Altered microbial community structure and organic matter composition under elevated CO2 and N fertilization in the duke forest

The dynamics and fate of terrestrial organic matter (OM) under elevated atmospheric CO2 and nitrogen (N) fertilization are important aspects of long-term carbon sequestration. Despite numerous studies, questions still remain as to whether the chemical composition of OM may alter with these environmental changes. In this study, we employed molecular-level methods to investigate the composition and degradation of various OM components in the forest floor (O horizon) and mineral soil (0-15 cm) from the Duke forest free air CO2 enrichment (FACE) experiment. We measured microbial responses to elevated CO2 and N fertilization in the mineral soil using phospholipid fatty acid (PLFA) profiles. Increased fresh carbon inputs into the forest floor under elevated CO2 were observed at the molecular-level by two degradation parameters of plant-derived steroids and cutin-derived compounds. The ratios of fungal to bacterial PLFAs and Gram-negative to Gram-positive bacterial PLFAs decreased in the mineral soil with N fertilization, indicating an altered soil microbial community composition. Moreover, the acid to aldehyde ratios of lignin-derived phenols increased with N fertilization, suggesting enhanced lignin degradation in the mineral soil. 1H nuclear magnetic resonance (NMR) spectra of soil humic substances revealed an enrichment of leaf-derived alkyl structures with both elevated CO2 and N fertilization. We suggest that microbial decomposition of SOM constituents such as lignin and hydrolysable lipids was promoted under both elevated CO2 and N fertilization, which led to the enrichment of plant-derived recalcitrant structures (such as alkyl carbon) in the soil.