Stocking rate impacts on tallgrass prairie landscape carbon fluxes

Carbon sequestration in ecosystems reduces atmospheric CO2, a major forcing factor in global warming. We aimed to determine the ecosystem C balance under heavy and moderate stocking rates or ungrazed (UG) pastures during the first 2 yr of a 3-yr period and to determine if changing to moderate stocking on a previously heavily stocked pasture would result in compensatory ecosystem C retention. During 2003 and 2004, net ecosystem C exchange (NEEc) was measured with eddy correlation towers on pastures that were UG, stocked at 2 acres per steer until the middle of the grazing season [intensive early stocking (IES)], and season-long stocking at the high rate of 2 acres per steer (SLS-H). In 2005, the SLS-H pasture changed to a season-long moderate stocking rate of 4 acres per steer (SLS-H-M). Net ecosystem C exchange was summed over an entire year from the burn date to the burn date the following year minus the C lost from burning and livestock export to estimate C balance. During the growing season, SLS-H reduced ecosystem C retention compared with UG and IES in 2003 and 2004 (+223, +212, and -669 lb acre(-1) for UG, IES and SLS-H). In 2005, when the stocking rate on the SLS-H pasture was changed to SLS-H-M, C retention was greater than under UG and IES (+107, +89, and +205 lb acre(-1) for UG, IES and SLS-H-M). We conclude that following stocking at an abusive rate, there is compensatory C retention resulting from greater nutrient and water availability.