Dynamic exchange and remobilization processes as controls of soil organic carbon turnover

Dissolved organic matter (DOM) can be a major source of organic carbon (OC) stocks in mineral soils of high-leaching forest ecosystems due to its high affinity towards reactive mineral phases, thus forming mineral-associated organic matter (MAOM). However, there is considerable dispute on the quantitative role of litter-derived DOM in the formation of MAOM in deeper mineral soils. There is also lacking proof, whether DOM is transported through soil via repeated sequences of sorption, microbial processing, and remobilization as is conceptualized by the `cascade model’ of Kaiser and Kalbitz (DOI: 10.1016/j.soilbio.2012.04.002).

We investigated into these processes by a combination of monitoring dissolved organic carbon (DOC) and CO₂ fluxes in a ¹³C field labelling experiment at subsoil observatories, field manipulation experiments, and laboratory studies to disentangle the effects of different sources and microbial turnover on the cycling of organic matter (OM) from the top mineral soil to the subsoil.

From ¹³C monitoring of DOM, CO₂ and OM it appeared that particularly litter-derived OM leached into the soil is quickly decomposed and contributes only little to subsoil OM at a time scale of 2-3 years. The pattern of the ¹³C pulse in DOM and OM through the soil profile indicated a cascade-type transport of the litter-derived OM and that the MAOM formed is quite labile. The use of segmented suction plates showed that there are preferential flow paths to the subsoil that persist for years. Large DOC fluxes along these flow paths likely create hotspots where microbial processing may dominate the formation of MAOM, opposed to regions of low OM fluxes, where rather direct sorption prevails. The cascade model was also clearly supported by experiments investigating OM exchange processes of artificial mineral-organic associations exposed to field conditions. The highly OM-loaded minerals were microbial hotspots and, besides the selective retention of more strongly adsorbable DOM molecules, microbial assimilation appeared to be largely involved in the release of OM back into solution.

In conclusion, repeated sorption, microbial processing and remobilization cycles appear to control the formation of MAOM during migration of OM at long time scales. While these processes partly explain concentration, age, and composition of OM within the soil profile, horizontal variability in DOM fluxes are likely the key for different processes in the formation of MAOM.