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Understanding Nitrous Oxide – the Greenhouse Gas of Most Significance in Agriculture

September 1, 2014

In the Abbotsford Agricultural Waste Strategy (on this blog), we identified one of the solutions for a sustainable agriculture is to develop “strategies to reduce ammonia and greenhouse gas emissions from agriculture through incentives or tax reduction”.

In the last post, we identified nitrous oxide as the most significant greenhouse gas emitted from agriculture (40-50% of greenhouse gas emission), and recommended implementing mandatory nutrient management planning as being the strategy that has the greatest potential for mitigation.

In this post, we will review the science and microbiology of nitrous oxide production and emission, so we can better understand how implementing a nitrogen management planning process will reduce GHG emissions.

Nitrous oxide has the radiative forcing 296 times that of carbon dioxide, which means that it doesn’t take much of this gas to have a big effect on greenhouse gas contributions. Crutzen (1970) was one of the first scientists to link nitrous oxide with ozone depletion. He also connected it with fertilizer use.

“Over the past 100,000 years, concentrations of nitrous oxide in the atmosphere have rarely exceeded 280 ppb. Levels have risen since the 1920s, however, reaching a new high of 324 ppb in 2011. This increase is primarily due to agriculture.” (US EPA 2012)

Davidson (2009) analyzed trends in atmospheric nitrous oxide concentrations and concluded the following: “Before 1960, agricultural expansion, including livestock production, may have caused globally significant mining of soil nitrogen, fuelling a steady increase in atmospheric nitrous oxide. After 1960, the rate of the increase rose, due to accelerating use of synthetic nitrogen fertilizers. Using a regression model, I show that 2.0% of manure nitrogen and 2.5% of fertilizer nitrogen was converted to nitrous oxide between 1860 and 2005; these percentage contributions explain the entire pattern of increasing nitrous oxide concentrations over this period.”

Park et al. (2011) used naturally occuring 15N to distinguish between natural and anthropogenic sources of nitrous oxide, and confirmed that increased atmospheric nitrous oxide concentrations is primarly due to increased use of nitrogen-based fertilizer.

Nitrous oxide is a greenhouse gas that is important to consider in agriculture in Abbotsford for three reasons.

1. Nitrous oxide “leakage” in the nitrogen cycle can’t be avoided.

2. Nitrous oxide emission occurs during nitrification – the conversion of ammonium to nitrate, a microbially mediated process in the presence of oxygen in soil amended with manure or fertilizer, in soil following deposition of ammonia, or in surface waters contaminated with ammonium (IPCC 2007) .

3. Nitrous oxide emission occurs during denitrification – the conversion of nitrate back to atmospheric N2, a microbially mediated process in the absence of oxygen in soil or manure, following manure or fertilizer application to the field, or in surface or groundwater contaminated with nitrate (IPCC 2007).

Nitrous Oxide Emission Naturally Occurs as Part of the Nitrogen Cycle

The best way to understand the importance of nitrous oxide is to show it using a simplified nitrogen cycle.

Simplified nitrogen cycle showing where nitrous oxide emission occurs.

Simplified nitrogen cycle showing where nitrous oxide emission occurs.

All of the nitrogen fixed from the atmosphere either through fertilizer manufacture, N fixation by legumes, or nitrate production by lighting enters the nitrogen cycle is returned to the atmosphere via a process called denitrification. There is significant nitrogen cycling within this simplified cycle, especially between organic nitrogen, ammonium and nitrate. As we can see from the diagram above, nitrous oxide emission occurs during internal cycling as well, during the transformation from ammonium to nitrate. This process is called nitrification.

Nitrous Oxide is Emitted During Denitrification

Denitrification is defined as the “mircobial reduction of nitrate or nitrite to gaseous nitrogen, either as molecular nitrogen or as an oxide of nitrogen” (Soil Science Society of America 1979).  When nitrous oxide was first considered an important greenhouse gas, it was considered that denitrification was the main contributor because N2O was a known intermediate gas in the microbially mediated denitrification process. The final product of denitrification is atmospheric nitrogen, which represents the final step in the nitrogen cycle.

Nitrogen species change from nitrate to dinitrogen during denitrification, showing nitrous oxide as an intermediate.

Nitrogen species change from nitrate to dinitrogen during denitrification, showing nitrous oxide as an intermediate.

The amount of N2O emitted during denitrification depends on process conditions. There are three key factors required for denitrification to occur.

1. Nitrate (NO3-) needs to be present

2. Denitrifying bacteria require a carbon source as an energy source

3. No oxygen (O2) present, otherwise the microbes use O2 as the electron acceptor

Nitrous Oxide is Emitted During Nitrification

Nitrification is the biological conversion of reduced nitrogen in the form of ammonia or ammonium to organic N or oxidized N in the form of nitrate or nitrite (Norton and Stark 2011). The understanding that nitrous oxide emission could occur as a result of nitrification was relatively recent (Bremner and Blackmer 1981). The contribution of nitrification to total nitrous oxide emission appears to be increasing. Park et al. (2011) distinguished that the relative contribution of nitrification to global microbial N2O production increased from 13% in 1750 to 23% in 2005.

Intermediates in the nitrification pathway from ammonium to nitrate, showing where nitrous oxide is emitted.

Intermediates in the nitrification pathway from ammonium to nitrate, showing where nitrous oxide is emitted.

There are two primary conditions that are required for nitrification to occur.

1. Nitrifying bacteria require oxygen

2. Ammonium must be present, which is used as an energy source by the bacteria.

In addition, nitrification is signficantly reduced at temperatures greater than 40 C.

Nitrous oxide emission has also been reported via a process known as heterotrophic nitrification, where the nitrifying bacteria utilize carbon as their energy source (Norton and Stark 2011). Cai et al. (2010) reported that heterotrophic nitrification increased with increasing carbon availability. Paul et al. (1993) also measured increased nitrous oxide emission during nitrification when additional carbon was added to the soil. Poth and Focht (1985) also introduced the concept of nitrifier denitrification, where heterotrophic nitrifiers are able to first oxidize ammonia to nitrite, then reduce the nitrite to dinitrogen, with nitrous oxide as the intermediate product. It is important to note that in the nitrifier denitrification process, the first step is an oxidative step and hence requires oxygen.

Addition of carbon does two things, both of which may increase potential nitrous oxide emission. The first is that it provides readily available carbon which may increase heterotrophic and denitrifier nitrification, and it also increases oxygen demand, which increases the potential for anaerobic microsites in manure, compost or soil. This was also concluded by Petersen and Sommer (2011) where they state: “Readily degradable components serve as an O2 sink, as well as a C and energy source for heterotrophic denitrification at oxic-anoxic interfaces”

There appears to be a number of controlling factors that have been suggested for the amount of N2O emitted during nitrification. They include:

1. Oxygen supply – which may include anaerobic microsites

2. Carbon supply – which feeds heterotrophic nitrifiers and reduces oxygen supply (Sommer et al. 2004)

3. Carbon supply – which changes microbial population in favor of heterotrophic nitrifiers (Paul et al. 1993).

4. Ammonia concentration – increases N2O emissions due to higher N cycling rates (Hwang et al. 2006)

It appears that all of the above factors are important. The fourth factor – the carbon ammonium ratio appears to be contradictory to the second and third, however, it appears that the carbon/ammonium ratio is controlled more by the ammonium concentration in this case. In some preliminary research, I measured N2O emissions from dairy cattle manure that had been fed diets varying in protein and found that N2O emissions from the dairy cattle manure amended to soil was correlated with the ammonium concentration in the manure (unpublished data).

Manure application to soil creates ideal conditions for N2O emission:

1. the soil oxygen supply is reduced because soil pores are blocked and oxygen in the soil is used up by the microbes feeding on the carbon in the manure.

2. the manure provides carbon for microbes that reduces the oxygen content in soil and supplies food for the denitrifying bacteria.

3. the manure contains ammonium, part of which is converted to N2O as a byproduct of nitrification to soil nitrate.

Excess manure application (more application than is needed by the crops) is of concern because it enhances the conditions for N2O emission. Given that N2O emission comprises 40-50% of GHG emissions from animal agriculture, the most likely and potentially achievable strategy to reduce greenhouse gas emissions from animal agriculture is to implement mandatory nutrient management planning.

We measured N2O emission from manured soils at Agriculture and Agri-Food Canada in 1996.

We measured N2O emission from manured soils at Agriculture and Agri-Food Canada in 1996.


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