Effect of repeated drying-rewetting cycles on microbial biomass carbon in soils with different climatic histories

There are contrasting results regarding the effect of repeated drying-rewetting (DRW) cycles on microbial biomass carbon (MBC), and two fundamentally different mechanisms have been postulated. The first is a microbial stress mechanism which will reduce MBC as stress-sensitive microbes die, and the second is a substrate supply mechanism which will increase MBC through the release of a microbial substrate from non-biomass soil organic carbon (C). However, the balance of these two mechanisms has not been fully examined for various soils with different climatic histories, especially for soils from humid areas. We hypothesized that soils subjected to fewer DRW events would be largely affected by the stress mechanism. Therefore, the effect of repeated DRW cycles on MBC and C dynamics was investigated and compared to that of a moist control treatment for four soils with different DRW histories. The first DRW significantly reduced the MBC for soils with less DRW but not for soils with more DRW. However, when comparing the sizes of MBC after 28 days of four DRW cycles and the moist treatment, the result was not related to the microbial resistance of each DRW. Cumulative respired CO2-C over a 4-day moist period after each DRW was always significantly greater than that in the moist treatment even when the MBC was not reduced by the DRW. The results with no change in MBC suggested that the substrate supply mechanism rather than the stress mechanism would be essential for the effect of repeated DRW cycles on MBC and C dynamics.