Resistant soil carbon is more vulnerable to priming effect than active soil carbon

The newly added exogenous organic matter may change the decomposition rate of native soil organic matter (SOM) by the priming effect (PE) and further impact the terrestrial carbon (C) balance. Earth system models have not yet considered the distinct responses of soil C pools with different stability and turnover rates to PE. We addressed this knowledge gap by incubating three soils (from the same site but at different stages of SOM decomposition) developed under C3 vegetation with or without the addition of maize straw (C4) and its pyrolysis product-biochar (C4) in a 180 d-incubation at 22 and 32 °C. The three soils were sampled from 15-year old-field, 15-year bare-fallow, and 23-year bare-fallow plus additional laboratory 815-d incubation, representing active (old-field soil) and relatively resistant (two bare-fallow soils, with more stable fraction and less labile fraction) soil C pools, respectively. Soil-derived CO2 was distinguished by a two-component mixing model using a natural 13C isotopic tracing method (C3 vs. C4). The PE was by 30%–239% higher in two bare-fallow soils than in old-field soil regardless of types of new C input (straw and biochar) or temperatures (22 and 32 °C), and the straw-induced PE was by 90%–297% higher than the biochar-induced PE regardless of soils or temperatures. Moreover, the straw-induced PE was not altered with warming in the three soils, but the biochar-induced PE was significantly decreased by warming, especially in old-field soil. These results suggested that the PE varies with soil C pools with different stability. The relatively resistant soil C pool is more vulnerable to PE than the active soil C pool, but the resistant soil C pool can sequestrate more exogenous C and result in higher soil C storage than the active soil C pool, at least in the short term. In addition, more biochar-C can be retained in these soils than straw-C, but this difference would decrease with warming. Overall, this study suggests that future experimental and modeling studies should pay attention to the distinct vulnerability of soil C pools with different stability to PE and warming for accurately predicting soil C dynamics.