A Sphagnum incubation study using 15N-labelled atmospheric N2 reveals contrasting potential for biological N2 fixation at three medium-polluted Central European peat bogs

Microbial N₂-fixation helps to sustain carbon accumulation in pristine peatlands and to remove CO₂ from the atmosphere. Recent work has provided evidence that this energetically costly process is not completely downregulated at sites with higher availability of reactive nitrogen (N_r). We studied nitrogen (N) cycling at three high-elevation, mainly rain-fed, Sphagnum-dominated peat bogs in the northern Czech Republic receiving medium to high amounts of reactive nitrogen (N_r) via atmospheric deposition. ¹⁵N/¹⁴N isotope ratios were determined in N_r deposition, along vertical peat profiles, and in a laboratory incubation study using fresh Sphagnum and ¹⁵N-enriched atmospheric N₂. Our objective was to assess the potential for biological N₂-fixation at the selected study sites in light of various biogeochemical parameters. Historically, all the peat bogs experienced similar changes in atmospheric N_r (mainly NO₃^--N and NH₄-N) inputs. N_r depositions at all three sites peaked between 1980 and 1990. During that time period, the highest annual depositions were close to 10 kg ha^-1 yr^-1 at the slightly more polluted site Uhlirska (UHL) than at Male mechove jezirko (MMJ) and Brumiste (BRU). Since ca. 1990, atmospheric deposition of N_r has been steadily decreasing. Living Sphagnum had variable N concentrations with similar means for all three sites (1.1, 1.0 and 0.9 wt. % at MMJ, BRU and UHL, respectively). Downcore, peat density remained nearly constant at MMJ but increased at BRU and UHL. Ash contents were below 10 wt. % at least to the depth of 20 cm. With an increasing peat depth, both N concentration and δ¹⁵N values generally increased, while C/N ratios tended to decrease. At depths > 10 cm, N/P ratio was lower at UHL than at the other two sites and remained nearly constant downcore. N/P ratio at MMJ increased from ~10 to ~20 with an increasing depth, whereas the N/P ratio exhibited a zigzag vertical pattern at BRU, reaching a value of 40 in deeper segments. The potential for biological N₂-fixation was investigated using a replicated laboratory incubation of fresh Sphagnum in a closed system following an application of 98 % enriched atmospheric N₂. The experiment lasted for 7 days. The control Sphagnum samples had δ¹⁵N values of -4.0 ‰ (BRU and UHL) and -3.7 ‰ (MMJ). At the end of the incubation, the δ¹⁵N significantly increased only in MMJ moss reaching + 70 ‰, while it remained unchanged in BRU and UHL moss. Biological N₂ fixation was thus recorded at only at MMJ, a site with the lowest N/P ratio in the topmost 2-cm thick sections. Potential N₂ fixation rates at MMJ were similar to values previously reported for Finland (Leppänen et al. 2015) but ~7 times lower than at sites located in Patagonia, Chile (Knorr et al. 2016).

References

Leppänen et al., 2015. Plant and Soil, 389, 185-196.

Knorr et al., 2016, Global Change Biology 21, 2357–2365.