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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. |
