Leaving crop residues to rot could be an unexpected boon for climate mitigation

Leaving agricultural fields in an untidy state could go a long way to help mitigate climate change, a new study shows. 

If farmers left crop residues to rot on the ground instead of clearing them away or turning them into compost, they could sequester larger amounts of carbon in the soil—one of the most promising pathways to bringing down global greenhouse gas emissions, the authors on the new Nature Communications study say. 

“Plant residues are oftentimes disregarded as ‘fast food’ for microorganisms that are instantly decomposed,” says Kristina Witzgall, a Ph.D. Candidate at the Technical University of Munich, and lead author on the study. “But our results show that these plant residues are more than that. They are functional components with an important role for soil carbon storage.” 

The findings follow on the heels of research earlier this year, which showed that even though global soils have been touted as one of our planet’s major and most essential carbon stores, they may actually be storing substantially less carbon than we’d always believed. The new research offers one potential route to counteract that trend and lock in more—and it’s all founded on an intriguing relationship between organic matter and fungi in the soil. 

The researchers based their study on the common understanding that all living matter contains carbon—including rotting plant residues. They wondered if this vast and continuously renewing agricultural carbon store could more effectively be captured in the soil. To find out, they took several soil samples from agricultural fields, and then sprinkled some with residues left over from maize farming—specifically in this case, with maize stalks. For three months, the researchers left natural processes to unfold in their samples, and then examined the carbon content of the soils.

First off, they found that compared to samples with no residual matter, the soils containing crop fragments were richer in soil fungi, suggesting that their increase was related to the presence of the leftover organic matter. But what’s the significance of these fungi? Well, not only do they eat leftover material and lock the carbon away partially by consuming it, they also help to sequester carbon in a different way—by essentially enclosing it into the soil.

Because of the way that fungi grow —with long, reaching filaments that wrap around things in their path—they are able to aggregate soil and organic matter together into clumps. When fungi do this in soil that contains crop residues, they incorporate these carbon-rich leftovers into those clumps. As they consume organic matter, the fungi also create a sticky byproduct that helps to further bind these bits of soil and crop residues together. 

The upshot of this process is that by enclosing raw organic matter away safely inside these large chunks of soil, the fungi reduce its exposure to other microorganisms that would ordinarily decompose these residues, and then release their carbon dioxide into the atmosphere. “Within these clumps, the soil carbon is protected against further decomposition,” Witzgall explains. “And [so] these soil clumps are crucial for stabilizing soil carbon for a longer time in the soil.” 

This photo shows the long tendrils of the fungi aggregating bits of soil with the crop residues (the striated material in the background) to form a large, enclosed chunk of material. Courtesy of Carsten W Mueller. 

In fact, the researchers estimate that leaving crops residues to rot on the ground enables carbon to be locked into the soil for four times longer than if these residues were cleared away.

The experiments also revealed that samples containing chunkier soils harbored almost double the carbon concentrations of finer-grained soils. The researchers think that’s because the coarser the soil, the larger those soil clumps will be, and the more carbon they can therefore keep locked away in the form of residual plant matter. What’s more, chunkier soils seem to better-support fungi, because their long tendrils allow them to bridge the larger air gaps between fragments of soil. So it’s an ideal scaffold for their growth—and a win-win for both fungi and the organic carbon they’re so good at packaging up into the soil. 

All this suggests that abandoning crop leftovers on farms with chunky soils is an effortless shortcut to storing more carbon in the ground. And luckily, many farmers already do this: “Leaving plant residues in the field to decompose is not new – farmers have implemented this strategy for a long time,” Witzgall says—explaining that plant residues are often used to boost the health and yields of crops, which is why many farmers opt to leave them out. 

But now there is a new compelling reason to keep up this habit—and to do it more intentionally, she says. “What our research shows is that crop residues are absolutely central to carbon storage, and that we should use them in a much more calculated way in the future.”

Source: Witzgall et. al. “Particulate organic matter as a functional soil component for persistent soil organic carbon.” Nature Communications. 2021. 

Image: Pixabay