Manter Daniel K, Kavanagh Kathleen L, Rose Cathy L
USDA Forest Service, PNW Research Station, 3200 SW Jefferson Way, Corvallis, OR, USA.
Tree Physiol. 2005 Aug;25(8):1015-21. doi: 10.1093/treephys/25.8.1015.
High foliar nitrogen concentration ([N]) is associated with high rates of photosynthesis and thus high tree productivity; however, at excessive [N], tree productivity is reduced. Reports of excessive [N] in the Douglas-fir forests of the Oregon Coast Range prompted this investigation of growth and needle physiological responses to increasing foliar N concentrations in 1-year-old Douglas-fir seedlings. After 1 year of N fertilization, total seedling biomass increased with each successive increase in N fertilizer concentration, except in the highest N fertilization treatment. Of the many physiological responses that were analyzed, only photosynthetic capacity (i.e., Vcmax), respiration rates and leaf specific conductance (KL) differed significantly between N treatments. Photosynthetic capacity showed a curvilinear relationship with foliar [N], reaching an apparent maximum rate when needle N concentrations exceeded about 12 mg g(-1). In vitro measurements of ribulose-1,5-bisphosphate carboxylase (Rubisco) activity suggested that photosynthetic capacity was best related to activated, not total, Rubisco content. Rubisco activation state declined as foliar [N] increased, and based on its significant correlation (r2= 0.63) with foliar Mn:Mg ratios, it may be related to Mn inactivation of Rubisco. Respiration rates increased linearly as foliar N concentration increased (r2= 0.84). The value of K(L) also increased as foliar [N] increased, reaching a maximum when foliar [N] exceeded about 10 mg g(-1). Changes in K(L) were unrelated to changes in leaf area or sapwood area because leaf area to sapwood area ratios remained constant. Cumulative effects of the observed physiological responses to N fertilization were analyzed by modeling annual net CO2 assimilation (Anet) based on treatment specific values of Vcmax, dark respiration (Rdark) and KL. Estimates of Anet were highly correlated with measured total seedling biomass (r2= 0.992), suggesting that long-term, cumulative effects of maximum Rubisco carboxylation, Rdark and KL responses to N fertilization may limit seedling production when foliar N exceeds about 13 mg g(-1) or is reduced to less than about 11 mg g(-1).
高叶氮浓度（[N]）与高光合速率相关，进而与高树木生产力相关；然而，当[N]过高时，树木生产力会降低。俄勒冈海岸山脉花旗松森林中过高[N]的报告促使对1年生花旗松幼苗增加叶氮浓度时的生长和针叶生理反应进行了这项研究。经过1年的氮肥施用，除了最高氮肥处理外，随着氮肥浓度的逐次增加，幼苗总生物量增加。在分析的众多生理反应中，只有光合能力（即Vcmax）、呼吸速率和叶比导率（KL）在不同氮处理之间存在显著差异。光合能力与叶[N]呈曲线关系，当针叶氮浓度超过约12 mg g⁻¹时达到明显的最大速率。1,5-二磷酸核酮糖羧化酶（Rubisco）活性的体外测量表明，光合能力与活化的而非总Rubisco含量最相关。随着叶[N]增加，Rubisco活化状态下降，基于其与叶锰:镁比率的显著相关性（r² = 0.63），它可能与Rubisco的锰失活有关。呼吸速率随着叶氮浓度的增加呈线性增加（r² = 0.84）。KL的值也随着叶[N]的增加而增加，当叶[N]超过约10 mg g⁻¹时达到最大值。KL的变化与叶面积或边材面积的变化无关，因为叶面积与边材面积的比率保持不变。通过基于Vcmax、暗呼吸（Rdark）和KL的处理特定值对年净CO₂同化（Anet）进行建模，分析了观察到的对氮肥生理反应的累积效应。Anet的估计值与测量的幼苗总生物量高度相关（r² = 0.992），这表明当叶氮超过约13 mg g⁻¹或降至低于约11 mg g⁻¹时，Rubisco羧化、Rdark和KL对氮肥的长期累积效应可能会限制幼苗生长。
