Want to make your barren yard lush again? Just add a bit of soil from your local meadow. A new study reveals that the addition of foreign soil—and more importantly, the organisms it contains—can shape which plants will grow in the future. Such “inoculations” could even help bring back fallow farmlands and turn deserts green.
"This is a really cool and remarkable study," says Harsh Bais, a root biologist at the University of Delaware, Newark, who was not involved in the work. "Dirt matters."
Soil isn’t as simple as it seems. It contains microscopic bacteria and fungi, as well as tiny worms called nematodes and other invertebrates. Ecologists have long known that these underground communities build critical partnerships with the plants growing nearby. But many of these partnerships remain a mystery. Small-scale studies in greenhouses have shown that adding the right soil can promote the growth of a particular plant community, and some researchers have even tried soil transplantation—replacing one soil with another—to get certain endangered plants to grow.
Such need is great across the globe, where many once-fertile lands are turning into desert, and a significant amount of agricultural land is lost every year. What’s more, when governments and nonprofit organizations try to bring back grasslands, forests, and other ecosystems destroyed by agriculture and other human uses, they are often disappointed: Restoration can take decades. It sometimes fails altogether.
E. R. Jasper Wubs, an ecologist at the Netherlands Institute of Ecology in Wageningen, hoped to find a better way. But instead of doing wholesale transplantation of soil—which can be expensive—he wanted to see what would happen with a booster shot.
He and colleagues set up a series of 2- to 5-hectare plots on abandoned, degraded farmland in the Netherlands. They removed about 60 centimeters of top soil from part of each plot and spread a 1-centimeter-thick layer of soil in others. The soil was taken from either a heathland—rolling hills dominated by heather and small shrubs—or a grassland. They then added seeds from 30 plant species from a variety of habitats and waited—for 6 years.
When their waiting was up, they compared the seeded areas with and without the added soil layers, looking at which species were thriving and which were not. The source of the added soil greatly influenced what grew where, they report today in Nature Plants. Plots with heathland soil were covered with heather and gorse, whereas plots with grasslands soil were overflowing with a variety of grasses. The added soil made the existing land richer—as the researchers found more nematode worms, more bacteria, and more fungi in those sections of the plots. Those with heathland soil also had a greater diversity of springtails and mites.
Experts are impressed. "This is the first large-scale experiment of its kind," says Xue-Xian Zhang, a microbial ecologist at Massey University in Palmerston North, New Zealand. "It's pretty impressive that they did it on the scale that they did," adds Noah Fierer, a microbial ecologist at the University of Colorado, Boulder. "It's a real-world demonstration that by altering soil, you can alter vegetation."
The impact of the added soil was greatest where top soil had been removed, but it still had a beneficial effect even in places where it was simply layered over the existing soil. That may sidestep the need for costly removal of top soil, says Nicolás Rascovan, a microbial ecologist at Aix-Marseille University in France, who was not involved with the work.
Still, there’s much more to understand about how the process actually works and whether it works in wet as well as dry places like grasslands and heathlands. Fierer wonders whether it's not the microbes but just some extra organic carbon—a nutrient—or other characteristic of the soil that matters. Zhang predicts that it's not just the soils' microbes but also the seeds in the added soil that are important. Rascovan would like to learn more details about what's happening underground. "It's not really clear which mechanisms and processes take place after soil inoculation [that] end up triggering the establishment of a particular plant community," he says.
Already, land managers in the Netherlands are trying soil inoculation to promote restoration at 15 different sites. Given that the European Union seeks to restore 15% of Europe’s degraded land by 2020, Wubs expects more trials. And because inoculation requires much less soil than transplantation, it can be used to restore much larger areas. This approach could yield new ecosystems in a matter of years, not decades, Wubs says. “Natural succession takes much, much longer.”
But what about the home gardener? “The right soil community is key to the success of your plants,” Wubs says. But being successful is in reach, Rascovan adds: "Since no complex techniques are required, anyone could potentially use this approach."