There has been plenty of media coverage of the current extremely hot and dry weather. The drought is revealing archaeological features, (see also here), and we can even see the browning of our landscape from space. But this drought is not good news for our ecosystems at all, and one example of that are the recent wildfires in the Peak District. These fires are not just bad news for the plants and animals that live there, but also they make large amounts of carbon that have been sequestered over many years go up in the air as CO2, and this can amplify climate change. Drought also affects our ecosystems more subtly than that, but the long-term consequences might be as severe.
We can all see the effect the drought is having on plants: lawns are turning yellow, corn leaves are rolling, and in extreme cases, trees even lose their leaves. They become inactive, and can even die. This is not just bad news for the plants, it is also bad news for the soil. When plants stop growing, they are not photosynthesing, and when they are not photosynthesising, they are not removing CO2from the atmosphere. A large part of this photosynthesised CO2 goes straight into the soil to fuel the activities of microbes, which carry out important functions such as the decomposition of plant litter and the release of nutrients for plant growth.
This means that drought is not just affecting those tiny but important soil microbes directly (you can read more about this in my post from 5 years ago), but also indirectly through its effects on plant growth. In extreme cases, when drought kills certain plants and promotes the growth of others, drought can cause plant communities to change, and these changes can persist long after the rainfall has returned. Because different plants, through pumping carbon into the soil, can select for different communities of soil bacteria and fungi, this can also permanently change the properties of those soil microbial communities. When this happens, it can have unexpected consequences for our ecosystems.
And this is exactly what we found in our recent paper. When we subjected big pots with different grassland communities to extreme summer drought, we found that the fast-growing grass Dactylis glomerata (cock’s-foot) completely took over once the rainfall had returned. The effect of this strong shift in plant community composition on the soil bacteria was as important as the effect of the drought itself! But rather than focussing on the diversity or specific species of bacteria and fungi, we looked at the interactions within bacterial and fungal communities, which can inform about their stability under disturbances.
We found that bacterial networks had more destabilising properties, and that these destabilising properties were further promoted by drought. Importantly, it was the abundance of D. glomerata that seemed to drive these changes in network topology. We uncovered a novel mechanism through which this fast-growing grass continued to affect bacterial networks long after the drought had ended: because of its increased growth, it increased evapotranspiration (the loss of water from the plants into the atmosphere), and this in turn decreased soil moisture, affecting the bacteria in the soil.
So why is this bad news? Because we also found a link between changes in bacterial community composition and ecosystem functioning. Specifically, these changes in bacterial communities were linked to changes in the production of CO2and N2O – both potent greenhouse gases. But also, these bacterial networks might be destabilised for a long time, which means that they might be more vulnerable to a future drought. Finally, in a follow up experiment, we found that these changes in soil bacterial communities had consequences for future plant community composition, too: drought-induced changes in bacterial communities promoted the future abundance of D. glomerata even further. This could mean that the aboveground and belowground changes we saw as a result of drought leaves this ecosystem permanently changed and more vulnerable to future disturbances.
So next time you see scorched grassland, also spare a thought about what is happening to the microbes belowground – this could provide crucial information about the ability of our ecosystems to cope with climate change.