Seasonal effects of soil microclimate on microbial activities depend on the understory in two forest ecosystems with contrasting water regimes

Forest management is a promising solution to mitigate climate change due to the ability of forests to store large amounts of carbon in the soil and their biomass, however the understory vegetation is often overlooked. While some studies have underlined that understory can be a key component of forest productivity and soil functioning, the seasonal effects of understory on microbial activity, mediated by changes in soil microclimate, are still unclear. In this study, we investigated the effects of understory management on soil functioning every season in two French forests, characterised by contrasting levels of water supply (wet and dry moorlands). In the wet moorland, we found that understory removal increased microbial biomass (+58.55 %), but had no effect on soil temperature and C-related enzyme activities, and varying effects on soil moisture depending on the season. Seasonal fluctuations were mostly influenced by an upwelling event that occurred during winter in this site and resulted in the immersion of the soil for several consecutive weeks. In the dry moorland, understory removal increased significantly temperature (+6.11 %) and microbial biomass (+77.26 %), but decreased C-related enzyme activities (−23.77 %) and soil moisture (−28.5 %) consistently throughout the seasons. The seasonal fluctuations were more important during the growing season in this site, particularly in summer when soil moisture reached its lowest point. Overall, understory removal effects on soil microbial activity were dependent on soil water in both systems, but were consistently more important in the dry moorland. These findings highlight the necessity for improved comprehension of seasonal fluctuations in the mechanisms governing soil functioning under varying understory managements.