Biological nitrogen fixation in polluted Central European peat bogs: 15N2 incubation experiments, 210Pb-derived nitrogen accumulation rates, and the role of phosphorus availability

Microbial N₂ fixation (BNF) helps to sustain C accumulation in pristine peatlands and to remove CO₂ from the atmosphere. However, a combination of high anthropogenic inputs of reactive nitrogen (N_r) and sustained N₂ fixation may accelerate the invasion of vascular plants into the peat bogs, leading to a reduction of C–N stocks. Recent work in peatlands of polluted regions has indeed documented measurable BNF rates. Such data indicate partial adaptation of diazotrophs to increasing N_r deposition, instead of rapid downregulation of this energy-intensive microbial process. In addition to overall N_r availability and the NH₄⁺/NO₃^- ratio in atmospheric deposition, BNF controls include diazotrophic community structure, moss identity, temperature, moisture, phosphorus availability, bog water pH, and molybdenum availability. We present the results of a BNF study in Central European peat bogs that historically received as much as 20 kg N_r ha^-1 yr^-1 from the atmosphere. At five sites, we compared total N accumulation in peat and cumulative N_r deposition since 1950 and 1900. We took advantage of existing extrapolations of historical NH₄⁺ and NO₃^- emissions, and quantified the role of horizontal N_r deposition via fog interception and dry deposition. Eleven peat cores were ²¹⁰Pb-dated. At all sites, the amount of N accumulated in peat exceeded the cumulative atmospheric N_r input. At one site in the industrially polluted north of the Czech Republic, atmospheric N_r input appeared to explain only 41% of N accumulation in peat. One possible explanation would be that the found “excess” N in peat was, at least partly, a result of N₂-fixation. However, at least two sets of empirical data suggest that such high BNF rates in the studied central European peat bogs are not ecologically plausible: (i) in direct measurements of N₂-fixation rates using ¹⁵N₂ labelling, d¹⁵N values of Sphagnum significantly increased, but could explain only a small part of the “excess” N in peat that had been estimated by ²¹⁰Pb-dating; (ii) literature data on phosphorus deposition rates at various Central European sites suggest P limitation. Consequently, the ²¹⁰Pb-derived N accretion rates violate reasonable ranges of peatland C:N:P stoichiometry. Our new measurements of N:P ratios in atmospheric deposition at three peat bogs situated near the borders between the Czech Republic, Poland, Germany and Austria confirm the P limitation: The total N:P molar ratios were ~200 at Kunštátská kaple Bog (Eagle Mts.), ~100 at Černý potok (Slakovský les Mts.), and ~80 at Žďárecká slatˇ (Šumava Mts.). The hypothetical breaking point between N and P limitation in plant biomass is close to the N:P molar ratio of 35 (P limitation at higher N:P). In the paper, we will discuss uncertainties in ²¹⁰Pb dating and cumulative BNF rates in Central European peat bogs based on new ¹⁵N₂ laboratory incubations of peat substrate collected from several peat depths in different seasons.