Mixture enhances microbial network complexity of soil carbon, nitrogen and phosphorus cycling in Eucalyptus plantations

Mixed-species plantations have the potential to promote soil nutrient cycling, productivity, and ecosystem function. However, the microbial mechanisms underlying the mixing effect in different stand types remain poorly understood. We studied the effect of mixtures with either one of the three N2-fixing or one of the three non-N2-fixing tree species on Eucalyptus growth. Furthermore, we employed metagenomics to investigate co-occurrence networks of soil microbial community, and carbon (C), nitrogen (N), and phosphorus (P)-transformation gene in the Eucalyptus rhizosphere and bulk soil of the monospecific or mixed Eucalyptus plantations. Mixing with either N2-fixing or non–N2-fixing species can promote the network complexity rather than diversity of soil microbial community. Mixture always significantly increased Eucalyptus growth and the network complexity and stability of C, N, and P-transformation genes, but decreased their gene abundance, suggesting a trade-off between functionality and abundance stimulated by mixed-species afforestation. The positive mixing effects on Eucalyptus growth were related with the interactions between C, N, and P cycling, depending on N2-fixing functional traits. Mixtures of N2-fixing trees intensively promoted correlations between Eucalyptus growth and N and P functional genes, while non-N2-fixing trees extensively stimulated the significant correlations with both C, N, and P genes and soil total P. Our findings highlight the importance of rapid responses of soil functional microbes for nutrient cycling, for favoring productivity in mixed-species forest plantations.