How organic matter traps water in soil — even in the driest conditions

From lifelong farmers to backyard gardeners, most plant-lovers know that adding organic matter to a field, vegetable plot or flowerpot increases the soil’s moisture.

Now, for the first time, Northwestern University scientists have uncovered the molecular mechanisms that enable organic matter to boost soil’s ability to retain water — even in desert-like conditions.

Carbohydrates — key components of plants and microbes — act like a molecular glue, using water to form sticky bridges between organic molecules and soil minerals, the team found. These bridges lock in moisture that otherwise might evaporate. The discovery sheds light onto how soils stay moist during drought and even how water might have survived for billions of years trapped in otherworldly rocks, including on Mars and in meteorites. 

The study was published on Saturday (Aug. 9) in the journal PNAS Nexus.

“The right amount of minerals and organic matter in soils leads to healthy soils with good moisture,” said Northwestern’s Ludmilla Aristilde, who led the study. “It’s something everyone has experienced, but we haven’t fully understood the physics and chemistry of how that works. By figuring this out, we could potentially engineer soil to have the right chemistry, turning it into long-term sponges that preserve moisture.”

An expert in the dynamics of organics in environmental processes, Aristilde is an associate professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering and is a member of the Center for Synthetic Biology, International Institute for Nanotechnology and Paula M. Trienens Institute for Sustainability and Energy. Recent Ph.D. graduate Sabrina Kelch and postdoctoral researcher Benjamin Barrios-Cerda — both from Aristilde’s laboratory — are the paper’s first and second authors, respectively.

Water-trapping bridges

To conduct the study, Aristilde’s team mixed a common clay mineral (smectite) found in soils with three types of carbohydrates: glucose, amylose and amylopectin. While glucose is a simple carbohydrate or sugar, amylose and amylopectin are complex polymers in starch, made from linking glucose units together. Amylose is a long, linear chain of glucose; amylopectin also is a long chain but has tree-like branches.

“We decided to use carbohydrates as a type of organic matter because it exists everywhere,” Aristilde said. “Cellulose, which is the most abundant biopolymer on Earth, is made of glucose, and plants and microbes secrete different, simple to complex carbohydrates into soil. We also selected carbohydrates because they have simple chemistry to avoid complicating our results with certain side reactions.”

Using a combination of molecular dynamics simulations, quantum mechanics and laboratory experiments, Aristilde and her team examined the nanoscale interactions among clay minerals, water molecules and the three types of carbohydrates compounds. The scientists found that hydrogen bonds provided a key mechanism that enables clays and carbohydrates to hold onto water. 

A weak, attractive force, hydrogen bonds make water molecules “stick” together to form a droplet or flow through a faucet. Aristilde’s team discovered water also forms hydrogen bonds with the surface of clay minerals and carbohydrates at the same time, creating bridges of water between the two entities. These bridges lock in water more tightly, making it less likely to be lost through evaporation.

“When a water molecule is retained via a hydrogen bond with a carbohydrate and a hydrogen bond with the surface of a mineral, this water has a strong binding energy and is stuck between the two things it’s interacting with,” Aristilde said.

Complex sugar quintuples bond strengths 

Using molecular simulations, the researchers found that water molecules lodged between the clay mineral surface and the carbohydrates had stronger binding energy compared to water bound to clay alone. In fact, complex sugar polymers helped clay bind water up to five times more tightly than clay without an associated carbohydrate. Even in extremely dry conditions, water bound to clay and carbohydrates was far less likely to evaporate and more likely to remain trapped within the nanopores of the clay.

“We increased the temperature to measure water loss in both the presence and absence of carbohydrates,” Aristilde said. “Compared to the clay by itself, it required higher temperatures for water to leave the matrix with the presence of the clay and carbohydrates together. This means the water was retained more strongly in the presence of the carbohydrates.” 

The branched and long-chain carbohydrates also prevented the clay’s pores from completely collapsing in dry conditions. Typically, as clay dries out, its nanoscale pores shrink with increasing loss of water from the pores. But the complex carbohydrates can prevent full collapse of the clay nanopore. This may help preserve the retention of moisture associated with the trapped organics in the pores for long periods of time, including during droughts.

Not only will this new information help us understand soil on our own planet, it also could provide new insights about neighbors in our solar system and beyond.

“Even though our goal was to understand how soil on Earth holds on to its moisture, the mechanisms we uncovered here may have implications in understanding phenomenon beyond our planet,” Aristilde said. “There is a lot of interest in how this relationship between organics and water might play out on other planets — especially those that are considered to have once harbored life.”

The study, “Mechanisms of water retention at carbohydrate-clay interfaces,” was supported by supported by the U.S. Department of Energy (DE-SC0021172) and Northwestern’s International Institute for Nanotechnology.