Going deeper underground – unravelling microbial activity and carbon cycling in deep soils in the Central Amazon

Tropical forest soils are important carbon stocks, despite often being highly weathered and depleted in mineral nutrients. In these soils, microbial communities play a crucial role in carbon, nitrogen and phosphorus cycling, and contribute largely to the soils’ nutrient pools. However, as tropical primary forests are remote, and deep soil layers are difficult to access, little is still known about the role of microbial activity affecting carbon cycling beyond the more frequently studied top layers.We conducted a carbon and nutrient stock inventory in soils down to two meters at the experimental site of the AmazonFACE program, located in Central Amazonia, near Manaus, Brazil. Additionally, we investigated microbial community composition with phospholipid fatty acids (PLFA) and measured microbial respiration and potential extracellular enzyme activity rates in short-term incubations. We found that the Amazon FACE site harbors 180.0 (±6.50) Mg C ha-1 in the upper two meters of soil, with almost 60% already being stored in the first 50 cm. Below the organic upper 5 cm of soil, C:N ratios remained constant at around 14.5, however, δ13C signatures of soil organic carbon increased, indicating more often turned-over carbon at deeper layers. We found a faster decrease in fungal than bacterial PLFA markers with depth, and no 16:1w5 markers (representing arbuscular mycorrhizal fungi) below 20 cm of soil. In contrast, our results showed less strong declines in microbial respiration rates.Overall, our data shows that the upper 50 cm of soil have a crucial function in forest carbon storage and turnover, likely related to plant nutrient inputs by roots, facilitating higher microbial activity, making these upper layers more prone to environmental changes. As in deeper soil layers fine root biomass and microbial activity are relatively low, these layers can play an important role in forest resilience. However, normalized by microbial biomass, carbon mineralization is still high in deeper layers, suggesting that they are not static and could be sensitive to climate change.