Assessment of the contribution of aquatic carbon fluxes to carbon losses from UK peatlands
Background to research
Global carbon cycles are highly topical since changes in carbon cycling have interactions with policy for soils and land management, climate and energy, water and ecology. The rises in dissolved organic carbon, which have been widely reported for rivers of the northern hemisphere over the last decade, have caused concern about an increasing net loss of stored terrestrial carbon. Whilst the fact that these increasing concentrations are widespread is not disputed, both the reasons for this and the impact on the global carbon balance are not certain. The relative impacts of environmental and land use change on carbon destabilisation in organic soils, and for the transport of the carbon from soils to waters, are of topical debate. Added to this it is not known how reactive different forms of carbon are in the aquatic environment, as this governs whether the carbon is lost back to the atmosphere, or transported and ultimately buried in the oceans. There have been previous reviews of aquatic carbon that have focused more on single forms such as dissolved organic carbon. This report aims to satisfy a need to examine carbon forms together; by reviewing the literature, determining the extent of the data resources across the UK and by making spatial and temporal analyses of available data.
Objectives of research
Three specific aims were set out to guide these analyses:
Key findings and recommendations
Our review found a shortage of studies which have addressed multiple forms of carbon in aquatic systems. The current order of data and knowledge on different C forms appears to be: dissolved organic >> particulate organic > CO2 ~ dissolved inorganic >> particulate inorganic C forms. The aquatic pathway is a key way in which carbon is lost from soils; hence variation in the aquatic loss component can determine the net sink or source condition of a peatland with respect to its carbon balance. There is especially a need to place aquatic carbon fluxes in the context of net ecosystem carbon budgets. This is something that has been done for a very few sites and restricts us in making informed decisions about the net impacts of management actions (forestry, wind developments, peatland restoration etc) on carbon budgets. A further key part of this uncertainty is in the knowledge of the reactivity of the carbon forms in the aquatic ecosystem which governs their fate. To have a net ‗climate forcing‘ effect requires the aquatic carbon forms to be out gassed from the rivers and lakes as CO2. The extent of this, via respiration or photo-reactions, is not known and this may change considerably if rising DOC concentrations are attributed mainly to certain forms of carbon only.
This lack of information on these key factors was reflected in the extent of the meta-data collected from across the UK on different forms of carbon. The meta-data indicated that dissolved organic carbon was a parameter measured by researchers either at high temporal resolutions but for a small number of sites, or by regulatory agencies or industry at a larger number of sites but with limited temporal resolution. There was a shortage of records for alternate forms of carbon in aquatic systems. This is something that should be addressed by the research and regulatory communities together. Potentially, surrogate data could be useful in improving the knowledge base (e.g. turbidity for particulate carbon, or spectral properties for certain forms of dissolved organic matter), but care needs to be taken in their calibration.
The analysis of data took two forms; spatial analyses and a temporal analysis. However, both were necessarily limited to one principal carbon form by the available data. One aspect was the spatial analysis of dissolved organic carbon fluxes (compiled with total organic carbon data) for rivers using an export modeling approach explained by simple catchment factors. This was done for England, Wales and Scotland in one chapter and for Northern Ireland in another chapter (due the different nature of data that were available). Organic soils had the greater export coefficients and these were considerably greater in Northern Ireland. This was partly explained by geo-climatic and partly by land-management pressures. A further analytical study was undertaken of the trends in dissolved/total organic carbon for Scotland only. A novel non-linear trend analysis approach was adopted exploring compiled datasets from SEPA and Marine Science Scotland. A very consistent non-linear increasing trend was found across Scotland. Catchment attributes were then explored as explanatory variables against deviations in the gradient of specific sites from the common trend. Atmospheric deposition terms were highly significant in overall models but the considerable unexplained variance could be due to management and climatic factors that could not be explored in detail due to spatial data limitations. Therefore, as well as river data on concentrations and fluxes of different carbon forms, the scientific and policy-advising community require an improved selection of catchment data to drive the understanding that will allow us to make decisions regarding management of peatland systems and predictions of changes in the carbon balance for the future.
Key words: aquatic carbon; dissolved organic carbon (DOC); particulate organic carbon (POC); carbon dioxide; methane; dissolved inorganic carbon (DIC); catchments; organic soils; peatland; atmospheric deposition; land management; climate
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