Soil nutrients and season drive fine root traits, soil microbial community and their interaction in a Pinus koraiensis seed orchard

Fine root biomass, morphology, and soil microbial biomass were examined in a Pinus koraiensis orchard under four fertilization levels (0, 0.75, 1.25, 2.0 kg compound fertilizer per tree representing control, low-, moderate- and high-level) across three seasons. Fertilization increased soil available nitrogen, phosphorus, and potassium contents, enhanced soil cation exchange capacity, and reduced soil pH from 5.56 to 4.80. Accompanied by the significant shifts in fine root traits and soil microbial community. Fertilization increased specific root length (SRL: 57–77 %) and specific root area (SRA: 30–39 %) across growing season. High-level fertilization reduced soil total microbial (17–27 %) and bacterial PLFAs (21–22 %) in July and September. High- and moderate-level fertilization decreased soil fungal PLFAs (20–26 %) in July. Moderate-level fertilization increased the fungi to bacteria ratio (F:B) by 34 % (May), 16 % (July), and 17 % (September). Fine root biomass negatively correlated with F:B, soil total microbial, bacterial, and fungal biomass. F:B was negatively related to fine root diameter, positively associated with SRL and SRA. Taking the response of fine root traits, soil microbial assembly, soil properties to fertilization together, moderate-level fertilization (1.25 kg per tree) may represent a threshold rate, optimizing nutrient availability while maintaining microbial balance. Seasonal variations further revealed that fine roots shifted from acquisitive strategies in spring and summer to conservative strategies in autumn, impacting soil microbial community composition. Our findings highlight the interconnected responses of fine roots and soil microorganisms to fertilization and seasonal dynamics, providing insights for sustainable forest management.