Effects of elevated CO2 on soil microbial communities in a tropical understory forest in the Central Amazon

Soil microbial communities are central to understanding interactions between soil and climate change by controlling  major carbon and nutrient fluxes, such as organic matter formation and decomposition. In addition, soil microbial communities respond sensitively to climate change and  can control the magnitude and direction of potential soil feedback to climate. Tropical forests have a crucial role as carbon sink to reduce elevated atmospheric CO2 (eCO2), but they rely on soil microbial communities to access nutrients from organic matter. However, especially in tropical areas, the effects of increasing atmospheric CO2 on soil microbial community composition are still not fully understood. Studies, primarily conducted in temperate regions, demonstrate that eCO2 concentrations can cause changes in the structure and activity of soil microbial communities responsible for decomposing organic matter. Additionally, the richness of some microorganism species is relatively sensitive and decrease under eCO2, potentially leading to changes in the abundance of specific microbial taxa. Consequently, this could change nutrient mineralization rates, affecting primary production rates, and subsequently plant organic matter quality and inputs to soil. Here we used an Open-Top Chamber (OTC) experiment to expose the understory vegetation in a tropical lowland forest to eCO2 (+200 ppm above ambient), and tested the impacts of eCO2 on soil microbial (fungal, bacterial, and archaeal) community structure using 16SrRNA amplicon sequencing. Two soil collections were conducted in eight OTCs (four eCO2 and four ambient control OTCs), in September 2019 before the CO2 increase and in September 2021 after two years of CO2 increment. Our results showed a significant decrease in microbial biomass carbon and phosphorus pools in response to eCO2 and a strong tendency of decreased DNA concentrations corroborating a potential decrease in soil microbial biomass. In addition, our data will provide insights in the diversity of microorganisms in Amazonian soil, and allow to better understand soil microbial community responses and feedbacks of tropical forests to eCO2 and consequences for soil carbon and nutrient cycling.