Dinâmica da matéria orgânica do solo em ecossistemas de floresta secundária sobre solos antrópicos e solos não-antrópicos (adjacentes) na amazônia central

Abstract

The Amazonian Dark Earths are characterized by high contents of stable organic matter which are stored in soil for hundreds or thousands of years, even under tropical edaphoclimatic conditions, which favor fast soil carbon (C) mineralization. Understanding the dynamics of this organic matter and the mechanisms of C stabilization are essential for the establishment of agro-ecological systems that foster C sequestration in the soil, as an alternative to mitigate the increase of CO2 in the atmosphere. The objectives of this study were: i) to investigate the dynamics of organic matter in anthropic and adjacent soils; 2) to identify key factors that control the dynamic of organic matter, and 3) to determine the stocks of total-C in soil, as well as the stocks associated to each organic matter physical fraction in the Indian Dark Earth, adjacent soil and transition soil. The study was carried out in five different locations in central Amazonia, where anthropic soils under old secondary forest were located. The experimental design had randomized blocks with five replications and three treatments: Indian Dark Earth (TPI), the transition soil between TPI and adjacent soil (ST), and adjacent soil (SA). Soil and litter samples were evaluated for C stored in soil, in microbial biomass-C and in litter, the proportion of C in each fraction of soil organic matter, the mineralization of C, and the fluxes of soil CO2. The organic matter dynamics was similar in different soils, showing seasonal effects, with faster cycling in the rainy season and slow than in the dry season. Higher amounts of microbial biomass were found in the TPI, which may indicate that these soils are benefiting the microbial population, which become an important storage pool of labile C these systems. In the natural environments of secondary forests, the main factors influencing the dynamics of soil organic matter (SOM) in the three soil types were the season, the temperature, and soil texture. However, under controlled temperature and moisture (in the incubation in laboratorial experiments), the SOM dynamics was slower in the TPI, indicating that other factors may strongly influence the processes of SOM decomposition in these soils. The TPI contains high stocks of humified C, but there was neither no evidence of accumulation of new C in these systems, no evidence that the presence of labile C stimulates the decomposition of stable soil C. The largest proportion of C was found in the fraction associated with the silt and clay, stressing the importance of the physical protection mechanisms in the stabilization of the C in soils. The application of the principles of C stabilization in the antropogenic Dark Earths to develop new dark earths in the tropics is an interesting alternative to sequester carbon in soils thus mitigating part of the CO2 emissions to the atmosphere.