Soil microbial diversity and network complexity promote phosphorus transformation – a case of long-term mixed plantations of <i>Eucalyptus</i> and a nitrogen-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 how mixed plantations of Eucalyptus and N-fixing tree species affect P transformation and microbiota interactions remains unknown. Therefore, we conducted a 17 y field experiment comparing pure Eucalyptus plantations (PPs) and mixed plantations (MPs) with Eucalyptus and an N-fixing tree species to assess their effects on soil P transformation, using data collected from two soil layers (0–10 and 10–20 cm). The results showed that α-diversity indices (ACE, Chao1, and Shannon indices) were significantly higher in MPs than in PPs for both bacteria and fungi. Furthermore, MPs exhibited significantly higher relative abundances of bacterial phyla Proteobacteria (0–10 cm), Verrucomicrobia, and Rokubacteria, as well as fungal phyla Mortierllomycota, Mucoromycota, and Rozellomycota. Conversely, MPs showed lower abundances of the bacterial phyla Chloroflexi, Actinobacteria, and Planctomycetes and the fungal phylum Ascomycota. Gene copy numbers of functional genes were also elevated in MPs, including 16S rRNA, internal transcribed spacer (ITS), N functional genes [nifH (0–10 cm), AOB-amoA, narG, nirS, and nosZ (0–10 cm)], and P functional genes [phoC, phoD (0–10 cm), BPP, and pqqC]. The findings indicate that MPs can enhance soil microbial diversity, network complexity, and the relative abundance of functional genes, which involved N and P transformation, 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.