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.首先，相较于暖温带样地，亚热带样地的本底氮有效性（N availability）更高，因此我们提出如下假设：氮添加（N addition）会降低亚热带样地的细根生物量（fine root biomass），却会提升暖温带样地的细根生物量，且两地的差异程度可能存在显著区别。其次，细根对氮有效性高度敏感，因此我们假设氮添加的模式与速率均会对亚热带及暖温带样地的细根生物量产生显著影响。最后，依据最优资源分配理论（optimal resource allocation theory），氮有效性的提升会改变细根的生存策略。我们提出假说：氮添加会提升细根的功能多样性（functional diversity），而功能多样性可解释细根生物量的变异，这一效应在氮有效性较低的暖温带样地中尤为显著。 随后，我们用清水轻柔冲洗掉残留土壤，再用去离子水（deionized water）将各优势种的细根再次冲洗干净。将细根置于盛有去离子水的亚克力托盘内，确保其均匀分布且无重叠。使用爱普生扫描仪（Epson scanner, Epson America, Inc. California, USA）扫描托盘中的细根，并通过WinRHIZO Pro（2012b，加拿大公司）软件获取细根的形态学数据，包括根长、根体积、根直径、根尖数量及根分叉数量。将扫描后的细根置于65℃下烘干48小时，以测得各优势种的细根生物量。