Mandan grazing

Journal of Environmental Quality 39:799-809, May June 2010 reports that at the USDA's Northern Great Plains Research Lab near Mandan, North Dakota, grazing treatments were established in 1916 on western wheatgrass and blue grama pastures. In 1932, they seeded crested wheatgrass into plowed native range. In 1959, soil samples were taken and archived for two native pastures and the crested wheatgrass seeding. In 2003, 44 years later, soils were sampled again with a hydraulic probe at multiple locations within each pasture.

All pastures were grazed from May to October with yearling steers: one pasture with .39 animals per hectare, another with 1.1, and the crested wheatgrass seeding was grazed with .4 animals per hectare in spring and early summer, and 2.3 animals per hectare for the remainder of the grazing season.

Over 44 years, the more heavily grazed native pasture's measured soil organic carbon content went from 117 tons C per hectare to 135 tons to a depth of about 60 cm, about .41 tons C per hectare per year. The more lightly grazed native grass pasture gained .39 tons C per hectare per year, and the crested wheat seeding gained .46 tons C per hectare per year. This study recorded substantial soil carbon gains over a generation or two, on all three pastures, under (continuous) grazing.

In addition, the researchers measured the flux of methane and nitrous oxide to and from soils, from 2003 to 2006. Soils in all three pastures were found to be sinks for methane, that is, they absorbed more methane than they released, with the greatest absorption occurring in the growing season (presumably due to the activity of methanotrophic bacteria in the soil). Soils in all three pastures were minor to moderate sources of nitrous oxide, with the crested wheatgrass seeding a moderate emitter because of artificial N being applied.