Chemical Warfare and Soil Saviors: The Secret Life of Leaves

Editor’s note: This is a guest post by Erin McKenney, an assistant professor of applied ecology at NC State.

As an evolutionary ecologist who specializes in gut microbes, I think about leaves in terms of diet and nutrition. Leaves provide protein in addition to fiber, expanding the feeding strategy of plant-eating animals beyond grass to more specialized niches. And folivores (leaf-eating species) are particularly interesting.

That’s because folivores depend on microbial fermentation to not only digest fiber but to also help break down the chemicals found in leaves that plants produce to defend themselves.

Microbes digest complex carbohydrates (e.g., amylose, amylopectin, cellulose – otherwise known as starches and fiber, respectively) to produce short chain fatty acids (SCFAs). No vertebrate on the planet can digest cellulose on their own, but the microbes in a folivore’s gut produce SCFAs that satisfy up to 90% of an herbivore’s energy needs.

Hidden Plant Defense Systems

But plants don’t want to be eaten. So, many plants produce secondary metabolites (SMs) to defend themselves against herbivores. Some SMs are released into the air when a plant is eaten, attracting predators who can take out the herbivores (e.g., drawing wasps to eat caterpillars). Other SMs directly attack the herbivore from within. Tannins, for example, bind proteins and trace elements, preventing the herbivore from getting the nutrition it needs – and eventually starving the herbivore. Phytoestrogens are another example – they can interfere with an herbivore’s reproductive cycle. In other words, they can make it less likely for the herbivore to reproduce successfully, limiting the threat of future generations of leaf eaters.

But gut microbes have evolved to consume anything the host can’t – including secondary metabolites. They essentially detoxify the diet, maximizing nutritional benefits and preventing a variety of detrimental impacts to the host.

Even the leaves that escape folivores while still attached to a plant ultimately become a meal in the end. Fallen leaves are a primary food source for earthworms, whose specialized gut microbes digest the tough dead leaves into castings (poop) that contribute to topsoil restoration.

The Wonders of Worm Poop

Worm castings are remarkable in several ways. First, castings contain readily available nutrients for plants, like nitrogen, phosphorus and potassium. What’s more, those nutrients are released slowly, promoting sustained plant growth. Second, castings also enhance soil structure by improving aeration and draining. Lastly, castings (like all poop) contain microbes from the worm’s gut, which suppress soil-borne diseases and repel certain pests.

So those fallen leaves you see every autumn are playing critical roles, first as worm food, and then as a key ingredient in keeping soils healthy.