Fluxos de CO2, água e energia em pastagens e caatinga no semiárido pernambucano

Abstract

The natural vegetation of Caatinga is relatively well adapted and tolerant to drought and to high temperatures. It is known that forest conversions to pasture and/or crop cultivation are human interventions, which affect energy exchanges, water and carbon between land surface and the atmosphere. However, up to the present moment, there is no study measuring these fluxes in the caatinga and pastures areas in the Brazilian semiarid lands. Thus, this paper aimed at determining the fluxes of energy, H2O and CO2 in areas of caatinga and pastures, as well as at analyzing the seasonal variability patterns of these fluxes. The land activities were carried out in two areas, one of them located on the Buenos Aires farm (7° 59’ 31’’ S and 38° 17’ 59’’ O), and the other located on the Lagoinha farm (07° 56’ 50,4’’ S and 38° 23’ 29’’ O), cultivated with pasture of grass chain (Urochloa mosambicensis), both properties located in the city of Serra Talhada, state of Pernambuco, Brazil. In order to determine the fluxes of energy, H2O and CO2, a methodology of correlation of swirls was adopted, by means of micrometeorological towers, installed in the center of each experimental area. Along with the index of foliar area, we have also measured the aboveground phytomass of both the pasture and the caatinga herbaceous vegetation, as well as water storage in the soil, by means of TDR sensors. On the basis of the results, it has been found that the maximum production of dry pasture mass was in the order of 2,208 kg ha-1 and annual average of 832 kg ha-1. On the other hand, the caatinga dry mass was in the order of 2,559 kg ha-1 and the annual average was 626 kg ha-1. Water storage in the soil (0-40 cm) of pasture was 29% greater than the one of the caatinga. This fact was possibly attributed to the interception of rain by the caatinga canopy. In relation the fluxes of energy, radiation balance (Rb) was used mainly as sensitive flow of heat (H), with 51% in the pasture and 47% in the caatinga. The fraction of Rb used as flow of latent heat (LE) was of 23% in the pasture and 32% in the caatinga. In relation to the evapotranspiration, the caatinga had total values (523 mm) and average values (1,4 mm d-1) greater than the pasture (389 mm and 1,1 mm d-1), possibly due to the greater depth of its radicular system. During the experimental period, the fluxes of daily average CO2 were ˗ 0,91 and ˗ 0,68 μmol m-2 s-1 for the caatinga and the pasture, respectively. Not only during the rainy season but also the dry season, the vegetation acted as atmospheric CO2 sink. The caatinga was more efficient than the pasture, sequestrating in average 14,6 kg of C ha-1 d-1, during the rainy season and 4,3 kg of C ha-1 d-1, in the dry season, while the pasture sequestrated 11,7 kg of C ha-1 d-1, in the rainy season and 2,5 kg of C ha-1 d-1, in the dry season. The caatinga has proved more efficient than the pasture in using soil water (greater ET) and in sequestrating atmospheric CO2.