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

Abstract

Availability of reactive nitrogen (Nr; mainly ammonium, NH4+, and nitrate, NO3−) via precipitation and upwelling groundwater affects carbon (C) accumulation in peat deposits and biological greenhouse gas emissions. Isotope composition of Nr 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 Nr ha−1 yr−1), and possible additional Nr 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 δ15Nr signatures of vertical precipitation were negative (mass-weighted means of −7.2 and −8.1‰ at WS and GL, respectively) despite reports that NOx emitted by coal-burning power plants, a precursor of deposited nitrate, had been extremely positive (up to +26‰). Mean δ15N values at both sites increased in the order: vertical precipitation < living Sphagnum < atmospheric N2 < surface bog water < maturating peat. Intermediate δ15N values of living Sphagnum (−1.5‰ at both sites) suggested simultaneous incorporation of isotopically light atmospheric Nr and isotopically heavy Nr from bog water in the newly formed biomass. The δ15N 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 δ15N values of bulk peat at greater peat depths. Nitrogen concentration gradients and δ15N 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 Nr.