Chronic Atmospheric Reactive Nitrogen Deposition Suppresses Biological Nitrogen Fixation in Peatlands

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.

Chronic atmospheric reactive N deposition has breached the N sink capacity of a northern ombrotrophic peatbog increasing the gaseous and fluvial N losses

“…This suppression was found to be close to the reduction in BNF (by 64%) caused by experimental fertilisation treatments in plots of Degerö Stormyr receiving 30 kg N ha −1 y −1 of Nr for more than 20 years. The magnitude of the suppression in BNF activity was shown to be higher in more pristine peatlands rather than bogs that have been exposed to long-term elevated Nr deposition (Saiz et al, 2021), as in the Migneint, and this could be ascribed to adaptations of the physiological response of the mosses to inorganic N supply. Specifically, it has been shown that Sphagnum is capable of regulating its uptake of ammonium, in a way that maximises N use efficiency while avoiding intoxication (Fritz et al, 2014).…”

“…Despite the relatively high atmospheric deposition of Nr above the critical load of 10 kg N ha −1 y −1 that in the long-term could favour the dominance of vascular plants over Sphagnum mosses (Dore et al, 2012), biological nitrogen fixation associated with mosses and in bulk surface peat (asymbiotic) has not completely shut down but accounted for~2% of the N inputs in the Migneint bog. Saiz et al (2021) have shown that BNF activity in the Migneint is suppressed by 69% compared to the more pristine Swedish Degerö Stormyr peatland receiving~2 kg N ha −1 y −1 of background Nr atmospheric deposition. This suppression was found to be close to the reduction in BNF (by 64%) caused by experimental fertilisation treatments in plots of Degerö Stormyr receiving 30 kg N ha −1 y −1 of Nr for more than 20 years.…”

“…Measurements of BNF derived in situ are rare in the literature. While BNF was not measured during the 2013-14 study period, for the purpose of constructing our N budget without relying on literature values from elsewhere, we have used in situ measurements of BNF from the Migneint from 2016 -17 reported in Saiz et al (2021). BNF was measured in 4 Sphagnum spp.…”

“…High rates of atmospherically deposited Nr could potentially negate the need for a 'costly' investment on BNF. Experimental studies with N additions above ambient levels have shown an inconsistent change of BNF activity with the added N (Kox et al, 2016;van den Elzen et al, 2018), while BNF rates under a natural gradient of Nr deposition in the UK have been shown to reduce but not completely shut down (Saiz et al, 2021). This can potentially have important implications when estimating the N economy of peatlands and should be quantitatively accounted for in N budget studies.…”

Structural design and control of photocatalytic nitrogen-fixing catalysts

“…[1][2][3] Only through microbial action, can plants convert free nitrogen in the atmosphere into ammonia or other nitrogenous organic matter for their own growth, which is called biological nitrogen xation. 4,5 In nature addition, there is another method of nitrogen xation: under the action of lightning, N 2 is directly converted into NO, and then into NO 2 . This process is called high-energy nitrogen xation.…”