The importance of meso-scale anoxic spots and events in oxic environments for carbon dynamics and greenhouse gas emissions from terrestrial, riparian and aquatic ecosystems

Local environments such as soils and waterways, but also landscape-scale environments such as agricultural areas, are often classified as oxic based on the dominant conditions in such systems. Still, anoxic conditions do occur within these “oxic” landscapes, their upland and lowland soils, sediments and creeks at a wide range of spatiotemporal scales, and can represent hot spots and hot moments for greenhouse gas emissions but also carbon storage due to their strongly deviating biogeochemical character.

Within the project “Bavarian landscapes under climate change”, located in the crystalline Bavarian Forest, Germany, we characterize and quantify the role of anoxic spots for greenhouse gas emissions and carbon storage at the scale of agricultural landscapes, including managed grassland and cropland soils, riparian soils and creeks including their special features such as beaver dams. Our focus is on mesoscale anoxic events, at the 1-10 m² scale, that are the result of temporary conditions, such as flooding or fine sediment deposition. We place these mesoscale events into the context of the whole agricultural landscape and watershed, and characterize the carbon species, gaseous emissions, carbon stocks, and organic carbon degradation dynamics in the different oxic and anoxic parts of our larger system, both aquatic and terrestrial. First results indicate that the enhanced greenhouse gas emissions, but also carbon storage, can be large enough to be relevant offsets of the larger scale (whole stream, riparian zone) carbon dynamics. Ignoring these spatially or temporally relatively small events or locations in upscaling or modeling can thus lead to important underestimations of carbon storage, as well as to offsets of the greenhouse gas balance of an area.