Carbon dioxide exchange over agricultural landscape using eddy correlation and footprint modelling

Within an agricultural landscape of western Denmark, the carbon dioxide exchange was studied throughout a year (April 1998–March 1999). During the growing season, five eddy correlation systems were operated in parallel over some of the more important crops (winter wheat, winter barley, spring barley, maize and grass). A sixth system was mounted on top of a 48 m mast to enable landscape-wide flux measurements both in summer and winter. The spatial distribution of the different crop types was mapped by use of satellite images (Landsat TM and SPOT). A very large diversity in carbon functioning is observed when comparing the carbon dioxide fluxes from the different fields. In the middle of the growing season, May–June, the daytime CO2 fluxes range from a net emission of 5 g C m−2 per day to a carbon assimilation of 12 g C m−2 per day. Due to differences in canopy development this range is maintained almost until the end of the growing season. It is demonstrated that daily CO2 fluxes can be simulated by a combined photosynthesis and soil respiration model. By this approach, it is concluded that the photosynthetic capacity is nearly equal for all the grain crops (120–140 μmol m−2 s−1) which is moreover 30–40% higher than that of maize and grass. To estimate landscape CO2 fluxes, the measurements from the individual fields are weighted according to their areal contribution. These estimates are found to be in good agreement with the direct measurements conducted from the 48 m mast when using a three-dimensional (3-D) footprint to define the source area. Spatial integration is also used to calculate the annual net ecosystem exchange (NEE) which proves to be −31 g C m−2 per year for the landscape as a whole, i.e. there is a net transport of CO2 from the atmosphere to the biosphere. This value holds, however, only for the specific year and for a measurement level close to the ground. The sensitivity of the annual NEE to temperature variations is discussed. By comparing flux measurements at two levels (48 and 2.5 m), it is demonstrated that the CO2 budget for an agricultural area may be affected by anthropogenic CO2 sources.