BioCycle February 2011, Vol. 52, No. 2, p. 52
The sky is not falling. Nitrous oxide (N2O) emissions are not the death knell for composting. Relax, take a deep breath. I will admit that in the past, I too, have lost sleep over N2O emissions from organics, both during composting and after land application of finished products. What has helped me relax is reading lots and lots of literature. To save you that torture and still let you sleep, this column is going to provide a N2O 101.
While N2O emissions have been detected from compost piles and while N2O is 296 times more potent a greenhouse gas as CO2, enough is understood about the processes that result in the formation of this gas that measures can and need to be taken to minimize its release. Our knowledge is not perfect on this, and believe me more work needs to be done. But some basics can certainly help to minimize emissions. First you have to understand where this gas comes from.
Nitrous oxide is never produced on purpose. It is the incidental by-product of two stages in the nitrogen cycle. For those of you without schooling in soils and agronomy, you may not have had the nitrogen cycle drummed into your head repeatedly, semester after semester. So here it goes. Nitrogen is an essential plant, animal and person nutrient. It is the building block of proteins. Although 77 percent of the gas in the atmosphere is nitrogen, nitrogen deficiencies are the most common plant deficiency (not counting lack of water). Not enough nitrogen is what makes plants turn yellow.
If you take a soil fertility class, you can learn about all sorts of really slick nutrient deficiency symptoms including striped leaves in corn (manganese) and mushy apple centers (boron). But those are rare and exciting – nitrogen deficiencies are commonplace. The reason nitrogen is so often in short supply is that, even though the atmosphere is full of it, the only way to make N plant-available is to convert the gaseous N into ammonia. This conversion is done by soil microorganisms (rhizobia and frankia) or the Haber Bosch process (an industrial process invented by two German scientists to make explosives).
Nitrogen, just like carbon, cycles between these plant available forms and the atmosphere. The first part of the nitrogen cycle is mineralization: the conversion of organic N (in proteins in animal or plant tissue) first to ammonia and then to nitrate. In the second part of the cycle, denitrification, that ammonia is transformed back into nitrogen gas. And so it goes, around and around.
Nitrous oxide can be produced both during the first part of the cycle, mineralization, as well as in the second part of the cycle, denitrification. The big emphasis on N2O production from agriculture has traditionally been the denitrification portion of the cycle. During denitrification, electrons released from soil microbes eating carbon are dumped onto nitrate nitrogen. This only happens when there isn’t enough oxygen around to take the electrons (a much more efficient reaction), i.e., in waterlogged conditions. For agriculture, N2O emissions are high when there are fine textured, wet soils. In a sandy dry soil, you get nothing, just wilted and thirsty lettuce. Put that same lettuce in a wet clay soil, you get enough N2O to keep you up at night.
N2O FORMATION DURING COMPOSTING
When I first starting learning about N2O emissions and how it is formed during denitrification, I thought this would be no problem for composting. For N2O to form, conditions need to be anaerobic. And anaerobic conditions during composting generate bad odors and the neighbors will shut you down way faster than your greenhouse gas emissions. That was a very lovely and relatively short-lived period in my N2O awareness. Why? Most N2O formed during composting is not there during the initial phases when you see methane. Those initial phases of composting, as the piles are just heating up, are generally the wettest, with the lowest air flow and highest potential for anaerobic conditions.
However, N2O is most often found in the middle of the composting process. In the wettest, initial stage of composting, most of the N in the system is present as organic N. The protein compounds in the feedstocks have to be broken down – mineralized to form first ammonia and then nitrate – before denitrification processes can transform them to N2O. So you wouldn’t expect to see N2O from denitrification during those first few days as there is almost no nitrate to denitrify.
When you do see N2O is in the middle of the composting process. Here, a portion of the organic N has probably transformed to ammonia. The N2O release is likely happening during the mineralization portion of the reaction, the part when ammonia is being oxidized to form nitrate. When I first realized this, I started to get upset again. How are you supposed to control this?
First, understand how the reaction works. The bacteria responsible for this reaction get energy from getting rid of electrons on ammonia and transforming it into nitrate. This is a two-part reaction. In the first part, the ammonia (NH3) is transformed to N2O – with the help of some carbon dioxide and oxygen. This reaction can also be accomplished without the N2O. The second part is transformation of the N2O – to NO3-, here with a little oxygen and N2O thrown in. It is thought that the N2O starts forming if there is a build up of nitrite (N2O -) because the nitrite is toxic to the bacteria responsible for Part 2 of the transformation.
Well it turns out that you can control this -once again by following standard practices of good composting. Soil scientists are figuring out that N2O release in soils can be controlled by including nitrification inhibitors with the fertilizers, microbial inhibitors that slow down the mineralization of ammonia to nitrate. In a compost pile, you can do this by slowing down the formation of ammonia. In one study, keeping the pile at 55°C instead of 65°C not only reduced ammonia emissions by about 50 percent, it also accelerated decomposition (Eklind et al., 2007). A more recent study showed that with less ammonia, you in fact, get much less N2O (Shen et al., 2010). What the researchers did to make sure that there was less ammonia was to increase aeration.
In another study, researchers added either finished compost or microbes that oxidize nitrite to piles. The N2O emissions were reduced by over 75 percent, from 88 g N- N2O kg-1 total nitrogen to 17-20. A range of other studies have shown that good practices, e.g., a feedstock C:N ratio greater than or equal to 30:1, keeping moisture content low to maintain aerobic conditions, and covering a pile with finished compost or mixing finished compost into a pile, will all effectively control N2O emissions.
In other words, by composting properly, you get very low N2O emissions. And if you don’t, you may not only increase N2O emissions, you can slow the composting process and make nasty odors at the same time. Take for example a study by Amlinger et al. published in 2008. They composted yard trimmings, food scraps and biosolids – not as a mixture but in individual piles. The biosolids compost feedstocks had a C:N ratio of 2 or 1:1 while the food waste had a C:N ratio of 16.5:1; green waste was 27:1. Moisture content and odors of the piles were not reported, just how much water researchers ended up adding to each pile. The biosolids didn’t really decompose during the study; they just sat there, most likely in a wet, smelly pile. For all of their piles, where ammonia was high N2O emissions were also high. Where piles required low water addition (suggesting that they were saturated to begin with), N2O emissions were high. Low N2O emissions were observed in the drier piles with higher C:N ratios.
The results from this study do not mean that you should give up sleeping in place of worrying. Rather, they suggest that good composting practices – practices that accelerate decomposition, limit ammonia loss and odors – will also result in minimal N2O emissions. Here is a helpful bullet list:
• Pile temperature of a maximum of 55°C
• High aeration
• C:N ratio of 30:1
• Mix in finished compost with feedstocks
• Cover initial pile with finished compost (will get mixed in with turns in a windrow)
You do these things and your neighbors will like you. The atmosphere also will like you. Now have a cup of tea and get some sleep.
Sally Brown – Research Associate Professor at the University of Washington in Seattle – authors this regular column. E-mail Dr. Brown at firstname.lastname@example.org.
Amlinger, F., S. Peyr, and C. Cuhls. 2008. Greenhouse gas emissions from composting and mechanical biological treatment. Waste Mangement & Research, 26:47-60.
Eklind, Y., C. Sundberg, S. Smårs, K. Steger, I. Sundh, H.Kirchmann, and H. Jönsson. 2007. Carbon turnover and ammonia emissions during composting of biowaste at different temperatures. J. Environ. Qual., 36:1512-1520.
Shen, Y., L. Ren, G. Li, T. Chen, and R. Guo. 2011. Influence of aeration on CH4, N2O and NH3 emissions during aerobic composting of a chicken manure and high C/N waste mixture. Waste Management, 31:33-38.
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BioCycle February 2011, Vol. 52, No. 2, p. 52