Air Pollutants: Elevated Carbon Dioxide

Authored by: Hans-Joachim Weigel , Jürgen Bender

Atmosphere and Climate

Print publication date:  June  2020
Online publication date:  May  2020

Print ISBN: 9781138339675
eBook ISBN: 9780429440984
Adobe ISBN:

10.1201/9780429440984-4

 

Abstract

The assessment of the potential combined effects of air pollutants and elevated atmospheric concentrations of carbon dioxide (CO2) on vegetation is of critical importance during the next decades. The interactive effects of these atmospheric compounds on crops, trees, and other types of vegetation have been shown. Existing evidence on such interactions is almost entirely restricted to CO2 and ozone (O3), the concentrations of which are increasing globally. Results from a number of studies indicate that elevated CO2 may reduce the adverse effects of O3 on plant growth and productivity, but the available information is inconsistent as several studies show that elevated CO2 did not ameliorate the negative effects of O3. The future interactions of elevated CO2 and enhanced atmospheric nitrogen (N) deposition are of concern in many ecosystem types with respect to carbon sequestration and biodiversity. The overall impact of climate change, including elevated CO2, on future air pollutant effects is difficult to predict because of the largely uncertain influence and feedback of other growth variables such as plant genotype, soil water deficit, nutrient availability, or temperature.

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Air Pollutants: Elevated Carbon Dioxide

Introduction

The concentrations of various compounds in the atmosphere have changed during the last century, and they continue to change. Most of these compounds interact with the terrestrial biosphere as they are part of the overall biogeochemical cycling of, e.g., carbon, oxygen, nitrogen, and sulfur.[1] For example, depending on their concentrations, gaseous compounds [sulfur dioxide (SO2), nitrogen monoxide and dioxide (NO2/NO)] may be beneficial to terrestrial ecosystems or remain inert (O3) at low concentrations, whereas at higher levels, they may act as air pollutants affecting these systems in an adverse manner. Although atmospheric CO2 is the basic plant resource for photosynthesis, its current concentration is still limiting C3 plant growth. The rapid increase of the global atmospheric CO2 concentration [CO2], along with the overall changes in climate and atmospheric chemistry, require an assessment of the potential future interactive effects of air pollutants and elevated [CO2] on terrestrial ecosystems.

Atmospheric Change: Concentrations and Trends

On a global scale, the concentrations of a variety of gaseous and particulate compounds in the atmosphere, including CO2, NO/NO2, SO2, O3, ammonia (NH3), heavy metals, and volatile organic compounds (VOC) have undergone temporal and spatial changes during the last century.[2] After peak emissions in the 1960s to the 1980s in industrialized countries particularly, the concentrations of SO2, and to a smaller extent of NOx (NO/NO2), VOCs, and particulate matter, have declined during the past decades in Europe and North America. NH3, which is the most important reduced N species, is of importance as a direct air pollutant in the vicinity of local emitters. However, wet and dry N deposition from oxidized and reduced N species are predicted to increase in other regions of the world.[3] The occurrence and distribution of airborne VOCs are difficult to assess because there are both anthropogenic and biogenic sources. With respect to heavy metals such as lead (Pb), cadmium (Cd), nickel (Ni), mercury (Hg), and zinc (Zn), a decline in emission and subsequent deposition was observed in most of Europe since the late 1980s. Unlike the development in Europe and North America, emissions and consequently atmospheric concentrations of many of the above-mentioned compounds have been increasing over the last two decades particularly in the rapidly growing regions of Asia, Africa, and Latin America.[4] For example, China and India are now the leading emitters of SO2 in the world. Also, the predicted further increase in global nitrogen oxide (NOx) emissions may be attributed largely to these countries. On the other hand, concentrations of ground-level O3 and atmospheric CO2 have increased and continue to increase on a global scale. In most industrialized countries, O3 concentration [O3] has nearly doubled during the last 100 years. Current background [O3] in the Northern hemisphere is within the range of 23–34 ppbv (parts-per-billion by volume). Although at least in most parts of Western Europe there is a clear trend of decreasing O3 peak values (“photosmog episodes”), models predict that background [O3] will continue to increase at a rate of 0.5% to 2% per year in the Northern Hemisphere during the next several decades, and that global surface [O3] is expected to be in the range of 42–84 ppbv by 2100.[5] O3 pollution has also become a major environmental problem in many of the countries with rapidly developing population and related economic growth, respectively. O3 is currently considered the most important atmospheric pollutant that has direct negative effects on vegetation worldwide. Its concentrations vary considerably in time and space and show distinct annual and diurnal patterns.

Since the beginning of the 19th century, the CO2 in the atmosphere has increased globally from approximately 280 ppmv (parts-per-million by volume) to current values of about 395 ppmv. It is expected that CO2 will continue to increase even more rapidly and may reach about 550–650 ppmv between 2050 and 2070.[6] CO2 is the substrate for plant photosynthesis, and its current atmospheric concentration is limiting for photosynthesis and growth of C3 plants. It is expected that the increase in CO2 will have far-reaching consequences for most types of vegetation.

Effects of O3 and CO2 Alone

Due to their global importance and their contrasting effects on vegetation, plant growth responses to either O3 or CO2 alone are briefly described. Primary O3 effects include subtle biochemical and ultrastructural changes in the plant cell, which may result in impaired photosynthesis, alterations of carbon allocation patterns, symptoms of visible injury, enhanced senescence, reduced growth and economic yield, altered resistance to other abiotic and biotic stresses, or reduced flowering and seed production at the whole-plant level[7,8] At the ecosystem level, this may result in a loss of competitive abilities of plant species in communities along with shifts in biodiversity and impaired ecosystem functions and services like reduced carbon sequestration and altered hydrology.[9] For example, current ambient (O3) in many industrialized areas has been shown to suppress crop yields of sensitive species and to retard growth and development of trees and other plant species of the non-woody (semi)natural vegetation. Overall quantification of O3 effects on vegetation is complicated by large inter- and intra-specific variability in the O3 susceptibility of plants.[10]

By contrast, plants of the C3 type most frequently respond to elevated CO2 with a stimulation of photosynthesis accompanied by a reduced stomatal conductance and transpiration rate, an enhanced concentrations of soluble carbohydrates, and a stimulation of biomass production and economic yield.[11] Similarly, in C4 plants, higher CO2 concentrations reduce stomatal conductance and transpiration, i.e., both C3 and C4 plants may benefit from elevated CO2 by improved water-use efficiency and a reduced demand for water. Under well-watered conditions, no significant growth stimulation has been found so far in C4 plants, because C4 photosynthesis is saturated under ambient CO2.[12,13] Growth and yield enhancements of up to 25–35% as compared to ambient CO2 have been observed when crop plants were exposed to 550–750 ppmv CO2. Experiments with tree species ranging from short-term studies with seedlings to long-term whole-stand manipulations have also shown that elevated CO2 stimulated net photosynthesis and resulted in enhanced tree growth in almost all cases.[14,15] As with O3, plant species differ widely in their response to high CO2, which makes an overall assessment of its potential effects on vegetation difficult.

Interactive Effects of Air Pollutants and CO2

Along with the ongoing and predicted further changes of global climate and atmospheric chemistry, there is considerable interest in how terrestrial ecosystems will respond to these multiple environmental changes and particularly how the individual changes in atmospheric constituents may interact with each other when they impact vegetation. The majority of studies dealing with this issue have addressed two-way interactions of O3 and elevated CO2, although there is much less information on how other air pollutants interact with high CO2. There are no studies describing three-way interactions, i.e., in which two air pollution components and elevated CO2 are combined together. In a biological sense, the combined action of multiple factors in comparison to single-factor effects can be described as additive (effect directly predictable from single-factor treatment) or as interactive. Interactive effects can be synergistic (effect > than expected from single-factor treatment) or antagonistic (effect < than expected from single-factor treatment).[16]

CO2 and O3

A great number of previous and more recent studies using different experimental approaches ranging from controlled environment to free-air O3- and CO2-enrichment systems have been carried out on the combined effects of the two gases. The bulk of these studies has shown that high CO2 in the range of 200–400 ppmv above current ambient CO2 levels either partially or totally compensates for adverse O3 effects, whether these effects have been addressed at the biochemical and physiological level or at the whole-plant level including growth and yield. This has been demonstrated for crop (e.g., wheat, soybean, potato, rice) as well as for tree (e.g., trembling aspen, paper birch, sugar maple) species, although little information is available for grassland species.[17–20] For example, elevated CO2 reduces O3 effects, such as a loss in root and main stem biomass, a decrease in leaf area and mass, general foliar damage, lower growth and yield, lower starch levels, and an altered carbon balance. Results from recent free-air concentration enrichment (FACE) studies, however, have indicated that the mitigating effect of elevated CO2 against O3 damage might be less than predicted from earlier chamber studies.[12]

The proposed mechanisms to explain the protective effect of elevated CO2 against the phytotoxic effects of O3 include the following: i) reduced uptake or flux of O3 through the stomata due to a CO2-induced stomatal closure, ii) improved supply of carbon skeletons supporting the synthesis of antioxidants involved in the scavenging of O3 and its toxic products, iii) protection of the RuBisCo protein from O3-induced degradation, and iv) CO2-induced changes in the cell surface/volume ratio.[9,21,22] However, it has been shown that in spite of decreased stomatal conductance under elevated CO2, adverse effects of O3 may still occur.[8,19] Additionally, elevated O3 has been found to impair stomatal responsiveness to CO2, i.e., O3 causes less-sensitive (“sluggish”) stomatal responses to elevated CO2.[23] As CO2 effects on stomatal conductance may be species specific, it is not yet possible to support a general concept of a CO2-induced reduction in the flux of O3 into the plant. Nevertheless, a reduction in stomatal conductance and thus in the O3 uptake may increase atmospheric O3 in the boundary layer.[24] Moreover, in a given plant species, protection by high CO2 from a particular adverse effect is not necessarily associated with the protection against another adverse effect. For instance, in wheat plants, elevated CO2 provided full protection from effects of O3 on total plant biomass, but not on grain yield. From the available database of studies that have examined the interactive effects of O3 and CO2, the information is not entirely consistent, as several studies revealed that elevated CO2 may not always protect plants from the adverse effects of O3 (Table 3.1).

Table 3.1   Selected Examples of the Effects of Elevated O3 and CO2, Alone or in Combination, on Plant Responses (Examples with Significant Adverse Effects of O3 on Visible Injury, Photosynthesis, Growth, and Yield)

Species

O3 Effect

CO2 Effect

O3/CO2 Effect

Potato

Decreased chlorophyll content; visible foliar leaf injury

n. e.

Adverse effect of O3 on chlorophyll content unchanged; reduced degree of visible O3-induced leaf injury

Wheat

Visible leaf injury; reduced photosynthesis; reduced growth; reduced yield

Increased photosynthesis; increased growth; increased yield

Reduced degree of visible O3-induced leaf injury; amelioration of negative O3 effects on photosynthesis, growth, and yield

Soybean

Reduced photosynthesis; reduced growth; reduced seed yield

Increased photosynthesis; increased growth; insignificant increase of seed yield

O3 impact on photosynthesis lessened; amelioration of negative O3 effects on photosynthesis, growth, and yield

Cotton

Reduced leaf area per mass; reduced starch contents

Increased leaf area per mass and starch contents

Prevention of adverse effects of O3 by CO2

Norway spruce

Visible leaf injury (chlorotic mottling)

n.e.

No effect of CO2 on the degree of O3-induced leaf injury

Trembling aspen (different O3-sensitive and -tolerant clones)

Reduced tree growth parameters (height, diameter, volume)

Enhancement of growth parameters

No effect of CO2 on the degree of O3-induced growth reductions

Paper birch

Reduced photosynthesis; decreased dry matter production

Increased photosynthesis; increased dry matter production

Decrease in photosynthesis and dry matter production similar to O3 alone

White clover (sensitive clone)

Visible leaf injury

n.e.

Little effect on the degree of O3-induced foliar injury

Source:Adapted from Karnosky et al.,[19] Vandermeiren et al.,[20] Polle & Pell,[21] and Runeckles.[26] Abbreviations: CO2, carbon dioxide; n.e., No effect; O3, Ozone.

CO2 and Other Air Pollutants

Very few studies have addressed the combined effects of elevated CO2 and of other air pollutants. SO2 has long been known to adversely affect agricultural crops and forest plants above a certain threshold concentration.[25] Reduced photosynthesis, altered water relations, growth retardations, yield losses, and altered susceptibilities to other stresses are common plant responses observed under SO2 stress. Due to the diminishing importance of SO2 as a widespread air pollutant, few studies have been conducted on the combined action of SO2 and elevated CO2. In earlier studies, it was shown for some crop species that elevated CO2 reduced the sensitivity of the plants to SO2 injury or protected them from the negative effects of SO2 on growth and yield.[26] With the combined exposure of crop species to both gases, the yield increments were sometimes even larger when compared to the stimulation observed with exposure to elevated CO2 alone, suggesting that the plants were able to use the airborne sulfur more effectively under the conditions of enhanced carbon availability. Low-to-moderate SO2 concentrations may confer a nutritional benefit to plants, particularly under conditions of low sulfur availability in the soil.

Studies on the interactive effects of elevated CO2 and nitrogen oxides (NO and NO2) are confined to commercial greenhouses under conditions of horticultural crop production under very high CO2 and are not considered here. However, it has been shown repeatedly that positive plant growth responses to elevated CO2 are smaller at low relative to high soil N supply. This is related to the question on the role of future atmospheric N deposition and “aerial carbon fertilization” by elevated CO2 in shaping the size of the terrestrial carbon sink and how plant biodiversity might be affected by these inputs. Assuming that aerial N supply via enhanced N deposition causes similar effects as soil N fertilization, a few experimental and modeling studies addressed the question of how elevated CO2 interacts with N deposition. For example, it has been shown that N addition enhanced the CO2 stimulation of plant productivity in the first phase of a multiyear CO2–N manipulation study with a herbaceous wetland plant community. But in the longer term, the observed N-induced shift in the plant community composition suppressed the CO2 stimulation of plant productivity, indicating that plant community shifts can act as a feedback effect that alters ecosystem responses to elevated CO2.[27] In a long-term study with simulated grassland systems with 16 species, high N supply reduced species richness by 16% under ambient CO2 but only by 8% under elevated CO2, i.e., high CO2 ameliorated negative N effects.[28,29] Elevated CO2 and N addition have been found to affect above and belowground C allocation in temperate forest trees in an opposite way, i.e., elevated CO2 increases belowground allocation, whereas N increases aboveground allocation; however, the ratio of above vs. belowground C flow does not change in the combination of both treatments.[30]

Conclusion

The assessment of the potential combined effects of air pollutants and elevated atmospheric concentrations of CO2 on vegetation is of critical importance during the next decades. Interactive effects of these atmospheric compounds on crops, trees, and other types of vegetation have been shown. Existing evidence on such interactions is almost entirely restricted to CO2 and O3, the concentrations of which are increasing globally. Although rising CO2 will be mostly beneficial to plants, current ambient O3 are high enough to impair plants in many regions of the world. There is prevailing information that elevated CO2 may protect plants from adverse effects of O3, but this has not been demonstrated unequivocally. There is also some information that rising CO2 may protect plants against phytotoxic SO2 concentrations. The future interactions of elevated CO2 and enhanced atmospheric N deposition are of concern in many ecosystem types with respect to carbon sequestration and biodiversity. Overall, our understanding has to be improved about how other growth variables, such as plant genotype, soil water deficit, nutrient availability, or temperature, may modify the interaction between air pollutants and elevated CO2.

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