Temperton V M, Grayston S J, Jackson G, Barton C V M, Millard P, Jarvis P G
Institute of Ecology and Resource Management, University of Edinburgh, Darwin Building, King's Buildings, Mayfield Road, Edinburgh EH9 3JU, UK.
Tree Physiol. 2003 Oct;23(15):1051-9. doi: 10.1093/treephys/23.15.1051.
Nitrogen-fixing plant species may respond more positively to elevated atmospheric carbon dioxide concentrations ([CO2]) than other species because of their ability to maintain a high internal nutrient supply. A key factor in the growth response of trees to elevated [CO2] is the availability of nitrogen, although how elevated [CO2] influences the rate of N2-fixation of nodulated trees growing under field conditions is unclear. To elucidate this relationship, we measured total biomass, relative growth rate, net assimilation rate (NAR), leaf area and net photosynthetic rate of N2-fixing Alnus glutinosa (L.) Gaertn. (common alder) trees grown for 3 years in open-top chambers in the presence of either ambient or elevated atmospheric [CO2] and two soil N regimes: full nutrient solution or no fertilizer. Nitrogen fixation by Frankia spp. in the root nodules of unfertilized trees was assessed by the acetylene reduction method. We hypothesized that unfertilized trees would show similar positive growth and physiological responses to elevated [CO2] as the fertilized trees. Growth in elevated [CO2] stimulated (relative) net photosynthesis and (absolute) total biomass accumulation. Relative total biomass increased, and leaf nitrogen remained stable, only during the first year of the experiment. Toward the end of the experiment, signs of photosynthetic acclimation occurred, i.e., down-regulation of the photosynthetic apparatus. Relative growth rate was not significantly affected by elevated [CO2] because although NAR was increased, the effect on relative growth rate was negated by a reduction in leaf area ratio. Neither leaf area nor leaf P concentration was affected by growth in elevated [CO2]. Nodule mass increased on roots of unfertilized trees exposed to elevated [CO2] compared with fertilized trees exposed to ambient [CO2]. There was also a biologically significant, although not statistically significant, stimulation of nitrogenase activity in nodules exposed to elevated [CO2]. Root nodules of trees exposed to elevated [CO2] were smaller and more evenly spaced than root nodules of trees exposed to ambient [CO2]. The lack of an interaction between nutrient and [CO2] effects on growth, biomass and photosynthesis indicates that the unfertilized trees maintained similar CO2-induced growth and photosynthetic enhancements as the fertilized trees. This implies that alder trees growing in natural conditions, which are often limited by soil N availability, should nevertheless benefit from increasing atmospheric [CO2].
固氮植物物种可能比其他物种对大气二氧化碳浓度（[CO₂]）升高的反应更积极，因为它们有能力维持较高的内部养分供应。树木对升高的[CO₂]生长反应的一个关键因素是氮的有效性，尽管升高的[CO₂]如何影响田间条件下生长的结瘤树木的N₂固定速率尚不清楚。为了阐明这种关系，我们测量了在开放式气室中生长3年的固氮欧洲桤木（Alnus glutinosa (L.) Gaertn.）（普通桤木）树的总生物量、相对生长速率、净同化率（NAR）、叶面积和净光合速率，实验设置了两种大气[CO₂]水平（环境水平或升高水平）以及两种土壤氮素状况（全营养液或不施肥）。通过乙炔还原法评估未施肥树木根瘤中Frankia spp.的固氮情况。我们假设未施肥树木对升高的[CO₂]会表现出与施肥树木相似的积极生长和生理反应。升高的[CO₂]环境下的生长刺激了（相对）净光合作用和（绝对）总生物量积累。仅在实验的第一年，相对总生物量增加，且叶片氮含量保持稳定。在实验接近尾声时，出现了光合适应的迹象，即光合机构的下调。相对生长速率并未受到升高的[CO₂]的显著影响，因为尽管NAR增加了，但叶面积比的降低抵消了其对相对生长速率的影响。升高的[CO₂]环境下的生长对叶面积和叶片磷浓度均无影响。与暴露于环境[CO₂]的施肥树木相比，暴露于升高的[CO₂]的未施肥树木根瘤质量增加。暴露于升高的[CO₂]的根瘤中，固氮酶活性也受到了生物学上显著的刺激，尽管在统计学上不显著。暴露于升高的[CO₂]的树木的根瘤比暴露于环境[CO₂]的树木的根瘤更小且间距更均匀。养分和[CO₂]对生长、生物量和光合作用的影响之间缺乏相互作用，这表明未施肥树木与施肥树木一样，维持了相似的由CO₂诱导的生长和光合增强。这意味着在自然条件下生长且通常受土壤氮有效性限制的桤木，仍应能从大气[CO₂]增加中受益。
