Fig. 7: Proposed mechanisms for the priming effect and its linkage to mineral weathering.

a Depth effect: Mechanisms for linkage between mineral weathering and primed soil organic matter (SOM) decomposition, which is dependent on decreasing dissolved organic matter (DOM) molecular size and complexity and increasing DOM bioavailability with depth. Mineral weathering congruency diminishes with depth because of decreasing DOM molecular size and complexity. The flux and bioavailability of DOM jointly result in a decrease of energy availability with depth, which partly explains vertical changes in weathering intensity (SRO: short-range-order minerals). b Time effect: At the temporal scale, weathering congruency generally increases with diminishing DOM molecular size and complexity (i.e., increasing DOM bioavailability). Mineral weathering intensity (fluxes of mineral nutrients) closely controls organic matter decomposition rates, and the priming effect is closely linked to weathering congruency. A likely coupling mechanism is: chemical weathering increases the content of mineral nutrients in porewater for microbial activity, which primes SOM biodegradation. Weathering congruency can exert an important control on the priming effect through influencing the formation of reactive secondary minerals and protecting organo-mineral associations from microbial decomposition (see Fig. 1 for more details). c Two possible process sequences of DOM input to soil, which are summarized from Critical Zone observations.