On a sunny, sultry summer day, Joe Blastick, a land steward with The Nature Conservancy, scans the hilly pastures north of Clear Lake, South Dakota, and rattles off the names of plants he sees. Silver leaf scurf pea, purple prairie clover, green needle grass, and dozens of others blend into a cacophony of color and texture. Monarch and regal fritillary butterflies sample their nectar; meadowlarks and eastern kingbirds glide through the thick afternoon air.

 As impressive as the visible display is, the real action here is underground. Roots of grasses and other prairie perennials may descend 10 feet into the soil. Powered by the sun, these grasslands have for millennia pulled carbon out of the air and stashed it deep in