Time between two partial rewetting events influences the respiration flush and microbial growth after the final rewetting

The effect of rapid rewetting of dry soil to a water content optimal for microbial activity on soil respiration and nutrient cycling has been extensively studied. However, with smaller rainfall events, dry soil may also rewet more slowly with an interval between partial rewetting events. In this experiment, soil unamended or amended with faba bean residue (C/N 9) was incubated at 50% water-holding capacity (WHC) for 2 weeks, then the soil of the rewet treatments was air-dried and incubated dry for 2 weeks. Then, the air-dried soil was exposed to six rewetting treatments: rewet rapidly to 50% WHC once on day 0 (RR50), rewet partially to 25% WHC once on day 0 (PR25), or rewet slowly by two applications of 25% WHC each with the first rewetting on day 0 and the second after 6 h (SR6), 12 h (SR12), 36 h (SR36), or 60 h (SR60). Constantly moist (CM) soil was kept at 50% WHC throughout the experiment. The flush of respiration after the final rewetting followed the order SR6 > RR50 > SR12 > SR36 > PR25, SR60. Microbial biomass C on the first day after the final rewetting decreased in the order RR50 > PR25 > SR6 > SR12 > SR 36 > SR60, CM. We conclude that slow rewetting can increase the flush of respiration compared to rapid rewetting, but only if the second partial rewetting occurs 6 h after the first. The size of the flush decreased with increasing time between the two rewetting events.