Microbial growth dynamics form a poor link between litter quality and mineral-associated organic matter formation

Microbial products are a primary constituent of mineral-associated organic matter (MAOM) - which accounts for the majority of soil carbon (C). As such, recent concepts predict that enhanced microbial growth, carbon use efficiency (CUE), or turnover should promote the transfer of plant inputs into the MAOM pool. Because high-quality plant litters are hypothesized to promote faster or more efficient growth, this mechanism is often invoked to explain the observation of greater MAOM formation in soils receiving fast-decomposing litters. However, the proposed microbial linkage between litter quality and MAOM formation is rarely evaluated. Moreover, there is a need to concurrently evaluate how microbial growth traits influence MAOM decomposition, as such effects could offset or accentuate litter quality effects on total MAOM stocks. To test the importance of microbial growth traits in mediating MAOM formation and decomposition during plant litter decomposition, we incubated leaf litters from 16 different temperate broadleaf tree species with isotopically distinct soil and tracked the fate of C for 185 days. We also quantified the growth traits of microbes during early- and mid-stage litter decomposition using a substrate non-specific approach. In agreement with common hypotheses, we found that rapid leaf litter decomposition promoted faster microbial growth and turnover, and higher CUE. Rapid decomposition also promoted faster and more efficient MAOM formation, but microbial growth traits did not explain this effect suggesting a poor mechanistic link between plant inputs and MAOM formation. We did, however, observe a positive association between microbial growth rates and the decomposition of native MAOM which led to a muted effect of litter quality on total MAOM. Collectively, our results point toward a need for 1) concepts and models of soil organic matter to consider alternative mechanisms linking litter quality to MAOM formation and 2) measurements of MAOM decomposition to accompany measurements of MAOM formation.