Promoted phosphorus transformation by increasing soil microbial diversity and network complexity – A case of long-term mixed-species plantations of Eucalyptus with N-fixing tree species

Abstract. Increased nitrogen (N) availability influences soil phosphorus (P) cycling through multiple pathways. Soil microorganisms are essential facilitating a wide range of ecosystem functions. However, the impact of mixed plantations of Eucalyptus and N-fixing tree species affect P transformation and microbiota interactions remains unknown. Therefore, we conducted a 17-year field experiment in pure Eucalyptus plantations (PPs) to assess the effects of soil P transformation in mixed plantations (MPs) of Eucalyptus and N-fixing trees species. The results showed that α-diversity indices for bacteria ACE and Chao1 as well as Shannon indices index for both bacteria and fungi were significantly higher in MPs than in PPs. Significantly higher relative abundances in MPs than in PPs were determined for the bacterial phyla Proteobacteria (0–10 cm soil layer only), Verrucomicrobia, and Rokubacteria and for the fungal phyla Mortierllomycota, Mucoromycota, and Rozellomycota. By contrast, those of the bacterial phyla Chloroflexi, Actinobacteria, and Planctomycetes and fungal phylum Ascomycota were significantly lower in MPs than in PPs. Gene copy numbers for 16S rRNA, internal transcribed spacer (ITS), N functional genes [nifH (0–10 cm soil layer only), AOB-amoA, narG, nirS, and nosZ (0–10 cm soil layer only)] and P functional genes [phoC, phoD (0–10 cm soil layer only), BPP, and pqqC] were also significantly higher in MPs than in PPs. The findings indicated that MPs can enhance soil microbial diversity, network complexity, and the relative abundance of functional genes associated with N and P cycling by optimizing soil nutrient levels and pH, thereby facilitating P transformation. Therefore, MPs of Eucalyptus and N-fixing tree species may represent a promising forest management strategy to improve ecosystem P benefits.