Root C investment to exchange for the nutrient is a critical nutrient acquisition strategy in regulating soil biogeochemical processes and maintaining the stability of forest structure and functions. However, the response patterns of root C cost-nutrient benefit strategies to N deposition in forests with different initial soil N availability are poorly understood. In this study, root exudation rates were measured by in situ collection during the growing season in two coniferous plantations (i.e., Pinus armandii and Picea asperata plantations with relatively lower and higher initial soil N availability, respectively) to assess whether two plantations differed in root C cost under N addition (25 kg N ha⁻¹ a⁻¹). Simultaneously, the differences in enzyme activity and related soil N mineralization rate between the rhizosphere and bulk soils were quantified to assess whether two plantations differed in N acquisition capacities under N addition. The results showed that N addition to some extent mitigated N-limitation by improving soil N availability, reducing root exudation rate per root biomass and rhizosphere effect of N mineralization by 36.43% and 38.13% in the P. armandii plantation, implying that plant nutrient acquisition strategy shifts from “high C cost-N benefit” to “low C cost-N benefit” strategy. In contrast, in the P. asperata plantation with relatively high initial soil N availability, N addition had little effect on root exudate inputs and related N mineralization capacity, i.e., plant root C cost-nutrient acquisition strategy was independent of N addition in the P. asperata plantation. Collectively, the results suggest that the response patterns of root C cost-N acquisition strategies to N addition between different tree species closely depend on soil N availability, which was more sensitive in forests with lower soil N availability. As a result, variable tradeoffs between root C investment and N acquisition provide new insight into the understanding of rhizosphere nutrient cycling for stability and productivity of forest ecosystem under environmental changes.

根C投入换取养分是调节土壤生物地球化学过程和维持森林结构和功能稳定性的关键养分获取策略。然而，人们对根系 C 成本-养分效益策略对具有不同初始土壤 N 可用性的森林中 N 沉积的响应模式知之甚少。在这项研究中，通过在生长季节在两个针叶林（即分别具有相对较低和较高的初始土壤氮可用性的华山松和云杉种植园）的原位采集来测量根系分泌物率，以评估两个种植园的根系是否存在差异。添加氮下的 C 成本（25 kg N ha ^-1 a ^-1）。同时，量化根际和大块土壤之间酶活性和相关土壤氮矿化率的差异，以评估两个人工林在氮添加下的氮获取能力是否不同。结果表明，施氮在一定程度上通过提高土壤氮的有效性、降低每根生物量的根系渗出率和氮矿化的根际效应在华山人工林中减轻了 36.43% 和 38.13% 的氮限制，这意味着植物养分获取战略从“高C成本-N收益”转向“低C成本-N收益”战略。相比之下，在P. asperata在初始土壤 N 有效性相对较高的人工林中，N 添加对根系分泌物输入和相关的 N 矿化能力几乎没有影响，即植物根 C 成本 - 养分获取策略与P. asperata人工林中的 N 添加无关。总的来说，结果表明根C成本-N获取策略对不同树种之间N添加的响应模式密切依赖于土壤N的有效性，这在土壤N有效性较低的森林中更为敏感。因此，根系 C 投资和 N 获取之间的可变权衡为理解根际养分循环对环境变化下森林生态系统稳定性和生产力的理解提供了新的见解。