Greenhouse gas losses from peatland pipes: a major pathway for loss to the atmosphere?

Peatland pipes are large natural macropores which contribute significantly to streamflow and represent a potentially important transport pathway between terrestrial and aquatic/atmospheric systems. Our study aimed to estimate the contribution of pipeflow to catchment-scale greenhouse gas (GHG) losses (CO2, CH4 and N2O) in a UK peatland using a combination of fortnightly spot and continuous sensor measurements. Inter-pipe variability was high for all GHGs. Mean pipe water concentrations ranged from 0.70 to 6.51 mg C L-1, 0.90 to 897 μg C L-1, and 0.36 to 1.36 μg N L-1 for CO2, CH4 and N2O respectively. High-resolution CO2 data showed temporal changes in the connectivity between pipes and the surrounding peat, with connectivity greatest when water table was high and lowest at low water table depths when discharge was associated with deeper, CO2-enriched sources. Total downstream export from the eight studied pipes represented 3%, 38% and 3% of CO2, CH4 and N2O export at the catchment outlet, whilst contributing only ~2% of total catchment runoff. Direct degassing of CO2 and CH4 to the atmosphere was evident from an intensively monitored pipe outlet. Upscaling evasion estimates from the pipe outlets gave conservative catchment-scale emission rates of 7.08 g CO2-eq m-2 yr-1 and 50.2 g CO2-eq m-2 yr-1 for CO2 and CH4, respectively. Although the catchment-scale estimates contain significant uncertainty, they highlight the potential importance of pipes as a pathway for the release of terrestrially-produced GHGs to the atmosphere.