No overall stimulation of soil respiration under mature deciduous forest trees after 7 years of CO2 enrichment

The anthropogenic rise in atmospheric CO₂ is expected to impact carbon (C) fluxes not only at ecosystem level but also at the global scale by altering C cycle processes in soils. At the Swiss Canopy Crane (SCC), we examined how 7 years of free air CO₂ enrichment (FACE) affected soil CO₂ dynamics in a ca. 100‐year‐old mixed deciduous forest. The use of ¹³C‐depleted CO₂ for canopy enrichment allowed us to trace the flow of recently fixed C. In the 7th year of growth at ～550 ppm CO₂, soil respiratory CO₂ consisted of 39% labelled C. During the growing season, soil air CO₂ concentration was significantly enhanced under CO₂‐exposed trees. However, elevated CO₂ failed to stimulate cumulative soil respiration (R_s) over the growing season. We found periodic reductions as well as increases in instantaneous rates of R_s in response to elevated CO₂, depending on soil temperature and soil volumetric water content (VWC; significant three‐way interaction). During wet periods, soil water savings under CO₂‐enriched trees led to excessive VWC (>45%) that suppressed R_s. Elevated CO₂ stimulated R_s only when VWC was ≤40% and concurrent soil temperature was high (>15 °C). Seasonal Q₁₀ estimates of R_s were significantly lower under elevated (Q₁₀=3.30) compared with ambient CO₂ (Q₁₀=3.97). However, this effect disappeared when three consecutive sampling dates of extremely high VWC were disregarded. This suggests that elevated CO₂ affected Q₁₀ mainly indirectly through changes in VWC. Fine root respiration did not differ significantly between treatments but soil microbial biomass (C_mic) increased by 14% under elevated CO₂ (marginally significant). Our findings do not indicate enhanced soil C emissions in such stands under future atmospheric CO₂. It remains to be shown whether C losses via leaching of dissolved organic or inorganic C (DOC, DIC) help to balance the C budget in this forest.     

Warming and free-air CO2 enrichment alter demographics in four co-occurring grassland species

Species differ in their responses to global changes such as rising CO(2) and temperature, meaning that global changes are likely to change the structure of plant communities. Such alterations in community composition must be underlain by changes in the population dynamics of component species. Here, the impact of elevated CO(2) (550 micromol mol(-1)) and warming (+2 degrees C) on the population growth of four plant species important in Australian temperate grasslands is reported. Data collected from the Tasmanian free-air CO(2) enrichment (TasFACE) experiment between 2003 and 2006 were analysed using population matrix models. Population growth of Themeda triandra, a perennial C(4) grass, was largely unaffected by either factor but population growth of Austrodanthonia caespitosa, a perennial C(3) grass, was reduced substantially in elevated CO(2) plots. Warming and elevated CO(2) had antagonistic effects on population growth of two invasive weeds, Hypochaeris radicata and Leontodon taraxacoides, with warming causing population decline. Analysis of life cycle stages showed that seed production, seedling emergence and establishment were important factors in the responses of the species to global changes. These results show that the demographic approach is very useful in understanding the variable responses of plants to global changes and in elucidating the life cycle stages that are most responsive.     

开放式空气CO2浓度增高(FACE)对稻田土壤微生物的影响

1　引　　言公元 175 0年前 ,大气ＣＯ2 浓度基本保持 2 80 μｍｏｌ·ｍｏｌ-1左右 .工业革命后 ,其浓度逐渐上升 ,上升速度在 196 0年后加快 ,其中 80年代以来上升最快 .从 80年代到 90年代期间 ,ＣＯ2 浓度从 330 μｍｏｌ·ｍｏｌ-1增加到 35 4 μｍｏｌ·ｍｏｌ-1,平均每年递增 1.8μｍｏｌ·ｍｏｌ-1[2 ] .据ＩＰＣＣ(1995 )估计 ,到 2 1世纪末 ,ＣＯ2 浓度将由目前的 35 5 μｍｏｌ·ｍｏｌ-1上升到 70 0 μｍｏｌ·ｍｏｌ-1.这势必对整个生物界和地球生态环境产生深刻的影响 .因此 ,国内外已开展了大量的研究工作 ,获得了许多研究结…     

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland,but it shifts the vertical gradient in xylem [CO2]

To quantify stem respiration (R_S) under elevated CO₂ (eCO₂), stem CO₂ efflux (E_A) and CO₂ flux through the xylem (F_T) should be accounted for, because part of respired CO₂ is transported upwards with the sap solution. However, previous studies have used E_A as a proxy of R_S, which could lead to equivocal conclusions. Here, to test the effect of eCO₂ on R_S, both E_A and F_T were measured in a free‐air CO₂ enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced E_A and R_S, which were unaffected by eCO₂, likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO₂ concentration measured near the stem base was higher under eCO₂, and decreased along the stem resulting in a negative contribution of F_T to R_S, whereas the contribution of F_T to R_S under ambient CO₂ was positive. Negative F_T indicates net efflux of CO₂ respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of R_S on eCO₂ and suggest stimulated root respiration under eCO₂ that may shift vertical gradients in xylem [CO₂] confounding the interpretation of E_A measurements.     

Soil warming and CO2 enrichment induce biomass shifts in alpine tree line vegetation

Responses of alpine tree line ecosystems to increasing atmospheric CO₂ concentrations and global warming are poorly understood. We used an experiment at the Swiss tree line to investigate changes in vegetation biomass after 9 years of free air CO₂ enrichment (+200 ppm; 2001–2009) and 6 years of soil warming (+4 °C; 2007–2012). The study contained two key tree line species, Larix decidua and Pinus uncinata, both approximately 40 years old, growing in heath vegetation dominated by dwarf shrubs. In 2012, we harvested and measured biomass of all trees (including root systems), above‐ground understorey vegetation and fine roots. Overall, soil warming had clearer effects on plant biomass than CO₂ enrichment, and there were no interactive effects between treatments. Total plant biomass increased in warmed plots containing Pinus but not in those with Larix. This response was driven by changes in tree mass (+50%), which contributed an average of 84% (5.7 kg m^?2) of total plant mass. Pinus coarse root mass was especially enhanced by warming (+100%), yielding an increased root mass fraction. Elevated CO₂ led to an increased relative growth rate of Larix stem basal area but no change in the final biomass of either tree species. Total understorey above‐ground mass was not altered by soil warming or elevated CO₂. However, Vaccinium myrtillus mass increased with both treatments, graminoid mass declined with warming, and forb and nonvascular plant (moss and lichen) mass decreased with both treatments. Fine roots showed a substantial reduction under soil warming (?40% for all roots <2 mm in diameter at 0–20 cm soil depth) but no change with CO₂ enrichment. Our findings suggest that enhanced overall productivity and shifts in biomass allocation will occur at the tree line, particularly with global warming. However, individual species and functional groups will respond differently to these environmental changes, with consequences for ecosystem structure and functioning.     

大气CO2浓度升高对农田土壤微生物及其相关因素的影响

综述了大气CO2浓度升高条件下,农田土壤微生物区系、土壤呼吸、土壤微生物生物量;植物-微生物共生体--内生菌根、根瘤及其与农田土壤微生物活动相关因素发生的变化。该方面的研究虽然受实验条件限制,在国内外开展研究的持续时间较短,但现有的研究表明,大气CO2浓度升高主要通过影响植物生长而间接影响农田土壤微生物活性。     

草地土壤呼吸研究进展

土壤呼吸是生态系统物质循环和能量流动的重要过程,具有多方面的生态意义。在综合比较了土壤呼吸的各种测定方法,概括了草地土壤呼吸的速率和年排放量,并就影响草地土壤呼吸的一些重要因素进行了阐述,文章最后讨论了草地土壤呼吸今后的研究方向。     

辽东栎对大气CO2倍增的响应

 本文研究了CO2加浓对暖温带落叶阔叶混交林典型自然群落建群种辽东栎的影响,结果表明：在生理学方面,CO2倍增下气孔阻抗略增大,为对照的106％,蒸腾速率略下降,为对照的92％,暗呼吸速率与对照很接近,但略微下降为对照的98.9％。净光合速率、昼夜净光合量、水分利用效率都明显提高,分别为对照的155％,172％和179％。可以看出C02倍增对辽东栎的生理过程有促进作用,属正效应。其中以生长旺季6、7月增长更为明显。在生长方面,CO2倍增下生长各项指标增长也较明显,叶面积为对照的107％,叶干重为对照的140％,以植株高度增加最明显,为对照的331％,清楚的看出辽东栎的生长与生理过程的变化趋势是一致的、均属正效应。也就是说在其他环境资源满足植物要求时,CO2倍增对树木具有“施肥”作用,它可促进植物的生理过程和提高其生物生产力。     

开放式空气CO2浓度增高对水稻冠层微气候的影响

利用位于江苏省无锡市安镇的我国唯一的农田开放式空气CO2 浓度增高 (FACE)系统平台 ,于2 0 0 1年 8月 2 6日至 10月 13日 (水稻抽穗至成熟期 )进行水稻作物冠层微气候连续观测 ,以研究FACE对水稻冠层微气候特征的影响 .结果表明 ,FACE降低了水稻叶片的气孔导度 ,FACE与对照水稻叶片气孔导度的差异上层叶片大于下层叶片 ,生长前期大于生长后期 .FACE使白天水稻冠层和叶片温度升高 ,这种差异生长前期大于生长后期 ;但FACE对夜间水稻冠层温度的影响不明显 .在水稻旺盛生长的抽穗开花期 ,晴天正午前后FACE水稻冠层温度比对照高 1.2℃ ;从开花至成熟期 ,FACE水稻冠层白天平均温度比对照高 0 .4 3℃ .FACE对冠层空气温度也有影响 ,白天水稻冠层空气温度FACE高于对照 ,这种差异随太阳辐射增强而增大且冠层中部大于冠层顶部 ;冠层中部空气温度FACE与对照的差异 (Tface-Tambient)日最大值在 0 .4 7～ 1.2℃之间 ,而冠层顶部的Tface-Tambient日最大值在 0 .37～ 0 .8℃之间 .夜间水稻冠层空气温度FACE与对照差别不大 ,变化在± 0 .3℃之内 .而FACE对水稻冠层空气湿度无显著影响 ,表明FACE使水稻叶片气孔导度降低 ,从而削弱了植株的蒸腾降温作用 ,导致水稻冠层温度和冠层空气温度升高 ,改变了整个水稻冠层的温度环     

Water-use efficiency of four native trees under CO2 enrichment and N addition in subtropical model forest ecosystems

Aims We aimed to evaluate the changes in water-use efficiency (WUE) in native tree species in forests of subtropical China, and determine how coexisting species would be responding to increases in atmospheric carbon dioxide (CO₂) concentrations and nitrogen (N) deposition.Methods We used model forest ecosystems in open-top chambers to study the effects of elevated CO₂ (ca. 700 μmol mol^-1) alone and together with N addition (NH 4 NO 3 applied at 100kg N ha^-1 year^-1) on WUE of four native tree species (Schima superba, Ormosia pinnata, Castanopsis hystrix and Acmena acuminatissima) from 2006 to 2010.Important findings Our result indicated that all species increased their WUE when they were exposed to elevated CO₂. Although higher WUE was shown in faster-growing species (S. superba and O. pinnata) than that of slower-growing species (C. hystrix and Acmena acuminatissima), the increased extent of WUE induced by elevated CO₂ was higher in the slower-growing species than that of the faster-growing species (P < 0.01). The N treatment decreased WUE of S. superba, while the effects on other species were not significant. The interactions between elevated CO₂ and N addition increased intrinsic WUE of S. superba significantly (P < 0.001), however, it did not affect WUE of the other tree species significantly. We conclude that the responses of native tree species to elevated CO₂ and N addition are different in subtropical China. The species-specific effects of elevated CO₂ and N addition on WUE would have important implications on species composition in China's subtropics in response to global change.     

大气CO2浓度升高对稻田水体细菌及大肠菌群数量的影响

稻田水体中细菌(尤其是其中的大肠菌群)数量的多少及活性深刻影响着水体质量和物质循环,然而大气CO2浓度升高对它们的影响至今鲜有报道.为此,借助国际上唯一的稻麦复种FACE(free air CO2 enrichment)试验(位于江苏省江都市,始于2004年),于2006年对稻田水体中细菌数量、大肠菌群数量、总有机碳量和总氮量等进行了动态监测.结果表明,大气 CO2浓度升高显著提高了以上各指标在稻田水体中的含量(P <0.01),在整个水稻生育期,与对照相比,水体中的细菌数量、大肠菌群数量、总有机碳量和总氮量平均分别提高了45.9%、68.8%、31.2%和25.9%,不同生育期之间上述各指标存在显著差异(P<0.01).可见,大气CO2浓度升高不仅可通过改变稻田水体质量的方式来影响水稻的安全生产,而且还可能通过田间排水尤其是水稻生长前期的暴雨导致的洪涝来加重稻田生态系统向周边居民井水和其它水域的细菌和大肠菌群的输出量,从而可能影响周边水体质量及人体健康.     

Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange,biochemistry and optical techniques

Lifelong exposure to elevated concentrations of atmospheric CO₂ may enhance carbon assimilation of trees with unlimited rooting volume and consequently may reduce requirements for photoprotective pigments. In early summer the effects of elevated [CO₂] on carboxylation and light utilization of mature Quercus pubescens trees growing under chronic [CO₂] enrichment at two CO₂ springs and control sites in Italy were examined. Net photosynthesis was enhanced by 36 to 77%. There was no evidence of photosynthetic downregulation early in the growing season when sink demand presumably was greatest. Specifically, maximum assimilation at saturating [CO₂], electron transport capacity, and Rubisco content, activity and carboxylation capacity were not significantly different in trees growing at the CO₂ springs and their respective control sites. Foliar biochemical content, leaf reflectance index of chlorophyll pigments (NDVI), and photochemical efficiency of PSII (ΔF/F_m′) also were not significantly affected by [CO₂] enrichment except that starch content and ΔF/F_m′ tended to be higher at one spring (42 and 15%, respectively). Contrary to expectation, prolonged elevation of [CO₂] did not reduce xanthophyll cycle pigment pools or alter mid‐day values of leaf reflectance index of xanthophyll cycle pigments (PRI), despite the enhancement of carbon assimilation. However, both these pigments and PRI were well correlated with electron transport capacity.     

Comparative response of seedlings of selected native dry tropical and alien invasive species to CO2 enrichment

Aims Global climate change and ongoing plant invasion are the two prominent ecological issues threatening biodiversity world wide. Among invasive species, Lantana camara and Hyptis suaveolens are the two most important invaders in the dry deciduous forest in India. We monitored the growth of these two invasive species and seedlings of four native dry deciduous species (Acacia catechu, Bauhinia variegata, Dalbergia latifolia and Tectona grandis) under ambient (375–395 μ mol mol^-1) and elevated CO₂ (700–750 μ mol mol^-1) to study the differential growth response of invasive and native seedlings.Methods Seedlings of all the species were exposed to ambient and elevated CO₂. After 60 days of exposure, seedlings were harvested and all the growth-related parameters like plant height; biomass of root, stem and leaves; total seedling biomass; R/S ratio; allocation parameters; net assimilation rate (NAR) and relative growth rate (RGR) were determined.Important findings Biomass, RGR and NAR of all the species increased under elevated CO₂ but the increase was higher in invasive species and they formed larger seedlings than natives. Therefore under the CO₂ -enriched future atmosphere, competitive hierarchies could change and may interfere with the species composition of the invaded area.     

Continuous monitoring of rhizosphere respiration after labelling of plant shoots with 14CO2

The present work describes an original method to follow rate of ¹⁴CO₂ and total CO₂ production from rhizosphere respiration after plant shoots had been pulse-labelled with ¹⁴CO₂. We used a radioactivity detector equipped with a plastic cell for flow detection of beta radiation by solid scintillation counting. The radioactivity detector was coupled with an infrared gas analyser. The flow detection of ¹⁴CO₂ was compared to trapping of ¹⁴CO₂ in NaOH and counting by liquid scintillation. First, we demonstrated that NaOH (1 M) trapped 95% of the CO₂ of a gaseous sample. Then, we determined that the counting efficiency of the radioactivity flow cell was 41% of the activity of gaseous samples as determined by trapping in NaOH (1 M) and by counting by static liquid scintillation. The sensitivity of the ¹⁴CO₂- flow detection was 0.08 Bq mL⁻¹ air and the precision was 2.9% of the activity measured compared to 0.9% for NaOH trapping method. We presented two applications which illustrate the relevance of ¹⁴CO₂-flow detection to investigations using 14C to trace photoassimilates within the plant-soil system. First, we examined the kinetics of 14CO₂ production when concentrated acid is added to NaH¹⁴CO₃. This method is the most commonly used to label photoassimilates with ¹⁴C. Then, we monitored ¹⁴CO₂ activity in rhizosphere respiration of 5-week old maize cultivated in soil and whose shoots had been pulse-labelled with ¹⁴CO₂. We conclude that alkali traps should be used for a cumulative determination of ¹⁴CO₂ because they are cheap and accurate. On the other hand, we demonstrated that the flow detection of ¹⁴CO₂ had a finer temporal resolution and was consequently a relevant tool to study C dynamics in the rhizosphere at a short time scale. This revised version was published online in June 2006 with corrections to the Cover Date.     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999