In the vast steppes of northern Utah, where shrubs of sagebrush stretch as far as the eye can see in every direction, water is scarce during most of the summer. From the surface, the ground looks cracked and dry. But underground, what little water is present is being handily redistributed through the soil by the roots of sagebrush that have adapted to thrive in this environment. That movement of water, researchers report in a PNAS Early Edition paper, doesn’t just help plants maximize their own water intake, but helps support the soil microbes that plants rely on for nutrients.
25 years ago, scientists discovered that sagebrush are among the plants that have natural hydraulic lift systems. Deep roots, which often extend to wetter soil than is at the surface, absorb water. Then, the water travels upward. But at night, it doesn’t all go straight to the plant’s shoots and leaves. Instead, some of the water is released in shallower soil, where it can later be reabsorbed by smaller roots the next day.
“Plant roots do more than just take up water,” says biologist Zoe Cardon of the Marine Biological Laboratory in Woods Hole, first author of the new paper. “Water actually moves into roots where the soil is wet and then it comes out of the roots in areas where the soil is dry. So the roots are acting as pipes.”
Over the past few decades, scientists had studied how this movement of water occurs and how it benefits a plant’s water intake. But Cardon and her colleagues wondered whether there was more to the story. Did the movement of water also affect the soil microbes living nearby the plant roots?
“In a lot of these arid lands, the productivity of plants is not just limited by water,” Cardon explains. “It’s limited by nitrogen or phosphorus availability as well.” And plants rely on soil microbes to convert these chemicals into forms that they can use.
To test the role of sagebrush’s hydraulic lift in supporting populations of these soil microbes, Cardon’s team set up an experiment in the sagebrush steppes of Utah. In one plot of plants, they provided a small amount of extra water only to the deepest roots, through buried pipes. In another plot, the plants were left to grow as usual, without any extra water. The experiment continued for an entire summer. Then, at the end of the summer, Cardon’s team used labeled molecules of nitrogen to track how efficiently the plant was taking in nutrients.
In the plants that had received deep doses of water throughout the summer, more overall nitrogen accumulated in the newly produced inflorescences—the seed-containing part of the plant. The observation told the researchers that the plants were taking in more nutrients; tracking the labeled nitrogen in soil, too, showed microbes were working harder in the surrounding soil to make nutrients available. Direct measurements of the microbial community haven’t been made yet, but the initial findings are a strong indicator that the movement of water through plants’ hydraulic lift affects the activity, numbers, or perhaps even the types of microbes in surrounding soil.
“What we saw out in nature was the effect of altered microbes,” says Cardon. “Whatever the microbes are doing, more nutrients are ending up in the plant. Now we want to understand better what those microbes are doing.”
Future studies, she says, will aim to understand in more detail how the microbial population is altered at a molecular and genetic level, and whether the same effect is seen in all environments and for all plants with deep roots.
“Understanding how this interaction between plant roots and microbes evolved over the past 400 million years may help us as we try to maintain food and fuel and wood production on the planet,” says Cardon.
By adding deep-rooted plants to fields of crops, for example, scientists may be able to take advantage of the natural hydraulic lift to boost both the water levels and microbial activity in the shallow soil used by the crops. Such an application of the research on plant hydraulic lift could provide an entirely new way of fertilizing plants that lack deep roots and thus access to deep water and nutrients.