Trace gas cycling and soil carbon budgets in

rewetted upland mires of Exmoor National Park

Introduction The River Exe and its tributaries supply water to >500,000 people and support an extensive tourism and recreation industry. Significant areas of upland blanket mire in the Exe catchment were damaged in the 19th and 20th centuries by drainage and peat cutting. South West Water plc has initiated and funded a 5-year programme to restore ~2,500 ha of degraded upland mire. Landowners presently do not receive monetary benefit for improvements to water supply or net reduction in greenhouse gas fluxes resulting from peatland restoration. The principal goal of this study is to quantify atmospheric and fluvial fluxes of CO2, CH4, and N2O in two test catchments situated in Exmoor National Park (Fig. 1) before and after grip-blocking to determine net changes in CO2(eq) resulting from mire rewetting. Findings will be used by South West Water plc to determine whether remuneration for landowners should be negotiated in 2015-2020 water prices with the Water Services Regulation Authority (Ofwat).

Adam McAleer1, Edward Hornibrook1 and David Smith2

1Bristol Biogeochemistry Research Centre, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK

2South West Water Ltd, Peninsula House, Rydon Lane, Exeter, Devon, EX2 7HR, UK

Figure 2 (Above). A - Ground collars and pore water equilibrator installed at the Spooners catchment; B - assembly of a 40-cm depth pore water membrane equilibrator.

Hypothesis: Rewetting of Exmoor mires will increase CH4 emissions but the net change in CO2(eq) will be beneficial because of decreases in CO2 and N2O fluxes.

Figure 1 (Below). The Spooner and Acland catchments are situated at the headwaters of the Barle River and cover areas of 47 and 13.6 hectares, respectively. Both areas are isolated headwater catchments drained by a single first order stream. Ground cover is primarily degraded blanket bog crosscut by networks of closely spaced drainage ditches. The Environment Agency, Exeter University and South West Water plc are conducting a concurrent detailed hydrological study to quantify water input, retention and outflow characteristics in the catchments. Peat depths shown in the figure are taken from Bowes (2006).

Spooners catchment

References Bowes, A.C. (2006). Exmoor Blanket Bog inventory and restoration plan for English Nature. Unpublished Thesis. MSc. University of Calgary. Chambers, F.M. Beilman, D.W., and Yu, Z. (2011). Methods for determining peat humification and for quantifying peat bulk density, organic matter and carbon content for palaeostudies of climate and peatland carbon dynamics. Mires and Peat. 7. Hesslein, R. H. (1976). An in situ sampler for close interval pore water studies. Limnology and Oceanography, 912-914. Hope, D., Palmer, S.M., Billett, M.F., and Dawson, J.J.C. (2001). Carbon dioxide and methane evasion from a temperate peatland stream. Limnology and Oceanography 46, 847-857.

Aclands catchment

Hummock (Ground Collar)

Hollow (Ground Collar)

Pore Water Equilibrator

A

Polyurethane tubing

PVC sheet with troughs sealed with

a membrane

B Exmoor National Park

It is anticipated that fluvial transport of CO2, CH4 and N2O will comprise a significant proportion of annual export. Rates of gaseous evasion will be measured in stream reaches within each catchment using propane and a non-volatile conservative tracer (Hope et al. 2001). Valuation of net changes in CO2(eq) will need to consider the time period required to achieve a new equilibrium state post grip-blocking. Inter-annual variations in pre-rewetting surface fluxes and export via streams will be assessed through concurrent measurements in several adjacent unrestored catchments. Additionally, several grips will remain unblocked within the two test catchments to serve as controls.

Work Programme Soil water concentrations and atmospheric fluxes of CO2, CH4 and N2O will be measured in relation to key landscape features and differences in vegetation cover using pore water equilibrators and conventional flux chambers / ground collars (Fig. 2). The pore water equilibrators are a modified version of the design of Hesslein (1976) (i.e., ‘peepers’) that do not require removal from the soil to extract high purity water that has been equilibrated with pore water solutes. Changes in the soil pool of CO2, CH4 and N2O will be quantified from concentration data and peat depth and porosity. Peat depth in the catchments has been characterised previously by Bowes (2006). Bulk density and carbon content will be determined using approaches described by Chambers et al. (2011).

Simonsbath