Soils act as both sources and sinks of N2O and NO. However, on the global scale the N2O and NO emissions dominate the sink activity. The production and consumption of N2O and NO in soils involves both biotic and abiotic processes. Numerous groups of micro-organisms contribute to the production and consumption of NO and N2O, but biological nitrification and denitrification are the dominant processes involved.
Biological denitrification is the reduction of nitrate (NO3-) or nitrite (NO2-) to gaseous N oxides and molecular N2 by essentially aerobic bacteria. Nitrification is the biological oxidation of ammonium (NH4+) to NO2- or NO3- under aerobic conditions. Under oxygen limited conditions nitrifiers can use NO2- as a terminal electron acceptor to avoid accumulation of the toxic NO2-, whereby N2O and NO are produced.
The bacterial processes of denitrification and nitrification are the dominant sources of N2O and NO in most systems. Only denitrification is recognized as a significant biological consumptive fate for N2O and NO. The chemical decomposition of HNO2 (or chemical denitrification), that is, the reduction of NO2- by chemical reductants under oxygen limited conditions and at low pH, can also produce N2, N2O and NO. Chemical denitrification generally occurs when NO2- accumulates under oxygen limited conditions, which may occur when nitrification rates are high, e.g. after application of NH4+-based mineral fertilizers or animal manure. This process may account for 15-20 percent of NO formation.
Plant uptake of NO and NO2 is a further important biological process determining the exchange of NOx at the Earth's surface. Some other abiotic processes may also produce and consume trace amounts of NO.
The simplifying conceptual `hole in the pipe' model is useful in understanding the processes of NO, N2O and N2 emissions. In this model, gas production and exchange with the atmosphere depend on: (i) factors controlling the amount of N flowing through the pipe (that is those affecting denitrification and nitrification rates e.g. N availability and temperature); and (ii) the size of the holes in the pipe through which N gases leak. Factors controlling the partitioning of the reacting N species to NO, N2O or more reduced or oxidized forms regulate the size, while the rate at which N moves through the pipes determines the importance of the leaks. In addition to soil and climate characteristics, agricultural management may also interfere with soil processes and thus influence N availability and the relative rates of N2O and NO.
In agricultural soils N inputs from various sources determine the amount of N flowing through the pipe. N fertilizer, N inputs in agricultural soils, animal manure, crop residues, deposition and biological N fixation are strong stimulants of denitrification and associated N oxide fluxes.
The magnitude of NH3 volatilization also determines the availability of N for nitrification and denitrification. Studies have found denitrification to be lower when NH3 losses are high in both upland soils and wetland rice systems.
Whether denitrification or nitrification dominates depends on many different factors. Nitrification is a relatively constant process across ecosystems, whereas denitrification rates are temporally and spatially variable. In most soils, availability of NH4+ and oxygen are the most important factors controlling nitrification. The major controls on biological denitrification include the availability of organic carbon, oxygen and nitrate (NO3-) or other N oxides.
Various studies have found high denitrification activity in `hot spots' created by decomposing organic matter which generates anaerobic microsites. This phenomenon may explain some of the high spatial variability of soil denitrification commonly observed.
N2/N2O ratios observed under different conditions in laboratory experiments were found extremely variable and dependent on the available C and NO3-, and on the moisture content of the soil. The NO/N2O emission ratio has been proposed as an indicator of the importance of nitrification and denitrification. Laboratory studies indicate that for nitrifiers the NO/N2O ratio is close to unity while for denitrifiers this ratio is less than unity. As denitrification and nitrification often occur simultaneously, it is not possible to extrapolate these results to field conditions. However, even in field situations the NO/N2O ratio may provide an indication of the dominant process responsible for NO emission.
Soil moisture and temperature control soil processes at all levels by governing organic matter decomposition rates, denitrification and nitrification. Some studies have shown that emissions of NO increase with increasing soil temperatures. However, the relationship between temperature and NO fluxes is subject to considerable uncertainty with numerous exceptions in temperate and tropical systems where no clear relationship was found between temperature and NO fluxes. During denitrification the ratio of N2O/N2 generally increases with decreasing temperatures.
In systems where soil N temporarily accumulates due to wet-dry or freeze-thaw cycles, then early spring, winter and autumn in temperate climates may account for an important part of the annual N2O emission from agricultural or any other land.
Soil-water content influences N2O and NOx emissions from all types of soil. In general, microbial activity peaks at 30-60 percent water-filled pore space. Nitrification and associated N2O and NOx production also show maximum activity at 30-60 percent water-filled pore space, while optimum conditions for denitrification may occur at 50-80 to 60-90 percent water-filled pore space.
The oxygen and moisture status and gas diffusion in agricultural soils depend on soil texture and drainage. Fine-textured soils have more capillary pores within aggregates than do sandy soils, and so hold soil water more tightly. As a result, anaerobic conditions may be more easily reached and maintained for longer periods within aggregates in fine-textured soils than in coarse-textured soils.
The wetting of dry soils causes pulses in N mineralization, nitrification and NO and N2O fluxes. The alternating drying and wetting of soils enhances the release of N2O and NO from the soil to the atmosphere although peaks in N2O production may decline with subsequent wetting events.
Soil pH has a marked effect on the products of denitrification. Denitrification rates are slower under acid than under slightly alkaline conditions, but the N2O fraction may be larger at low soil pH, particularly with an adequate nitrate supply. This is commonly attributed to the sensitivity of N2O reductase to proton activity. Emissions of N2O and NO decrease with increasing pH in acid soils, and increase when the pH of alkaline soils decreases. Soil pH is a major factor in determining the mechanism of NO formation. In one study, in an alkaline loamy clay soil (pH 7.8) nitrification was the main source of NO, whereas in an acid sandy clay loam (pH 4.7) denitrification dominated the NO production.
Before escaping from the soil to the atmosphere the N gases diffuse through the soil pore system, where denitrifiers may consume NO and N2O or plants may take up NO. In situations where high soil water content, impeded drainage, shallow groundwater, soil structure, soil compaction, fine soil texture or soil surface sealing limit gas diffusion, denitrification activity is high. However, such conditions greatly enhance the probability of denitrifiers re-consuming N2O and NO. Soils close to saturation show high denitrification activity but low N2O and NO emission. Under such conditions the oxygen supply is low and N2O and NO diffusion is limited; in the absence of oxygen as an electron acceptor, denitrifiers consume more N2O and NO than under more aerobic conditions. Under wet conditions N2O uptake from the atmosphere may occur.
Nitrous oxide fluxes from flooded rice systems during the growing season are generally lower than those from rainfed fields. The cause of this may be the anaerobic conditions prevailing in wetland rice systems. However, aerobic conditions generally prevail during the fallow period after the rice crop, allowing nitrification of NH4+ mineralized from soil organic matter and residues of crops and aquatic biota. Observed N2O emission from drained soils during the post-harvest fallow period is much higher than during the crop season.
Biological N fixation by leguminous crops, such as alfalfa, soybeans, pulses, and clovers, provides an important input of N in many agricultural systems. Although these crops generally receive no or only small N fertilizer inputs as a starter, studies have demonstrated that leguminous crops show emissions of N2O that are of the same level as those of fertilized non-leguminous crops.
The type of fertilizer influences the magnitude of nitrification and denitrification. Furthermore, the type of fertilizer may influence the magnitude of NO and fluxes. For example, anhydrous ammonia (AA), which is generally injected, commonly shows higher losses than other fertilizers.
Total N gas production and in particular emission may increase with the N application rate. However, denitrification and production may be more closely related to the amount of unused N than to the total amount. Therefore, the timing of fertilizer application is an important factor. Any prolongation of the period when NH4+-based fertilizers can undergo nitrification or NO3--based fertilizers undergo denitrification, without competition from plant uptake, is likely to increase emissions of NO and N2O.
Under tropical conditions the potential for high and NO emission rates exists, but management plays an important role in determining the magnitude of the fluxes. In a study that adjusted N fertilizer inputs to plant demand, N2O losses were low. Where fertilizer application is excessive, as in some cash crops such as banana, the N availability for nitrification and denitrification is high, and so the O2 supply becomes the primary control of NO and N2O emissions.
Fertilizer application mode and timing influence NH3 volatilization and the efficiency of plant uptake, and hence the availability of N for nitrification and denitrification. Generally, emissions from subsurface applied or injected N fertilizers are higher than from broadcast mineral fertilizers and animal manure. Compared to broadcasting, subsurface applied N has resulted in higher N2O but lower NO losses.
Inputs from crop residues in agricultural fields are important sources of C and N for nitrification and denitrification. In addition, the action of incorporation may also stimulate mineralization of soil organic matter. Some studies have observed higher denitrification activity and N2O fluxes after retaining residues on the land compared to removing them. This effect may be related to the effect of residues on the soil surface on moisture conditions of the topmost soil layer. The incorporation of residues also causes accelerated NO emissions.
The effects of tillage on mineralization of soil organic matter are well established. Tillage may also affect the conditions for N2O and NO emissions from soils. Some studies have observed higher N2O losses for no-tillage systems compared to conventional tillage. This may be related to higher denitrification activity. However, N2O losses from no-tillage systems may be lower than from tilled soils where fields have remained uncultivated for a number of years. Some studies report an increase in NO emission caused by ploughing in a temperate climate and under tropical conditions.
Ammonia is formed constantly in soils because of the biological degradation of organic compounds and NH4+ yielding mineral and organic fertilizers. As it is a gas, any NH3 present in soils, water or fertilizers can volatilize to the atmosphere. However, NH3 reacts with protons, metals and acidic compounds to form ions, compounds or complexes of varying stability. Ammonia has a strong affinity for water, and its reactions in water are fundamental to regulating the rate of loss.
After its application to the soil, the NH4+ can remain on the exchange sites, nitrify to NO3-, or decompose to NH3, depending on soil and environmental conditions:
In fertilized fields the input of NH4+ depends on: fertilizer type; the rate and mode of fertilizer application; soil moisture content, infiltration rate, and CEC; and urease activity (in the case of urea). The difference in NH3 partial pressure between the ambient atmosphere and that in equilibrium with moist soil, floodwater, or the intercellular air space of plant leaves drives ammonia volatilization. The partial pressure of NH3 in the soil is controlled by the rate of removal of ammonium or NH3 in solution, or by displacing any of the equilibria in some other way. Wind speed (regulating the exchange between soil and air), temperature, and the pH of the soil solution or irrigation water are important regulating factors, as all three variables affect the partial pressure of NH3.
Increasing temperature increases the relative proportion of NH3 to NH4+ present at a given pH, decreases the solubility of NH3 in water, and increases the diffusion of NH3 away from the air-water or air-soil interface.
In particular, the pH affects the equilibrium between NH4+ and NH3 so that the relative concentration of NH3 increases from 0.1 percent to 1, 10 and 50 percent as the pH increases from 6 to ~7, ~8 and ~9, respectively. The volatilization process itself produces acidity. The nitrification process can reduce NH3 volatilization in two ways: by decreasing NH4+ availability; and by producing acidity.
Other variables influencing NH3 volatilization include the pH buffer capacity and the CEC of the soil. The CEC is important as the negatively charged CEC absorbs the positively charged NH4+. A major part of the soil's NH3 holding capacity is attributable to soil organic matter.
Other factors include: the level of urease activity (in the case of urea application, or urine); the availability of moisture; soil texture; the nitrification rate; and the presence of plants or plant residues.
The presence of absorbed or added Ca and certain phosphate compounds may influence NH3 losses in upland systems. The loss of NH3 may be lower in the presence of Ca due to micro-environmental pH reduction, CaCO3 formation or Ca-urea double salt formation. Certain acidic phosphate compounds, such as H3PO4, NH4H2PO4 and Ca(H2PO4)2 may precipitate the micro-environmental Ca and change the environment to favour increased NH3. Both the formula (pH) and form of the phosphate added can influence reactions with Ca, and therefore NH3 losses.
Conditions in wetland rice systems require special attention. First, the measurement data in these systems suggest that NH3 volatilization rates are often higher than in upland systems, and globally they may represent important losses. Second, the conditions in inundated systems with respect to NH3 volatilization are different from those in upland systems.
In wetland rice systems, factors such as fertilizer type, rate, time and method of application, floodwater depth, and algal growth exert their influences through the primary variables (ammoniacal N concentration, the pH and temperature of floodwater, and wind speed).
The rate of NH3 loss depends on the equilibrium vapour pressure of NH3 in floodwater and on wind speed. Wind speed markedly affects NH3 volatilization from floodwater. For example, data from Philippines field studies show a linear relationship between wind speed and NH3 volatilization. This also explains why in some studies that used low or no air-exchange enclosure techniques or forced draught methods NH3 loss rates were low even with high N application rates.
The vapour pressure of NH3 in floodwater is a function of the ammoniacal N concentration, pH and temperature. The content of aqueous NH3 in floodwater increases by about a factor of 10 per unit increase in pH in the pH range 7.5-9. A linear function at a given total ammoniacal N concentration best describes the dependency of aqueous NH3 on temperature. Furthermore, water evaporation losses, which may be high under tropical conditions, will generally lead to increasing activity of NH4+, thus influencing the chemical equilibria.
Processes that potentially affect the ammoniacal concentration of floodwater and indirectly the NH3 loss include urease activity, cation exchange and N immobilization. Furthermore, assimilation of NH4+ by algae, weeds and rice plants can decrease the quantity of ammoniacal N. The competitiveness of rice plants for NH4+ varies with growth stage. One study found that plants contained negligible amounts of 15N-labelled urea 1 week after broadcast and incorporated urea, and less than 10 percent of urea broadcast to floodwater 2-3 weeks after transplanting, and about 40 percent of the 15N in urea applied at panicle initiation.
In sandy soils or soils with low CEC the fertilizer treatment may result in high ammoniacal concentrations in the floodwater, with large losses of applied N despite incorporation of the fertilizer.
In flooded rice fields the pH of floodwater appears to be synchronized with the cycles of photosynthesis and net respiration, i.e. the depletion and addition of CO2 to the floodwater. Carbon dioxide in solution, as a product of the photosynthesis and respiratory balance of aquatic organisms, and occurring as undissociated carbonic acid, bicarbonate and carbonate ions is the buffering system that regulates the water pH. Water pH values of 9.5-10 can occur in shallow floodwater populated by aquatic biota under high solar radiation, rising during the day and dropping at night. Diurnal pH variations of 2-3 units are not uncommon where fertilizer N is broadcast in rice fields.
The correlation between water pH and the carbonic acid system in water is complex, but in its simplified form it can be characterized by the electroneutrality condition:
[H+] = [HCO3-] = 2[CO32-]+[OH-]
and equilibrium activities: -log[H+] = -log[HCO3-] = 5.65; -log[CO2 aq.] = -log[H2CO3] = 5.0 (as a non-volatile acid); -log[H2CO3] ~7.8; -log[CO32-] = 8.5. In shallow-water systems found in flooded rice cultivation there is CO2 exchange between the various submerged aquatic plant species and the water. The major organisms are algal forms, which develop quickly into a large biomass. The generalized reaction involving the plant biochemistry of CO2 is as follows:
The photomineral process decreases the net activities of [CO2aq.] + [H2CO3] during favourable daylight periods, but when respiratory activity exceeds photosynthesis H2CO3, acidity and the total concentration of dissolved carbonic acid increase. A number of complex relationships exist in the system, such as reactions of CO2 with alkaline earth minerals, carbonates, and divalent cations. Factors that also affect water pH in flooded soil systems are: soil type, its electrical conductivity, and previous cropping history; soil management practices such as puddling; and the quality of the irrigation water.
Fertilizer management and the stage of crop development influence the magnitude of the diurnal fluctuation of the pH of floodwater, probably through their effect on the algal biomass. A strong increase in the maximum values of the floodwater pH occurs after urea application. Because phosphorous is also generally applied at this time, the algal biomass increases rapidly. In one study, in some fields the floodwater pH did not increase as much due to the absence of algal growth, a fact attributed to the weather conditions during the rice crop at that location in China in that particular year. Diurnal variations in floodwater pH found at panicle initiation when the crop shades the floodwater are generally smaller than in early stages of the growing season.
As NH3 volatilization produces acidity, alkalinity (primarily HCO3- in floodwater) must therefore be present to buffer the production of H+ to sustain NH3 volatilization. Where farmers use urea as a fertilizer, its hydrolysis is the major source of HCO3-. When ammonium sulphate (AS) is applied, the irrigation water is probably the major source of alkalinity. Even with low alkalinity in floodwater, evapotranspiration and repeated irrigation may cause a build up of alkalinity. Additional alkalinity may result from increased soil reduction at night coupled with CO2 production.
Fertilizer management, through its influence on the concentration of ammoniacal N in the floodwater, has a pronounced effect on overall NH3 loss. NH3 losses are generally highest with applications of urea or ammonium sulphate fertilizer 2-3 weeks after transplanting the rice. Volatilization rates are much lower for ammonium sulphate and urea applications a few days before panicle initiation of the rice crop (commonly ~50-60 days after sowing), or at booting (~65-70 days after sowing). The reason is that the rice crop reduces wind speed and thus NH3 exchange between the water surface and the air, the crop shades that floodwater and thus reduces algal growth. This causes lower pH levels and smaller amplitudes in the daily pH cycle than in fields with no or small rice plants.
The incorporation of urea to puddled soil, when done without standing floodwater, also reduces NH3 losses. The cause of this reduction may be a better contact of the fertilizer with the soil and an associated increase in the reaction of the ammonium ions formed with the cation exchange sites and the immobilizing micro-organisms. Another possible cause may be a reduction of algal growth as a result of lower ammoniacal concentration of the floodwater. The incorporation of urea in the presence of floodwater was effective only in soils with a high CEC.