Formation of necromass-derived soil organic carbon determined by microbial death pathways

Soil organic matter is the dominant carbon pool in terrestrial ecosystems, and its management is of increasing policy relevance. Soil microbes are the main drivers of soil organic carbon sequestration, especially through accumulation of their necromass. However, since the direct characterization of microbial necromass in soil is challenging, its composition and formation remain unresolved. Here we provide evidence that microbial death pathways (the distinct processes of microbial dying) in soil affect necromass composition and its subsequent fate. Importantly, the composition of derived microbial necromass does not equal that of microbial biomass. From biomass to necromass, distinct chemical transformations lead to increases in cell wall/cytoplasm ratios while nutrient contents and easily degradable compounds are depleted. The exact changes depend on environmental conditions and the relevance of different microbial death pathways, for example, predation, starvation or anthropogenic stresses. This has far-reaching consequences for mechanisms underpinning biogeochemical processes: (1) the quantity and persistence of microbial necromass is governed by microbial death pathways, not only the initial biomass composition; (2) efficient recycling of nutrients within microbial biomass presents a possible pathway of organic carbon sequestration that minimizes nitrogen losses; (3) human-induced disturbances affect the causes of microbial death and consequently necromass composition. Thus, new research focusing on microbial death pathways holds great potential to improve management strategies for soil organic carbon storage. Not only microbial growth but also death drive the soil microbial carbon pump.