南岭东部常绿阔叶林主要树种根际与根叶养分对磷添加的响应

Phosphorus is a key-limiting factor to the productivity of evergreen broadleaved forest, and rhizosphere phosphorus transformation, root and leaf nutrient dynamics are the key processes to understand the phosphorus limitation to tree growth. The response of tree to phosphorus addition varied with species and growth stage is unclear, which limits our understanding of phosphorus regulation on the productivity of evergreen broadleaved forest. We design a paired experiment of phosphorus addition (50 kg/ha/yr) and control with ten replications in a representative evergreen broadleaved forest in Jiunianshan Forest Ecological station located at the eastern of Nanling Mountain. The selected trees including six tree species (Michelia maudiae, Schima superba, Elaeocarpus sylvestris, Machilus thunbergii, Castanopsis faberi and C. fargesii) and their three growth stages (young, middle-aged and mature, represented respectively with 5, 10 and 25 cm diameter at breast height) are used to regularly collect rhizosphere, bulk soils, fresh absorptive, transportive, storagive and dead roots, fresh and litter leaves. Soils are mainly determined various functional phosphorus fractions and the diversity of phosphate-solubilizing microbes. Roots and leaves are mainly measured different functional phosphorus and other elements concentrations, and calculated their resorption efficiencies. The data will be used to reveal how the response pattern of nutrient internal cycles vary with tree species and growth stages to phosphorus addition, to clarify the adaption and feedback mechanisms of trees grown at different stages to phosphorus limitation, and to explore the interaction between the trade-off of leaf versus root traits and tree growth. The anticipated results will help enhance the understanding of subtropical forest phosphorus cycle and the productivity maintenance mechanism in evergreen broad-leaved forest.

磷是常绿阔叶林生产力的关键限制因子之一，根际磷转化及根叶养分动态是理解磷限制树木生长的关键过程。对亚热带不同树种及不同生长发育阶段树木响应磷肥的异同知之较少，限制了理解磷调控常绿阔叶林生产力的机制。在南岭东部九连山森林生态站典型常绿阔叶林布设施磷肥（50kg/ha/yr）与对照两种处理10次重复配对试验，选取6个树种（深山含笑、木荷、杜英、红楠、罗浮栲和丝粟栲）的幼年、中年和成熟体（胸径分别5、10和20cm左右）为研究对象，定期收集根际和非根际土壤，吸收、运输､贮藏根和死根，新鲜和凋落叶等，重点测定土壤不同形态磷含量及解磷微生物多样性，根和叶不同功能态磷及其他元素含量，计算其回收效率；揭示不同树种及不同龄级树木养分内循环对施磷的响应规律，阐明不同生长发育阶段的树木适应和反馈磷限制的潜在机理及根叶属性权衡与树木生长的耦合机制。增强对亚热带常绿阔叶林磷循环的理解及生产力维持机制的认知。

磷是亚热带常绿阔叶林的关键限制性养分，且氮沉降加剧磷限制性，但常绿阔叶林不同树种及不同生长发育阶段树木对氮磷供应变化的响应异同及其机制还存在争议。本项目依托位于南岭东部九连山次生常绿阔叶林氮磷添加长期野外实验平台，开展+P（50 kg P ha-1 a-1）、+N（100 kg N ha-1 a-1）、P+N和对照共4种处理对内生菌根树种（深山含笑、木荷、绒毛润楠等）和外生菌根树种（罗浮栲、丝粟栲和米槠等）根际土壤养分、根叶性状及养分、树木光合效率及生长的影响研究。结果表明，所有树种根际土壤平均铵态氮随+N显著提高（P<0.05），有效磷随+P显著提高，但硝态氮、全氮、全磷和有机碳含量不同处理无显著差异。所有树种细根全氮和全磷平均含量分别因+N和+P处理而显著提高。氮沉降驱动下内生菌根幼树增加叶面积、减少叶宽长，成年树增加叶厚度和比叶质量；而外生菌根树种无论幼树和成年树均倾向于增加比叶质量；磷添加对叶结构性状影响不显著。氮磷添加显著影响内外生菌根幼树叶片全氮、氨基酸和叶绿素含量，施肥和菌根对成年树全氮含量有显著交互作用；氮磷添加驱动下，内生菌根树种倾向于养分积累，增加叶片氮含量，外生菌根树种倾向于生理防御，提高叶片氮基酸含量。+N处理降低内生菌根树种而提高外生菌根树种叶片最大羧化速率和最大电子传递速率；氮磷添加分别提高外生菌根树种光合氮利用效率和光合磷利用效率，显著提高内生菌根树种水分利用效率；除最大羧化速率，其他叶片光合特性指标均为幼树响应比成年树敏感。+N和+P处理显著提高外生菌根幼树的平均相对生长速率，而P+N显著提高内生菌根树种成年树的平均相对生长速率。结构方程模型路径分析，氮磷添加和菌根类型通过介导根际土壤养分和叶片性状影响树木生长，内生菌根树种比外生菌根树种、成年树比幼树具有更强性状可塑性，树木生长对氮磷添加响应滞后于根叶性状。总体来看，常绿阔叶林主要树种对氮磷添加响应模式因菌根类型和生长阶段各异，全球变化背景常绿阔叶林的演变具有不确定性。