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The size of the soil carbon sink depends on the balance between soil organic matter (SOM) formation and decomposition. Our understanding of how SOM forms and is stabilized, however, is shifting. Traditional theory maintains the formation of SOM is due to chemical complexity: difficult to decompose plant inputs persist in the soil while easily decomposable inputs are respired as CO₂. However, consensus is now building around an alternative thesis, hypothesizing that the plant inputs most easily assimilated by soil decomposers are the ones stabilized as SOM because dead microbial biomass is now considered one of the primary components of stable SOM. As such, the efficiency with which the microbial community uses these plant inputs has direct implications for the amount and rate of SOM formation under both a constant and changing climate. Our study empirically tests and measures the effects of substrate …