Stabilized microbial necromass in soil is more strongly coupled with microbial diversity than the bioavailability of plant inputs

Microbial necromass carbon (C) can substantially contribute to stabilized soil organic matter (SOM), and effective management of this C may help mitigate climate change. However, factors important to the formation of microbial necromass are only partly understood. While bioavailable plant inputs may induce necromass formation by boosting microbial growth and C use efficiency, other microbial traits, such as those related to secretion systems or adhesion and motility, may also be relevant. These traits may be independent of the bioavailability of plant inputs and modulated by environmental factors such as soil depth or site age. Such links, however, have hardly been studied. Here, we used replicated plots of European alder (more bioavailable inputs) and Scots pine (less bioavailable inputs) to investigate links among plant inputs, soil depth, site age, microbial community composition, and microbial necromass C in stabilized SOM, i.e., particulate organic matter occluded within aggregates (oPOM) and mineral-associated organic matter (MAOM). We did not find evidence that bioavailable plant inputs, nor soil depth and site age, were major drivers of microbial necromass formation. Instead, certain microbial taxa, and microbial diversity in particular, were most tightly related to microbial necromass C in MAOM. Microbial necromass C also substantially contributed to oPOM (up to ∼57% of the C stored in that fraction), a C pool considered to largely derive from plant biomolecules. Combined, however, microbial necromass C in oPOM and MAOM only accounted for ∼23% of bulk C contents. Our results imply that effective C-focused research and management have to consider constraints on microbial community composition and diversity, microbial necromass in pools other than MAOM, and formation of plant-derived SOM.