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

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 NO3-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.