The IPCC suggests that N2O emissions during the composting process are 0.03 g per kg of waste. If we assume a nitrogen concentration of 1% (wet weight basis) in the waste, it works out to an emission factor of 3% of the nitrogen in the waste. This is the factor that we use in Canadian composting protocols as well – for example the one used in Alberta. The Alberta protocol does not apply to animal manures or to compost that contains animal manures. The reasons for this are that animal manures may be higher in nitrogen content and may emit greater quantities of nitrous oxide. One question that we have to ask is how the increasingly higher percentage of food waste (and higher nitrogen content of the composting material) will impact N2O emissions during composting?
Most of what we know about N2O emission during composting comes from animal manure. We have identified some of the microbes that produce N2O. We think we understand the several chemical pathways that produce it, but we are not confident of which pathway is more important, and less confident of which microbes are producing it. We have research that reported up to 9.9% of the total N lost as nitrous oxide and we have other studies that show that compost takes up N2O (you are welcome to read more about the science of N2O during composting at http://johnpaulprofessional.wordpress.com/2013/06/14/nitrous-oxide-emission-during-composting-of-animal-manures/).
It is important to understand more about N2O emission because we may have to rethink how we include it in our protocols. Understanding N2O emissions may also help establish policy direction about how we manage our waste, including how we may promote composting. For example, when I began Transform Compost Systems in 1998, I was already aware of N2O emissions during composting from previous research, and already suspected that:
- N2O emission may be reduced if we could oxidize available carbon before the ammonium was oxidized.
- N2O emission (and methane emission) may be reduced by increasing the oxygen concentrations in our composting material.
- Containing the composting process would allow potential capture of ammonia emission (which contributes to N2O emission), and perhaps also N2O emission.
In the early 2000s, when one of my former students prepared a comparison comparing GHG emissions from composting or landfilling, I suggested nitrous oxide emission be included. At that time, many scientists were aware that nitrous oxide was emitted during composting. In 2008, I suggested that nitrous oxide emission may be a “lurking laughing monster that we composters need to deal with”. (http://www.transformcompostsystems.com/articles/Nitrous%20Oxide%20Emission%20During%20Composting%20March%202008.pdf). When I was in Europe a few months ago, I learned that an increasing number of studies reported high nitrous oxide emissions during management of digestate following anaerobic digestion, which was significantly influencing the positive GHG reduction normally promoted by anaerobic digestion.
Composting protocols have to include ammonia emission as well, because although ammonia is not considered a greenhouse gas, the ammonia that is emitted is redeposited somewhere on land or water where it undergoes further transformation leading to N2O emission. Ammonia emission is now included in the IPCC protocols for GHG emission from manure management and is being considered in some European GHG protocols for composting and waste management.
What do we know about direct N2O emission during composting? We know that it is produced during either nitrification (oxidation of ammonium to nitrite and nitrate), and during denitrification (reduction of nitrite or nitrate to atmospheric N2). I have described these processes in greater detail in another blog (http://johnpaulprofessional.com/2014/09/01/understanding-nitrous-oxide-the-greenhouse-gas-of-most-significance-in-agriculture/).
We know that ammonium has to be oxidized at least to nitrite (the first step of the nitrification process) before N2O emission can occur. We know that high temperatures during composting will inhibit the nitrification process, and hence we may not expect high N2O emission rates during the thermophilic stage of composting. This assumes that the entire composting mass is thermophilic, which in most cases is incorrect because we have lower temperatures near the pile or windrow surface. The surface of the composting mass is where nitrous oxide is produced and emitted.
We suspect that the curing process may be a high source of N2O emission because it is during this period that much of the ammonium is oxidized. This is important to understand because we can read studies of low N2O emission during active composting, and low N2O emissions from soil following compost application, but we may have missed measuring N2O during the curing process.
We know that outdoor windrow processes that are more likely to become anaerobic emit more nitrous oxide (and methane) than aerated piles. We also know that there is a positive correlation between the nitrogen content of the waste we are composting and the N2O emissions.
So what is the magnitude of the N2O emissions? When I recommended that N2O emission be included in comparison of GHG from various waste management options in the early 2000s, it appeared that the N2O emission rate averaged about 0.5% of the total N in the composting material.
A recent report in Germany reviewed GHG emissions from a number of organic waste processing plants ( http://hss.ulb.uni-bonn.de/2012/3002/3002.pdf), and found that the net N2O emissions during composting was 0.055 g per kg of waste, significantly lower than the 0.3 g per kg of waste value used in the IPCC (2006) GHG emission estimates. It was also reported that open windrow composting of anaerobic digestate resulted in the highest N2O emissions, with the recommendation to implement aerated composting as a post digestion strategy to reduce N2O emissions.
It appears that the emission of laughing gas (N2O) during composting may not be as great a contributer to GHG emissions as previously had been considered. Management of the composting process is important, and must be considered for other environmental factors as well as GHG emissions.