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The Birch effect is a pulse in soil C and N mineralization caused by the wetting of dry soils, but the role of the soil moisture increment (Delta SWC) is still poorly understood. We quantified the relationship between Delta SWC and the Birch effect, and its interactions with pre-wetting soil moisture (preSWC) and substrate supply.

Two soils (clay loam and sandy loam) under a Pinus halepensis forest were subjected to rewetting in laboratory treatments combining different Delta SWC and preSWC values, with or without additional substrate (5 mg g(-1) P. halepensis needles). Respiration flush (Delta R), changes in microbial biomass C (MBC) and net N mineralization (NMIN) were measured.

Overall, we found a relationship with the form: Delta R = a Delta SWC + b, where the slope (a) was significant only when pre-wetting water potential was below a threshold value in the range of -100 to -1,200 kPa. However, the threshold alone does not fully describe the role of preSWC in slope variability. Substrate addition modified the Delta SWC sensitivity of Birch effect, enhancing it in the clay loam and suppressing it in the sandy loam.

The intensity of the wetting is a dominant factor regulating Birch effect, and Delta SWC is useful for its quantification.