Superiority of Pseudomonas fluorescens inoculation over nitrogen fertilization by altering phyllosphere microbiomes to promote Salix matsudana growth

Fertilizing soil or inoculating microbial agents can both promote plant growth, yet the differences between these methods and how they affect plant growth through changes in the phyllosphere microbiome of cuttings from different ages of mother trees have not been comprehensively studied. We applied nitrogen (N) fertilizer, inoculated Pseudomonas fluorescens, and a combination of both to cuttings of Salix matsudana from two different forest ages, measuring the composition of the phyllosphere microbial community, plant growth, and soil nutrient content. Our results showed that inoculation with P. fluorescens had a stronger growth-promoting effect than N fertilization alone, primarily reflected in the improvement of plant functional traits such as increased biomass, accumulation of nutrients in plants and soil, and optimization of root traits. Additionally, inoculation with P. fluorescens significantly impacted the beta diversity of the phyllosphere microbial community, with K-strategy fungi playing a major role in plant N fixation, which was also the key factors in enhancing the growth-promotion effect. Differences were also observed between cuttings from the two forest ages. Cuttings from middle-aged trees showed better growth performance than younger cuttings, a phenomenon further confirmed by differences in the composition of the phyllosphere microbiome. Specifically, inoculation with P. fluorescens affected the relative abundance of the Myriangiales and Capnodiales orders in the phyllosphere fungi of young and middle-aged cuttings, respectively, contributing to the stability of the phyllosphere environment and plant health. These findings deepen our understanding of how microbial inoculation influences plant growth at the phyllosphere level and provide a theoretical basis for environmentally friendly and sustainable measures in the development of Salix plantations at different stages.