Abstract
Biological nitrogen fixation (BNF) represents the natural pathway by which mosses meet their demands for bioavailable/reactive nitrogen (Nr) in peatlands. However, following intensification of nitrogen fertilizer and fossil fuel use, atmospheric Nr deposition has increased exposing peatlands to Nr loading often above the ecological threshold. As BNF is energy intensive, therefore, it is unclear whether BNF shuts down when Nr availability is no longer a rarity. We studied the response of BNF under a gradient of Nr deposition extending over decades in three peatlands in the U.K., and at a background deposition peatland in Sweden. Experimental nitrogen fertilization plots in the Swedish site were also evaluated for BNF activity. In situ BNF activity of peatlands receiving Nr deposition of 6, 17, and 27 kg N ha-1 yr-1 was not shut down but rather suppressed by 54, 69, and 74%, respectively, compared to the rates under background Nr deposition of ∼2 kg N ha-1 yr-1. These findings were corroborated by similar BNF suppression at the fertilization plots in Sweden. Therefore, contribution of BNF in peatlands exposed to chronic Nr deposition needs accounting when modeling peatland's nitrogen pools, given that nitrogen availability exerts a key control on the carbon capture of peatlands, globally.
Highlights
IntroductionThe input of anthropogenic reactive nitrogen (Nr) to the land has more than doubled due to three principal activities: agricultural intensification, fertilizer production and fossil fuel combustion.[1−3] This Nr consists of two major forms: reduced N (NHx) mainly in the forms of NH3 and NH4+, and oxidized N (NOy) mainly in the forms of NO2 and particulate NO3−.4 in Western countries the Nr deposition rates are expected to continue declining during the few decades,[5] in the developing countries of Asia, Africa, and South America, Nr deposition is expected to rise further by 20% between 2010 and 2100.6 In the U.K., the Nr deposition rates in peatlands range from
The biological nitrogen fixation (BNF) suppression ratios (Table 2) obtained for each of the British sites while using the Swedish Degerö peatland as reference, we observed that the suppression effect was 13.3 times higher in the Forsinard than in the Migneint, and 1.2 times higher in Migneint than in the Fenn’s & Whixall peatland
Nr deposition rates (e.g., Forsinard) than in areas with high Nr deposition rates (e.g., Fenn’s & Whixall) (Table 2). This suggest that BNF activity is more sensitive to Nr deposition in areas with a low Nr deposition rate, i.e., more pristine areas, and as the Nr deposition rate increases the suppression ratio decreases, suggesting the development of diazotrophic microbes tolerance to high rates of Nr deposition
Summary
The input of anthropogenic reactive nitrogen (Nr) to the land has more than doubled due to three principal activities: agricultural intensification, fertilizer production and fossil fuel combustion.[1−3] This Nr consists of two major forms: reduced N (NHx) mainly in the forms of NH3 and NH4+, and oxidized N (NOy) mainly in the forms of NO2 and particulate NO3−.4 in Western countries the Nr deposition rates are expected to continue declining during the few decades,[5] in the developing countries of Asia, Africa, and South America, Nr deposition is expected to rise further by 20% between 2010 and 2100.6 In the U.K., the Nr deposition rates in peatlands range from
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