Carbon dioxide fluxes over a grazed prairie and seeded pasture in the Northern Great Plains

Abstract

Temperate grasslands are vast terrestrial ecosystems that may be an important component of the global carbon (C) cycle; however, annual C flux data for these grasslands are limited. The Bowen ratio/energy balance (BREB) technique was used to measure CO 2 fluxes over a grazed mixed-grass prairie and a seeded western wheatgrass [ Pascopyrum smithii (Rybd) Löve] site at Mandan, ND from 24 April to 26 October in 1996, 1997, and 1998. Above-ground biomass and leaf area index (LAI) were measured about every 21 days throughout the season. Root biomass and soil organic C and N content were determined to 110 cm depth in selected increments about mid-July each year. Peak above-ground biomass and LAI coincided with peak fluxes and occurred between mid-July to early August. Biomass averaged 1227 and 1726 kg ha −1 and LAI 0.44 and 0.59, for prairie and western wheatgrass, respectively. Average CO 2 flux for the growing season was 279 g CO 2 m −2 for prairie and 218 g CO 2 m −2 for western wheatgrass (positive flux is CO 2 uptake and negative flux is CO 2 loss to the atmosphere). Using prior measured dormant season CO 2 fluxes from the prairie sites gave annual flux estimates that ranged from −131 to 128 g CO 2 m −2 for western wheatgrass and from −70 to 189 g CO 2 m −2 for the prairie. This wide range in calculated annual fluxes suggests that additional research is required concerning dormant season flux measurements to obtain accurate estimates of annual CO 2 fluxes. These results suggest Northern Great Plains mixed-grass prairie grasslands can either be a sink or a source for atmospheric CO 2 or near equilibrium, depending on the magnitude of the dormant season flux.