Year-round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie

An Ameriflux site was established in mid 1996 to study the exchange of CO2 in a native tallgrass prairie of north-central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid-July-early August 1996), moisture-stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid-day net ecosystem CO2 exchange (NEE), under well-watered conditions, reached a maximum magnitude of 1.4 mg CO2 m(-2) s(-1) (flux toward the surface is positive) during peak growth (mid-July 1997), with green leaf area index of 2.8. In contrast, under moisture-stress conditions in the same growth stage in 1996, mid-day NEE was reduced to near-zero. Average night NEE ranged from near-zero, during winter dormancy, to -0.50 mg CO2 m(-2) s(-1), during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y-intercept, so obtained, can be potentially used as an estimate of night-time CO2 emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO2 m(-2) d(-1) (8.4 g C m(-2) d(-1)) in mid-July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE I:on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m(-2) y(-1). After incorporating carbon loss during the prescribed burn, the net annual carbon exchange in this prairie was near-zero in 1998.