Effects of C and N application on Azotobacter and nitrogen cycle in farmland soils of central Guizhou, China

Abstract Carbon sources and nitrogen sources are two important groups of substances in analyses of the effects of exogenous additives on soil microorganisms. Glucose (present in soil) and urea (extensively used in agricultural production) are substances often used to study the specific effects of carbon and nitrogen addition on microorganisms. Azotobacter, a non-symbiotic nitrogen-fixing bacterium, has been characterized in the laboratory and applied to soil in numerous studies. However, soil microorganisms display considerable diversity, and the effects of in exogenous substances stimulation on various microorganisms are uncertain. The potential effects of forced exogenous substance stimulation on Azotobacter are not well understood. Here, the effects of C and N application on Azotobacter growth and nitrogen cycle metabolism in farmland soil in central Guizhou, China, were studied through analysis of four treatment groups: control (CK), glucose treatment (C), urea treatment (N), and glucose + urea treatment (CN). The results showed that the relative abundances of the Azotobacter genus and relevant species were increased in group C, indicating promotion of Azotobacter growth (P < 0.001). The relative abundances of the Azotobacter genus and relevant species in group CN were significantly different from the abundances in group N and CK (P < 0.05). Furthermore, the relative abundances of nif genes (i.e., nifH, nifD, and nifK) and nirD were significantly increased in group C. However, the relative abundances of the aforementioned four nitrogen cycle-related genes did not significantly differ between group CN and groups CK and N. The main source species of the aforementioned four nitrogen cycle-related genes were Azotobacter species. The total nitrogen and alkali-hydrolyzed nitrogen contents in soil did not significantly differ in a comparison between group C and groups N and CN. Piecewise structural equation modeling analysis revealed that bacterial α-diversity, Azotobacter, and nitrogen cycle genes had significant direct effects on the alkali-hydrolyzed nitrogen content in soil and had negligible direct effects on the total nitrogen content in soil. These findings improve the broader understanding of Azotobacter and provide theoretical support for reduced Azotobacter utilization in soil. Graphical Abstract