Monitoring Carbon, Heat, and Water Vapor turbulent fluxes over an Agricultural Field in Western Amazon

We present results from a micrometeorological point of view of the land-use change in Santarém, BR, located in the Western Amazon (3.0120S, 54.5370W). Since the 1970's the Amazonian deforestation resulted from forest to pasture conversion. However, in recent years, there has been an increase of the area of rice and soybean plantation in the Santarém region. In the Large scale Biosphere-Atmosphere experiment in Amazonia (LBA) there is a continuing effort to understand the consequences of this land use conversion by exploring a continuous data set. This study presents the results of turbulent flux measurements of carbon, heat, and moisture using an eddy covariance flux observation system and other micrometeorogical instruments located in an agricultural site in the Eastern Amazon. To estimate nighttime fluxes, a budget method was used due to the lack of mechanic turbulence in this site. During the last 6 years this field has been transformed from a pasture to a rice and soybean plantation. We will show the changes in turbulent fluxes (CO2, H20, and heat), radiative parameters (albedo and PAR-albedo), and other parameters due to changing landscape from a pasture to crop fields.

For instance, greenness of the vegetation, and differences in land cover type are clearly detected from changes in the PAR-albedo. Seasonal changes in these parameters follow those of the daytime evaporation and carbon uptake. These changes in the albedo do not only indicate changes the net radiation regime, but they also mirror changes in energy partition and CO2 fluxes. Lowest values of the Bowen ratio (b) were observed during wet periods, principally during rice plantation. During this period, b values were lower than the equilibrium value, probably due to the fact that the rice roots were taking more ground water than do grass roots. There is net uptake of carbon in this field and it is bigger than adjacent forest. Carbon exchange depends on the agricultural practices (figure 1). For instance, after plowing and tilling there is a small efflux of CO2, rice crops have the highest carbon uptake. Therefore, the decision to have one or both crops or none, rice and soybean, and its fallow in one year will impact on the annual carbon uptake.

Figure 1: The lines represent the cumulative carbon storage measured from the eddy covariance system for rice and soybeans crops in several years (in color). The circles represent the biomass results that were taken just before the harvest. The error bars are the standard deviation of the biomass measurements.