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• 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. Functional diversity strengthens species diversity and ecosystem function relationships; however, in the case of mixed forests, the relationship between fine root biomass and functional diversity is not completely understood. In this study, we compared fine root biomass and functional diversity of vegetation in response to canopy and understory nitrogen (N) addition rates of 0, 25, and 50 kg N ha−1 yr−1 in subtropical and warm-temperate forests. Canopy and understory N addition decreased the fine root biomass of the subtropical but increased the biomass of the warm-temperate forest during the growing season. The biomass and functional diversity of fine roots are more affected by N addition pattern than by rate. The functional richness index of fine roots was negatively correlated with fine root biomass at community level in the subtropical forest during the growing season, and NO 3 -N in the soil had the highest explanatory power for fine root biomass variation. Functional richness was positively correlated with fine root biomass at community level in the warm-temperate forest and had the highest explanatory power. In the warm-temperate forest, the synergistic increase in functional richness and fine root biomass promoted increases in nutrient uptake, whereas in the subtropical forest, either N restricted fine root growth or functional richness compensated for the reduction in fine root biomass. These findings indicate that the fine root biomass and functional diversity of subtropical and warm-temperate forests respond differentially to the addition of N. [ABSTRACT FROM AUTHOR]

• N treatment approach and sampling time affected fine root biomass and morphology. • Canopy N addition increased fine root biomass. • Fine root biomass and morphology were mainly affected in the growing season. • Fine root biomass was most affected by NH 4 -N and NO 3 -N in growing season. Global nitrogen (N) deposition patterns have profoundly affected the production and morphological structure of fine roots, which in turn changed the distribution of carbon in forests. However, traditional experiments of N addition in forests have ignored the ecological processes in the canopy, such as nitrogen retention, and the effects of these on N deposition and fine root functional traits remain unclear. In this study, we quantified the effects of the canopy (CAN) and understory N addition (UAN) on the biomass and morphology of fine roots in a temperate deciduous forest. Based on the three-way ANOVA, we found that the N treatment approach (CAN vs. UAN; p < 0.05) and sampling time (July, October, and January; p < 0.001) significantly affected the fine root biomass, specific root length, specific root surface area, and root tissue density and diameter. Through the summation of fine root biomass, it was found that CAN treatment increased fine root biomass, while UAN treatment decreased fine root biomass, and canopy N addition at 50 kg ha-1yr−1 (CAN50) significantly increased fine root biomass compared with understory N addition at 50 kg ha-1yr−1 (UAN50) in July and October (p < 0.05). Redundancy analysis (RDA) showed that fine root biomass was most affected by NH 4 -N and NO 3 -N in July. There was no consistent response of fine root morphology to the N application method and N addition rate; it was mainly affected by season (p < 0.001). Different results were obtained using different N treatment approaches. The effects of UAN on forest fine root biomass were likely overestimated compared to the effects of CAN. Our experimental results will provide a scientific basis for a more accurate prediction of the impacts of future global N deposition on fine roots in forest ecosystems. [ABSTRACT FROM AUTHOR]