Department of Aquatic Ecology & Environmental Biology, Institute for Water and Wetland Research, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.
Bosgroep Zuid-Nederland, Huisvenseweg 14, 5591 VD Heeze, The Netherlands.
Sci Total Environ. 2018 Jan 1;610-611:732-740. doi: 10.1016/j.scitotenv.2017.08.102. Epub 2017 Aug 17.
Pristine bogs, peatlands in which vegetation is exclusively fed by rainwater (ombrotrophic), typically have a low atmospheric deposition of reactive nitrogen (N) (<0.5kghay). An important additional N source is N fixation by symbiotic microorganisms (diazotrophs) in peat and mosses. Although the effects of increased total airborne N by anthropogenic emissions on bog vegetation are well documented, the important question remains how different N forms (ammonium, NH, versus nitrate, NO) affect N cycling, as their relative contribution to the total load strongly varies among regions globally. Here, we studied the effects of 11years of experimentally increased deposition (32 versus 8kgNhay) of either NH or NO on N accumulation in three moss and one lichen species (Sphagnum capillifolium, S. papillosum, Pleurozium schreberi and Cladonia portentosa), N fixation rates of their symbionts, and potential N losses to peat soil and atmosphere, in a bog in Scotland. Increased input of both N forms led to 15-90% increase in N content for all moss species, without affecting their cover. The keystone species S. capillifolium showed 4 times higher N allocation into free amino acids, indicating N stress, but only in response to increased NH. In contrast, NO addition resulted in enhanced peat N mineralization linked to microbial NO reduction, increasing soil pH, N concentrations and N losses via denitrification. Unexpectedly, increased deposition from 8 to 32kghay in both N forms did not affect N fixation rates for any of the moss species and corresponded to an additional input of 5kgNhay with a 100% S. capillifolium cover. Since both N forms clearly show differential effects on living Sphagnum and biogeochemical processes in the underlying peat, N form should be included in the assessment of the effects of N pollution on peatlands.
原始沼泽地是一种仅靠雨水（腐殖质营养型）来滋养植被的泥炭地，其大气中活性氮（N）的沉积量通常较低（<0.5kg/ha）。氮的另一个重要来源是泥炭和苔藓中共生微生物（固氮菌）的固氮作用。虽然人为排放增加的总大气氮对沼泽植被的影响已得到充分记录，但一个重要的问题仍然是不同的 N 形态（铵，NH，与硝酸盐，NO）如何影响 N 循环，因为它们对总负荷的相对贡献在全球范围内差异很大。在这里，我们研究了在苏格兰的一个沼泽地中，11 年实验性增加 NH 或 NO 沉积（32 与 8kgN/ha）对三种苔藓和一种地衣物种（Sphagnum capillifolium、S. papillosum、Pleurozium schreberi 和 Cladonia portentosa）中 N 积累、共生体固氮速率以及潜在的 N 损失到泥炭土壤和大气中的影响。两种 N 形态的输入增加都导致所有苔藓物种的 N 含量增加了 15-90%，而不会影响它们的覆盖率。关键物种 S. capillifolium 将 4 倍更多的 N 分配到游离氨基酸中，表明存在 N 胁迫，但仅对 NH 的增加有反应。相比之下，NO 的添加导致与微生物 NO 还原相关的泥炭 N 矿化增强，增加了土壤 pH 值、N 浓度以及通过反硝化作用的 N 损失。出乎意料的是，两种 N 形态从 8 增加到 32kgN/ha 都没有影响任何一种苔藓物种的固氮速率，并且对应于 100%的 S. capillifolium 覆盖率下增加了 5kgN/ha 的额外输入。由于两种 N 形态对活的 Sphagnum 和下面的泥炭中的生物地球化学过程都明显表现出不同的影响，因此在评估 N 污染对泥炭地的影响时应包括 N 形态。
