A Sphagnum incubation study using 15N-labelled atmospheric N2 reveals contrasting potential for biological N2 fixation at three medium-polluted Central European peat bogs
Authors: Stepanova Marketa , Novak Martin , Cejkova Bohuslava , Buzek Frantisek , Jackova Ivana , Prechova Eva , Veselovsky Frantisek , Curik Jan 
Published: March 2024

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 (Nr). 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 (Nr) via atmospheric deposition. 15N/14N isotope ratios were determined in Nr 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 Nr (mainly NO3--N and NH4-N) inputs. Nr 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 Nr 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 &#948;15N 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 &#948;15N values of -4.0 &#8240; (BRU and UHL) and -3.7 &#8240; (MMJ). At the end of the incubation, the &#948;15N significantly increased only in MMJ moss reaching + 70 &#8240;, 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 (Lepp&#228;nen et al. 2015) but ~7 times lower than at sites located in Patagonia, Chile (Knorr et al. 2016).ReferencesLepp&#228;nen et al., 2015. Plant and Soil, 389, 185-196.Knorr et al., 2016, Global Change Biology 21, 2357&#8211;2365.