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Does Anaerobic Digestion Reduce Nitrous Oxide Emissions?

October 8, 2015

We can expect nitrous oxide emissions to significantly increase following anaerobic digestion of animal manures, unless ammonia emission is being controlled and the digested manure is applied at a rate that optimizes the nitrogen use efficiency by the crop.

Nitrous oxide is an important greenhouse gas to consider, as more than 60% of the world’s nitrous oxide emissions come from agriculture (Smith et al. 2007). In Canada, the estimate is higher at 65% (Kebreab et al. 2006). Agriculture and Agri-Food Climate Change Table (2000) identified farm nutrient management plans as a mechanism for optimizing nitrogen applications and a strategy to reduce N2O emissions. Smith et al. (2007) concluded that:

“Improving N use efficiency can reduce nitrous oxide emisions and indirectly reduce GHG emissions from N fertilizer manufacture. By reducing leaching and volatile losses, improved efficiency of N use can also reduce off-site nitrous oxide emissions.”

European modelling suggests a 40% reduction in N2O emission by implementing anaerobic digestion (Leip et al. 2010) and a Canadian literature review suggests up to 70% reduction (Vanderzwaag et al. 2011).  Given that N2O emissions are almost 3 x higher than methane emissions from manure storages on farms (Dairy Farmers of Canada 2010), and N2O emission reduction is not included in the GHG emission protocol for dairy farms in Canada (ADFI 2008), we would conclude that there is a substantial untapped GHG emission reduction potential! We need to be cautious with this modelling.

The European modelling assumes that digested manure is applied at rates that match crop uptake. There is no mandatory nutrient management planning currently required in British Columbia, and we can expect additional nitrogen to be imported to farms that are digesting manure. This is required for economic reasons.

Excess nutrients is already a concern in British Columbia, as it is in many areas in the US.  Kruger and Frear (2008) reported that co-digestion may result in significant import of nutrients, with USDA already reporting that 36% and 55% of larger US dairies already have N and P overloads on their farmland, respectively. Frear (2010) reported that 15-20% addition of food waste would increase the total amount of nitrogen on the farm by 57%, and increase ammonia by 23%.

When digested manure is utilized as a plant nutrient source, the research comparing N2O emissions from undigested manure with digested manure shows mixed results. Important factors involved in N2O emission include the following:

  1. Carbon content in the manure – increased carbon increases N2O emission. Anaerobically digested manure contains less available carbon
  2. Ammonium concentration – higher ammonium concentrations increase N2O emission rates. Anaerobically digested manures have a higher ammonium concentration than undigested manures.
  3. Manure pH – higher pH in digestates increase NH3 losss following application to soil, which decreases direct N2O emissions.

Available carbon in manure or digestate has a significant impact on N2O emission potential. Paul et al. (1993) observed increased N2O emission from manured soils when additional carbon was added. Sommer et al. (2004) developed a model describing how additional available carbon in the manure increased the oxygen demand and hence created lower oxygen concentrations, thereby increasing N2O emission. The logical conclusion was that if available carbon was removed via anaerobic digestion, N2O emission would decrease.

Ammonium concentration in the manure or digestate also has a significant impact on N2O emission potential. Heller et al. (2010) measured a significant correlation between soil NH4+ and N2O emission. Chadwick et al. (2011) observed that N2O emission rates were correlated with readily available N in manure, and not on total N. Paul et al. (1993) measured higher N2O emission rates following addition of ammonium and acetate to soil, than following addition of nitrate and acetate to soil. This suggests that N2O emission from soil is not simply due to increased anaerobic conditions in the soil.  Digested manures typically have higher ammonium concentrations than non-digested manure.

Manure pH also has a significant impact on direct N2O emission potential from soil mostly because it will affect NH3 losses following manure application. Patni and Jui (1985) observed an increase in manure pH following disappearance of volatile fatty acids in manure slurry. Paul et al. (1993) explained how the pH of manure is balanced by volatile fatty acids that lower the pH, and ammonium that increases pH. When the volatile fatty acids are utilized to produce methane during digestion, the pH of the manure naturally increases. This also increases the potential for ammonia emission following application of digestate. This has been observed in several studies. Paul et al. (1998) observed that NH3 emission rates from dairy cattle increased with increasing manure pH.

A review of research comparing N2O emission from digested or non-digested manures shows mixed results. Amon et al. (2006) measured higher NH3 emission and N2O emission was higher from the digested manure than from the undigested manure following application to soil.

Wulf et al. (2002) measured higher N2O emission on grassland from digested manure slurry than from non-digested slurry. They observed the opposite on arable land. They too, measured higher ammonium concentration and higher pH in the digested slurry. They also measured higher NH3 emission from digested manure on grassland, and suggested that indirect N2O emissions resulting from NH3 emission may be very important in estimating greenhouse gas potential.

The 20-40% potential reduction of N2O resulting from anaerobic digestion in Europe is derived primarily from the research of Petersen (1999), who measured significantly lower N2O emissions from digested slurry compared with undigested slurry. There are two notable observations in this study. The first is that the slurry application rates were based on the NH4+ content of the slurry, not on total N, or equal application rates. This meant that the application rate of undigested slurry was significantly higher than the rate of digested slurry. The second observation was that the soil NH4+ measured shortly after manure application appeared to be significantly higher with the undigested manure. This suggests that N2O emission is very much related to NH4+ in the manure and a balancing manure or digestate application based on NH4+ concentrations becomes a very important part of waste management to reduced N2O emissions.

There are three studies with swine manure that clearly demonstrate a dramatic reduction in N2O emission following anaerobic digestion. Vallejo et al. (2006) measured a 40% reduction in N2O emission from soil following digested and separated pig slurry compared with raw pig slurry. In both of these cases, the primary factor affecting N2O emission reduction is the loss of carbon and the decreased solids associated with raw manure. In a three year study, (Chantigny et al. 2007) measured 54 to 69% and 17 to 71% lower N2O emission with digested swine manure than with raw swine manure in loam soil and in sandy loam soil cropped to forage. Bertora et al. (2008) measured a 55% reduction in N2O emission following application of digested pig slurry to soil compared with raw pig slurry. They concluded that available carbon was the primary factor and NH4+ content was the secondary factor affecting N2O emissions from organic materials applied to soil.

Collins et al. (2010) reported reduced N2O emissions from soil following application of separated dairy manure digestate compared to the undigested manure. In a field study, they observed an almost 50% emission reduction in one year, but only a very small reduction in the second year. In a laboratory study measuring N2O emission during the first 48 hours, they observed higher N2O emission from the digested manure one year, and lower N2O emission the second year, compared to the undigested manure. Crolla (2011) measured significantly higher NH3 emissions from digested manure applied to soil in the spring during two consecutive years. They measured higher N2O emissions following application of digested manure in one year, and slightly lower N2O emissions compared to undigested manure in the following year. Herrmann (2012) concluded that in some cases, N2O emission was lower in soil following application of digestate compared with raw pig slurry, and in other cases, the opposite observation was made. Lemke et al. (2012) reported higher N2O emissions from undigested manure (4% of total N applied) compared with digested swine manure (1.4% of N applied) in Saskatchewan.  Joo et al. (2013) measured slightly less N2O emissions from soil following digested manure application than undigested manure application. They also reported significant CH4 and CO2 emission from the storage lagoons following anaerobic digestion, suggesting continued decomposition of carbon.

The N2O model developed described in Sommer et al. (2004) predicted that N2O emission could be reduced by more than 50% following anaerobic digestion. Using a modelling approach (CAPRI model) for all countries in the EU, Leip et al. (2010) concluded that N2O emission following field application was up to 40% lower following anaerobic digestion. The literature does not consistently support these predictions.

The potential for N2O emission reduction following anaerobic digestion exists, however, continued VFA and CH4 production in digestate fuel N2O emission during nitrification following field application. This means that we are not able to consistently assign an N2O emission reduction factor for anaerobic digestion.

The research suggests that net N2O emissions following manure or digestate application to soil is correlated with the ammonium content of the manure or digestate, and that N2O emission increases non-linearly with increased ammonium application to soil.


There is a potential to reduce nitrous oxide emissions by anaerobically digesting the manure. This potential only applies if ammonia emissions during digestate storage and following field application is minimized, and if the manure nitrogen is applied at agronomic rates.

Currently in British Columbia, because anaerobic digesters require off-farm waste to be economically viable, and there is no mandatory nutrient management planning, we can expect that anaerobic digestion of our animal manures will significantly increase nitrous oxide emissions.


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