Labile carbon input determines the direction and magnitude of the priming effect

•Five levels of glucose applied weekly to four different soils for seven weeks.•A positive linear relationship between glucose amounts and priming.•Three soils showed substrate thresholds – minimum C input required to stimulate positive priming.•Priming was not strongly related to soil microbial biomass.•Labile C input increased priming and accelerated soil C cycling.

Labile carbon (C) input to soil can accelerate or slow the decomposition of soil organic matter, a phenomenon called priming. However, priming is difficult to predict, making its relationship with C input elusive. To assess this relationship, we added 13C-glucose at five levels (8 to 1606 μg C g−1 week−1) to the soil from four different ecosystems for seven weeks. We observed a positive linear relationship between C input and priming in all soils: priming increased from negative or no priming at low C input to strong positive priming at high C input. However, the sensitivity of priming to C input varied among soils and between ways of expressing C input, and decreased with elevation. Positive substrate thresholds were detected in three soils (56 to 242 μg C g−1 week−1), suggesting the minimum C input required to trigger positive priming. Carbon input expressed as a fraction of microbial biomass explained 16.5% less variation in priming than did C input expressed as a fraction of dry soil mass, indicating that priming is not strongly related to the size of the soil microbial biomass. We conclude that priming increases with the rate of labile C input, once that rate exceeds a threshold, but the magnitude of increase varies among soils. Further research on mechanisms causing the variation of priming sensitivity to increasing labile C input might help promote a quantitative understanding of how such phenomenon impacts soil C cycling, offering the potential to improve earth system models.