02449nas a2200181   4500008004100000245013300041210006900174300001000243490000700253520172600260653003401986653002302020653003202043653003202075100001402107700002002121856012602141       1998                        eng d00aFluxes of CO2, water vapor, and energy from a prairie ecosystem during the seasonal transition from carbon sink to carbon source0 aFluxes of CO2 water vapor and energy from a prairie ecosystem du  a1 -140 v893 aIn many temperate-zone ecosystems, seasonal changes in environmental and biological factors influence the dynamics and magnitude of surface-atmosphere exchange. Research was conducted to measure surface-layer fluxes of CO2, water vapor, and energy in a C4-dominated tallgrass prairie during the autumnal transition from carbon sink to carbon source. Data were collected between DOY 220 and 320, 1996 on the Konza Prairie Research Natural Area near Manhattan, KS, USA. Mass fluxes were measured with a tower-based conditional sampling (CS) system, and the surface energy balance was measured with Bowen ratio (BR) methods. Soil-surface CO2 fluxes were measured with a closed-chamber system. Carbon and energy fluxes decreased over the study period as the canopy senesced. When skies were clear, daily net CO2 exchange (NCE) varied from a maximum gain of 17.8 g CO2 m−2 day−1 on DOY 226 to a maximum loss of −10.3 g CO2 m−2 day−1 on DOY 290. Over the 100-day study period, the ecosystem had a net loss of −217 g CO2 m−2, with the change from sink to source occurring on about DOY 255. Soil-surface CO2 fluxes were −0.4 mg CO2 m−2 s−1 at the start of the study but declined to −0.04 mg CO2 m−2 s−1 on DOY 320. The Bowen ratio increased from 0.5 to 4 over the study period. The seasonal trend in NCE was governed by the senescence of the canopy and not abrupt changes in weather. Senescence also influenced canopy conductance, which caused a seasonal transformation in the surface energy balance. Data suggest that any climatic or management factors that affect the rate and timing of the autumnal sink-source transition can have a strong influence, on the carbon and water balance in the ecosystem.10aCarbon sink-source transition10aSeasonal variation10aSurface-atmosphere exchange10aTallgrass prairie ecosystem1 aHam, J.M.1 aKnapp, Alan, K.  uhttp://knz.lternet.edu/content/fluxes-co2-water-vapor-and-energy-prairie-ecosystem-during-seasonal-transition-carbon-sink