Fine root biomass and morphology in a temperate forest are influenced more by the nitrogen treatment approach than the rate

Abstract

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 NH4-N and NO3-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.