Amynthas spp. Earthworms Enhance Nitrogen Transfer Via Bacterial Community Shifts in Temperate Forest Soils

Earthworms function as ecosystem engineers by linking litter decomposition, microbial activity, and plant nitrogen (N) uptake. We conducted a 6-week mesocosm experiment using ¹⁵N-labeled Quercus acutissima litter to investigate the impact of Amynthas spp. earthworm effects on N transformation and partitioning of plant–microbe–earthworm intersactions in a temperate forest. The experiment comprised two treatments: (1) control with Q. acutissima seedlings and litter (-EW), and (2) earthworm treatment with seedlings, litter, and earthworms (+ EW). We traced litter-derived N transfer to seedling organs (roots, stems, leaves) and three soils [bulk soil from -EW (S), bulk soil from + EW (Se), and fresh earthworm casts from + EW (Ce)] and analyzed bacterial community composition using 16 S rRNA sequencing. Earthworms significantly enhanced litter-derived N (%) transfer to plant tissues in roots (34%), stems (41%), and leaves (39%), while plant biomass and total tissue N remained unchanged, indicating that earthworms accelerated N source turnover rather than net N uptake. Ce showed enrichment of fast-growing bacterial taxa (Bacteroidota, Bacillota) relative to the oligotroph-dominated S and Se, suggesting enhanced N availability and microbial growth potential in Ce. This was further supported by a strong positive correlation between litter-derived N in soil and community-weighted mean rRNA gene copy number, linking bacterial community composition to litter-derived N transformation and partitioning. Moreover, functional prediction based on 16 S rRNA gene profiles suggested a greater potential for denitrification in Se and nitrification in Ce. These findings suggest that Amynthas spp. earthworms accelerate litter decomposition, enhance litter-derived N transfer into seedlings and soil, and regulate microbial community structure, thereby promoting N turnover in temperate forests during the early stage of litter decomposition.