Faster soil organic carbon turnover in MAOM versus POM: straw input causes larger microbial driven soil organic carbon decomposition but higher straw accumulation in MAOM

Straw-related carbon (C) dynamics are central for soil organic C (SOC) accrual in soils. However, the underlying microbial groups driving straw decomposition and accumulation in particulate organic matter (POM) and mineral-associated organic matter (MAOM) remain elusive. This study effectively isolated POM and MAOM by using ultrasonic energy (kept below 80 J mL−1) and size-density fractionation that minimally impacts microbial activity and community. We further conducted an 87-day incubation to examine the transformation of added C4 straw and the involved bacterial mechanisms in POM and MAOM. Here, we showed that: i) SOC turnover was faster in MAOM compared to POM, as MAOM stabilized more straw C, likely through strong organic-mineral interactions, while exhibiting significantly higher SOC mineralization than POM over the incubation period; and ii) MAOM, versus POM, exhibited difference of bacterial community and metabolisms during incubation. For instance, microorganisms within MAOM were enriched with genes involved in i) decomposing easily utilized C sources (e.g., sugars, pectin) and ii) the pathways of microbial biomass synthesis. This led to faster SOC turnover via larger native SOC decomposition (possibly through co-metabolism mechanisms) and higher new SOC formation (possibly through biomass-necromass accumulation). Conversely, POM enriched with K-strategists and genes encoding enzymes decomposing recalcitrant C sources (e.g., cellulose, hemicellulose, lignin), possibly via nitrogen mining as nutrients were exhausted in the later stage. This study firstly reveals the bacterial drivers involved in straw-C transformation within POM and MAOM by proper separating approach and highlights the different bacterial community and their metabolisms underpinning added straw decomposition and consequent C accrual in POM and MAOM.