关帝山3个典型森林群落优势种的氮素利用策略差异

Differentiation of Nitrogen Use Strategies of Three Typical Forest Community Dominant Species in Guandishan Forest Region

【目的】研究关帝山林区华北落叶松林、白桦林和沙棘林3个典型森林群落优势种的新生枝叶氮含量、地上生物量和氮利用效率对5个土壤施氮梯度的差异化响应,阐明3个森林群落优势种不同的氮利用功能特性和氮利用策略,为氮沉降背景下的森林质量提升和精准经营提供理论依据。【方法】采用野外5梯度土壤氮(0、9、18、27和36 g·m^(−2) a^(−1))添加试验的方法,比较3个森林群落优势种和群落总体的新生枝叶氮含量、地上新增生物量和氮利用效率对土壤不同施氮量的响应;采用线性模型拟合地上新增生物量随新生枝叶氮含量、氮利用效率和土壤氮素供应水平的变化过程;通过构建结构方程模型,分析土壤氮添加影响地上新增生物量的作用路径。【结果】3个群落优势种的新生枝叶氮含量总体表现为华北落叶松<白桦<沙棘,新生叶氮含量显著高于新生枝,两者的差异表现为白桦>华北落叶松>沙棘,生长盛期的枝叶氮含量都高于末期,生长期之间的差异也表现为白桦>华北落叶松>沙棘;新生枝和新生叶在生长盛期和末期的氮含量都有随施氮量增加而增加的趋势,但3个优势种的变化表现出显著不同的趋势,饱和施氮量和饱和氮含量均表现为华北落叶松<白桦<沙棘;3个群落新生枝叶生物量都随施氮量的增大而升高,但递增率拟合结果表现为华北落叶松林>沙棘林>白桦林;群落氮利用效率都随施氮量增大而降低,以华北落叶松林的降幅最大,沙棘林最小,也随优势树种枝叶氮储量的升高而降低,以白桦林降幅最大,沙棘林最小;群落优势种和群落类型对群落新生枝叶氮储量和氮素利用效率直接作用显著,也受施氮量的影响,进而正向驱动群落新增生物量积累。【结论】华北落叶松林、白桦林和沙棘林3个森林群落新增生物量的差异主要取决于群落类型及其优势种的差异,直接受群落优势种的氮利用效率和新生枝叶氮含量的影响,并受土壤供氮水平的调节;三者的氮利用策略分异表现在氮需求、氮利用效率、环境氮敏感性、饱和氮供应水平4个方面;依次表现为“低需求-高效率-高敏感-低饱和”、“较低需求-低效率-较高敏感-较低饱和”和“固氮型-高需求-低效率-低敏感-高饱和”3个氮利用策略类型,可为不同氮沉降情境或林地氮含量水平下的林分优势树种选择、结构调控和林地精准管理提供依据。

【Objective】To study the differential responses of nitrogen contents(NCs)of new branches and leaves(NBL),above-ground biomass increment(AGBI),and nitrogen use efficiency(NUE)of the 3 dominant species in 3 typical forest communities of Larix principis-rupprechtii forest,Betula platyphylla forest and Hippophae rhamnoides forest to different soil nitrogen addition gradients in Guandishan Forest Region.The objectives are:elucidate the different functional characteristics and nitrogen use strategies,and improve the growth of the forests and precision forest management with the increasing global nitrogen deposition.【Method】Five levels of soil nitrogen addition(0,9,18,27,36 g·m^(−2) a^(−1))experimentation were implemented in field to compare the responses of nitrogen contents of new branches and leaves,above-ground biomass increment,and nitrogen use efficiency to the different soil nitrogen supplements,and linear models were used to simulated the changes of above-ground biomass increment,nitrogen contents and nitrogen use efficiency with the increment of the soil nitrogen additions.The effective pathway of soil nitrogen addition to above-ground biomass increment in the 3 forest communities was explored by constructional equation model analysis.【Result】The nitrogen contents of new branches and leaves of the 3 dominant species were totally presented as L.principis-rupprechtii>B.platyphylla>H.rhamnoides.The nitrogen contents of new branches and leaves in July were higher than those in September,and were all increase with nitrogen addition gradient.The differences sequence of the nitrogen contents of new branches and leaves in 3 dominant species between the two growth stages are B.platyphylla>L.principis-rupprechtii>H.rhamnoides.The sequence of the saturated soil nitrogen addition and saturated soil nitrogen contents for the nitrogen contents of new branches and leaves of the 3 forest communities are all L.principis-rupprechtii<B.platyphylla<H.rhamnoides.The new biomass of the 3 forest communities were all increased with increasing soil nitrogen addition,but the response patterns and increasing rates were significantly different as L.principis-rupprechtii>H.rhamnoides>B.platyphylla.The nitrogen use efficiency of the 3 forest communities decreased with increasing soil nitrogen addition,and the decrease sequence was L.principis-rupprechtii>B.platyphylla>H.rhamnoides.And there also decreased with the increase of nitrogen contents in branches and leaves in the 3 forest communities by the decreasing sequence of B.platyphylla>L.principis-rupprechtii>H.rhamnoides.Dominant species and community type determined by community composition directly effected the nitrogen storage in new biomass and community nitrogen use efficiency,also secondly effected by the soil nitrogen additions,and furthermore positively driving the new biomass accumulation in the community.【Conclusion】The new biomass increment of the three forest communities was mainly determined by the community type and its dominant species but directly affected by the nitrogen use efficiency of the dominant species and the nitrogen contents of the new branches and leaves both regulated by the soil nitrogen supply.Differential nitrogen use strategies of the three communities could be indicated by nitrogen requirement,nitrogen use efficiency,nitrogen sensitivity,and saturated nitrogen supply comprehensively.The three nitrogen use strategy types are in turn:“low demand-high efficiency-high sensitivity-low saturation”,“low demand-low efficiency-high sensitivity-low saturation”and“nitrogen fixing-high demand-low efficiency-low sensitivity-high saturation”.There could provide the principles for the dominant tree species selection,composition regulation and stand site cultivation in precise forest management under different nitrogen deposition scenario or soil nitrogen contents.