How sweet it is

Using sugar to help restore vanishing Puget prairie

Think about prairies, and the vast grasslands of Kansas or the Dakotas come to mind, but Washington is a prairie state, too, if a minor one. The Puget prairies of Western Washington are home to a wide variety of native wildflower species, which thrive in the dry, nutrient-poor soils left by the retreating glacier that carved Puget Sound.

Much of the historic area of prairie, open and perfect for strip malls and housing developments, has been destroyed, and what survives—only 2–3 percent of the original range—was subsequently invaded by Scotch broom.

That scourge of the Northwest, deceivingly pretty with yellow flowers in June and covering everything from pasture land to highway median strips, has been successfully controlled in many remaining prairie areas, but Scotch broom, it seems, deposits a little calling card. The broom roots associate with nitrogen-fixing bacteria, which leave the soil more fertile, even after the broom has been extracted. Higher nitrogen levels allow nonnative species, particularly invasive grasses, to crowd out native plants, a big problem for people working to restore the prairies. So how can excess nitrogen be removed from the soil, giving native flora a chance?

Puget Sound Associate Professor of Biology Betsy Kirkpatrick had two ideas. One was biomass removal. Plants absorb nitrogen when they grow, so it follows that removing a plant also removes nitrogen, as long as the plant is taken from the site. Otherwise it would eventually decompose and return the nitrogen to the soil.

The second involved what might be called a sweet relationship. Soil bacteria (and all living things) need carbon, but the bacteria are usually limited by the amount of carbon in the ground. Adding a carbon source, such as sugar, increases soil bacterial populations, causing them to use more nitrogen and make less available to plants.

In a greenhouse, Kirkpatrick tested the two methods. Both reduced soil nitrogen by about 50 percent, but sugar had an additional benefit: The native prairie grass she tested was much more successful competing with invasive grasses when sugar was added to the soil.

“I think this is because a soluble carbon source like sugar induces osmotic stress [a kind of physiological drought], and native prairie plants are much more drought tolerant than the invasive grasses,” she says.

Using sugar as an anti-fertilizer sounds expensive, but Kirkpatrick says sugar makes sense: “Herbicides are costly, too, both in dollars and environmental effects, and they only treat the symptom, not the cause.”

Whether sugar can both reduce nitrogen and give the native species an additional advantage in the field is what she and students working under UPS summer research stipends are testing this summer. You can read more about the experiments and see if the researchers celebrate the sweet taste of success at www.pugetsound.edu/academics/departments-and-programs/undergraduate/biology/research/faculty-research/betsy-kirkpatrick.