Changes in Soil Carbon Fractions in a Tropical Savanna Afforested with Acacia mangium

The Venezuelan oil industry has been developing afforestation plans in savannas to create “CO2 compensation forests” to offset emissions from its activities. Acacia mangium is a pilot species used for this purpose. This study aimed to compare the changes in soil C fractions, nutrient stocks and microbial ecophysiological indices between savanna and 10-year-old A. mangium forest. Soil samples were collected from A, Bt1, and Bt2 horizons in both ecosystems. Soil C fractions (i.e., total, soluble, microbial, light, and heavy fractions), nutrient stocks (i.e., exchangeable Ca, Mg, K, and Al, available P, and total N), and microbial ecophysiological indices (i.e., metabolic quotient, C mineralization microbial coefficient, and basal respiration) were analyzed between savanna and 10-year-old A. mangium forest. A. mangium forest had higher total C in the litter and organic matter heavy fraction, greater microbial biomass, total N, and exchangeable aluminum, and lower phosphorus and pH compared to the savanna. Afforestation increased soil microbial biomass, but basal respiration and microbial coefficient were not different. Metabolic coefficient, mineralization coefficient, and metabolic efficiency coefficient were lower in the Acacia forest than in the savanna. After 10 years, afforestation of savannas with A. mangium fulfills the purpose of promoting C sequestration, particularly within more stable fractions (i.e., carbon in heavy fraction) and labile forms of carbon microbial biomass. Furthermore, the N concentration is higher in Acacia forests due to the N-fixing ability of this species. However, acidity and exchangeable Al were higher while P availability was lower. Ecophysiological indices revealed a more efficient use of C by microorganisms and a less stressful condition under afforestation that could be related to a better quality of organic matter with a higher N concentration. More detailed studies on soil quality and other ecosystem indicators must be conducted to unveil the impacts of afforestation.