Seminar:Ingo Schöning

Soil microbial necromass is suggested to play an essential role in the formation of SOC. However, despite the recognised importance of the contribution of microbial necromass to soil organic carbon (SOC) sequestration, it’s uptake into the soil has not yet been studied with natural microbial biomass originating directly from the respective site. To fill this knowledge gap, we incubated soil samples from A horizons of managed beech forests in the Schwäbische Alb and the Schorfheide-Chorin with 2% (v/v) ¹³C labelled CO₂ for 2 weeks. This led to an uptake of ¹³C-labelled CO₂ by CO₂-fixing microorganisms. The soil samples were then further incubated after the addition of CO2-free synthetic air for 2, 4 and 9 weeks. Finally, the soil organic matter was separated in a light and mineral-associated fraction by density fractionation using sodium polytungstate with a density of 1.6 g cm⁻³. ¹³C analysis was used to determine where the 13C-labelled microbial biomass of the CO2-fixing microorganisms has remained. First results show that after 4 weeks of incubation we found between 10 and 36 % of the labelled microbial biomass in the light fraction and between 64 and 90 % in the mineral associated soil organic matter fraction. We show that significant amounts of microbial biomass from CO₂ fixing microorganisms can be stored in both, the light and the mineral associated soil organic matter fraction. Our results suggest, that the stabilization of microbial biomass in soils depends on their properties. While more microbial biomass might be stored in light fractions in sandy soils, the opposite might be true for clay rich soils.