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Vol. 8 No. 3 - July 2002

Air Pollutants, Plants Response, Soil Microbes and Ecosystem Biodiversity

By: J. H. B. Garner

INTRODUCTION

Energy in the form of carbon compounds (chemical energy) is required to sustain life by virtually all-living organisms. Energy keeps societies functioning. Electrical engineering produced by coal and oil heats, lights and air-conditions our homes. Energy derived from oil powers airplanes, cars, trucks, tractors and diesel engines. Most of the energy currently used by society results from the continuing action of plants today or in the past. Therefore, it can be said that, HUMANS INHABIT THIS PLANET AS GUESTS OF THE PLANT KINGDOM.

An increasing world population has increased the demand for energy. Unfortunately this does not come without cost. Scientific studies for more than three decades have shown that burning of coal and oil produces emissions that affect the growth and reproduction of crops, forests and the ecosystems on which life depends. Through the years there has been concern for the effects of air pollution on plants and above ground environment. However, little consideration has been given to the possibility that injury to life above ground might influence the unseen life in the soil environment.

PLANT EXPOSURE/RESPONSE

Plants require chemical energy in the form of carbon compounds (sugars) to sustain their life processes. Carbon compounds are produced during the process of photosynthesis. Production by terrestrial vegetation provided approximately half of the carbon that annually cycles between the earth and the atmosphere. With the aid of chlorophyll in their leaves, plants use the energy of sunlight to combine carbon dioxide from the atmosphere and water from the soil to form sugars, oxygen is given off into the atmosphere during the process. The sugars formed are moved about the plant, converted to carbohydrates for storage, or combined with minerals such as nitrogen, phosphorus, potassium, sulfur and other nutrients from the soil to form the organic compounds required for their maintenance and growth.

Sulfur dioxide (SO2), nitrogen oxides (NOx) and ozone (03), the most phytotoxic pollutants, result from the emissions of power plants, automobile exhausts and volatile organic compounds (VOCs) emitted from a number of sources. The effects of exposure to atmospheric concentrations of the three pollutants, sulfur oxides, nitrogen oxides and ozone, on crops and natural vegetation have been studied by scientists for more than 30 years. Study of the impact of the deposition of the acidic rain on crops, forests and ecosystem is more recent. Acidic rain is formed in the atmosphere from emissions of sulfur and nitrogen oxides combined with the acidic hydrogen ion (H+).

High concentration of sulfur dioxide, nitrogen oxides and ozone in the atmosphere can affect plant growth and reproduction. They are capable of inhibiting photosynthesis, carbon (sugar) production, and altering carbon allocation to roots and stems and reducing carbohydrate formation of mycorrhizae (a symbiotic fungus/root relationship), uptake of important minerals, e.g., nitrogen phosphorus, potassium and sulfur, and root and stem growth.

SOIL BIODIVERSITY AND PLANT GROWTH

The importance of soil organisms for plant growth has been recognized for more than a century. Today, greater recognition of the fact that soil organisms can influence ecosystem processes has lead to increase in the study of the soil and freshwater, and marine sediments. The soil environment, which is composed of minerals and organic matter, water, air, and vast array of bacteria, fungi, algae, actinomycetes, protozoa, nematodes, and arthropods, is one of the most dynamic sites of biological interactions in nature many of which are microscopic, are unknown. Their ecology is at least as important as the ecology of any community or ecosystem that has been studied above ground.

There is interrelatedness between the aboveground and belowground ecosystems. Soil organisms (the consumers), depend on aboveground vegetation (the producers), for the sugars and carbohydrates (carbon compounds) produced during photosynthesis. Without the soil ecosystem and the microorganisms involved in the mineral nutrient cycles, plant growth, agriculture, and life in general would not be possible. Soil biodiversity is a crucial factor in regulating how ecosystems function. Interest in soil biodiversity has increased with the awareness of the fact that many of the most important interactions between plants take place below ground. Especially in nutrient poor soils, the dynamic interactions between plant roots, animals and microbial processes appear to determine what grows where and how.

Bacteria and fungi are usually most abundant in the rhizosphere, the area around the root where exudates are most abundant. They benefit from the nutrients (chiefly simple sugars) exuded by plant roots into the soil. In turn their activities create chemical and biological changes in rhizosphere. By decomposing organic matter they play an essential role in controlling and making inorganic mineral nutrients available for plant uptake. Bacteria in the soil are essential in the cycling of nitrogen, carbon, phosphorus and sulfur. They also assist in making other major mineral nutrients such as potassium, magnesium, and iron available for plant uptake.

Mycorrhizae

Fungi, as integral components of soil ecosystems, are involved either directly or indirectly with all other organisms in the soil. Success of other organisms in the system, and even survival of the soil ecosystem itself, depends on fungal activity. Fungi are a source of food for many soil organisms, including bacteria, other fungi, nematodes, insects, earthworms and mammals.

Fungi form mycorrhizae, a mutualistic, symbiotic relationship with plant roots that is integral in the uptake of nutrients and may be one of the most important and least understood biological associations regulating community and ecosystem functioning, are formed by fungi attracted by energy-rich carbon compounds in the plant root exudates. The mycorrhizal relationship is widespread in the plant kingdom and is found in a variety of habitats from the tropics to boreal regions. The majority of plants cannot take up mineral nutrients and water from the soil and achieve optimum growth and reproduction without mycorrhizae. The few exceptions are aquatic plants, sedges and members of cabbage family. Most mycorrhizal fungi are obligately dependent on the host for their energy requirements. In return, their role in the uptake of mineral nutrients, make them essential to plant growth. The fungus-plant root relationship is of particular benefit to plants growing in nutrient-poor soils.

Mycorrhizae are sensitive to changes in the capacity of host plants to translocate the carbon compounds to the roots. An indirect effect can occur when the pollutant influences the allocation of the carbon from the plant leaves and reduces the supply of sugars to the roots. A direct effect occurs if deposition of the pollutant influences the growth and physiology of the root and reduces its capacity to absorb nutrients from the soil.

Soil Mediated Response

The impact of nitrates, sulfates and acidic deposition on soil ecosystems is determined by the impact they have on the growth of mycorrhizal fungi involved in plant nutrient uptake and on bacteria involved in nutrient cycling. The impact of ozone on the other hand, results from its effects on photosynthesis and altered carbon translocation to the roots. The following text discusses how the effects on above ground vegetation cited above can influence mycorrhizal relationship.

Effects of Nitrogen Deposition

Nitrogen is the element of greatest importance in plant growth and is usually in short supply in forests. To a large extent in all plants, it governs the utilization of phosphorus, potassium, sulfur and other nutrients as well as the process of photosynthesis itself. Approximately 75% of the nitrogen in a plant leaf is used during photosynthesis. The nitrogen-photosynthesis relationship, therefore, is critical to the growth of trees. Most plants must obtain nitrogen through absorption by the roots of inorganic ammonium and nitrate ions, and mycorrhizae play an important role in that process. In soils with low nitrogen and phosphorus availability, mycorrhizae directly transfer these elements to tree roots.

Today, human activities have greatly increased the amount of nitrogen cycling between the above ground and below ground ecosystems and subsequently have influenced the relationships between the two domains. Temperate regions of the world that traditionally were considered to be nitrogen limited are experiencing increased nitrogen deposition and accumulating nitrogen in excess of growth requirements of temperate region ecosystems, especially forests . Changes in nitrogen supply affect nutrient availability of an ecosystem and can alter biodiversity. Atmospherically deposited nitrogen can act as a fertilizer in nitrogen-poor soil. Not all plants, however, are capable of utilizing extra nitrogen. Most plant species growing in nutrient poor conditions are adapted to such habitats and only can complete successfully on soils low in nitrogen. Plants are capable of utilizing additional nitrogen.

The appearance of nitrogen in the soil solution is an early symptom of excess nitrogen. In the final stages, disruption of forest structure becomes visible. Plant succession, i.e. changes in community patterns, and biodiversity are affected significantly by chronic nitrogen additions in some North American ecosystems. Long-term nitrogen fertilization studies in both New England and Europe suggest that some forests receiving chronic inputs of nitrogen may decline in productivity and experience greater mortality.

Effects of Sulfur Deposition

Sulfur is a major component of plant proteins and, as such, is an essential plant nutrient. The most important source of sulfur, even though plants can utilize atmospheric SO2, is sulfate taken up from the soil by plant roots. The availability of organically bound sulfur in soils depends largely on microbial decomposition, a relatively slow process. The major factor controlling the movement of sulfur from the soil into vegetation is the rate of release from the organic to the inorganic compartment.

Atmospheric deposition is an important component of the sulfur cycle. This is true not only in polluted areas where atmospheric deposition is very high, but also in areas of low sulfur input. Additions of sulfur into the soil in the form of sulfate could alter the important organic-sulfur/organic-nitrogen relationship involved in protein formation in plants. The biochemical relationship between sulfur and nitrogen in plant proteins indicates that neither element can be assessed adequately without reference to the other. There is a regulatory coupling of sulfur and nitrogen metabolism. Nitrogen uptake in forests, therefore, may be loosely regulated by sulfur availability, however, sulfate additions in excess of needs do not necessarily lead to injury.

Effects of Acidic Deposition

Acidic deposition over the past quarter of a century has emerged as a critical environmental stress that affects forested landscapes and aquatic ecosystems in North America, Europe, and Asia. It can originate from transboundary air pollution and can affect large geographic areas. It is highly variable across space and time, links air pollution to diverse terrestrial and aquatic ecosystems and alters the interactions of many elements (e.g., calcium, magnesium, and aluminium). Acidic deposition has played a major role in recent soil acidification in some areas of Europe and, to a more limited extent, eastern North America. Acidic deposition is composed of ions, gases, and particles derived from gaseous emissions of sulfur dioxide, nitrogen oxides, ammonia, and particulate emissions of acidifying and neutralizing compounds. It contributes directly and indirectly to biological stress and the degradation of ecosystems.

A major concern has been that soil acidity would lead to nutrient deficiency. Calcium is essential in the formation of wood and the maintenance of cells, the primary plant tissues necessary for tree growth. Trees obtain calcium from the soil, but to be taken up by roots, the positively/ charged ion (Ca+), must be dissolved in soil water. Tree species may be adversely affected if high aluminium to nutrient ratios limit uptake of calcium and magnesium and create a nutrient deficiency. Acid deposition by lowering the acidity (pH) of aluminium-rich soil can increase aluminium concentrations in soil water through dissolution and ion-exchange processes. When in solution, aluminium can be taken up by roots transported through the tree and, eventually, deposited on the forest floor in leaves and branches. Aluminium is more readily taken up than is calcium because it has a higher affinity for negatively charged surfaces than does calcium. When present in the forest floor, aluminium tends to displace adsorbed calcium and causes it to be more readily leached. The continued buildup of aluminium in the forest floor layer, where nutrient uptake is greatest, can decrease the availability of calcium to the roots.

Mycorrhizal fungi have been suggested as possible biological indicators of changes resulting from acidic deposition. Mycorrhizas and fine roots are an extremely dynamic component of below ground ecosystems and can respond rapidly to stress. They have a relatively short life span, and their turn over appears to be strongly controlled by environmental factors. Changes in mycorrhizal species composition of the loss of dominant mycorrhizal species in areas where diversity is already low may lead to an increased susceptibility of plant to stress. Stress affects the total amount of carbon fixed by plants during photosynthesis and modifies carbon allocation to stems and roots. Because mycorrhizal fungi are dependant for their growth on the sugars from the host plants, stresses that shift the allocation of carbon reserves to the production of new leaves at the expense of supporting tissues will be reflected rapidly in decreased soil chemistry, particularly increasing acidity (pH) and the increase in availability of aluminium have been associated with decreased mycorrhizal formation and the uptake of nutrients by the plant host.

Ozone Exposures and Below-Ground Changes

Tropospheric ozone exposures of sensitive conifers have been shown to reduce the rate of photosynthesis and alter the production and translocation of sugars (carbon) to the stems and roots. Foliar injury and increased needle senescence (drop off) alters succession (community change) of the fungal microflora involved in leaf decomposition on the forest floor and significantly changes the movement of the nutrients from the leaf into the soil. Ozone exposures may affect the soil ecosystem through one or more of the following: (1) changes in litter quality and quantity, (2) decreased carbon allocation to roots, (3) altered root exudation and soil carbon dioxide (C02) flux, and (4) decreased root growth and possibly increased root mortality. Decreased carbon production in plants, disrupts carbon availability for the maintenance of the below-ground system, alters mycorrhizal colonization and compatibility. Changes in carbon allocation to roots, reduced root carbon exudation, and altered mycorrhizal colonization are important factors affecting carbon flux into and out of the forested ecosystems. In addition, carbon allocation affects the soil organisms associated with the root exudates.

The reduced translocation of sugars to tree roots resulting from ozone exposures has been observed to impact the formation of feeder roots. Ozone exposures, by inhibiting photosynthesis and reducing the allocation of sugars to the roots are a significant factor in influencing mycorrhizal formation, nutrient uptake and tree growth and reproduction.

ECOSYSTEM RESPONSE AND BIO-DIVERSITY CHANGE

Plants do not live alone. In nature they are members of ecosystems, structurally complex communities comprised of populations of plants, animals, insects, and microorganisms that interact with one another and with their non-living (abiotic) chemical and physical environment. The energy plants (the producers) obtain from sunlight during photosynthesis and the mineral nutrients taken up from the soil are transferred to other species (the consumers) within an ecosystem through food webs. Movement of mineral nutrients through an ecosystem is cyclic. Nutrients are used or stored and eventually returned to the soil. Energy, on the other hand, is transferred from organism to organism through an ecosystem in food webs, then dissipated into the atmosphere as heat. The flow of energy and cycling of nutrients provide the interconnectedness between ecosystem parts and transforms the community from a random collection of species into an integrated whole, an ecosystem, in which the biotic and abiotic parts are interrelated.

The relationship between structure and function is a fundamental one in ecosystem science. Ecosystem structure refers to the species present, their biodiversity, abundance, mass, and arrangement within an ecosystem. Biodiversity encompasses variation at all levels of biological organization, including individuals, populations, species, and ecosystems. Ecosystem functions of energy flow, nutrient cycling, and water and material flow are characterized by the way in which the components (e.g., plants, animals, and microorganisms), interact and the effect their activities have on the physical and chemical environment. Ecosystem properties such as community structure and the patterns of nutrient energy movement that emerge from the interactions among the various components can feed back to influence subsequent development of those interactions. Elucidating these interactions across the scales of time and space is fundamental to understanding the relationships between biodiversity and ecosystem functioning.

Ecosystem response to stresses begins at the population level with the response of individual plants or animals. Plant responses, both structural and functional, must be scaled in both time and space and propagated from the individual to the more complex levels of community interaction to produce observable changes in an ecosystem. Individual organisms within a population, based on their genetic constitution (genotype), stage of growth at time of exposure, and microhabitats in which they are growing, vary in their ability to withstand the stress of environmental changes. The range within which the sensitive organisms can exist and function determines the ability of the population to survive. Those organisms able to cope with the stresses survive and reproduce. Competition among the different species results in succession (community change in species variety and stratification over time) and ultimately produces ecosystems composed of populations of plant species having the capability to tolerate the stresses. The number of species in a community usually increases during succession in unpolluted atmospheres. Productivity, biomass, community height, and structural complexity increases. Severe stresses, on the other hand, divert energy from growth and reproduction to maintenance, and return succession to an earlier stage.

Ecosystems are subject to natural periodic stresses, such as drought, flooding, fire, and attacks by biotic pathogens (e.g., fungi, insects). These natural perturbations are seldom more than a temporary setback, and recovery can be rapid. Rapid recovery is enhanced by the presence of many different species in the ecosystems. In contrast, anthropogenic stresses usually are severe, debilitating stresses. Severely stressed ecosystems do not recover readily, but may be further degraded. Anthropogenic stresses can be classified into four main groups: (1) physical restructuring (e.g., changes resulting from land use); (2) introduction of exotic species; (3) over harvesting; and (4) discharge of toxic substances into the atmosphere, onto land, and into water.

Ecosystems lack the capacity to maintain their normal structure and functions and adapt to the anthropogenic stresses cited above, unless the stress is removed. Such stresses result in a process of degradation marked by a decrease in biodiversity, reduced primary (photosynthesis) and secondary (storage and growth) production, and a lower capacity to recover and return to its original state. In addition, there is an increased prevalence of disease, reduced nutrient cycling, increased dominance of exotic species, and increased dominance by smaller, short-lived opportunistic species. Once the stress is removed, a process of succession (community change) begins which ultimately may return the ecosystem to a semblance of its former structure. Air pollution stresses, if acute, are usually short term and its effects soon visible. Chronic stresses, on the other hand, are long-term stresses whose effects occur at different levels of ecosystem organization, appear only after long-term exposures, as in the case of acidic deposition in the northeast or ozone in California.

Human existence on this planet depends on ecosystems and the products (goods) and services they provide. These essential products (goods) and services provided by the planet's collective biodiversity (the earth's flora, fauna, and microorganisms) are clean air, clean water, clean soil, and clean energy. Today, governments around the world pursue a "bottom line" that is driven by an economy that is disconnected from the natural world and is fundamentally destructive of local ecosystems. For this reason, human society needs to be reconnected to the biologically diverse ecosystems and the natural world of which they are a part. There is a need to understand the biodiversity that encompasses all levels of biological organization, including populations, individuals, species and ecosystems. Populations, geographical entities within a species of organisms, usually distinguished ecologically or genetically, are essential to the conservation of species diversity. Their number and size influence the probability of the future existence of the entire species. The number, biodiversity, structure, and functions of ecosystem populations, provide ecosystem products (goods) and services. For any given population, the number of individuals, the genetic variation between individuals, and the area occupied affects ecosystem functioning and the delivery of ecosystem services and other benefits provided by that population. Loss of population diversity means loss of the benefits, in particular, with time, the loss of the life-support systems on which humanity relies.

Concern has risen in recent years regarding the consequences of changes in the biological diversity of ecosystems. The concerns arise because human activities are creating disturbances that are causing the loss of biodiversity (structure), altering the complexity and stability of ecosystems and producing changes in nutrient cycling (function). There are few ecosystems on earth today that are not influenced by humans. There is a need, therefore, to understand how ecosystems respond to both natural and anthropogenic stresses and, especially, the ways- that anthropogenic stresses are affecting ecosystem products and services.

Attempts have been made to value biodiversity and the world's ecosystem services and natural capital. Attempts to value ecosystems and their services are probably misplaced. "Economics cannot estimate the importance of natural environments to society: only biology cart do that." The role of economics is to help design institutions that will provide incentives to the public and policy makers for the conservation of important natural systems and for mediating human impacts on the biologically diverse ecosystems and the biosphere so that they are sustainable. There is a need to understand human impacts on ecosystems so that ecosystem management can define what ecological conditions are desired. The establishment of ecological goals involves a close linkage between scientists and decision makers, in which science informs decision makers and the public the ecological conditions that are achievable under particular management regimes that enable decision makers can make choices that reflect societal values, including issues of economics, politics, and culture.

SUMMARY

Soil is the forgotten environment. The foregoing discussion emphasizes the importance of the interaction of soil microorganisms, the growth of trees, other above-ground vegetation, and the interdependency of each on the other. Activities, specifically pollutants formed in the burning of the carbon compounds, coal and oil (e.g., nitrates and nitrogen oxides, sulfur oxides and secondary pollutants such as ozone and acid rain), can have impact on the growth of above-ground vegetation by impairing photosynthesis and translocation of carbon in the form of sugars to the roots or through deposition onto the soil, change the functioning of the soil microflora, particularly mycorrhizal fungi and bacteria, and alter ecosystem processes. Altering ecosystem processes can result in changes in ecosystem structure and function both above and below ground, and in turn adversely affect the formation of goods and services that are essential to life on Earth.

Dr. J. H. B. Garner is a Senior Scientist at the National Center for environmental Assessment, U. S. Environmental Protection Agency, Research  Triangle Park, NC, U.S.A.


This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.


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