The role of pipes in carbon export from peatlands

Lead Research Organisation: NERC CEH (Up to 30.11.2019)
Department Name: Billett

Abstract

Since the end of the last ice age large amounts of atmospheric carbon dioxide have been slowly locked up in peat soils in the cool, wet, northern regions of the world as partially decomposed plant remains. If this huge reservoir of carbon was to be released back in to the atmosphere it would cause a significant rise in carbon dioxide and release methane and result in further global warming. At the moment scientists are trying to unravel the mechanisms that control the losses and gains of carbon from this large area of the Earth's surface. Peat soils, which can be up to 5 m or more in thickness, consist of about 90 % water and are important resources for the companies that supply water to homes in the UK. Although it has been known that peat soils contain large numbers of 'pipes' (naturally occurring sub-surface tubes or voids of various lengths and sizes), it has recently been shown that they occur in extremely large numbers in many UK peatlands. Pipe development may also be encouraged by the drainage of peat bogs and also by periods of summer drought such as those which occurred in 1976 and 1995 in northern Europe. Since much of our understanding about water movement in peats tends to exclude the role of pipes, it is important to investigate the role that they play in delivering water and carbon (in various dissolved and gaseous forms and as small water-borne particles) to streams. Peat pipes could provide an important route to connect the large reservoir of carbon stored in peats to peatland pools and streams, and may act as pathways along which carbon can leak away. We aim to investigate how pipes control the loss of water and carbon from peatlands. Most of the research work, which will take three years to complete and involves collaboration between scientists in Leeds, London and Edinburgh, will be carried out at Moor House in the north Pennines, UK in a catchment that has been designated a carbon study catchment by the Centre for Ecology and Hydrology. Our work will involve measuring the concentrations and amounts of carbon and water flowing in pipes and streams. We will use techniques that allow us to determine the age of the carbon being released and therefore where in the peat the carbon is coming from. The overall aim of our work is to help scientists to understand more about the natural processes which affect this important global store of carbon.
 
Description All five objectives were achieved. This study is the first of the role of natural soil pipes in carbon (C) losses from deep peat systems. Pipes were found to be an important C loss pathway. Pipe contributions are dynamic and vary with time during storms, under different antecedent conditions and seasonally. Contributions of C are very different between pipes. In the study catchment we identified 84 separate pipe outlets which were spot sampled, with eight of these continuously gauged for discharge, C fluxes and chemistry. Our measurements suggest that there are three main types of pipes that have markedly different characteristics: i) ephemeral pipes that flow mainly during storm events; ii) perennial pipes dominated by surface waters and iii) perennial pipes with significant deep water sources. There are differences in water chemistry and organic C concentrations for any given pipe as discharge varies and this does not appear to be connected to discharge rates. Rather organic C concentrations in pipe waters are controlled by antecedent conditions and seasonal effects. Overall, pipes contributed around 35 % of the dissolved organic carbon (DOC) flowing through the stream system. Particulate organic carbon (POC) release was highly episodic and dominated by very occasional but large pulses which represent pipe wall failures within the network and occasional flushing by storm waters. Some pipe outlets became inactive and others opened up or changed in size during the study indicating an active geomorphology. The POC flux from pipes was greater than released by the stream at the catchment outlet showing that C may be temporarily stored or lost through decomposition in situ once removed from the peat mass by pipe transport. For greenhouse gas (GHG) losses from the pipes (CO2, CH4, N2O) we used a combination of fortnightly spot measurements and novel continuous sensors. Mean concentrations for individual pipes ranged from 0.70 to 6.51 mg C L-1, 0.90 to 897 µg C L-1, and 0.36 to1.36 µg N L-1 for CO2, CH4 and N2O respectively. High resolution CO2 data illustrated temporal changes in connectivity of pipes with the peat. Connectivity was greatest when water table was high and lowest at low discharge where deeper, CO2-enriched sources maintained base-flow. Total export from the pipes represented 4%, 49% and 769% of CO2, CH4 and N2O export at the catchment outlet, while contributing 15% of total river discharge. Direct degassing from the pipe outlets was discovered showing that the pipes themselves acted as hotspots for radiative forcing. Upscaling evasion from the pipe outlets gave conservative catchment-scale emission estimates of 18.5 g CO2-eq m-2 yr-1 and 13.7 g CO2-eq m-2 yr-1 for CO2 and CH4, respectively.

Our data shows that pipes can draw water from depth with significantly different chemistry which has a marked influence on low flow stream chemistry. As land management has been shown to influence pipe development in peatlands there is a wider chemical significance for aquatic ecosystems and water supply of the role of peatland pipes. Pipe networks in peat headwater catchments provide a significant portion of runoff during low flow periods, as much as 40 %, and therefore significantly affect stream water chemistry under these conditions. This has important implications for both natural ecosystems and water supply companies. The contribution of pipes under low flow conditions may, in some cases, provide discharge in headwater streams where very little would be available otherwise, hence providing further habitats for aquatic organisms and supply for water companies. The increased levels of base cations and higher pH found in pipe waters draining deep peats may also provide conditions suitable for wider ranges of aquatic organisms in peatland streams than would otherwise be so. The isotopic data during storm events showed that while the C sources were mainly modern, some pipes draw old C from depth. CO2 is predominantly derived from organic matter via aerobic decomposition (i.e. from decay of peat near the surface, which fits with the 14C values) for some pipes. In contrast, the very 13C-enriched values for CO2 evaded from other pipes suggest that this CO2 is predominantly derived via fermentation (anaerobic decay) which fits with the old 14C results (<500 years BP) implying a source deeper in the peat profile. Typically DOC was found to be modern being originally fixed from the atmosphere about 10-20 years ago. However, the highest 14C concentrations were determined from the samples collected at the peak flood level, suggesting that this was when the oldest DOC was being moved, when pipe network connectivity was at a maximum within the peat. The pipes that tap deep sources of C appear to be very strong hotspots for methane release which is a crucial finding since land management activity can encourage more deep-seated piping to develop and methane is a powerful GHG. Given the importance of pipes for peatland C budgets, and that land management and climate change may encourage enhanced pipe development, the findings of this project will be of great importance for national and international scientists and practitioners interested in the role of peatlands in the global C cycle and water quality.
Exploitation Route Including peat pipes in C budgets for peatlands and peatland restoration.

Including pipes when developing infrastructure on peatlands
Sectors Construction,Environment
 
Description Yorkshire Water used the findings to test peatland restoration impacts on C release via pipe blocking.
First Year Of Impact 2012
Sector Environment
Impact Types Economic