Influence of nitrogen addition on the functional diversity and biomass of fine roots in warm-temperate and subtropical forests

•Fine root biomass and functional diversity under canopy and understory N addition was compared.•Fine root biomass and functional diversity are influenced more by the N addition pattern than the rate.•Functional richness of fine roots negatively correlated with fine root biomass in the subtropical forest.•N addition increases the functional richness of fine roots.•Fine root of subtropical and warm-temperate forests respond differentially to N addition.