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Microbial necromass dominates soil organic matter. Recent research on necromass and soil carbon storage has focused on necromass production and stabilization mechanisms but not on the mechanisms of necromass retention. We present evidence from soil incubations with stable-isotope labeled necromass that abiotic adsorption may be more important than biotic immobilization for short-term necromass retention. We demonstrate that necromass adsorbs not only to mineral surfaces, but may also interact with other necromass. Furthermore, necromass cell chemistry alters necromass-necromass interaction, with more bacterial tracer retained when there is yeast necromass present. These findings suggest that the adsorption and abiotic interaction of microbial necromass and its functional properties, beyond chemical stability, deserve further investigation in the context of soil carbon sequestration. The fate of microbial necromass in mineral soils. Image 1 • Abiotic adsorption of isotopically-labeled necromass exceeds biotic immobilization. • Abiotic processes include mineral-organic sorption and organic-organic interactions. • Necromass chemistry matters: Gram-negative bacteria retained more with yeast. • Abiotic microbial biomass materials have properties important for global soil carbon sequestration. [ABSTRACT FROM AUTHOR]