Elevated atmospheric carbon dioxide concentration stimulates soil microbial activity and impacts water-extractable organic carbon in an agricultural soil

Carbon dioxide (CO2) enrichment and increased nitrogen (N) deposition can change microbial activity and dissolved organic carbon (DOC) turnover, consequently affecting carbon sequestration in soils. However, we do not have much available information on the relationship between soil DOC and microbial activity under CO2 enrichment and N addition in semi-arid agroecosystems. Using free air CO2 enrichment (FACE), soybean and winter wheat were grown in the field under ambient CO2 (350 μmol mol−1) and elevated CO2 (550 μmol mol−1) conditions subjected to two N fertilizer regimes (132 and 306 kg N ha−1 year−1). Rhizosphere soils and bulk soils at three depths, 0–10, 10–20 and 20–40 cm, were collected to determine water extractable organic matter (WEOM) characteristics with fluorescence spectroscopy and parallel factor analyses of excitation/emission matrix, as well as five extracellular enzymes activities. All significant effects were observed in the topsoil (0–10 cm): elevated CO2 decreased water extractable organic carbon concentration of the rhizosphere soils and bulk soils by 8.5 and 10.1 %, respectively. Furthermore, elevated CO2 changed the composition and structure of soil WEOM by increasing the plant- and microbial-derived components in the rhizosphere and solubilizing soil organic matter (SOM). The activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase were stimulated by elevated CO2 in the rhizosphere soils and bulk soils. Our findings suggest that the stimulation of microbial activity elicited by elevated CO2 increased the turnover of labile WEOM and the solubilization of SOM in the topsoils, which could be adverse to the accumulation and stability of soil carbon in the semi-arid agroecosystems in northern China.