The soil carbon dioxide (CO2) efflux corresponds to the principal flow of carbon from the biosphere to the atmosphere. Tropical soils, which usually show high moisture and temperature, offer very favorable conditions for the production of CO2 resulting from, the decomposition of organic matter, root respiration and microbial respiration, increasing soil CO2 emission to the atmosphere. Soil management practices influence the controlling factors of the soil carbon stock and CO2 emission. A greater understanding of this influence has stimulated studies on all ecosystems, which, among other things, seek to identify and/or develop practices that increase carbon storage and reduce the emission of soil CO2, as is the case of agroforestry. Due to the large global scientific interest in understanding the physical and biological processes involving CO2 fluxes between the surface and the atmosphere, the aim of this research was to evaluate the effects of the transition from secondary forest to oil palm-based agroforestry systems on the soil CO2 efflux and soil biotic and abiotic factors. The experiment was conducted in the municipality of Tomé Açu, northeast Pará, in a project which has been investigating models of palm oil production systems in agrosilvicultural arrangements in the context of smallholder agriculture. The soil CO2 efflux was measured with a portable photosynthesis measurement system (LI-6400) coupled to a soil respiration chamber. We also measured soil gravimetric moisture and temperature, soil respiration (SR), microbial biomass carbon (MBC), soil organic carbon, fine roots stock and litter stock in two AFS and a remaining secondary forest. Measurements of CO2 efflux and soil and plant sampling were taken between January and April 2015 and were carried out fortnightly. The soil CO2 efflux was mainly influenced by soil temperature and moisture in both agroforestry systems. The forest-AFS transition significantly affected the soil CO2 efflux, fine roots, MBC and SR. The transition from secondary forest to AFS with oil palm and the spatial variability of the species or arrangement of the systems affected the soil quality. These changes may be linked to impacts of changes in the structure and species composition of the systems and the management of soil organic matter.
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