Ciclagem de carbono e caracterização espectral em áreas de caatinga raleada e conservada

Abstract

The aim of this study was to evaluate the effects in the Caatinga biome of the technique of thinning on the dynamics of herbaceous phytomass productivity; fine roots; the deposition, accumulation and rates of decomposition of litter; aggregate stability; respirometry of the soil; stocks of carbon and nitrogen; and characterisation of changes in the spectral behaviour of the canopy. The study was carried out in two stages: the first in watersheds in the town of Iguatu, in the south central region of the State of Ceará, Brazil, and the second in the watershed of the Caxitoré River, in the northern region of the state. The Iguatu experimental area comprised two watersheds, one under thinning for 5 years (CR5) and the other preserved with natural vegetation for 35 years (CS35). The variables to be sampled were: productivity of herbaceous biomass; fine roots; gravimetric moisture; isotope δ13C (‰); aggregate stability; deposition, accumulation and rates of decomposition of litter; and stocks of total organic carbon and total nitrogen in the 0-20, 20-40 and 40-60 cm layers, from April 2013 to March 2014. The data were submitted to the analysis of means test and compared by t-test (p≤ 0.05). The increase in the intensity of herbaceous plants resulting from thinning contributed to an increase in stocks of total organic carbon (249% and 139% in the 20-40 cm and 40-60 cm layers respectively) and of total nitrogen (142% and 137% respectively), in relation to the area under preservation. The 0-10 cm layer of CR5 stored double the amount of fine roots found in CS35. In the topsoil (0-20 cm) of CR5, aggregates with a size 2.15 times greater than those found in CS35 were obtained. The greater conservation of litter on the soil of CR5 is associated with the greater input of lignified woody biomass from thinning, and consequently with the lower rates of decomposition and respirometry, which are evidenced by the low CO2 emissions into the atmosphere. Thinning in a Vertisol of the Caatinga biome has an influence on soil structure and on the stocks of carbon and nitrogen, making possible lower rates of CO2 emission and improving conditions for the infiltration of water. In the second phase of the experiment, SRTM (Shuttle Radar Topography Mission) data were used to evaluate the effect of the illumination geometry on the spectral characterisation of the canopy, as well as images from the OLI/Landsat 8 sensor, representative of the rainy and dry seasons of 2013. The images were converted into physical values (surface reflectance factors), the NDVI was calculated, and with the technique of principal component analysis images PC1 and PC2 were generated. Dispersion for the values of PC1 and PC2 from the different canopies was evaluated in a two-dimensional space. It was found that the reflectance intensity of the incident electromagnetic radiation in canopies of the caatinga biome is not only influenced by seasonality, but also by the illumination geometry arising from the topographical characteristics of the terrain. The effect of shading was predominant during the dry season, especially under low lighting conditions, irrespective of the structure of the plant cover. The NDVI proved to be unsuitable for detecting changes in the spectral behaviour of the Caatinga biome during the rainy season.