provide a more robust understanding of N cycling at the ecosystem level. In recent years, both sites experienced a moderate atmospheric pollution (3–9 kg of deposited N r ha −1 yr −1 ), and possible additional N r inputs via groundwater contaminated by fertilizers/livestock. Sampling of living Sphagnum , vertical peat profiles, atmospheric deposition, and surface bog water was carried out on October 22–24, 2017. The δ 15 N r signatures of vertical precipitation were negative (mass-weighted means of −7.2 and −8.1‰ at WS and GL, respectively) despite reports that NO x emitted by coal-burning power plants, a precursor of deposited nitrate, had been extremely positive (up to +26‰). Mean δ 15 N values at both sites increased in the order: vertical precipitation  Sphagnum 2 15 N values of living Sphagnum (−1.5‰ at both sites) suggested simultaneous incorporation of isotopically light atmospheric N r and isotopically heavy N r from bog water in the newly formed biomass. The δ 15 N values of bulk peat increased downcore by 4.5 and 4.9‰ at WS and GL, respectively. Gradual mineralization of organic N, mainly by nitrification and denitrification, is isotopically selective, leaving behind higher δ 15 N values of bulk peat at greater peat depths. Nitrogen concentration gradients and δ 15 N trends in vertical peat profiles at these locations therefore cannot be used as an archive of past changes in pollution levels and N isotope signatures of incoming N r .