Root order-dependent responses of fine root carbon, nitrogen and phosphorus stoichiometric characteristics to long-term nitrogen fertilization in <i>Metasequoia glyptostroboides</i> plantations

Fine root carbon (C), nitrogen (N) and phosphorus (P) stoichiometric characteristics are key indicators of plant nutrient acquisition strategies and environmental adaptation. Yet, their responses to long-term N deposition, especially the hierarchical variations across root orders, remain unclear, hindering a mechanistic understanding of root system plasticity. To assess root-order-specific responses of fine root C, N and P concentrations and stoichiometric ratios to long-term N fertilization, a field experiment was initiated in 2014 in coastal Metasequoia glyptostroboides Hu & W.C. Cheng plantations in Jiangsu Province, eastern China, involving five N fertilization levels (0, 56, 168, 280 and 336 kg ha-1 year-1). The results showed that N fertilization generally increased fine root N concentration, C/P and N/P ratios, and decreased P concentration and C/N ratio across root orders. Except for fine root C concentration, the absolute response ratios of fine root stoichiometric traits to N fertilization exhibited an increasing trend across root orders. The direct effects of N fertilization on the fine root stoichiometric characteristics were obviously higher than the indirect effects whether at the scale of the entire root system, functional module or individual root order. Significant associations between fine root functional traits and stoichiometric characteristics were observed at the scale of the entire root system, whereas such relationships were not evident at the scale of individual root order or functional module. Overall, the fine root stoichiometric characteristics responded more strongly with increasing root order under N fertilization, and interpretations of the drivers of these characteristics should be scale-explicit.