Nitrogen addition influences fine root growth and mycorrhizal symbiosis formation in trees with contrasting root morphology

Nitrogen (N) addition influences fine root growth and mycorrhizal symbiosis formation, the two dominant strategies for tree nutrient acquisition. Arbuscular mycorrhizae (AM) and ectomycorrhizae (ECM) are the two dominant types of mycorrhizal fungi that form symbioses with the roots of most trees. To reveal the divergent adaptive mechanisms and nutrient acquisition strategies of AM and ECM trees under N addition, we investigated the growth and nutrient acquisition of three dominant tree species in a subtropical forest with contrasting fine roots and mycorrhizal fungi symbioses. Castanopsis hystrix with thin fine roots forms associations with ECM. Michelia macclurei and Phoebe bournei with relatively thick fine roots form associations with AM. N addition facilitates the growth of AM trees more obvious than that of the EMC tree. Acid phosphatase activity was increased to overcome N addition-induced P deficiency. The increase in acid phosphatase activity in the rhizosphere of the ECM tree was greater than in AM trees. N addition decreased fine root length and mycorrhizal fungi colonization in all three tree species. The responses of fine roots and mycorrhizal fungi to N addition were species-specific: fine roots were more sensitive than mycorrhizal fungi for C. hystrix and P. bournei, while the opposite pattern (sensitivity of mycorrhiza > roots) was observed for M. macclurei. The divergent sensitivity of the two dominant nutrient acquisition strategies among tree species may facilitate ecological niche partitioning and inter-specific coexistence under limited nutrient availability.