The role of dissolved organic matter for C sequestration in soils - 13C tracer experiments to track the contribution of DOM for the physicochemical stabilisation of organic matter in soil

Soils store the major portion of the organic C that is present in terrestrial ecosystems. Part of that C resides for long periods in soil. Particularly old organic C can be found in the mineral subsoils which also contain a large share of the soil C. Besides of input by root exudates and litter, dissolved organic matter leached from organic soil layers and topsoils is an important source for C in subsoils. Here dissolved organic matter sorbs to soil mineral matrix, a process that seems to control accumulation and stabilisation of organic matter. Because of its proposed contribution to stable C, first attempts have been made to implement dissolved organic matter into the RothC model for soil C cycling.The reason for stabilisation of C in the mineral soil is the strong and largely irreversible binding to reactive clay-sized minerals such as Al and Fe hydrous oxides. The extent to which minerals can sorb and stabilise organic C depends on their properties such as surface area, porosity, and surface reactivity. However, the sorption of relatively young dissolved organic matter contrasts the radiocarbon age of subsoil organic matter.Neither the sorption capacity of minerals nor of soils is infinite. This induces competition between dissolved and bound organic matter and probably the consecutive translocation of older carbon down the soil profile. Also, it is possible that effective stabilisation by sorption is limited to certain reactive sites. Organic compounds occupying these sites may grow old while other ones sorbing to a more crowed surface receive much less protection and therefore are degraded.In the proposed collaborative research, we use dissolved organic matter produced from 13C-labelled deciduous and coniferous tree litter harvested in CO2-enrichments experiments run by WSL in sorption experiments with minerals with different properties and soils. The 13C signal in the equilibrium solution of minerals with beforehand absorbed organic matter (without label) and of the soils will inform on the possible exchange processes with dissolved and adsorbed organic C. Incubation of 13C-labelled organic matter adsorbed to minerals and soils with hydrolytic enzymes will allow to estimate the preservative effect of sorption and determination of 13CO2 will show to which extent fresh and old organic matter is decomposed.