Article (refereed) -postprintEvans, Chris D.; Renou-Wilson, Flo; Strack, Maria. 2016. The role of waterborne carbon in the greenhouse gas balance of drained and rewetted peatlands [in special issue: Carbon cycling in aquatic ecosystems] Aquatic Sciences, 78 (3). 573-590. 10.1007/s00027-015-0447-y Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner.The role of waterborne carbon in the greenhouse gas balance of drained and re-wetted peatlands AbstractAccounting for greenhouse gas (GHG) emissions and removals in managed ecosystems has generally focused on direct land-atmosphere fluxes, but in peatlands a significant proportion of total carbon loss occurs via fluvial transport. This study considers the composition of this 'waterborne carbon' flux, its potential contribution to GHG emissions, and the extent to which it may change in response to land-management. The work describes, and builds on, a methodology to account for major components of these emissions developed for the 2013 Wetland Supplement of the Intergovernmental Panel on Climate Change. We identify two major components of GHG emissions from waterbodies draining organic soil: i) 'on site' emissions of methane (and to a lesser extent CO2) from drainage ditches located within the peatland; and ii) 'off site' emissions of CO2 resulting from downstream oxidation of dissolved and particulate organic carbon (DOC and POC) within the aquatic system. Methane emissions from ditches were found to be large in many cases (mean 60 g CH4 m -2 yr -1 based on all reported values), countering the view that methane emissions cease following wetland drainage. Emissions were greatest from ditches in intensive agricultural peatlands, but data were sparse and showed high variability. For DOC, the magnitude of the natural flux varied strongly with latitude, from 5 g C m -2 yr -1 in northern boreal peatlands to 60 g C m -2 yr -1 in tropical peatlands. Available data suggest that DOC fluxes increase by around 60% following drainage, and that this increase may be reversed in the longer-term through re-wetting, although variability between studies was high, especially in relation to re-wetting response. Evidence regarding the fate of DOC is complex and inconclusive, but overall suggests that the majority of DOC exported from peatlands is converted to CO2 through photo-and/or bio-degradation in rivers, standing waters and oceans. The contribution of POC export to GHG emissions is even more uncertain, but we estimate that over half of exported POC may eventually be converted to CO2. Although POC fluxes are normally small, they can become very large when bare peat surfaces are exposed to fluvial erosion. Overall, we estimate that waterborne carbon emissions may contribute about 1 to 4 t CO2-eq ha -1 yr -1 of additional GHG emissions from drained peatlands. For a number of worked examples this repres...

Drained peat soils are a significant source of greenhouse gas (GHG) emissions to the atmosphere. Rewetting these soils is considered an important climate change mitigation tool to reduce emissions and create suitable conditions for carbon sequestration. Long-term monitoring is essential to capture interannual variations in GHG emissions and associated environmental variables and to reduce the uncertainty linked with GHG emission factor calculations. In this study, we present GHG balances: carbon dioxide (CO ), methane (CH ) and nitrous oxide (N O) calculated for a 5-year period at a rewetted industrial cutaway peatland in Ireland (rewetted 7 years prior to the start of the study); and compare the results with an adjacent drained area (2-year data set), and with ten long-term data sets from intact (i.e. undrained) peatlands in temperate and boreal regions. In the rewetted site, CO exchange (or net ecosystem exchange (NEE)) was strongly influenced by ecosystem respiration (R ) rather than gross primary production (GPP). CH emissions were related to soil temperature and either water table level or plant biomass. N O emissions were not detected in either drained or rewetted sites. Rewetting reduced CO emissions in unvegetated areas by approximately 50%. When upscaled to the ecosystem level, the emission factors (calculated as 5-year mean of annual balances) for the rewetted site were (±SD) -104 ± 80 g CO -C m yr (i.e. CO sink) and 9 ± 2 g CH -C m yr (i.e. CH source). Nearly a decade after rewetting, the GHG balance (100-year global warming potential) had reduced noticeably (i.e. less warming) in comparison with the drained site but was still higher than comparative intact sites. Our results indicate that rewetted sites may be more sensitive to interannual changes in weather conditions than their more resilient intact counterparts and may switch from an annual CO sink to a source if triggered by slightly drier conditions.

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Abstract. Drained peatlands are significant hotspots of carbon dioxide (CO 2 ) emissions and may also be more vulnerable to fire with its associated gaseous emissions. Under the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol, greenhouse gas (GHG) emissions from peatlands managed for extraction are reported on an annual basis. However, the Tier 1 (default) emission factors (EFs) provided in the IPCC 2013 Wetlands Supplement for this land use category may not be representative in all cases and countries are encouraged to move to highertier reporting levels with reduced uncertainty levels based on country-or regional-specific data. In this study, we quantified (1) CO 2 -C emissions from nine peat extraction sites in the Republic of Ireland and the United Kingdom, which were initially disaggregated by land use type (industrial versus domestic peat extraction), and (2) a range of GHGs that are released to the atmosphere with the burning of peat. Drainagerelated methane (CH 4 ) and nitrous oxide (N 2 O) emissions as well as CO 2 -C emissions associated with the off-site decomposition of horticultural peat were not included here. Our results show that net CO 2 -C emissions were strongly controlled by soil temperature at the industrial sites (bare peat) and by soil temperature and leaf area index at the vegetated domestic sites. Our derived EFs of 1.70 (±0.47) and 1.64 (±0.44) t CO 2 -C ha −1 yr −1 for the industrial and domestic sites respectively are considerably lower than the Tier 1 EF (2.8 ± 1.7 t CO 2 -C ha −1 yr −1 ) provided in the Wetlands Supplement. We propose that the difference between our derived values and the Wetlands Supplement value is due to differences in peat quality and, consequently, decomposition rates. Emissions from burning of the peat (g kg −1 dry fuel burned) were estimated to be approximately 1346 CO 2 , 8.35 methane (CH 4 ), 218 carbon monoxide (CO), 1.53 ethane (C 2 H 6 ), 1.74 ethylene (C 2 H 4 ), 0.60 methanol (CH 3 OH), 2.21 hydrogen cyanide (HCN) and 0.73 ammonia (NH 3 ), and this emphasises the importance of understanding the full suite of trace gas emissions from biomass burning. Our results highlight the importance of generating reliable Tier 2 values for different regions and land use categories. Furthermore, given that the IPCC Tier 1 EF was only based on 20 sites (all from Canada and Fennoscandia), we suggest that data from another 9 sites significantly expand the global data set, as well as adding a new region.

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Abstract. Temperate grasslands on organic soils are diverse due to edaphic properties but also to regional management practices and this heterogeneity is reflected in the wide range of greenhouse gas (GHG) flux values reported in the literature. In Ireland, most grasslands on organic soils were drained several decades ago and are managed as extensive pastures with little or no fertilisation. This study describes a 2-year study of the net ecosystem carbon balance (NECB) of two such sites. We determined GHG fluxes and waterborne carbon (C) emissions in a nutrient-rich grassland and compared it with values measured from two nutrient-poor organic soils: a deep-drained and a shallow-drained site. Carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) fluxes were determined using the chamber technique, and fluvial C fluxes were estimated by combining drainage water concentrations and flows.The nutrient-rich site was an annual source of CO 2 (233 g C m −2 yr −1 ), CH 4 neutral, and a small source of N 2 O (0.16 g N 2 O-N m −2 yr −1 ). Net ecosystem exchange (NEE) at the shallow-drained nutrient-poor site was −89 and −99 g C m −2 yr −1 in Years 1 and 2 respectively, and NEE at the deep-drained nutrient-poor site was 85 and −26 g C m −2 yr −1 respectively. Low CH 4 emissions (1.3 g C m −2 yr −1 ) were recorded at the shallow-drained nutrient-poor site. Fluvial exports from the nutrient-rich site totalled 69.8 g C m −2 yr −1 with 54 % as dissolved organic C. Waterborne C losses from the nutrient-poor site reflected differences in annual runoff totalling 44 g C m −2 yr −1 in Year 1 and 30.8 g C m −2 yr −1 in Year 2.The NECB of the nutrient-rich grassland was 663 g C m −2 yr −1 with biomass exports being the major component accounting for 53 %. The NECB of the nutrient-poor deep-drained site was less than half of the nutrient-rich site (2-year mean 267 g C m −2 yr −1 ). Although NEE at the nutrient-poor shallow-drained site was negative in both years, high biomass export meant it was a net C source (2-year mean NECB 103 g C m −2 yr −1 ). While the impacts of the nutrient and drainage status on NEE, biomass exports and fluvial C losses were confirmed, inter-regional differences in management practice and climate were also significant factors which impacted on the overall NECB of these ecosystems. Contrary to expectation, the NECB of nutrient-poor drained organic soils under grasslands is not necessarily a large C source and this has implications for Ireland's choice of national GHG inventory reporting methodologies. This study can also aid the development of strategies to deliver reduced emissions tailored to local grassland types.

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