Researchers Reveal Molecular Mechanisms That Bind Water and Soil
Water molecules form bridges between mineral and carbohydrates to lock in moisture, study finds.
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.
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.”
By figuring this out, we could potentially engineer soil to have the right chemistry, turning it into long-term sponges that preserve moisture.”
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.
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