Six Years of CO2 Flux Measurements for a Moderately Grazed Mixed-Grass Prairie
2004
Frank, A.B.
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 CO2 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 CO2 fluxes generally coincided with periods of maximum LAI and above-ground green biomass. The average time period for CO2 uptake was 5 May to 3 October. Annual CO2 fluxes ranged from a low of 13 g CO2/m2 in 1998 to a high of 247 g CO2/m2 in 2001, nearly a 20-fold difference, and averaged 108 g CO2/m2. The cumulative annual flux over all 6 years was 646 g CO2/m2 or 176 g CO2-C/m2. These results indicate that the strength of the carbon sink for this moderately grazed prairie site is about 30 g CO2-C/m2/yr, which is quite small, but considering that the site was grazed and still remains a sink for carbon, it is significant.
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