Nitrogen application influences the effect of bacteria on the belowground allocation of photosynthesized carbon under elevated CO2

Elevated carbon dioxide (eCO2) affects the translocation of photosynthesized carbon (C) from plants to soil, which is predominantly executed by soil microorganisms. However, the response strategy and mediation of the bacterial community on photosynthesized C allocation belowground remains elusive under eCO2 coupled with nitrogen (N) availability. Here, the spring wheat (Triticum aestivum L.) was continuously labeled with 13CO2 under ambient (aCO2, 350 ppm) or elevated (eCO2, 600 ppm) at three N application levels, and the incorporation of 13C into soil organic C was measured to indicate photosynthesized C accrual in soil. High-throughput MiSeq sequencing of 16S rRNA gene amplicons was used to explore soil bacterial response strategy. Compared with aCO2, eCO2 increased the relative abundance of Chloroflexi and Nitrospirae at the lowest N application level, while it increased that of Bacteroidetes at the highest N level. This shift of the response phyla from oligotrophic to copiotrophic species was closely associated with the increase in root biomass and its decomposability under eCO2 coupled with increasing N availability. The keystone taxa in the bacterial network might have strong ability to regulate such response of bacterial phyla to C and N availability. Specifically, the lower decomposability of the root-derived C caused by the intensive N deficiency under eCO2 stimulated oligotrophic bacteria, Saccharibacteria, Chloroflexi and Nitrospirae as keystone phyla, which could denote that the decomposition ability on recalcitrant substrate of bacterial phyla was enhanced before the activation of N transformation-related bacteria. Comparatively, sufficient N application under eCO2 facilitated copiotrophic bacteria as keystones for utilizing labile C derived from wheat roots, and thus the translocation efficiency of photosynthesized C belowground decreased. Our study sheds new light on the functional traits and mechanisms of the bacterial community in regulating C and N stoichiometry under eCO2.