The large area occupied by temperate grassland ecosystems makes it important to determine their strength as a carbon sink. The Bowen ratio/energy balance (BREB) technique was used to determine CO 2 fluxes over a moderately grazed mixed-grass prairie at Mandan, North Dakota, USA, over a 6-year period from 1996 to 2001. Above-ground biomass and leaf area index (LAI) were measured about every 21 days throughout the growing period. Root biomass was determined to 1.1 m depth in mid-July each year. Peak above-ground biomass typically occurred between mid-July to early August and ranged from 782 kg/ha in 1998 to 2173 kg/ha in 1999. Maximum LAI ranged from 0.4 in 1998 to 0.9 in 1999. Root biomass ranged from 11.8 Mg/ha in 1997 to 17.4 Mg/ha in 1996. Maximum daily CO 2 fluxes generally coincided with periods of maximum LAI and above-ground green biomass. The average time period for CO 2 uptake was 5 May to 3 October. Annual CO 2 fluxes ranged from a low of 13 g CO 2/m 2 in 1998 to a high of 247 g CO 2/m 2 in 2001, nearly a 20-fold difference, and averaged 108 g CO 2/m 2. The cumulative annual flux over all 6 years was 646 g CO 2/m 2 or 176 g CO 2-C/m 2. These results indicate that the strength of the carbon sink for this moderately grazed prairie site is about 30 g CO 2-C/m 2/yr, which is quite small, but considering that the site was grazed and still remains a sink for carbon, it is significant.
