Fine root biomass and morphology
First, due to the higher N availability background of the subtropical compared to warm-temperate site, we assume that N addition would decrease the fine root biomass in the subtropical site but increase the fine root biomass in the warm-temperate site, and the extent of discrepancy may be distinct at the two locations. Second, fine roots are highly sensitive to N availability and we assume that both the pattern and rate of N addition have a significant impact on the fine root biomass both in the subtropical and warm-temperate sites. Finally, according to the theory of optimal resource allocation, increasing the availability of N will change the survival strategy of fine roots. We hypothesize that the addition of N will increase the functional diversity of fine roots, and functional diversity can explain the variation of fine root biomass, especially in the warm-temperate site with low N availability. The remaining soil was gently washed away with clean water, and the fine roots of each dominant species were rinsed again with deionized water. The fine roots were placed in an acrylic tray filled with deionized water to ensure that they were evenly distributed with no overlaps. The fine roots in the tray were scanned using an Epson scanner (Epson America, Inc. California, USA), and the morphological data of the fine roots were obtained using WinRHIZO Pro (2012b, Canada Inc) software, which included the length, volume, diameter, number of root tips, and number of root forks. The scanned fine roots were dried for 48 h at 65 ℃ to obtain the fine root biomass of the dominant species.