Institute for Landscape Ecology, Hydrology Group, University of Münster, Heisenbergstr. 2, 48149, Münster, Germany.
Glob Chang Biol. 2015 Jun;21(6):2357-65. doi: 10.1111/gcb.12849. Epub 2015 Mar 6.
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 are mostly associated with the presence of Sphagnum mosses. Here, we report high nonsymbiotic N2 -fixation rates in two pristine Patagonian bogs with diversified vegetation and natural N deposition. Nonsymbiotic N2 fixation was measured in samples from 0 to 10, 10 to 20, and 40 to 50 cm depth using the (15) N2 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 (15) N2 assay, high nonsymbiotic N2 -fixation rates of 0.3-1.4 μmol N2 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 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 (15) N isotope of live Sphagnum (5.6 δ‰) from 0 to 10 cm points to solely N uptake from atmospheric deposition and nonsymbiotic N2 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 N2 fixation overcomes N deficiency in different vegetation communities and has great significance for N cycling and peat accumulation in pristine peatlands.
在原始泥炭地中，氮（N）营养依赖于通过大气沉积或生物固氮（N2）固定自然输入的无机 N。然而，N2 固定及其对 N 循环、植物生产力和泥炭积累的意义主要与泥炭藓的存在有关。在这里，我们报告了两个具有多样化植被和自然 N 沉积的原始巴塔哥尼亚沼泽中高的非共生 N2 固定率。使用 (15)N2 测定法和乙炔还原测定法 (ARA)，在 0 至 10、10 至 20 和 40 至 50 cm 深度的样品中测量非共生 N2 固定。ARA 大大低估了 N2 固定，因此不建议用于泥炭地研究。基于 (15)N2 测定法，在微氧条件（2 体积%）下，从被石莼或血管垫状植物覆盖的斑块的样品中，在 50 cm 深处发现了高的非共生 N2 固定率为 0.3-1.4 μmol N2 g(-1) day(-1)，后者的特点是密集和深的通气根。泥炭 N 浓度表明，在垫状植物下，非共生 N2 固定具有更大的潜力，这可能是由于根际中易分解的有机化合物和有氧条件的存在。在石莼斑块中，在 10 cm 深度以下高的 N2 固定更多地反映了在干燥期或低水位期间的潜力，此时氧气穿透顶部泥炭层并引发泥炭矿化。0 至 10 cm 处活石莼的（15）N 同位素自然丰度（5.6 δ‰）表明仅从大气沉积和非共生 N2 固定中吸收 N。来自垫状植物斑块的泥炭（-0.7 δ‰）的平均（15）N 特征表明，氮矿化提供了额外的氮供应。我们的研究结果表明，非共生 N2 固定克服了不同植被群落中的 N 缺乏，对原始泥炭地的 N 循环和泥炭积累具有重要意义。
