Researchers at Northwestern University have identified how organic matter enables soil to retain water, even in dry conditions. The team, led by Ludmilla Aristilde, found that carbohydrates from plants and microbes act as a kind of molecular glue. These carbohydrates use water to create bridges between organic molecules and soil minerals, which helps lock moisture into the soil.
The study was published on August 9 in PNAS Nexus. According to Aristilde, “The right amount of minerals and organic matter in soils leads to healthy soils with good moisture. 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.”
Aristilde is an associate professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering. She is also affiliated with the Center for Synthetic Biology, International Institute for Nanotechnology, and Paula M. Trienens Institute for Sustainability and Energy. Sabrina Kelch and Benjamin Barrios-Cerda from Aristilde’s laboratory are the first and second authors on the paper.
To investigate further, researchers mixed a common clay mineral called smectite with three types of carbohydrates: glucose (a simple sugar), amylose (a linear chain polymer), and amylopectin (a branched chain polymer). These carbohydrates are widely present in nature; cellulose—the most abundant biopolymer—is made from glucose.
“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 molecular dynamics simulations combined with laboratory experiments, the team observed interactions among clay minerals, water molecules, and carbohydrate compounds at the nanoscale level. They discovered that hydrogen bonds play a key role in binding water more strongly when both clay minerals and carbohydrates are present.
“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 explained.
Molecular simulations showed that these complex sugar polymers enabled clay to bind water up to five times more tightly than clay alone. Even under very dry conditions or higher temperatures intended to simulate evaporation loss, water bound by both clay and carbohydrate was less likely to evaporate.
“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.”
Additionally, branched or long-chain sugars helped prevent pores within clays from collapsing during dry periods—helping maintain some capacity for moisture retention over time.
Aristilde noted potential implications beyond Earth: “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. 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.”
This research received support from the U.S. Department of Energy (DE-SC0021172) as well as Northwestern University’s International Institute for Nanotechnology.