FLUXES OF CO2 WATER-VAPOR, AND ENERGY FROM A PRAIRIE ECOSYSTEM DURINGTHE SEASONAL TRANSITION FROM CARBON SINK TO CARBON SOURCE

In many temperate-zone ecosystems, seasonal changes in environmental a nd biological factors influence the dynamics and magnitude of surface- atmosphere exchange. Research was conducted to measure surface-layer f luxes of CO2, water vapor, and energy in a C-4-dominated tallgrass pra irie during the autumnal transition from carbon sink to carbon source. Data were collected between DOY 220 and 320, 1996 on the Konza Prairi e Research Natural Area near Manhattan, KS, USA. Mass fluxes were meas ured with a tower-based conditional sampling (CS) system, and the surf ace energy balance was measured with Bowen ratio (BR) methods. Soil-su rface CO2 fluxes were measured with a closed-chamber system. Carbon an d energy fluxes decreased over the study period as the canopy senesced . When skies were clear, daily net CO2 exchange (NCE) varied from a ma ximum 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 we re -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. Senes cence also influenced canopy conductance, which caused a seasonal tran sformation in the surface energy balance. Data suggest that any climat ic or management factors that affect the rate and timing of the autumn al sink-source transition can have a strong influence on the carbon an d water balance in the ecosystem. (C) 1998 Elsevier Science B.V.