|Knorr, KH; Horn, MA; Borken, W: Significant non-symbiotic nitrogen fixation in Patagonian ombrotrophic bogs, Global Change Biology, 21, 2357-2365 (2015), doi:10.1111/gcb.12849
Nitrogen (N) nutrition in pristine peatlands relies on the natural input of inorganic N through atmospheric deposition or biological dinitrogen (N2) fixation. However, N2 fixation and its significance for N cycling, plant productivity and peat buildup is mostly associated to the presence of Sphagnum mosses. Here, we report high non-symbiotic N2 fixation rates in two pristine Patagonian bogs with diversified vegetation and natural N deposition. Non-symbiotic N2 fixation was measured in samples from 0-10, 10-20 and 40-50 cm depth using the 15N2 assay as well as the acetylene reduction assay (ARA). The ARA considerably underestimated N2 fixation and can thus not be recommended for peatland studies. Based on the 15N2 assay, high non-symbiotic N2 fixation rates of 0.3-1.4 μmol N2 g−1 d−1 were found down to 50 cm under micro-oxic conditions (2 vol.%) in samples from plots covered by Sphagnum magellanicum or by vascular cushion plants, latter characterized by dense and deep aerenchyma roots. Peat N concentrations point to greater potential of non-symbiotic N2 fixation under cushion plants, likely because of the availability of easily decomposable organic compounds and oxic conditions in the rhizosphere. In the Sphagnum plots, high N2 fixation below 10 cm depth rather reflects the potential during dry periods or low water level when oxygen penetrates the top peat layer and triggers peat mineralization. Natural abundance of the 15N isotope of live Sphagnum (5.6 δ‰) from 0-10 cm points to solely N uptake from atmospheric deposition and non-symbiotic N2 fixation. A mean 15N signature of -0.7 δ‰ of peat from the cushion plant plots indicates additional N supply from N mineralization. Our findings suggest that non-symbiotic N2 fixation overcomes N deficiency in different vegetation communities and has great significance for N cycling and peat accumulation in pristine peatlands.