Isotopic evidence for nitrogen mobility in peat bogs

Abstract

Elevated nitrogen (N) input may reduce carbon (C) storage in peat. Under low atmospheric deposition, most N is bound in the moss layer. Under high N inputs, Sphagnum is not able to prevent penetration of dissolved N to deeper peat. Nitrogen may become available to the roots of invading vascular plants. The concurrent oxygenation of deeper peat layers, along with higher supply of labile organic C, may enhance microbial decomposition and lead to peat thinning. The resulting higher emissions of greenhouse gases may accelerate global warming. Seepage of N to deeper peat has never been quantified. Here we present evidence for post-depositional mobility of atmogenic N in peat, based on natural-abundance N isotope ratios. We conducted a reciprocal peat transplant experiment between two Sphagnum-dominated peat bogs in the Czech Republic (Central Europe), differing in anthropogenic N inputs. The northern site VJ received as much as 33kgNha−1yr−1 via spruce canopy throughfall. The southern site was less polluted (17.6kgNha−1yr−1). Isotope signatures of living moss differed between the two sites (δ15N of −3‰ and −7‰ at VJ and CB, respectively). After 18months, an isotope mass balance was constructed. In the CB-to-VJ transplant, a significant portion of original CB nitrogen (98–31%) was removed and replaced by nitrogen of the host site throughout the top 10cm of the profile. Nitrogen, deposited at VJ, was immobilized in imported CB peat that was up to 20years old. Additionally, we compared N concentration and N accumulation rates in 210Pb-dated peat profiles with well-constrained data on historical atmospheric N pollution. Nationwide N emissions peaked in 1990, while VJ exhibited the highest N content in peat that formed in 1930. This de-coupling of N inputs and N retention in peat might be interpreted as a result of translocation of dissolved pollutant N downcore, corroborating our δ15N results at VJ and CB. Data from a variety of peat bogs along pollution and climatic gradients would be needed to test to what extent the record of atmospheric N inputs in peat is overprinted by variable, locally-controlled decomposition rates.