Ren Hao, Gao Guoqiang, Ma Yaoyuan, Li Zuwang, Wang Siyuan, Gu Jiacun
Key Laboratory of Sustainable Forest Ecosystem Management-Ministry of Education, School of Forestry, Northeast Forestry University, Harbin 150040, China.
Sichuan Collegiate Engineering Research Center for Chuanxibei RHS Construction, Mianyang Normal University, Mianyang 621000, China.
Tree Physiol. 2023 Aug 11;43(8):1341-1353. doi: 10.1093/treephys/tpad047.
Root nitrogen (N)-uptake rate and uptake preference, and their association with root morphological and chemical traits are important to characterize root N-acquisition strategies of trees. However, how the root N-acquisition strategy varies with tree age, especially for those species that coexist at a common site, remains unknown. In this study, a field isotopic hydroponic method was used to determine the uptake rate and contribution of NH4+, NO3- and glycine, for three coexisting ectomycorrhizal coniferous species [Pinus koraiensis (Korean pine), Picea koraiensis (Korean spruce) and Abies nephrolepis (smelly fir)] at three age classes (young, middle-aged and mature) in a temperate forest. Concurrently, root morphological and chemical traits, as well as mycorrhizal colonization rate were determined. Our results show that the root uptake rate of total N and NH4+ gradually decreased across all three species with increasing tree age. The three species at all age classes preferred NH4+, except for middle-aged Korean spruce and mature smelly fir, which preferred glycine. In contrast, all three species showed the lowest acquisition of NO3-. According to the conceptual framework of 'root economics space', only a 'collaboration' gradient (i.e. dimension of root diameter vs specific root length or area) was identified for each species, in which root N-uptake rate loaded heavily on the side of 'do-it-yourself' (i.e. foraging N more by roots). Young trees of all species tended to exhibit the 'do-it-yourself' strategy for N uptake, and mature trees had an 'outsourcing' strategy (i.e. foraging N by a mycorrhizal partner), whereas middle-aged trees showed a balanced strategy. These findings suggest that shifts of root N-acquisition strategy with tree age in these species are mainly mediated by root traits along the 'collaboration' gradient, which advances our understanding of belowground competition, species coexistence and N cycling in temperate forests.
根系氮（N）吸收速率和吸收偏好，以及它们与根系形态和化学性状的关联，对于表征树木的根系氮获取策略至关重要。然而，根系氮获取策略如何随树龄变化，尤其是对于那些在同一地点共存的物种而言，仍然未知。在本研究中，采用田间同位素水培法，测定了温带森林中三个共存的外生菌根针叶树种[红松（Pinus koraiensis）、红皮云杉（Picea koraiensis）和臭冷杉（Abies nephrolepis）]在三个年龄阶段（幼龄、中年和成熟）对NH4+、NO3-和甘氨酸的吸收速率及贡献。同时，测定了根系形态和化学性状以及菌根侵染率。我们的结果表明，随着树龄增加，所有三个树种的总氮和NH4+根系吸收速率逐渐降低。除了中年红皮云杉和成熟臭冷杉偏好甘氨酸外，所有年龄阶段的这三个树种都偏好NH4+。相比之下，所有三个树种对NO3-的获取量最低。根据“根系经济空间”的概念框架，每个树种仅识别出一个“协作”梯度（即根直径与比根长或比根面积维度），其中根系氮吸收速率在“自己动手”（即更多地通过根系觅食氮）一侧负荷较重。所有树种的幼树在氮吸收方面倾向于表现出“自己动手”策略，而成熟树具有“外包”策略（即通过菌根伙伴觅食氮），而中年树则表现出平衡策略。这些发现表明，这些树种根系氮获取策略随树龄的变化主要由沿“协作”梯度的根系性状介导，这增进了我们对温带森林地下竞争、物种共存和氮循环的理解。
