Nitrous Oxide and California Agriculture
- Fertilizer Rates
- Fertilizer Placement
- Fertilizer Type
- Fertilizer Timing
- Irrigation Systems
- Nitrification and Urease Inhibitors
- Cover Crops
What is nitrous oxide?
Nitrous oxide (N2O) is a powerful greenhouse gas (GHG) produced by microbes that live in the soil. The so-called laughing gas is no laughing matter, as one molecule of N2O released into the atmosphere contributes almost 300 times more to climate change than a single molecule of carbon dioxide (CO2). This means that N2O can have a strong influence on the climate despite being produced in relatively smaller quantities.
Where does nitrous oxide come from?
Nitrification and denitrification are the two main ways that microbes produce N2O. Nitrification and denitrification are both stimulated by nitrogen (N) fertilization.
Nitrification refers to the conversion of ammonia/ammonium (NH3/NH4+) into nitrate (NO3-). This conversion is performed by nitrifying microbes. These microbes consume ammonia/ammonium and “breathe” oxygen. Microbes are not able to convert ammonia/ammonium into nitrate at 100% efficiency, and some N is lost as N2O gas. While overall rates of nitrification decrease as oxygen becomes less available, which can occur following soil wetting or compaction, the amount of N2O produced per molecule of ammonium increases. The nitrification process has two steps: 1) ammonia oxidization, when ammonia is converted into nitrite (NO2-) and 2) nitrite oxidization, when nitrite is converted into nitrate.
Nitrification occurs readily in soils with moderate water contents, as higher levels of soil water limit the amount of oxygen in the soil. However, the relative amount of N2O produced per unit of ammonia processed increases as oxygen levels continue to decrease. This means that significant amounts of N2O can be produced from nitrification even when oxygen levels are low, but above zero. Rising soil temperatures increases the rate of nitrification. Nitrifying microbes prefer a pH between 6.5 and 8.8, so nitrification rates are highest in this pH range.
Denitrification can be thought of as the complementary process to nitrification. Denitrification refers to the conversion of nitrate into nitrogen gas (N2), which is unreactive and harmless in the atmosphere. The term denitrification generally refers to the complete conversion process from NO3- to N2 gas, though individual steps in the process are often referred to as denitrification as well. N2O forms during an intermediate stage of denitrification when, under certain soil conditions, microbes are not able to fully convert nitrate to N2. As N2O is a gas, it can be released from the soil before it has the chance to be converted into N2, especially when it is produced close to the surface. As opposed to nitrifying microbes, denitrifying microbes “eat” carbon containing compounds produced by other microbes and “breathe” nitrate instead of oxygen. In fact, oxygen damages the enzymes used in denitrification, so the process is extremely limited when oxygen is available.
What does nitrous oxide have to do with agriculture?
In particular, N fertilization practices contribute significantly to N2O production, as fertilizers increase the availability of ammonia to nitrifying microbes and nitrate to denitrifiers. N2O emissions are highest when fertilizer applications exceed crop needs.
What conditions result in nitrous oxide emissions?
Soil microbes jump into action from the moment that fertilizer is applied to a field, feasting on this new source of nutrients. These microbes will continue to consume the fertilizer nitrogen, and release N2O, as long as it remains available and conditions such as temperature and moisture level allow for continued microbial activity.
The application of ammonia or ammonium based fertilizers results in high rates of nitrification and therefore losses of N in the form of N2O. Application of other N fertilizers, including manure and urea, also associated increase rates of nitrification and N2O production.[4, 5] When availability of ammonia becomes limited, less substrate for nitrification is available and N2O production decreases.
Denitrification occurs when there is available nitrate and water levels from irrigation or precipitation are high enough to expel most oxygen from the soil. For soils that are more compacted, and therefore have less space for air, less water is needed before oxygen levels are low enough for denitrification to occur.[1, 6, 7]
While irrigation and rainfall can result in decreases in soil oxygen, microbes breaking down organic matter also consume oxygen. If enough oxygen is consumed, oxygen levels can be lowered to the point at which denitrification occurs.
When water levels are near saturation, when the soil cannot hold any more water, the main product of denitrification is N2 instead of N2O. Even when this is the case, the denitrification process still represents a loss of N that could otherwise be used by crops.
The rate, timing, source, and placement of fertilizer all impact how microbes process fertilizer N. The relationship between other factors, including irrigation systems and cover crop utilization, also influence N2O emitting microbes.
How can I reduce nitrous oxide emissions without sacrificing returns?
By maintaining maximum fertilizer efficiency, N2O emissions can be limited without reducing yields. Applying fertilizer of the right type, at the right rate, in the right place, and at the right time, also known as the “Four Rs,” ensures that crops receive the nutrients they need while limiting excess nitrogen available to microbes that produce N2O.
N2O emissions can also be reduced by efficient irrigation management, especially the implementation of drip irrigation, which limits the time that conditions ripe for denitrification occur.
A more comprehensive guide on N management can be found on the nitrogen management page. Comprehensive fertilization guidelines for a variety of crops grown in California can be found on this CDFA website.
Management Considerations: Effective nutrient monitoring is the gold standard of a sustainable growing operation, and limiting N2O emissions is one of many benefits that results from informed fertilization practices. Pre-plant soil nutrient tests help determine appropriate N fertilization rates and are a critical component of developing a nutrient management plan. Pre-sidedress soil nitrate testing “quick tests” can be used to adjust fertilizer application rates mid-season. A detailed nutrient management plan helps to increase fertilizer efficiency and limit N2O production.
Background: After a concentrated source of N has been added to the soil, microbes are not able to process the added nutrients quickly enough. This can create a backlog of nitrite, which microbes can utilize in place of oxygen, producing N2O in the process. Applying N fertilizer in concentrated bands, as opposed to being broadcast or delivered as part of a drip based fertigation system, therefore stimulates greater N2O losses. With high water levels or low oxygen availability this becomes a significant source of N2O produced from soil.
Management Considerations: Concentrating fertilizer below the soil surface allows for increased plant root uptake. The advantages of increased yields must be balanced with attempts to limit N2O emissions.
Changing the type of fertilizer applied may influence N2O emissions. Anhydrous ammonia, a more concentrated source of N than urea based fertilizer, increases soil pH, which can lead to an increase in soil nitrite and N2O production. Potential differences in fertilizer driven N2O emissions are strongly influenced by soil conditions, such as temperature, moisture, and pH.
Management Consideration: Anhydrous ammonia and aqua ammonia are currently the two least expensive N fertilizer options (as of 2016).
Other studies performed outside of California also show that ammonia based fertilizers stimulate more N2O emissions than other N fertilizers. [15-18]
Field studies comparing fertilization timing and N2O emissions have yet to be carried out in California. Research from other locations shows that splitting fertilizer application between two events can reduce N2O emissions by 17-30% without reducing yields.[20, 21]
Management Considerations: For crop specific fertilization guidelines, visit the CDFA fertilization guidelines.
Two main factors are at play in explaining why targeted irrigation strategies help to reduce N2O emissions. First of all, high levels of soil moisture stimulate N2O production from both nitrification and denitrification. Second, drastic shifts in soil water content cause peak emissions events. In other words, when dry soil gets wet suddenly, either due to heavy irrigation or the first winter rains, a significant amount of N2O can be emitted in a matter of days. Targeted irrigation systems help prevent the conditions ripe for N2O production by keeping the soil consistently moist. Additionally, when N2O is produced deep in the soil near the drip line, there are more opportunities for denitrifying organisms to “catch” N2O and convert it into N2.[22, 23] Furthermore, plant root water uptake can help limit high levels of moisture in shallow soil layers, helping prevent large N2O emissions. 
Drip systems are more targeted than furrow irrigation in the delivery of water and nutrients (through fertigation) to crop roots. This can reduce both the time and area over which the conditions that promote N2O production occur. In annual cropping systems, subsurface drip reduces N2O emissions when compared to flood or furrow irrigation. For almond orchards, however, a number of studies suggest that micro sprinkler systems decrease N2O emissions when compared to surface drip. Drip irrigation has also been shown to reduce water use and decrease nitrate leaching. 
Management consideration: Implementing drip irrigation has associated costs and specific challenges that must be addressed. For more information about implementing drip irrigation in your cropping system, contact your local UC Cooperative Extension adviser.
Background: Nitrification inhibitors are chemical products that inhibit ammonia oxidation in soil (the first step of nitrification). This limits N2O production from both nitrification, by slowing the rate at which it occurs, and denitrification, by limiting nitrate availability.
Nitrification inhibitors, when applied together with N fertilizer, can reduce N2O emissions by 35%. [35, 36] Several field experiments in California are currently evaluating the efficacy of nitrification inhibitors in reducing N2O emissions.
Urease inhibitors, on the other hand, limit the rate that urea breaks down and becomes available to both plants and microbes. Current research suggests that urease inhibitors are not effective at either reducing N2O production or increasing yields in California. [35, 47]
Management Considerations: Currently dicyandiamide (DCD) and nitripyrin are the only nitrification inhibitors approved in California, but various slow release fertilizers are also being used. To maximize the benefits of nitrification inhibitors, the fertilizer with nitrification inhibitor should be applied as late as possible to maximize crop N uptake as ammonium. For example, a side-dress application should take place when plants are large enough to take up significant amounts of ammonium N. Plant performance may suffer if nitrification inhibitors are applied in excess of their recommended rates.
The large majority of nitrogen in the atmosphere exists in the form of dinitrogen gas (N2). This gas is unavailable to plants, and must be “fixed” before it can be used. This fixation is performed by a group of microbes known as N-fixers, who convert N2 into ammonia (NH3). Leguminous plants, such as beans or alfalfa, house N-fixing microbes in nodules in their roots. These microbes give some of this fixed nitrogen to plants in exchange for carbon compounds the microbes use as food. This N now stored in plant biomass can become accessible to future crops after the plants are incorporated into the soil, reducing the need for fertilization. However, elevated soil N from legume cover crops can also result in increased N2O emissions. On the other hand, grasses such as ryegrass or millet take up needed N from the soil, potentially limiting losses from nitrate leaching and nitrous oxide production.[31, 32]
The type of cover crop is a major factor driving differences in N2O production rates. While a recent study summarizing current findings showed that cover crops increased N2O emissions 60% of the time, these observations were split strongly along cover crop species lines. While leguminous cover crops, which introduce additional N to the soil, were frequently found to increase N2O emissions, the use of non-leguminous grasses largely did not cause significant production of N2O.  A recent analysis of legume based fertilization systems in Mediterranean climates, like those in California, found that N2O emissions in legume based systems were 60-80% lower when compared to other climate types. 
Management considerations: The choice of cover crop and timing of incorporation can be important factors that influence N2O production. Legume cover crops often increase N2O emissions, though the extent of this effect depends on irrigation practices, tillage, and a handful of other factors that are outside the control of growers, including precipitation and soil texture. Incorporating N fertilization credits from leguminous cover crops into nutrient management plans can help reduce N2O emissions stimulated by the use of cover crops. On the other hand, improper management of non-leguminous cover crops can reduce the availability of soil N due to immobilization.
The advantages of cover cropping stand in contrast to concerns about N2O emissions. For more information about the benefits of cover crops, check out the UC SAREP cover crop database and our cover crops page.
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