Contrasting δ15N values of atmospheric deposition and sphagnum peat bogs: N fixation as a possible cause
Abstract
Nitrogen (N) isotope systematics were investigated at two high-elevation ombrotrophic peat bogs polluted by farming and heavy industry. Our objective was to identify N sources and sinks for isotope mass balance considerations.
For the first time, we present a time-series of d15M values of atmospheric input at the same locations as d15M values of living Sphagnum and peat. The mean d15M values systematically increased in the order: input NH4 + (-10.0&) < input NO3- (-7.9&) < peat porewater (-5.6&) < Sphagnum (-5.0&) < shallow peat (-4.2&) < deep peat (-2.2&) < runoff (-1.4&) < porewater N2O (1.4&).
Surprisingly, N of Sphagnum was isotopically heavier than N of the atmospheric input (P < 0.001). If partial incorporation of reactive N from the atmosphere into Sphagnum was isotopically selective, the residual N would have to be isotopically extremely light.
Such N, however, was not identified anywhere in the ecosystem. Alternatively, Sphagnum may have contained an admixture of isotopically heavier N.
Ambient air contains such N in the form of N2 (d15MN2 = 0&). Because high energy is required to break the triple bond, microbial N fixation is likely to proceed only under limited availability of pollutant N.
Also for the first time, a d15M comparison is presented between anoxic deeper peat and porewater N2O. Isotopically light N is removed from anoxic substrate by denitrification, whose final product, N2, escapes into the atmosphere.
Porewater N2O is an isotopically heavy residuum following partial N2O reduction to N2.