During early afforestation stages, biotic and abiotic soil characteristics change at different paces. However, the extent that each of these characteristics contribute to plant performance and subsequent herbivory remains unclear. This study aimed to study the effects of biotic and abiotic characteristics of forest soil on Alnus glutinosa performance and their subsequent impact on foliar herbivory during early afforestation. Soils were collected from a series of replicated forests of 10, 15 or 25 years old, planted in agricultural soils. Two experiments were conducted, focusing on the effect of soil microbiome (live vs. sterilized forest soil, and bulk sterilized soil vs. bulk inoculated with forest soil) and forest age on tree performance, root-associated microbial communities, and plant-herbivore interactions. In 10-year-old forest soil, A. glutinosa stems were thinner when grown in sterilized soil than when grown in live soil. In 15- and 25-year-old soil, trees exhibited lower fine root percentages and thicker stems in sterilized than in live soils, suggesting age-dependent responses possibly arising from plant and microbe nutrient competition. Overall root-associated microbial communities showed no significant differences in their composition based on forest ages. Streptomyces sp. and Rokubacteriales were differentially more abundant in the roots of trees growing in 15-year-old soils and their relative abundance was correlated positively with aboveground biomass, suggesting that effects of forest age on tree performance are contingent on the unique microbiome of each forest. The herbivory assay using Mamestra brassicae larvae revealed a positive correlation between leaf nitrogen content and leaf area consumption in the live vs. sterilized soil experiment, but not in the inoculated soil experiment. Trees in 10-year-old forest soils exhibited the highest herbivore performance, suggesting heightened herbivore susceptibility in early afforestation. Our findings underscore that intricate relationships between soil conditions, microbial communities, and plant-herbivore interactions affect tree performance and herbivory during early afforestation. •Alder growth depends on the soil microbiome but there is no effect of soil age.•Root nodule density in alder decreased with increasing age of the forest soil.•The effect of soil sterilization on tree growth depended on the age of the soil.•Streptomyces and Rokubacteriales enhanced tree performance in 15-year-old soils.