Long-Term Drought Persistently Shifts Plant and Soil Microbial Communities but Has Limited Impact on CO₂ Fluxes Under Subsequent Drought

Droughts are increasing with climate change, affecting the functioning of terrestrial ecosystems and limiting their capacity to mitigate rising atmospheric CO₂ levels. However, there is still large uncertainty on the long-term impacts of drought on ecosystem carbon (C) cycling, and how this determines the effect of subsequent droughts. Here, we aimed to quantify how drought legacy affects the response of a heathland ecosystem to a subsequent drought for two life stages of Calluna vulgaris resulting from different mowing regimes. We imposed a subsequent drought in a long-term (20 years) drought field experiment combined with different mowing years. We hypothesised that drought legacy would reduce the impact of a subsequent drought on ecosystem respiration (ER) through shifts in microbial community composition, and we expected a stronger effect of drought legacy on building stage Calluna (mowed in 2013) than on seedlings (mowed in 2020), with knock-on effects for net ecosystem exchange (NEE) and ER. We found that drought legacy persistently shifted soil bacterial and fungal communities, but the subsequent drought had minimal effect. Drought legacy also shifted plant community composition, with the strongest effect of subsequent drought on the building stage of Calluna. Subsequent drought reduced all CO₂ fluxes independent of drought legacy, and this effect was most pronounced in the building stage of Calluna. The observed strong and persistent shifts in soil microbial communities as a result of 20 years of summer drought did not explain ecosystem CO₂ fluxes, which were determined by changes in plant communities. Thus, our findings show a mismatch between aboveground and belowground responses to drought, and highlight that older heathlands are more vulnerable to drought, reducing their CO₂ uptake capacity in the crucial phase of ecosystem C stock accumulation. These findings give insight into the consequences of long-term drought for ecosystem C cycling and its response to future drought.