Fungal and bacterial growth responses to drought and drying-rewetting cycles

Background/Question/Methods

Drought and drying - rewetting cycles are frequent physiological stressors for soil microbial communities; a stress that is predicted to grow more influential with future climate change. Understanding how the microbial community controls biogeochemical cycles under these dynamic events is instrumental to enable predictive power for C and N cycling in soils. We investigated the effect of repeated drying–rewetting cycles on bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth, on the biomass concentration and composition (PLFA), and on the soil respiration, in laboratory microcosms. Then, we investigated the influence of the drought legacy of soils on the microbial and soil C dynamics induced by a drying-rewetting cycle in soil from a two-year field experiment on summer drought in temperature forest soils. Finally, we extended our analysis of the influence of the legacy of drought on the active microbial community, by assessing the balance between fungal and bacterial growth on an inter-continental set of long term (>8 yrs) drought experiments including soils from across all of Europe (the INCREASE project).

Results/Conclusions

In our laboratory microcosm experiment we found that drying–rewetting decreased bacterial growth while fungal growth remained unaffected, resulting in an elevated fungal:bacterial growth ratio. Many drying–rewetting cycles did not, however, affect the fungal:bacterial growth ratio compared to few cycles. The biomass response of the microbial community differed from the growth response, with fungal and total biomass only being slightly negatively affected by the repeated drying–rewetting. We found that experimental summer droughts in temperate forest soils decreased the C quality (respiration per DOC) of the flush of resources released by the rewetting event, resulting in a slower peak in respiration, and a reduction in the total C mineralized within one week of rewetting. Bacterial growth mirrored the respiration dynamics, but the cumulative growth during one week was not affected by drought, indicating a higher bacterial C-use efficiency in drought-exposed soils. The drought effects were probably due to differences in tree growth (i.e. plant-C input). The cross-continental study on drought effects in Europe suggests a small shift toward fungal dominance, probably driven by C quality reductions. Overall, we see evidence for consistent drought induced effects on the actively growing soil microbial community, that subtly increase the C use efficiency of soil microbial communities.