Contrasting potential for biological N 2 fixation at three polluted central European Sphagnum peat bogs: combining the 15 N 2 -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 Sphagnumin pristine rain-fed bogs, diazotrophs supply additional Nrvia 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 N2fixers to changing N deposition. Our aim was to quantify BNF in high-elevation peatlandslocated 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 assessthe roles of individual BNF drivers. High short-term BNF rates (8.2 +/- 4.6 g N m(2)d(-1)) were observed at Mal & eacute;mechov & eacute; jez & iacute;rko, which receives similar to 17 kg Nrha(-1)yr(-1). The remaining two peat bogs, whose recent atmospheric Nrinputs differed from Mal & eacute; mechov & eacute; jez & iacute;rko by only 1-2 kg ha(-1)yr(-1)(Uhl & iacute;rsk & aacute; and Brumi & scaron;te), showed zeroBNF. The following parameters were investigated to elucidate the BNF difference: the NH+4-N/NO3--N ratio, temperature, wetness, Sphagnumspecies, organic-N availability, possible P limitation, possible molybdenum(Mo) limitation, SO(4)(2-)deposition, and pH. At Malmechov & eacute; jez & iacute;rko and Uhl & iacute;rsk & aacute;, 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 explainthe between-site differences in BNF. The N : P stoichiometry in peat and bog water indicated that Brumi & scaron;emay have lacked BNF due to P limitation, whereas non-detectable BNF at Uhl & iacute;rsk & aacute; may have been related tothe 70-fold higher SO2-4concentration in bog water. Across the sites, the mean natural-abundance delta 15N valuesincreased in the following order: atmospheric deposition (-5.3 +/- 0.3 parts per thousand)<Sphagnum(-4.3 +/- 0.1 parts per thousand)<bog water (-3.9 +/- 0.4 parts per thousand)<atmospheric N2(0.0 parts per thousand). Only at Brumi & scaron;te 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