Fluxes of carbon dioxide and water vapour over a C4 pasture in southwestern Amazonia (Brazil)

Abstract

In Brazil, pastures for cattle ranching are being established in areas that were previously forested. To investigate some consequences of this change in land use we measured fluxes of CO₂ and water vapour over a typical pasture, dominated by the introduced C₄ grass Brachiaria brizantha. In addition, we compared the CO₂, water vapour fluxes and canopy stomatal conductances observed with those obtained simultaneously over a nearby undisturbed rain forest. Measurements were made near the end of the wet season under conditions of ample soil moisture. Leaf area index of the pasture was 3.9.

The pasture had a lower canopy stomatal conductance than the forest (typically 0.2–0.3 mol m^-2 s^-1 versus 0.4–0.9 mol m^-2 s^-1 at high photon irradiance) and was less responsive to the canopy-to-air vapour pressure difference. As a consequence of these lower canopy stomatal conductances, the pasture used much less water than the forest with average values over the period examined being 153 mol H₂O m^-2 d^-1 and 249 mol H₂O m^-2 d^-1 for pasture and forest respectively (2.74 and 4.48 mm d^-1 respectively). This was also reflected by differing fractions of the absorbed energy being dissipated as evaporation. This proportion was typically 0.56 for the pasture and 0.74 for the forest.

After allowing for soil and plant respiration, average daily photosynthetic rates were 0.67 mol C m^-2 d^-1 for the pasture and 0.57 mol C m^-2 d^-1for the forest (8.0 and 6.8 g C m^-2 d^-1, respectively). Thus, despite an appreciably lower rate of water use the pasture assimilated more carbon on a daily basis. Nevertheless, Brachiaria displayed a somewhat lower rate of photosynthesis than expected for a C₄ grass, perhaps because of a low nutrient status. Indeed, at low and medium photon irradiance the pasture and forest showed remarkably similar photosynthetic performance. There was, however, less tendency for CO₂ assimilation rates of the pasture canopy to saturate at high photon irradiance. The respiratory fluxes from the two ecosystems at night were quite similar, 6–8 µmol m^-2 s^-1.

The ratio of intercellular CO₂ concentration to ambient CO₂ concentration was usually 0.4 to 0.6 for the pasture, a range which is higher than that often reported for C₄ plants but possibly not unusual for tropical grasses in their natural environment.