Microbial necromass ≠ microbial biomass: Microbial death pathways affect soil organic carbon sequestration

The last two decades soil organic matter research developed rapidly, uncovering a central role of soil microorganisms in the sequestration and storage of soil organic carbon (C), especially through accumulation of their necromass. However, despite strong evidence that the so-called soil microbial carbon pump is an important process, the direct characterization of microbial necromass in soil is difficult to achieve, leaving the actual chemical composition and formation of necromass unresolved. To fill this knowledge gap, we compiled evidence from microbiological literature on the processes of microbial dying, here referred to as microbial death pathways (MDPs). We discuss how fungi and bacteria die in soil, regarding the causes of death but also the consequences for chemical composition of microbial necromass. Evidence from existing literature clearly shows that MDPs in soil microorganisms represent relevant processes that affect necromass composition and its subsequent fate. Depending on environmental conditions and the relative significance of different MDPs, cell wall : cytoplasm ratios increase, while nutrient contents and easily degradable compounds are depleted. Thus, microbial necromass does not equal microbial biomass. These insights on microbial necromass are relevant for our understanding of mechanisms underpinning biogeochemical processes: (i) the quantity and persistence of microbial necromass is also governed by MDPs, not only the initial  biomass composition; (ii) efficient recycling of nutrients in microbial biomass during MDPs may minimize nitrogen losses during the process of C sequestration; (iii) human-induced disturbances do not only affect microbial activity, but also necromass quantity and composition. We present evidence for this novel concept of MDP, showing that not only microbial growth, but also death drive the soil microbial carbon pump. Additionally, we show some first data on actual experiments with “real” microbial necromass based on these principles, and discuss possibilities to explore this topic in future research studies.