Abstract
Nitrogen (N) nutrition in pristine peatlands relies on the natural input of inorganic N through atmospheric deposition or biological dinitrogen (N 2 ) fixation. However, N 2 fixation and its significance for N cycling, plant productivity, and peat buildup are mostly associated with the presence of Sphagnum mosses. Here, we report high nonsymbiotic N 2 ‐fixation rates in two pristine Patagonian bogs with diversified vegetation and natural N deposition. Nonsymbiotic N 2 fixation was measured in samples from 0 to 10, 10 to 20, and 40 to 50 cm depth using the 15 N 2 assay as well as the acetylene reduction assay ( ARA ). The ARA considerably underestimated N 2 fixation and can thus not be recommended for peatland studies. Based on the 15 N 2 assay, high nonsymbiotic N 2 ‐fixation rates of 0.3–1.4 μmol N 2 g −1 day −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 nonsymbiotic N 2 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 N 2 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 15 N isotope of live Sphagnum (5.6 δ ‰) from 0 to 10 cm points to solely N uptake from atmospheric deposition and nonsymbiotic N 2 fixation. A mean 15 N signature of −0.7 δ ‰ of peat from the cushion plant plots indicates additional N supply from N mineralization. Our findings suggest that nonsymbiotic N 2 fixation overcomes N deficiency in different vegetation communities and has great significance for N cycling and peat accumulation in pristine peatlands.