Land-use change effects on local energy, water, and carbon balances in an Amazonian agricultural field

To study how changing agricultural practices in the eastern Amazon affect carbon, heat and water exchanges, a 20 m tower was installed in a field in August 2000. Measurements include turbulent fluxes (momentum, heat, water vapor, and CO2) using the eddy covariance (EC) approach, soil heat flux, wind, and scalar profiles (T, q, and CO2), soil moisture content, terrestrial, total solar radiation, and photosynthetically active radiation (PAR, 400-700 nm). At the beginning of the measurements, in September 2000, the field was a pasture. On November 2001, the pasture was burned, plowed, and planted in upland (nonirrigated) rice. Calm nights were the norm in this site. Anomalously low values of net ecosystem exchange (NEE) were found using the EC method, even when the common criterion u*<0.2 m s(-1) was used to identify and exclude poor performance nights. We observed more plausible values of NEE using criterion u(*)<0.08 m s(-1), indicating that the criterion must be revised downward for flow over surfaces smoother than forests. However, even using the lower threshold, u(*) was lower than this limit for 82% of nights, and this led to nocturnal respiration underestimates. We compensate for this difficulty by estimating the respiration rate using the nocturnal boundary layer budget method. Land-use change from pasture to rice cultivation strongly affected both diurnal rates of turbulent exchange but also the pattern of seasonal variation. Seasonal wet and dry season differences in vegetation state were clearly detected in the albedo and PAR-albedo. These reflectivity changes were accompanied by modified net radiative flux, turbulent heat flux and evaporation rates. The highest evaporation rate was observed during the rice crop, when the field had total evaporation approximately half the precipitation input, less than that of the surrounding forest. Effects of the land-cover changes were also detected in the carbon budget. For the pasture, the maximum CO2 uptake occurred in May, appreciably delayed from the start of the rainy season. After the field was plowed and the soil was exposed and there was efflux of CO2 to the atmosphere day and night for an extended period. Highest values of carbon uptake occurred during the rice plantation. Although the upland rice took up carbon at double the rate of the pasture that it replaced, the field was left fallow for much of the year, during the dry season.