Effect of drought legacy and tree species admixing on bacterial growth and soil respiration rates upon drying-rewetting in a young tree plantation

Drought events are expected to increase as a consequence of climate change, which will most likely lead to increased frequency of drying-rewetting events in soils. Further, the establishment of mixed species forests may be an option to mitigate drought stress to plants. Bacterial growth and respiration rates exhibit two kinds of response upon drying-rewetting. The influence of drought legacy and tree species mixing on the microbial responses upon drying-rewetting remains unknown, but could play important role in terrestrial carbon balance in the context of climate change. Using soils from a young experimental plantation in Belgium, we investigated whether mixed planting (oak monoculture, and oak in combination with 1-3 other tree species) under simulated drought (~50 % precipitation reduction for 2 years) influenced soil microbial activity, biomass, community composition (PLFA) and microbial response upon drying-rewetting. Bacterial growth and respiration rates were lower in drought exposed soils, but fungal growth was unaffected. Drought legacy resulted in a higher fungal-to-bacterial growth ratio in the one and two tree species mixtures, but no difference in the three and four tree species mixtures. Microbial biomass was consistently lower in drought-exposed soils compared to ambient controls with no effect of tree species admixing. Upon drying-rewetting, all soils exhibited similar growth and respiration patterns with lower bacterial growth and respiration rates in drought legacy soils. Tree species mixing did not influence bacterial growth rates, but the respiration rates were higher in four tree species mixtures between 18-30 hours after drying-rewetting.
Overall, our findings demonstrate that drought can have lasting effects on microbial biomass, with consequences for microbial function. Results suggest that tree species admixing to oak may alleviate the drought legacy effect on fungal:bacterial ratio, but does not modulate the bacterial growth and respiration rates upon drying-rewetting.