Wetlands are the primary hotspots of carbon accumulation in proglacial areas

Glacial retreat is a well-known effect of global warming. Where glaciers retreat, land becomes available for soil formation. The water that is produced by the melting of the glaciers forms a stream system in the newly available land, and together these form the proglacial area, or glacial forefield. Proglacial areas are interesting study areas for a negative feedback loop of global warming: where land becomes available, microbial and plant biomass are formed, taking up CO₂ from the atmosphere. For inland glaciers, dry soils generally cover most of the surface of proglacial areas, with only a very small fraction covered by wetlands.

Using detailed carbon stock data, CO₂ flux measurements, and GIS methods, we assessed the contribution of soils and wetlands to the valley-wide carbon storage in a proglacial valley in the Martellertal, Parco Nazionale dello Stelvio, Italy. We explored the relationship among the CO₂ flux, soil carbon content, and location factors such as slope steepness, rock and vegetation cover, and litter layer thickness. Furthermore, we studied the relationship between the soil age, or time since deglaciation, and carbon stocks and fluxes. Our data shows that wetlands are major carbon storage hotspots: not only was the carbon stock significantly higher at wetland sampling locations, also the CO₂ uptake per surface area was significantly higher than in dry soils. These findings suggest that despite their small spatial coverage, wetlands are key areas to consider when assessing proglacial carbon budgets, both from a carbon storage as well as a carbon flux viewpoint.