Contrasting potential for biological N2 fixation at three polluted central European Sphagnum peat bogs: combining the 15N2-tracer and natural-abundance isotope approaches.

Availability of reactive nitrogen (N r) is a key control on carbon (C) sequestration in wetlands. To complement the metabolic demands of Sphagnum in pristine rain-fed bogs, diazotrophs supply additional N r via biological nitrogen fixation (BNF). As breaking the triple bond of atmospheric N 2 is energy-intensive, it is reasonable to assume that increasing inputs of pollutant N r will lead to BNF downregulation. However, recent studies have also documented measurable BNF rates in Sphagnum-dominated bogs in polluted regions, indicating the adaptation of N 2 fixers to changing N deposition. Our aim was to quantify BNF in high-elevation peatlands located in industrialized central Europe. A 15 N 2 -tracer experiment was combined with a natural-abundance N-isotope study at three Sphagnum-dominated peat bogs in the northern Czech Republic in an attempt to assess the roles of individual BNF drivers. High short-term BNF rates (8.2 ± 4.6 g N m 2 d -1) were observed at Malé mechové jezírko, which receives ∼ 17 kg N r ha -1 yr -1. The remaining two peat bogs, whose recent atmospheric N r inputs differed from Malé mechové jezírko by only 1–2 kg ha -1 yr -1 (Uhlí r̆ská and Brumiště), showed zero BNF. The following parameters were investigated to elucidate the BNF difference: the NH 4+ -N / NO 3- -N ratio, temperature, wetness, Sphagnum species, organic-N availability, possible P limitation, possible molybdenum (Mo) limitation, SO 42- deposition, and pH. At Malé mechové jezírko and Uhlí r̆ská, the same moss species (S. girgensohnii) was used for the 15 N 2 experiment; therefore, the host identity could not explain the difference in BNF at these sites. Temperature and moisture were also identical in all incubations and could not explain the between-site differences in BNF. The N : P stoichiometry in peat and bog water indicated that Brumiště may have lacked BNF due to P limitation, whereas non-detectable BNF at Uhlí r̆ská may have been related to the 70-fold higher SO 42- concentration in bog water. Across the sites, the mean natural-abundance δ15 N values increased in the following order: atmospheric deposition (- 5.3 ± 0.3 ‰) < Sphagnum (- 4.3 ± 0.1 ‰) < bog water (- 3.9 ± 0.4 ‰) < atmospheric N 2 (0.0 ‰). Only at Brumiště was N in Sphagnum significantly isotopically heavier than in atmospheric deposition, possibly indicating a longer-term BNF effect. Collectively, our data highlight spatial heterogeneity in BNF rates under high N r inputs as well as the importance of environmental parameters other than atmospheric N r pollution in regulating BNF. [ABSTRACT FROM AUTHOR]