Carbon dioxide fluxes over a northern, semiarid, mixed-grass prairie

Temperate grassland ecosystems are an important component of the global carbon (C) cycle. The Bowen ratio/energy balance (BREB) technique was used to measure CO2 fluxes over a mixed-grass prairie at Mandan, ND from 24 April to 26 October in 1996–1999. Above-ground biomass and leaf area index (LAI) were measured about every 21 days throughout the season. Root biomass and soil organic C and nitrogen (N) contents to 1.1 m depth were measured in mid-July each year. Peak above-ground biomass typically occurred between mid-July to early-August and ranged from 782 kg ha−1 in 1998 to 1692 kg ha−1 in 1996. Maximum LAI ranged from 0.3 in 1998 to 0.5 in 1999. Root biomass ranged from 12.7 mg ha−1 in 1996 to 16.2 mg ha−1 in 1997. Maximum daily CO2 fluxes generally coincided with periods of maximum above-ground biomass. Total CO2 flux for the 24 April to 26 October period ranged from 181 g CO2 m−2 in 1998 to 474 g CO2 m−2 in 1999 (positive flux is CO2 movement from atmosphere to plants or soil). The 4-year average flux for this period was 345 g CO2 m−2. Fluxes during the 1999–2000 dormant season were estimated from BREB (−349 g CO2 m−2) and soil flux (−265 g CO2 m−2) measurements and when combined with the growing season fluxes, resulted in estimated annual fluxes in 1999 of 125 and 209 g CO2 m−2 for BREB and soil flux dormant season measurements, respectively. These results suggest that the C budget of Northern Great Plains mixed-grass prairie grasslands may be near equilibrium.