There are all kinds of interactions between plants and the soils they grow in, and little by little we’re understanding more about what’s going on. Take root exudate for example. That’s the cocktail of carbohydrates – sugars, amino acids and organic acids – that result from photosynthesis and end up in the plant’s roots and the soils surrounding them. Root exudate is food for microbes, and the plant feeds the microbes in exchange for the nutrients they bring to the plant. It was considered a passive process. But now, researchers have discovered that the plants can actually manipulate the recipe of the carbohydrate cocktail to promote more micro-organism nutrient release and promote their own growth.
We know that plants can increase microbial activity by increasing the volume of root exudate. For example, when a plant is grazed, it produces more exudate, which increases microbial activity. This in turn releases more nutrients and thus the plant promotes its own regrowth.
But other stressors, like drought for example, can reduce the amount of exudate a plant produces. In that case, how does it manage to feed itself? By changing the composition of the cocktail. When researchers tested post-drought exudate from two plants common to managed hay meadows, they found that though there was less produced, it did increase the activity of micro-organisms in the soil.
“This came as a surprise to us,” says Franciska de Vries, professor of Earth Surface Science at the University of Amsterdam. “In this case, the increased activity must be due to changes in the composition of the carbohydrate cocktail.”
The team of researchers working on this project don’t yet know how the cocktail of carbohydrates change or how long the stimulating effect on the microbial community lasts. “We have some ideas about this,” says de Vries. “For instance, we suspect the exudate from plants who suffer from drought contains an increased amount of simple sugars.”
What Can We Do With This?
The team’s work contributes to a better understanding of the carbon cycle in ecosystems and how it is affected by climate change. De Vries says, “The increased microbial activity will also lead to more carbon being released from the soil. Soils are an important carbon sink; but our study shows that droughts might alter this function. Understanding relationships like these will help us better understand the feedback mechanisms in a climate that is predicted to become increasingly hit by drought.”
We probably don’t have the ability to change the cocktail our pasture forages are serving up to their below-ground microbes. But it’s something to consider when we’re going through drought and thinking about how we manage our pastures.
For more on this study, read the full journal article.