There is a heat wave in the UK, and at least in the north, where I live, not a single drop of rain has fallen for at least three weeks. I quite like it, especially since last year was basically one long, wet, windy autumn and I was craving for a real summer. But, with temperatures this high, and with this little rainfall, many plants are starting to look a bit poorly. Grass is turning brown, and forbs are hanging their heads. Especially in the north of England, where normally everything is lush and green around this time, this is an unusual sight.
I know this all too well, because I am running a drought experiment – our drought pots have been tortured to the max and we wouldn’t have needed the sturdy roofs, while we had to water our control pots.
So, plants are having a hard time, and I can imagine farmers are becoming worried. Because summer droughts are expected to increase in the UK, and when crops are stressed to their limit, this will lead to yield reductions. Modern agricultural crops have evolved to be adapted to high-resource, low risk environments, and have very different properties than their wild ancestors (read this great paper by García-Palacios et al.) – properties that are not much good for resisting drought conditions.
However, if you think that carnage is going on aboveground, then take a look belowground.
Bacteria, which inhabit the soil by millions and millions and are crucial for breaking down organic matter and releasing nutrients for plant growth, don’t cope very well with drought. Because they have semi-permeable cell walls, they have to accumulate solutes when the soil is drying to avoid dehydration and dying, which is energetically costly. When they can’t maintain their water potential anymore, they die. However, rewetting after a drought poses the real challenge for bacteria – if they can’t adjust quick enough, their cell walls will pop and they die, releasing all their accumulated solutes and causing a massive loss of valuable carbon and nitrogen from the soil (for more on microbial stress responses read this classic paper by Schimel et al.).
Fungi, which perform many of the same functions as bacteria in the soil, are better able to cope with extreme drought and rewetting than bacteria: they have stronger cell walls and are slower growing, which makes them more likely to resist these conditions. Many recent studies, ranging from very mechanistic laboratory experiments to field-scale studies, show that indeed fungi are more resistant to drought. And, importantly, they also retain valuable carbon and nitrogen better under drought.
Now we go back to our crops that are bred for maximum growth and yield. The plant properties that come with these changed growth strategies, such as high litter nitrogen content, actually select for soil microbial communities that are dominated by bacteria rather than by fungi, and are thus less resistant to drought!
It is important to look at the response of soil microbial communities to drought, not just because they govern carbon and nutrient cycling and are thus important for soil quality and for crop production in the long term, but also because they can directly and indirectly feed back to plant performance via a myriad of mechanisms.
So, if we want to increase the resistance of our crops to drought, it is very important to take a look belowground (see my earlier post on how to do this), and foster the tiny creatures that populate the soil.