Fate of N in a peatland, Whim bog: immobilisation in the vegetation and peat, leakage into pore water and losses as N<sub>2</sub>O depend on the form of N

Abstract

Abstract. Peatlands represent a vast carbon reserve that has accumulated under conditions of low nitrogen availability. Given the strong coupling between the carbon and nitrogen cycles, we need to establish the consequences of the increase in reactive nitrogen deposition for the sustainability of peatlands, and whether the form in which the nitrogen is deposited makes a difference. We have addressed these questions using a globally unique field simulation of reactive N deposition as dry deposited ammonia and wet deposited reduced N, ammonium and oxidised N, nitrate, added as ammonium chloride or sodium nitrate, to an ombrotrophic peatland, Whim bog in SE Scotland. Here we report the fate of 56 kg N ha −1 yr −1 additions over 10 yr and the consequences. The effects of 10 yr of reactive N additions depended on the form in which the N was applied. Ammonia-N deposition caused the keystone Sphagnum species, together with the main shrub Calluna and the pleurocarpous mosses, to disappear, exposing up to 30 % of the peat surface. This led to a significant increase in soil water nitrate and nitrous oxide emissions. By contrast wet deposited N, despite significantly reducing the cover of Sphagnum and Pleurozium moss, did not have a detrimental effect on Calluna cover nor did it significantly change soil water N concentrations or nitrous oxide emissions. Importantly 10 yr of wet deposited N did not bare the peat surface nor significantly disrupt the vegetation enabling the N to be retained within the carbon rich peatland ecosystems. However, given the significant role of Sphagnum in maintaining conditions that retard decomposition, this study suggests that all nitrogen forms will eventually compromise carbon sequestration by peatlands through loss of some keystone Sphagnum species.

Nitrous oxide emissions from a peatbog after 13 years of experimental nitrogen deposition

“…Our results confirm the early findings of Sheppard et al (2013) Lund et al (2009) found no effect of experimental N addition (NH 4 NO 3 ) on N 2 O emissions from two Swedish bogs, and peak fluxes were less than 1 nmol m −2 s −1 , when 40 kg N ha −1 yr −1 was applied in only three relatively large doses. Nykanen et al (2002) found no response of N 2 O emission to additions of up to 100 kg N ha −1 yr −1 (NH 4 NO 3 ) to a Sphagnum fuscum pine bog in Finland, over a 6-year study.…”

“…Here, we examine the effects of the dose and form of N deposition on emissions of N 2 O. Preliminary data on N 2 O fluxes were reported by Sheppard et al (2013), showing an increase with NH 3 , but no effect of NH + 4 and NO − 3 . Here, we analyse an additional 5 years of data collected at a wider range of locations, and with further time for any treatment effects to accumulate.…”

Inertia in an ombrotrophic bog ecosystem in response to 9 years' realistic perturbation by wet deposition of nitrogen, separated by form

“…Biomass assessments, together with nutrient concentrations in 2009, inferred most N should be in the soil (Sheppard et al ., ,b). Thus, we used soil pore water chemistry as the sensitive indicator (Sheppard et al ., ,b) for the path analysis.…”

Differential response of carbon cycling to long-term nutrient input and altered hydrological conditions in a continental Canadian peatland

“…For these normally nutrient-limited ecosystems, eutrophication is a major threat, as demonstrated in several long-term fertilization experiments. A decade of fertilizer applications to bogs in Canada (Mer Bleue), in the UK (Whim bog), and in Sweden (Degerö Stormyr) and 7 years of high nitrogen deposition to a bog in the Italian Alps caused a loss of mosses and an increase in vascular plant biomass (Bubier et al, 2007;Wang et al, 2016;Sheppard et al, 2013;Wiedermann et al, 2007;Bragazza et al, 2012). In the Mer Bleue bog, nutrient addition mostly increased the abundance of shrubs, whereas at Whim bog, Degerö Stormyr, and an Italian mire, both shrub and graminoid cover increased.…”