How soil fauna affects carbon fluxes in forest floors: Insights from size compartmentalized communities

The forest floor (FF) represents the interface between the production of aboveground biomass and the belowground cycling and storing of C. The conditions within the FF, including its structure, faunal community composition, and microbial activity, may influence the pathway, quantity, and stability of organic matter (OM) transferred and stored in the mineral soil beneath. C inputs like litter are either mineralized to CO₂, leached as dissolved organic C (DOC) into deeper soil, transformed into stable soil organic matter (SOM) by microbial communities or transferred into the mineral soil through soil fauna. While the impact of certain macrofauna like earthworms is well-studied, the role of mesofauna remains less understood despite evidence of their contribution to SOM cycling. To address this the DFG research group “Forest Floor” established a field mesocosm experiment across elevation gradients within temperate mixed forests in Germany and Switzerland. Four gradients were set up across different types of parent material including basalt, paragneiss, and limestone, resulting in differing FF types across and within gradients. Within each site, mesocosms were installed under beech- and maple-dominated canopies, respectively. The mesocosms had vertical openings on the sites covered with different mesh sizes (4 mm, 1 mm, and 0.045 mm) to allow horizontal movement in and out of the mesocosm. These mesh sizes create three size compartmentalized soil fauna communities with increasing limitations due to body size. A set of mesocosms with 4 mm mesh size is non-rotated while all other mesocosms are regularly rotated to limit root ingrowth. Site-specific FF was placed into the mesocosm undisturbed, then defaunated and its Ol horizon was replaced with beech or maple litter highly enriched with ¹³C, ¹⁵N, and ²H. We measure CO₂ and ¹³CO₂ 1 ,2 ,4 ,6 ,12, and 16 months after the mesocosm were placed in the field. DOC is collected bi-weekly in suction plates below the non-rotated mesocosms. This will allow us to establish a mass balance of beech and maple litter turnover as affected by different soil fauna communities in contrasting FF types. We expect the FF to accumulate with decreasing mesocosm accessibility resulting in a shift in C fluxes. A lower CO₂ flux due to unfavorable conditions might be counterbalanced by increased DOC production.