Workshop-report "Sediment and carbon fluxes under human impact and climate change"

Bern, Switzerland, 28-30 July 2011

Thomas Hoffmann, G. Erkens, G. Verstraeten, Hans Middelkoop, Andreas Lang

Sediment-burden carbon fluxes represent a considerable yet insufficiently understood component of the global carbon cycle. The contribution of soil erosion and sediment transport on hillslopes and in channels to greenhouse gas emission and/or long-term carbon storage remains largely unknown. Fundamental research questions remain: What is the fate of soil organic carbon that has been removed through soil erosion? What happens with carbon during transport on hillslopes and in channels? What are the major sources of carbon in fluvial and limnic sedimentary sinks? How does allochthonous and autochthonous carbon differ in these sinks? How stable is carbon after deposition? How can such fluxes be quantified to disentangle the long term carbon cycle?

The major aim of the workshop was to bring together leading and early-career scientists focusing on lateral carbon and sediment fluxes from different viewpoints (e.g. soils, hillslopes, fluvial- and limnic systems). Perspectives for unravelling long-term sediment-burden carbon fluxes were discussed, prospects to establish the relationships between carbon and sediment fluxes developed, and concepts, methods and regions identified that are best suited for establishing Holocene sediment-carbon fluxes. The workshop was supported by PAGES and organised by the PAGES-Focus 4 group LUCIFS (Land Use and Climate Impacts on Fluvial Systems). During the last decades LUCIFS has developed an in-depth understanding of fluxes of clastic sediment during the period of agriculture and now aims at integrating carbon budgets. 19 researchers from different disciplines (ecology, geology, geomorphology, and limnology) and countries (Belgium, Germany, Japan, Switzerland, The Netherlands, UK and USA) contributed to the workshop.

The first day focused on lateral carbon fluxes in different components along the sediment flow path. The contribution and importance of single system components (e.g. hillslopes, floodplains and lakes) and their interconnections were discussed. Keynotes were given by Kirstof van Oost  (Luvian, BE), Rolf Aalto (Exeter, UK), John Boyle (Liverpool, UK) on eroding and storing organic carbon on hillslopes, in floodplains and lakes.  The second day´s session focused on the wider implications of environmental changes (including climate change and human impacts) on sediment-burden carbon fluxes. Simon Mudd (Edinburgh, UK) and Jed O. Kaplan (Lausanne, CH) presented ideas on quantifying long term soil production and biogeochemistry for use as baseline information for modern soil processes and on Holocene carbon and land use changes, while Jane Willenbring (Pennsylvania, USA) ended the keynote talks with a presentation on quantifying long-term carbon loading from anthropogenic erosion. During the third day, Fritz Schlunegger and Fabian Van den Berg (Bern, CH) lead a fieldtrip to the Entlebuch area (Canton Luzern, CH) on subglacial erosion and the formation of inner valley gorges as well as on knickpoint retreat in glacial hanging valley.

Figure 1: Sediment-burden carbon fluxes on hillslopes and in floodplains; representing key mechanisms 1-3 as mentioned in the text.

All presentations highlighted the current overly simplistic consideration of hillslopes and channels in models of the global carbon cycle; hillslopes are often represented as simple engines that release carbon through soil formation and subsequent erosion, and fluvial channels are often viewed as simple pipes that passively transport carbon from the hillslopes to the oceans. During the workshop it became clear that sediment-burden carbon fluxes are altered by various processes that act at different spatial and temporal scales. Sediments eroded at hillslopes may be stored several times under different environmental conditions along the flow path to the oceans. During storage organic carbon can either decompose and/or build up depending on the specific environmental conditions that are themselves dependent on sediment flux and local hydrology. Four key mechanisms determine the coupled sediment/carbon flux: i) dynamic replacement of eroded C due to soil formation, ii) increased mineralization of C due to aggregate breakdown during transport, iii) protection of eroded organic material through burial and iv) organic matter complexation (Figure 1). On short time scales (~50 years) soil erosion seems to have only limited effects on atmospheric CO2-concentrations. On longer time scales (centuries to millennia) the protection through burial will dominate showing the clear need to define the relative importance of each key mechanism for specific time scales and certain environmental conditions. Furthermore, the discussion groups identified major gaps in understanding of the importance of feedback mechanisms between soil erosion and C-fluxes, such as interdependence of soil productivity and ecological diversity. Such feedbacks may irreversibly alter the carbon cycling and have the potential of being much more important than direct fluxes. The participants of the workshop highlighted the need for comparative studies on direct and indirect effects of soil erosion on the carbon cycle. These are needed to evaluate the effects of human-induced soil erosion in areas with different geology, climate (e.g. from humid to arid) and land use history (e.g. Europe vs. North America).

The given presentations provided a very good overview of the recent understanding as well as knowledge gaps in sediment-burden carbon fluxes on Holocene time scales. We only have a fragmented picture of the key mechanisms and the importance of different environments to store and emit C along the sediment/carbon flow path, and an integrated picture is still missing. Such integrated understanding will require knowledge on the coupling between different C-storage compartments (e.g. hillslope-channel-coupling) and on the wider implications of human-induced environmental changes (e.g soil degradation, and changing ground water table) on sediment-burden carbon fluxes during the period of agriculture.

The organisers and participants of the workshop are thankfully acknowledge the financial support offered by the PAGES-office.