Abstract
Peatlands play an important role in modulating the climate, mainly through sequestration of carbon dioxide into peat carbon, which depends on the availability of reactive nitrogen (Nr) to mosses. Atmospheric Nr deposition in the UK has been above the critical load for functional and structural changes to peatland mosses, thus threatening to accelerate their succession by vascular plants and increasing the possibility of Nr export to downstream ecosystems. The N balance of peatlands has received comparatively little attention, mainly due to the difficulty in measuring gaseous N losses as well as the Nr inputs due to biological nitrogen fixation (BNF). In this study we have estimated the mean annual N balance of an ombrotrophic bog (Migneint, North Wales) by measuring in situ N2 + N2O gaseous fluxes and also BNF in peat and mosses. Fluvial N export was monitored through a continuous record of DON flux, while atmospheric N deposition was modelled on a 5 × 5 km grid. The mean annual N mass balance was slightly positive (0.7 ± 4.1 kg N ha−1 y−1) and varied interannually indicating the fragile status of this bog ecosystem that has reached N saturation and is prone to becoming a net N source. Gaseous N losses were a major N output term accounting for 70% of the N inputs, mainly in the form of the inert N2 gas, thus providing partial mitigation to the adverse effects of chronic Nr enrichment. BNF was suppressed by 69%, compared to rates in pristine bogs, but was still active, contributing ~2% of the N inputs. The long-term peat N storage rate (8.4 ± 0.8 kg N ha−1 y−1) cannot be met by the measured N mass balance, showing that the bog catchment is losing more N than it can store due its saturated status.
Highlights
Despite covering about 3 % of the earth's surface, northern peatlands are globally significant sinks of carbon (C), accumulating an estimated ~500 Gt C (Loisel et al, 2014), much of which occurred during the Holocene
This finding is broadly in line with a recent spatially distributed nitrogen budget for the whole of Great Britain, which indicated that 34% of areas, located predominantly in the western part of the country, are identified as N sinks (see Fig. 4 in (Fan et al, 2020). It corroborates the few recent N budget studies in peatland catchments (Hill et al, 2016; Vogt et al, 2013; Worrall et al, 2012) that have shown that these northern peatlands can often behave as N sources, a common conclusion has been the high uncertainty of estimates regarding key N cycle processes such as biological nitrogen fixation and soil denitrification that are difficult to measure and for which there is scarcity of available field data
The modelled atmospheric deposition for the Migneint bog catchment (Table 1) falls in the middle of the range reported for the UK (7.6–27.6 kg N ha−1 y−1;(Fan et al, 2020), while the uncertainty typically associated with the UK FRAME model is between 20 and 30% (Tipping et al, 2017; Vogt et al, 2013)
Summary
Despite covering about 3 % of the earth's surface (or 4,000,000 km2), northern peatlands are globally significant sinks of carbon (C), accumulating an estimated ~500 Gt C (Loisel et al, 2014), much of which occurred during the Holocene. Global atmospheric N deposition increased from 86.6 to 93.6 Tg N y−1 between 1984 and 2016, an increase of 8% (Ackerman et al, 2019) Against this backdrop of increasing global N deposition, due to extensive regulation targeting oxidised N compounds (Fowler et al, 2007), Europe has experienced in the last three decades a steady decline in deposition rates. Despite this reduction, the United Kingdom, still receives Nr deposition in the region of 25 kg N ha−1 y−1, which is considerably more than background deposition of ~2 kg N ha−1 y−1 (Payne, 2014)
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