Effects of soil nitrogen addition on photosynthetic limitations in Fraxinus mandshurica Rupr. and Quercus mongolica Fish. ex Ledeb
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摘要:
本文以试验地常年大气氮（N）沉降量（23 kg·ha−1·year−1）为依据，设计了低度（LN，23 kg·ha−1·year−1）、中度（MN，46 kg·ha−1·year−1）和高度（HN，69 kg·ha−1·year−1）3种氮添加水平以模拟大气氮沉降，以无氮添加处理为对照（CK），探究过度氮沉降对森林阔叶树种水曲柳（Fraxinus mandshurica Rupr.）和蒙古栎（Quercus mongolica Fish. ex Ledeb）的生理生态效应。结果显示：（1）两树种CO2扩散性限制作用（即气孔限制lsc和叶肉限制lm）在氮添加后减弱，而后随氮量的增加先减弱后增强；其生化限制lb则在氮添加后增强，后随氮量增加先增强后减弱；（2）3种光合限制作用均在MN下达到最值， 中度土壤氮添加量对植株光合的促进效应最高；（3）土壤氮添加期间植株光合能力的增强主要源于CO2扩散性限制作用的减弱，而gsc变化（即lsc）为扩散性限制的主角作用因子；（4）3种光合限制作用（lsc、lm和lb）在不同生长期（7月和8月）均未表现出显著差异，表明lsc的光合主角限制“角色”无季节性差异；（5）一定范围内的土壤氮添加不会对植株的水分利用潜力产生显著影响。
Abstract:Based on natural nitrogen deposition in the field (23 kg·ha−1·year−1), this research employed low (LN, 23 kg·ha−1·year−1), moderate (MN, 46 kg·ha−1·year−1), and high (HN, 69 kg·ha−1·year−1) nitrogen levels to simulate natural nitrogen deposition, using no nitrogen addition used as a control (CK). The goal was to explore the physiological and ecological effects of excessive nitrogen deposition on two broad-leaved forest species, i.e., Manchurian ash (Fraxinus mandshurica Rupr.) and Mongolian oak (Quercus mongolica Fish. ex Ledeb). Results showed that 1) CO2 diffusional limitations (i.e., stomatal limitation, lsc, mesophyll limitation, lm) of both species decreased by more than 10% after nitrogen addition, then increased with increasing nitrogen supply, while biochemical limitation (lb) increased by more than 10% after nitrogen addition, then decreased with increasing nitrogen supply. 2) Both lsc and lm reached minimum values of 18.4% and 18.0% (Manchurian ash-August), 21.6% and 19.7% (Mongolian oak-July), and 21.6% and 20.1% (Mongolian oak-August), while lb reached a maximum value of 63.6% (Manchurian ash-August) and 59.7% and 58.3% (Mongolian oak-July and August) under MN treatment, indicating that soil nitrogen addition of 46 kg·ha-1·year-1 had the greatest photosynthesis-promoting effect. 3) The enhancement of plant photosynthetic capacity during soil nitrogen supply predominantly resulted from the weakening of CO2 diffusional limitations, in which stomatal conductance to CO2 (gsc, i.e., lsc) was the primary limiting factor affecting plant photosynthesis. 4) The three photosynthetic limitations (lsc, lm, and lb) did not show any significant differences between July and August, indicating that the primary photosynthetic role of lsc may lack seasonal variation. 5) Soil nitrogen addition within a certain content range did not significantly affect the water use potential of plants.
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图 1 水曲柳（A、C）和蒙古栎（B、D）相对限制作用对土壤氮含量变化的响应
lsc为气孔限制，lm为叶肉限制，lb为生化限制。CK为对照（0 kg·ha−1·year−1），LN为低氮添加（23 kg·ha−1·year−1），MN为中氮添加（46 kg·ha−1·year−1），HN为高氮添加（69 kg·ha−1·year−1）。下同。
Figure 1. Reponses to relative limitations of soil nitrogen content in Fraxinus mandshurica (A, C) and Quercus mongolica (B, D) saplings
lsc, stomatal limitation; lm, mesophyll limitation; lb, biochemical limitation. CK, control (0 kg·ha−1·year−1); LN, low nitrogen addition (23 kg·ha−1·year−1); MN, medium nitrogen addition (46 kg·ha−1·year−1); HN, high nitrogen addition (69 kg·ha−1·year−1). Same below.
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图 2 水曲柳（A、C）和蒙古栎（B、D）gsc、gm及生化能力对植株饱和光下碳同化相对贡献率随土壤氮含量的变化
SCL为gsc对叶片dPn/Pn的贡献值，MCL为gm对叶片dPn/Pn的贡献值，BL为生化能力对叶片dPn/Pn的贡献值。
Figure 2. Changes in contributions of gsc, gm, and biochemical capacity to light-saturated carbon assimilation (dPn/Pn) with soil nitrogen addition in Fraxinus mandshurica (A, C) and Quercus mongolica (B, D) saplings
SCL, contribution of gsc to dPn/Pn; MCL, contribution of gm to dPn/Pn; BL, contribution of biochemical capacity to dPn/Pn.
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图 3 水曲柳（A、C）和蒙古栎（B、D）叶片潜在水分利用效率对土壤氮含量变化的响应
Figure 3. Reponses of leaf intrinsic water-use efficiency (WUEi) to soil nitrogen content in Fraxinus mandshurica (A, C) and Quercus mongolica (B, D) saplings
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图 4 土壤氮添加下水曲柳（A、C、E）和蒙古栎（B、D、F）叶片WUEi与光合限制作用间的线性相关关系
Figure 4. Linear correlations between leaf WUEi and relative photosynthetic limitations in Fraxinus mandshurica (A, C, E) and Quercus mongolica (B, D, F) saplings
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图 5 不同土壤氮添加水平下水曲柳（A~D）和蒙古栎（E~H）叶片气孔变化
Figure 5. Changes in leaf stomata under different soil nitrogen addition levels in Fraxinus mandshurica (A-D) and Quercus mongolica saplings (E-H)
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