Freeze-thaw cycles: how not to mix soil.

The repeated cycles of ground freezing and thawing that occur in many places don't do a good job of churning the soil, a new study suggests.

Freeze-thaw cycles and the burrowing of animals are among the many natural phenomena that can mix the upper layers of Earth's soil, says James M. Kaste, a geologist at Dartmouth College in Hanover, N.H. To study the relative effectiveness of these mechanisms, he and his colleagues measured beryllium-7 and other radioactive elements that fall to the ground in precipitation and immediately attach themselves to particles at the surface. By documenting the concentrations of these short-lived elements at various depths, the researchers inferred how quickly surface soil mixes downward.

At forested sites in southeastern Australia, where burrowing insects, worms, and wombats are common, the uppermost 35 centimeters of the soil get thoroughly mixed every 1,200 years. At gopher-ridden grassland sites in Matin County, Calif., the same depth of soil gets churned, on average, every 660 years, the team reports in the March Geology.

In New Hampshire's White Mountains, however, where freeze-thaw cycles occur yearly but relatively acidic conditions make soil-dwelling organisms rare, radioactive tracers didn't infiltrate the soil well. The uppermost 35 em of soil there seems to get mixed up only once every 5,000 years or so, the team calculates.

The sluggishness of the soil turnover in the New Hampshire forests "was a big surprise to us," says Kaste.