- Email: [email protected]
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bioavailability in biosolid-amended soils, the US Environmental Protection Agency (USEPA) recommends the application of alkaline-stabilized biosolids and other liming agents to increase the soil pH to 6.5 or more. The primary purpose of liming arable soils is to overcome the chemical problems associated with soil acidity that include high concentrations of acid ions (Hþ and Al3þ) and toxic elements (Mn2þ), and low concentrations of basic cations (Ca and Mg) and other nutrient ions such as Mo and P. The hydrolysis of the basic cations in lime produces OH ions, which neutralize the Hþ ions, thereby decreasing the activity and bioavailability of Al and Mn. Liming also increases the solubility of Mo and P, thereby increasing their availability. Lime provides the basic nutrient cations (Ca2þ and Mg2þ) and also reduces the solubility of heavy metals, thereby minimizing their bioavailability and mobility in soils.
17
Further Reading Adams F (1984) Soil Acidity and Liming. Madison, WI: Soil Science Society of America Publishing. Evangelou VP (1995) Pyrite Oxidation and its Control. New York, NY: CRC Press. Haynes RJ (1984) Lime and phosphate in the soil–plant system. Advances in Agronomy 37: 249–467. Longhurst JWS (1991) Acid Deposition: Origin, Impacts and Abatement Strategies. New York, NY: Springer-Verlag. Marschner H (1995) Mineral Nutrition of Higher Plants, 2nd edn. London, UK: Academic Press. Rengel Z (ed.) (2003) Handbook of Soil Acidity. New York, NY: Marcel Dekker. Robson AD (ed.) (1989) Soil Acidity and Plant Growth. New York, NY: Academic Press. Ulrich B and Sumner ME (eds) (1991) Soil Acidity. New York, NY: Springer-Verlag. Wright RJ, Baligar VC, and Murrmann RP (eds) (1990) Plant–Soil Interactions at Low pH. Dordrecht, The Netherlands: Kluwer Academic Publishers.
AERATION D E Rolston, University of California–Davis, Davis, CA, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction In a general sense, aeration is the interchange of various gases between the atmosphere and the Earth and the various reactions that either consume or produce gases in the soil. The interchange results from concentration gradients established within soil by respiration of microorganisms and plant roots, by production of gases associated with biological reactions such as fermentation, nitrification, and denitrification, reduction–oxidation reactions of soil chemicals, and by soil incorporation of materials such as fumigants, anhydrous ammonia, pesticides, and various volatile organic chemicals from toxic waste sites. The two major gases associated with aeration are oxygen (O2) and carbon dioxide (CO2), where O2 moves from the atmosphere to soil and is consumed, and CO2 is produced in soil and moves from the soil to the atmosphere. Figure 1 indicates the general direction of the flow of gases within soil profiles. Soil aeration has been reviewed extensively over the years.
Soil-Air Composition The amount of air or soil-air content is directly related to the bulk density of the soil and the amount of water in the soil profile. The bulk density of natural soil varies from approximately 1.0 Mg m3 to 1.7–1.8 Mg m3. Thus, the relative amount of void or pore space in the soil varies between approximately 30 and 60%. The soil pores or voids can be filled with either air or water. Therefore the soil-air content or air-filled porosity can vary between approximately 30 and 60%.
Figure 1 Schematic indicating the general flow directions of important gases within soil.
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The composition of soil air depends on the relative magnitude of both the sources and the sinks of the various gas components, the interchange between soil air and atmospheric air, and the partitioning of the gases between the gaseous, liquid, and solid (mineral and organic matter) phases of the soil. If a soil were completely ‘aerated’ the concentrations of the gases in the soil air would be similar to that in the atmosphere. Oxygen concentrations in the soil air will be somewhat below that in the atmosphere (approximately 20% by volume), since O2 is consumed in soil by plant root and microbial respiration and through chemical reactions. Under some conditions, O2 concentrations can fall to zero and the soil becomes anaerobic (anoxic). It is now widely accepted that under some conditions soil profiles do not have to be either fully aerated or fully anaerobic but may be partially aerobic and partially anaerobic. Anaerobic pockets or ‘hot spots’ may exist within the soil due to pockets of very high O2 consumption such as around incorporated carbon materials and/or due to very slow diffusion to regions of O2 consumption. For example, the interior of large aggregates may be anaerobic for these reasons. CO2 concentrations in the soil air can be as high as 10 times more than in the atmosphere (0.036% by volume). Since nitrogen gas (N2) is more abundant than other gases in the atmosphere (approx. 78%) and there are generally no sources or sinks for N2 in the soil (except N2 absorbed during nitrogen fixation or produced during denitrification), the concentration of N2 in the soil air will be similar to that in the atmosphere, varying only slightly depending on the production and consumption of other soil gases. The soil air will also contain varying amounts of nitric oxide and nitrous oxide (from nitrification and denitrification); methane, hydrogen sulfide, and ethylene (from anaerobic processes); water vapor; and trace amounts of inert gases such as argon (Figure 1). Human activities also result in the accidental or intentional introduction of gases in the soil profile such as fumigants, anhydrous ammonia, pesticides, and various volatile organic chemicals that exist partially in the vapor phase.
coefficient of each gas pair. General equations for steady transport of a multicomponent mixture of gases have been developed based on gas kinetic theory. If gravity effects are ignored or diffusion occurs only horizontally, the well-known Stefan– Maxwell equations provide the theoretical framework for diffusion of gases in soils. Ficks law is generally applicable for only a few special cases. One of these cases is for the diffusion of a trace gas in a binary mixture, meaning that the mole fraction of the tracer gas is small. Since the binary diffusion coefficients of N2 in air and O2 in air are very similar and CO2 may be considered to exist in trace amounts, the diffusion of the two major gases associated with aeration may come under this case. Diffusion of some of the other gases existing in soil may not meet this criterion, however. Assuming that the special case conditions are met, Ficks law is given by: Mg dCg ¼ fg;d ¼ Dp At dx
½1
where Mg is the amount of gas diffusing (kilograms of gas), A is the cross-sectional area of the soil (square meters of soil), t is time (seconds), fg,d is the gas flux density (kilograms of gas per square meter of soil per second) due to molecular diffusion, Cg is concentration in the gaseous phase (kilograms of gas per cubic meter of soil air), x is distance (meters of soil), and Dp is the soil-gas diffusion coefficient (cubic meters of soil air per meter of soil per second). The soil-gas diffusion coefficient is the main variable controlling the degree of soil aeration. It is highest when the soil is dry and approaches zero as the soil becomes very wet or saturated. The Dp has been related both empirically and theoretically to the soil-air content by a number of authors. Since discrepancies between measured soil-gas diffusion coefficients stemming from eqn [1] and those calculated from the many empirical equations occur, it is often desirable to measure the soil-gas diffusion coefficient for particular situations. Laboratory methods using soil cores and field methods for measuring soil-gas diffusion coefficients have been developed. Convection
Gas Exchange Mechanisms Diffusion
Diffusion is considered to be the principal mechanism in the exchange of gases between the soil and the atmosphere. The diffusion velocities of gas mixtures in soil are related to each other in a complex manner dependent upon the mole fraction of each gas, the molar fluxes of each gas, and the binary diffusion
In addition to molecular diffusion processes, soil gases may also exchange with the atmosphere through convection (advection). Convective flow of gases means that the whole air parcel is moving through soil pores due to a pressure difference (gradient) between the soil and the atmosphere. Pressure gradients may develop due to barometric pressure changes in the atmosphere; wind blowing across the soil surface or against a hill or other landscape
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feature; infiltration of rainfall or irrigation water into the soil, soil-water redistribution, and evaporation; temperature differences across the upper part of the soil profile; and density differences due to high concentrations of gases that have densities much different from air. Convective gas flow is described by a form of Darcy’s Law: Ka dP fg;c ¼ ½2 dx where fg,c is the flux density of gas due to convection (cubic meters of gas per square meter of soil per second), Ka is the air permeability (cubic meters of gas per meter of soil), is the viscosity of the gas mixture (pascal-seconds), P is pressure (pascals), and x is distance (meters of soil). Air permeability is strongly influenced by soilwater content. Air permeability is a maximum in dry soil and decreases as the soil becomes wet, until it reaches zero at saturation. This is caused by progressive blockage of the soil pores by water. Several field and laboratory methods for measuring Ka have been developed, involving either steady-state or nonsteady-state flow, though steady-state measurements of gas permeability are preferred. Most field methods are based on the same principles as the laboratory methods, i.e., they involve measuring the flow rate of air through a soil column under known pressure differences across the column. Convective processes occur rapidly and sporadically. Thus, it is very difficult to observe, measure, and predict the gas exchange that occurs by convection. During infiltration into soil, there is ample evidence that air pressure increases ahead of the water wetting front, and convection of gases occurs. It is often assumed that diffusion is the main gas exchange process overall, because it is operating continuously, whereas convection occurs episodically. Of the convective processes, rainfall and irrigation may contribute the most to aeration of soils, with estimates that rainfall may account for 7–9% of total aeration. Adequately quantifying these processes and being able to predict and model convective flow processes is a goal yet to be fully attained.
19
respiration. The rates of O2 consumption and CO2 production are directly related to the rate of plant and microbial growth, which in turn is related to several environmental factors, including air and soil temperature, substrate availability, and soil moisture. For aerobic conditions, the amount of CO2 produced and O2 consumed tends to be about equal. For anaerobic conditions, the CO2 production will tend to be larger than the O2 consumption because other reactions are occurring. The concentrations of O2 and CO2 that occur in the soil pore space vary widely, especially for CO2, and depend on the rate of consumption and production and upon the rate that the soil is able to exchange these gases between the soil and the atmosphere through diffusion and convection. The diurnal and annual variability in the soil-gas concentrations are generally much greater for clayey soils than for sandy soils owing to the ability of sandy soils to transmit gases at a higher rate (larger soil-gas diffusion coefficients and air permeabilities) and maintain more constant concentrations. Oxidation–Reduction Processes
Gas Reactions
Oxidation and reduction are connected with the transfer of electrons from soil organic matter (or organic contaminants) to oxidized inorganic compounds catalyzed by enzymes produced by soil microorganisms. For well-aerated conditions (aerobic), O2 is the electron acceptor. When O2 becomes limiting (anaerobic), other substances will accept electrons or be reduced. Examples of compounds that can be reduced under anaerobic conditions are nitrate (denitrification), manganic manganese, ferric iron, sulfate, and perchlorate (a natural and anthropogenic contaminant). The reduction of nitrate and sulfate results in N2 (and N2O) and hydrogen sulfide gases, respectively. Another gas produced from reduction processes is methane (CH4). Both CH4 and N2O are strong greenhouse gases and contribute to global warming. In waterlogged or very wet soils and sediments that are unable to transmit O2 sufficiently fast through the profile, O2 is the first to disappear, followed by nitrate and then sequentially by the reduction of manganese, iron, and sulfate. Perchlorate is reduced at about the same point as nitrate. Several toxic organic chemicals also undergo redox reactions that greatly affect the kind and toxicity of the reaction products.
Respiration
Production and Consumption of Other Gases
O2 is continuously consumed and CO2 produced by plant roots and by soil microorganisms. Even plants such as rice that grow best in water-submerged soil transport O2 from the leaves to the roots for
There are a few gases produced in soils that are not necessarily associated with redox reactions. When fertilizer materials such as urea and ammonium salts are applied to the soil, reactions can occur, produce
20 AERATION
ammonia gas (NH3), particularly in soils with a pH of more than about 7.5 or 8, that fairly large emissions of NH3 can occur, for instance, if urea is applied to the soil surface, since urea hydrolysis results in a large increase in pH near the fertilizer granules, with NH3 being emitted. The deeper the material is placed, the less NH3 will be emitted. Ammonia may also be produced in soil from the incorporation of animal waste. Under aerobic conditions, ammonium-based materials either from fertilizer or mineralization of organic materials will be oxidized to nitrite and then to nitrate by microbial processes (nitrification). During nitrification, some N2O and nitric oxide (NO) can be produced and emitted to the atmosphere. Nitric oxide enters into the tropospheric ozone cycle and can contribute to very small particle (less than 10 m) generation in the atmosphere. Besides O2, gases including hydrocarbons, N2O, NO, CH4, and some sulfur gases may move from the atmosphere into the soil and be consumed by biological processes. Thus, the soil may act as a significant sink for atmospheric gases under some circumstances.
Aeration Requirements Plants
The plant response to inadequate aeration or lack of O2 is highly dependent upon the plant species, stage of growth, and upon several soil and environmental conditions such as temperature, water relations, and occurrence of toxic by-products of anaerobic conditions. The response of plants to inadequate aeration may be due to either direct or indirect effects. The direct effect is because of the lack of oxygen for root respiration. The indirect effect is due to changes in redox conditions that affect nutrient and water availability, soil pH, buildup of toxic concentrations of metabolites and metals, and the viability of pests and diseases. The direct effect of lack of O2 or slow diffusion of O2 to plant roots has been characterized by a measurement called the oxygen diffusion rate (ODR). The ODR is measured by placing a cylindrical platinum electrode into the soil. Oxygen is reduced at the electrode surface and creates a current that can be measured. Diffusion of O2 to the water-covered electrode is meant to mimic the diffusion through the water film around roots. Many studies have attempted to relate the ODR to plant response. ODR values smaller than approximately 0.2 g cm2 min1 indicate potentially poor aeration for many plant species. Values of more than approximately 0.4 g cm2 min1 are indicative of relatively good aeration for growth of most plants.
The plant symptoms of poor aeration are poor root growth or death, negative geotropism of roots, depressed shoot growth, wilting, leaf senescence, abortion of flowers, and termination of shoot apex growth. Poor aeration may also result in decreased transpiration; accumulation of ethylene, ethanol, and other metabolites; and decreased nutrient uptake. Reduced conditions in soil may also result in increased solubility of some chemicals that are toxic to plants. Remediation of Contaminated Soils
With the large use of chemicals in modern society, both inorganic and organic chemicals, either intentionally or accidentally, end up in soil as contaminants with varying degrees of toxicity. Large amounts of petroleum products end up contaminating soil, both as point and nonpoint sources of pollution. The hydrocarbons in crude petroleum include alkanes, cycloalkanes, aromatics, polycyclic aromatics, asphaltines, and resins. In addition, chlorinated solvents, pesticides, detergents, metals, and other kinds of chemicals may pollute soil. Microbial processes in soil can degrade many of the organic compounds, but the biodegradability of various compounds is greatly influenced by their physical state and toxicity, as well as soil environmental conditions, including soil water content, soil pH, temperature, levels of inorganic nutrients, levels of electron acceptors, and aeration. Organic contaminants provide a source of carbon to microorganisms or they provide electrons, which the organisms can use to obtain energy. Metal contaminants may undergo redox reactions affecting their solubility and toxicity, which are also dependent upon the aeration status of the soil. In most cases, biodegradation of organic contaminants depends on the activities of aerobic organisms. Thus, the presence of an adequate supply of O2 in the soil is essential for biodegradation or bioremediation to occur. In general, it takes 2–3 kg of O2 to degrade 1 kg of petroleum hydrocarbon. On the other hand, some compounds like the highly chlorinated chemicals are not easily degraded and may be broken down more effectively under anaerobic conditions. Some chemicals, perchlorate for instance, are only broken down under anaerobic conditions. Table 1 gives a list of some chemicals, indicating whether they are degradable by aerobic or anaerobic processes. For more information on biodegradation and bioremediation of contaminated soils, see Pollutants: Biodegradation in this encyclopedia.
Summary Aeration is the interchange of various gases between the atmosphere and soil and the various reactions that
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21
Table 1 Organic chemicals and their biodegradability Chemical class
Examples
Biodegradability
Aromatic hydrocarbons Ketones and esters Petroleum hydrocarbons Chlorinated solvents
Benzene, toluene Acetone, methylethyl ketone Fuel oil Trichloroethylene, perchloroethylene
Polyaromatic hydrocarbons Polychlorinated biphenyls Organic cyanides
Anthracene, benzo[a]pyrene, creosote Arochlors
Aerobic and anaerobic Aerobic and anaerobic Aerobic Aerobic (methanotrophs), anaerobic (reductive dechlorination) Aerobic ? Aerobic
Adapted from Baker KH and Herson DS (1994) Bioremediation. New York: McGraw-Hill. ß 1994 with permission.
either consume or produce gases in the soil. The composition of soil air depends on the relative magnitude of both the sources and sinks of the various gas components, the interchange between soil air and atmospheric air, and the partitioning of the gases between the gaseous, liquid, and solid (mineral and organic matter) phases of the soil. The two major gases associated with aeration are O2 and CO2, where O2 moves from the atmosphere to soil and is consumed by plant roots and microorganisms and CO2 moves from the soil, where it is produced by plant and microbial respiration, to the atmosphere. In addition to root and microbial respiration, O2 may also be consumed by reaction with metals and other compounds. The two transport mechanisms that result in aeration of soils are molecular diffusion and convection. Although convection can result in significant transport under certain situations, such as infiltration of rainfall or irrigation water into soil, diffusion is considered to be the dominant mechanism of exchange over the long term. The plant symptoms of poor aeration are poor root growth or death, negative geotropism of roots, depressed shoot growth, wilting, leaf senescence, abortion of flowers, and termination of shoot apex growth. Aeration is also important for the soil’s ability to degrade pollutants. In most cases, biodegradation of organic contaminants depends on the activities of aerobic organisms. Thus, the presence of an adequate supply of O2 in the soil is essential for biodegradation or bioremediation to occur. On the other hand, some chemicals will only degrade under anaerobic conditions. Knowledge
of a chemical’s biodegradability and whether the chemical will degrade under aerobic or anaerobic conditions plays a major role in design of effective bioremediation schemes. See also: Anaerobic Soils; Carbon Emissions and Sequestration; Diffusion; Greenhouse Gas Emissions; Hydrocarbons; Oxidation–Reduction of Contaminants; Pollutants: Biodegradation; Remediation of Polluted Soils; Vadose Zone: Hydrologic Processes
Further Reading Baker KH and Herson DS (1994) Bioremediation. New York: McGraw-Hill. Dane JH and Topp GC (eds) (2002) Methods of Soil Analysis, part 4, Physical Methods. Soil Science Society of America Book Series 5. Madison, WI: Soil Science Society of America. Eweis JB, Ergas SJ, Chang DPY, and Schroeder ED (1998) Bioremediation Principles. Boston: McGraw-Hill WCB. Gerstl Z, Chen Y, Mingelgrin U, and Yaron B (eds) (1989) Toxic Organic Chemicals in Porous Media. New York: Springer-Verlag. Glinski J and Steniewski W (1985) Soil Aeration and its Role for Plants. Boca Raton, FL: CRC Press. Hillel D (1998) Environmental Soil Physics. San Diego, CA: Academic Press. Jury WA, Gardner WR, and Gardner WH (1991) Soil Physics. New York: John Wiley. Scott HD (2000) Soil Physics, Agricultural and Environmental Applications. Ames, IA: Iowa State University Press.
bioavailability in biosolid-amended soils, the US Environmental Protection Agency (USEPA) recommends the application of alkaline-stabilized biosolids and other liming agents to increase the soil pH to 6.5 or more. The primary purpose of liming arable soils is to overcome the chemical problems associated with soil acidity that include high concentrations of acid ions (Hþ and Al3þ) and toxic elements (Mn2þ), and low concentrations of basic cations (Ca and Mg) and other nutrient ions such as Mo and P. The hydrolysis of the basic cations in lime produces OH ions, which neutralize the Hþ ions, thereby decreasing the activity and bioavailability of Al and Mn. Liming also increases the solubility of Mo and P, thereby increasing their availability. Lime provides the basic nutrient cations (Ca2þ and Mg2þ) and also reduces the solubility of heavy metals, thereby minimizing their bioavailability and mobility in soils.
17
Further Reading Adams F (1984) Soil Acidity and Liming. Madison, WI: Soil Science Society of America Publishing. Evangelou VP (1995) Pyrite Oxidation and its Control. New York, NY: CRC Press. Haynes RJ (1984) Lime and phosphate in the soil–plant system. Advances in Agronomy 37: 249–467. Longhurst JWS (1991) Acid Deposition: Origin, Impacts and Abatement Strategies. New York, NY: Springer-Verlag. Marschner H (1995) Mineral Nutrition of Higher Plants, 2nd edn. London, UK: Academic Press. Rengel Z (ed.) (2003) Handbook of Soil Acidity. New York, NY: Marcel Dekker. Robson AD (ed.) (1989) Soil Acidity and Plant Growth. New York, NY: Academic Press. Ulrich B and Sumner ME (eds) (1991) Soil Acidity. New York, NY: Springer-Verlag. Wright RJ, Baligar VC, and Murrmann RP (eds) (1990) Plant–Soil Interactions at Low pH. Dordrecht, The Netherlands: Kluwer Academic Publishers.
AERATION D E Rolston, University of California–Davis, Davis, CA, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction In a general sense, aeration is the interchange of various gases between the atmosphere and the Earth and the various reactions that either consume or produce gases in the soil. The interchange results from concentration gradients established within soil by respiration of microorganisms and plant roots, by production of gases associated with biological reactions such as fermentation, nitrification, and denitrification, reduction–oxidation reactions of soil chemicals, and by soil incorporation of materials such as fumigants, anhydrous ammonia, pesticides, and various volatile organic chemicals from toxic waste sites. The two major gases associated with aeration are oxygen (O2) and carbon dioxide (CO2), where O2 moves from the atmosphere to soil and is consumed, and CO2 is produced in soil and moves from the soil to the atmosphere. Figure 1 indicates the general direction of the flow of gases within soil profiles. Soil aeration has been reviewed extensively over the years.
Soil-Air Composition The amount of air or soil-air content is directly related to the bulk density of the soil and the amount of water in the soil profile. The bulk density of natural soil varies from approximately 1.0 Mg m3 to 1.7–1.8 Mg m3. Thus, the relative amount of void or pore space in the soil varies between approximately 30 and 60%. The soil pores or voids can be filled with either air or water. Therefore the soil-air content or air-filled porosity can vary between approximately 30 and 60%.
Figure 1 Schematic indicating the general flow directions of important gases within soil.
18 AERATION
The composition of soil air depends on the relative magnitude of both the sources and the sinks of the various gas components, the interchange between soil air and atmospheric air, and the partitioning of the gases between the gaseous, liquid, and solid (mineral and organic matter) phases of the soil. If a soil were completely ‘aerated’ the concentrations of the gases in the soil air would be similar to that in the atmosphere. Oxygen concentrations in the soil air will be somewhat below that in the atmosphere (approximately 20% by volume), since O2 is consumed in soil by plant root and microbial respiration and through chemical reactions. Under some conditions, O2 concentrations can fall to zero and the soil becomes anaerobic (anoxic). It is now widely accepted that under some conditions soil profiles do not have to be either fully aerated or fully anaerobic but may be partially aerobic and partially anaerobic. Anaerobic pockets or ‘hot spots’ may exist within the soil due to pockets of very high O2 consumption such as around incorporated carbon materials and/or due to very slow diffusion to regions of O2 consumption. For example, the interior of large aggregates may be anaerobic for these reasons. CO2 concentrations in the soil air can be as high as 10 times more than in the atmosphere (0.036% by volume). Since nitrogen gas (N2) is more abundant than other gases in the atmosphere (approx. 78%) and there are generally no sources or sinks for N2 in the soil (except N2 absorbed during nitrogen fixation or produced during denitrification), the concentration of N2 in the soil air will be similar to that in the atmosphere, varying only slightly depending on the production and consumption of other soil gases. The soil air will also contain varying amounts of nitric oxide and nitrous oxide (from nitrification and denitrification); methane, hydrogen sulfide, and ethylene (from anaerobic processes); water vapor; and trace amounts of inert gases such as argon (Figure 1). Human activities also result in the accidental or intentional introduction of gases in the soil profile such as fumigants, anhydrous ammonia, pesticides, and various volatile organic chemicals that exist partially in the vapor phase.
coefficient of each gas pair. General equations for steady transport of a multicomponent mixture of gases have been developed based on gas kinetic theory. If gravity effects are ignored or diffusion occurs only horizontally, the well-known Stefan– Maxwell equations provide the theoretical framework for diffusion of gases in soils. Ficks law is generally applicable for only a few special cases. One of these cases is for the diffusion of a trace gas in a binary mixture, meaning that the mole fraction of the tracer gas is small. Since the binary diffusion coefficients of N2 in air and O2 in air are very similar and CO2 may be considered to exist in trace amounts, the diffusion of the two major gases associated with aeration may come under this case. Diffusion of some of the other gases existing in soil may not meet this criterion, however. Assuming that the special case conditions are met, Ficks law is given by: Mg dCg ¼ fg;d ¼ Dp At dx
½1
where Mg is the amount of gas diffusing (kilograms of gas), A is the cross-sectional area of the soil (square meters of soil), t is time (seconds), fg,d is the gas flux density (kilograms of gas per square meter of soil per second) due to molecular diffusion, Cg is concentration in the gaseous phase (kilograms of gas per cubic meter of soil air), x is distance (meters of soil), and Dp is the soil-gas diffusion coefficient (cubic meters of soil air per meter of soil per second). The soil-gas diffusion coefficient is the main variable controlling the degree of soil aeration. It is highest when the soil is dry and approaches zero as the soil becomes very wet or saturated. The Dp has been related both empirically and theoretically to the soil-air content by a number of authors. Since discrepancies between measured soil-gas diffusion coefficients stemming from eqn [1] and those calculated from the many empirical equations occur, it is often desirable to measure the soil-gas diffusion coefficient for particular situations. Laboratory methods using soil cores and field methods for measuring soil-gas diffusion coefficients have been developed. Convection
Gas Exchange Mechanisms Diffusion
Diffusion is considered to be the principal mechanism in the exchange of gases between the soil and the atmosphere. The diffusion velocities of gas mixtures in soil are related to each other in a complex manner dependent upon the mole fraction of each gas, the molar fluxes of each gas, and the binary diffusion
In addition to molecular diffusion processes, soil gases may also exchange with the atmosphere through convection (advection). Convective flow of gases means that the whole air parcel is moving through soil pores due to a pressure difference (gradient) between the soil and the atmosphere. Pressure gradients may develop due to barometric pressure changes in the atmosphere; wind blowing across the soil surface or against a hill or other landscape
AERATION
feature; infiltration of rainfall or irrigation water into the soil, soil-water redistribution, and evaporation; temperature differences across the upper part of the soil profile; and density differences due to high concentrations of gases that have densities much different from air. Convective gas flow is described by a form of Darcy’s Law: Ka dP fg;c ¼ ½2 dx where fg,c is the flux density of gas due to convection (cubic meters of gas per square meter of soil per second), Ka is the air permeability (cubic meters of gas per meter of soil), is the viscosity of the gas mixture (pascal-seconds), P is pressure (pascals), and x is distance (meters of soil). Air permeability is strongly influenced by soilwater content. Air permeability is a maximum in dry soil and decreases as the soil becomes wet, until it reaches zero at saturation. This is caused by progressive blockage of the soil pores by water. Several field and laboratory methods for measuring Ka have been developed, involving either steady-state or nonsteady-state flow, though steady-state measurements of gas permeability are preferred. Most field methods are based on the same principles as the laboratory methods, i.e., they involve measuring the flow rate of air through a soil column under known pressure differences across the column. Convective processes occur rapidly and sporadically. Thus, it is very difficult to observe, measure, and predict the gas exchange that occurs by convection. During infiltration into soil, there is ample evidence that air pressure increases ahead of the water wetting front, and convection of gases occurs. It is often assumed that diffusion is the main gas exchange process overall, because it is operating continuously, whereas convection occurs episodically. Of the convective processes, rainfall and irrigation may contribute the most to aeration of soils, with estimates that rainfall may account for 7–9% of total aeration. Adequately quantifying these processes and being able to predict and model convective flow processes is a goal yet to be fully attained.
19
respiration. The rates of O2 consumption and CO2 production are directly related to the rate of plant and microbial growth, which in turn is related to several environmental factors, including air and soil temperature, substrate availability, and soil moisture. For aerobic conditions, the amount of CO2 produced and O2 consumed tends to be about equal. For anaerobic conditions, the CO2 production will tend to be larger than the O2 consumption because other reactions are occurring. The concentrations of O2 and CO2 that occur in the soil pore space vary widely, especially for CO2, and depend on the rate of consumption and production and upon the rate that the soil is able to exchange these gases between the soil and the atmosphere through diffusion and convection. The diurnal and annual variability in the soil-gas concentrations are generally much greater for clayey soils than for sandy soils owing to the ability of sandy soils to transmit gases at a higher rate (larger soil-gas diffusion coefficients and air permeabilities) and maintain more constant concentrations. Oxidation–Reduction Processes
Gas Reactions
Oxidation and reduction are connected with the transfer of electrons from soil organic matter (or organic contaminants) to oxidized inorganic compounds catalyzed by enzymes produced by soil microorganisms. For well-aerated conditions (aerobic), O2 is the electron acceptor. When O2 becomes limiting (anaerobic), other substances will accept electrons or be reduced. Examples of compounds that can be reduced under anaerobic conditions are nitrate (denitrification), manganic manganese, ferric iron, sulfate, and perchlorate (a natural and anthropogenic contaminant). The reduction of nitrate and sulfate results in N2 (and N2O) and hydrogen sulfide gases, respectively. Another gas produced from reduction processes is methane (CH4). Both CH4 and N2O are strong greenhouse gases and contribute to global warming. In waterlogged or very wet soils and sediments that are unable to transmit O2 sufficiently fast through the profile, O2 is the first to disappear, followed by nitrate and then sequentially by the reduction of manganese, iron, and sulfate. Perchlorate is reduced at about the same point as nitrate. Several toxic organic chemicals also undergo redox reactions that greatly affect the kind and toxicity of the reaction products.
Respiration
Production and Consumption of Other Gases
O2 is continuously consumed and CO2 produced by plant roots and by soil microorganisms. Even plants such as rice that grow best in water-submerged soil transport O2 from the leaves to the roots for
There are a few gases produced in soils that are not necessarily associated with redox reactions. When fertilizer materials such as urea and ammonium salts are applied to the soil, reactions can occur, produce
20 AERATION
ammonia gas (NH3), particularly in soils with a pH of more than about 7.5 or 8, that fairly large emissions of NH3 can occur, for instance, if urea is applied to the soil surface, since urea hydrolysis results in a large increase in pH near the fertilizer granules, with NH3 being emitted. The deeper the material is placed, the less NH3 will be emitted. Ammonia may also be produced in soil from the incorporation of animal waste. Under aerobic conditions, ammonium-based materials either from fertilizer or mineralization of organic materials will be oxidized to nitrite and then to nitrate by microbial processes (nitrification). During nitrification, some N2O and nitric oxide (NO) can be produced and emitted to the atmosphere. Nitric oxide enters into the tropospheric ozone cycle and can contribute to very small particle (less than 10 m) generation in the atmosphere. Besides O2, gases including hydrocarbons, N2O, NO, CH4, and some sulfur gases may move from the atmosphere into the soil and be consumed by biological processes. Thus, the soil may act as a significant sink for atmospheric gases under some circumstances.
Aeration Requirements Plants
The plant response to inadequate aeration or lack of O2 is highly dependent upon the plant species, stage of growth, and upon several soil and environmental conditions such as temperature, water relations, and occurrence of toxic by-products of anaerobic conditions. The response of plants to inadequate aeration may be due to either direct or indirect effects. The direct effect is because of the lack of oxygen for root respiration. The indirect effect is due to changes in redox conditions that affect nutrient and water availability, soil pH, buildup of toxic concentrations of metabolites and metals, and the viability of pests and diseases. The direct effect of lack of O2 or slow diffusion of O2 to plant roots has been characterized by a measurement called the oxygen diffusion rate (ODR). The ODR is measured by placing a cylindrical platinum electrode into the soil. Oxygen is reduced at the electrode surface and creates a current that can be measured. Diffusion of O2 to the water-covered electrode is meant to mimic the diffusion through the water film around roots. Many studies have attempted to relate the ODR to plant response. ODR values smaller than approximately 0.2 g cm2 min1 indicate potentially poor aeration for many plant species. Values of more than approximately 0.4 g cm2 min1 are indicative of relatively good aeration for growth of most plants.
The plant symptoms of poor aeration are poor root growth or death, negative geotropism of roots, depressed shoot growth, wilting, leaf senescence, abortion of flowers, and termination of shoot apex growth. Poor aeration may also result in decreased transpiration; accumulation of ethylene, ethanol, and other metabolites; and decreased nutrient uptake. Reduced conditions in soil may also result in increased solubility of some chemicals that are toxic to plants. Remediation of Contaminated Soils
With the large use of chemicals in modern society, both inorganic and organic chemicals, either intentionally or accidentally, end up in soil as contaminants with varying degrees of toxicity. Large amounts of petroleum products end up contaminating soil, both as point and nonpoint sources of pollution. The hydrocarbons in crude petroleum include alkanes, cycloalkanes, aromatics, polycyclic aromatics, asphaltines, and resins. In addition, chlorinated solvents, pesticides, detergents, metals, and other kinds of chemicals may pollute soil. Microbial processes in soil can degrade many of the organic compounds, but the biodegradability of various compounds is greatly influenced by their physical state and toxicity, as well as soil environmental conditions, including soil water content, soil pH, temperature, levels of inorganic nutrients, levels of electron acceptors, and aeration. Organic contaminants provide a source of carbon to microorganisms or they provide electrons, which the organisms can use to obtain energy. Metal contaminants may undergo redox reactions affecting their solubility and toxicity, which are also dependent upon the aeration status of the soil. In most cases, biodegradation of organic contaminants depends on the activities of aerobic organisms. Thus, the presence of an adequate supply of O2 in the soil is essential for biodegradation or bioremediation to occur. In general, it takes 2–3 kg of O2 to degrade 1 kg of petroleum hydrocarbon. On the other hand, some compounds like the highly chlorinated chemicals are not easily degraded and may be broken down more effectively under anaerobic conditions. Some chemicals, perchlorate for instance, are only broken down under anaerobic conditions. Table 1 gives a list of some chemicals, indicating whether they are degradable by aerobic or anaerobic processes. For more information on biodegradation and bioremediation of contaminated soils, see Pollutants: Biodegradation in this encyclopedia.
Summary Aeration is the interchange of various gases between the atmosphere and soil and the various reactions that
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Table 1 Organic chemicals and their biodegradability Chemical class
Examples
Biodegradability
Aromatic hydrocarbons Ketones and esters Petroleum hydrocarbons Chlorinated solvents
Benzene, toluene Acetone, methylethyl ketone Fuel oil Trichloroethylene, perchloroethylene
Polyaromatic hydrocarbons Polychlorinated biphenyls Organic cyanides
Anthracene, benzo[a]pyrene, creosote Arochlors
Aerobic and anaerobic Aerobic and anaerobic Aerobic Aerobic (methanotrophs), anaerobic (reductive dechlorination) Aerobic ? Aerobic
Adapted from Baker KH and Herson DS (1994) Bioremediation. New York: McGraw-Hill. ß 1994 with permission.
either consume or produce gases in the soil. The composition of soil air depends on the relative magnitude of both the sources and sinks of the various gas components, the interchange between soil air and atmospheric air, and the partitioning of the gases between the gaseous, liquid, and solid (mineral and organic matter) phases of the soil. The two major gases associated with aeration are O2 and CO2, where O2 moves from the atmosphere to soil and is consumed by plant roots and microorganisms and CO2 moves from the soil, where it is produced by plant and microbial respiration, to the atmosphere. In addition to root and microbial respiration, O2 may also be consumed by reaction with metals and other compounds. The two transport mechanisms that result in aeration of soils are molecular diffusion and convection. Although convection can result in significant transport under certain situations, such as infiltration of rainfall or irrigation water into soil, diffusion is considered to be the dominant mechanism of exchange over the long term. The plant symptoms of poor aeration are poor root growth or death, negative geotropism of roots, depressed shoot growth, wilting, leaf senescence, abortion of flowers, and termination of shoot apex growth. Aeration is also important for the soil’s ability to degrade pollutants. In most cases, biodegradation of organic contaminants depends on the activities of aerobic organisms. Thus, the presence of an adequate supply of O2 in the soil is essential for biodegradation or bioremediation to occur. On the other hand, some chemicals will only degrade under anaerobic conditions. Knowledge
of a chemical’s biodegradability and whether the chemical will degrade under aerobic or anaerobic conditions plays a major role in design of effective bioremediation schemes. See also: Anaerobic Soils; Carbon Emissions and Sequestration; Diffusion; Greenhouse Gas Emissions; Hydrocarbons; Oxidation–Reduction of Contaminants; Pollutants: Biodegradation; Remediation of Polluted Soils; Vadose Zone: Hydrologic Processes
Further Reading Baker KH and Herson DS (1994) Bioremediation. New York: McGraw-Hill. Dane JH and Topp GC (eds) (2002) Methods of Soil Analysis, part 4, Physical Methods. Soil Science Society of America Book Series 5. Madison, WI: Soil Science Society of America. Eweis JB, Ergas SJ, Chang DPY, and Schroeder ED (1998) Bioremediation Principles. Boston: McGraw-Hill WCB. Gerstl Z, Chen Y, Mingelgrin U, and Yaron B (eds) (1989) Toxic Organic Chemicals in Porous Media. New York: Springer-Verlag. Glinski J and Steniewski W (1985) Soil Aeration and its Role for Plants. Boca Raton, FL: CRC Press. Hillel D (1998) Environmental Soil Physics. San Diego, CA: Academic Press. Jury WA, Gardner WR, and Gardner WH (1991) Soil Physics. New York: John Wiley. Scott HD (2000) Soil Physics, Agricultural and Environmental Applications. Ames, IA: Iowa State University Press.