Abating Agricultural Nitrous Oxide Emissions
Impact: 1-2% of Global Warming Potential
Technology Maturity: Scaled/High Cost
This is the fourth article of a series on the climate technologies shown in my ClimateTech Market Map, with a deeper dive into their technical maturity and potential to reduce global warming. Here I'll cover the maturity and potential impact of technologies that can help reduce Nitrous Oxide (N?O) emissions from agriculture, particularly emissions from the application of nitrogen fertilizers to cropland and emissions from cattle pastures.
Many management and technical solutions exist to reduce N?O emissions from agriculture, but with the exception of Germany, no country has even partly mandated their use - even on a subsidized basis. Management actions to reduce emissions are purely voluntary and are marketed by government advisor networks under the overall message of smarter nitrogen use efficiency rather than climate change mitigation. Overall, tackling N?O emissions from a policy standpoint seems to be at the starting block, despite the availability of many technical solutions.
What is Nitrous Oxide?
N?O, which you may know as "laughing gas", is the third largest greenhouse gas (GHG) after carbon dioxide and methane. Atmospheric N?O emissions from all sources, man-made and natural, are responsible for about 6% of total global warming. About 30% of that number - 1-2% of total global warming, is from global agriculture. N?O is a powerful greenhouse gas because although its emissions volumes are low, its Global Warming Potential per unit is 265x as large as carbon dioxide's. Once in the atmosphere, it takes an average of 116 years to break down.
Current atmospheric N?O was estimated to be 332 parts per billion (ppb) by the IPCC 2021 assessment - compared to 267 ppb in our pre-industrial baseline. Half of that increase has been since 1980, which coincides with the vast expansion of chemical fertilizer usage in the Green Revolution and the heavy use of nitrogen fertilizers by Chinese agriculture.
N?O emissions do not get much airtime in the public conversation about climate change, compared to carbon dioxide from fossil fuels, or methane emissions. That's partly because N?O emissions are only a high share of total emissions for a few countries like Ireland and New Zealand with large pasture based livestock systems. It's also partly because N?O emissions mechanisms are complex and context dependent, and mitigation options are not straightforward.
Despite that, I'm going to attempt to summarize the mechanisms and the current technical mitigation options here, as well as some of the opportunities for innovation. (It's worth noting here that N?O is not the same as nitrogen dioxide (NO?) which is typically produced during the combustion of fossil fuels and is a major contributor to air pollution.)
How Is Nitrous Oxide Produced?
The Nitrogen in agricultural N?O emissions starts life either as a component of Nitrogen fertilizer or as a component of lifestock feed. Nitrogen fertilizer generally comes in three varieties: urea (CO(NH?)?), ammonium fertilizer (NH?) and nitrate fertilizer (NO?). Nitrate is what plants crave, and nitrate fertilizer can be used immediately by plants up to their capacity to absorb it. However, ammonium has to be converted (or "nitrified") to NO? before it can be used by crops. And urea has to go through two conversions: first converted to ammonium (by microbes with a urease enzyme) and then nitrified to nitrate.
Once fertilizer is applied, N?O can be emitted via multiple chemical and biological pathways that are modulated by a bewildering array of soil and environmental conditions. At least nine distinct pathways for N?O production along with several additional precursor pathways have been identified, some of them quite recently (1). The trifecta of soil microbes - bacteria, fungi and archaea - as well as purely chemical processes, have all been shown to be variably responsible for emissions, depending on ambient conditions such as soil pH, water saturation, temperature, soil texture, and more.
Of these potential pathways, two pathways have traditionally been considered most important for cropped systems: nitrification and denitrification mediated by soil bacteria. The relative contribution of each pathway varies by geography and land use. In the US Midwest, for example, bacterial de-nitrification is the dominant emissions source from arable land, and archaeal de-nitrification is the secondary contributor, with nitrification sources relatively minor. However, other pathways can dominate in certain soils and certain conditions. For example, fungal co-denitrification processes are the dominant N?O producers from hot spots of animal deposited nitrogen (urine) on grazed grasslands, producing up to 98% of N?O emissions (2).
Nitrification is the process whereby microbial groups take ammonia (NH?) or ammonium ions (NH??) and convert them to nitrate ions (NO?) through a varying number of intermediary steps that depend on the microbes involved. In some cases, an intermediate product, hydroxylamine (NH?OH), can be diverted into a side reaction that produces N?O directly.
Denitrification is a multi-step process whereby nitrate is converted back into atmospheric nitrogen (N?). N?O is an intermediate product in this multi-step process, and it is essentially a fugitive emission: escaping from the soil before it can be converted into gaseous nitrogen (N?). Denitrification is triggered by excessive nitrate in the soil and ecologically it serves the valuable purpose of "detoxifying" excessive nitrate to a tolerable concentration.
Co-denitrification occurs when soil nitrates (NO?) are used as an alternative oxygen source for respiration, releasing N?O as a waste product.
Abatement Strategies
Before summarizing technical inhibition strategies, I should state up front that the standard farm advisory recommendation for reducing nitrous oxide emissions is to change fertilizer application practice, not to add inhibitors. Changing from batch to continuous fertilizer application, using controlled release (polymer coated) formulations, avoiding spreading on water-saturated soils and during high temperatures, and spacing out organic and chemical fertilizer applications are some of the many practices proven to reduce N?O emissions - sometimes by up to 50%.
Other management practices that show effectiveness include biochar amendment (~20-54% reduction) and soil pH adjustment on acidic soils (up to 35% reductions). The final denitrification step by which N?O is converted to N? is inhibited by low pH, so raising the soil pH of acidic soils (~5.6-6.0) to 6.7-7.0 through the application of agricultural lime or other alkaline amendment can promote the conversion of N?O to N?, and thereby reduce N?O emissions. This could be particularly effective in some regions such as Central Africa where highly acidic soils are ubiquitous.
For livestock-linked emissions, behavioral modification approaches have been shown to be viable. For example, cattle can be trained to urinate on a collection grid using an automatic treat dispenser. This avoids the creation of high concentration nitrogen hot-spots on grassland that result in emissions bursts.
Chemical and Biological Inhibitors
In addition to management practices, inhibitors can also serve to reduce N?O emissions. Inhibitors have been traditionally used for economic reasons to increase fertilizer retention (and hence nitrogen uptake by crops) on soils that are prone to high losses from ammonia volatilization or N?O emissions. But with the exception of New Zealand's DCD pilot programs (terminated in 2012), and Germany's mandate requiring urease inhibitors (effective as of February 2020), no country has yet required inhibitor use with the aim of reducing global warming.
Urease inhibitors (UI's) slow down the conversion of urea to ammonium by the urease enzyme, and were first introduced in the 1970's. Urease inhibitors are highly effective at preventing gaseous ammonia losses (up to 90%) and can also reduce N?O emissions by shrinking the soil nitrate pool available for denitrifiers. However exceptions exist, such as on temperate pastureland (3). A meta-analysis by Grados et al. (2022) showed that UI's can reduce N?O emissions by an average of ~25%: although effect variance was high reflecting the influence of site specific factors.
Nitrification inhibitors (NI's) slow down the conversion of ammonium to nitrate by blocking the activity of the ammonium mono-oxygenase enzyme - the first step in nitrification. Like urease inhibitors, their common effect is to reduce N?O emissions by reducing the soil nitrate pool that is available to denitrifiers. Some also directly block the nitrification hydrozylamine side reaction that produces N?O directly. A meta-analysis by Fan et al. (2022) reported an average NI effectiveness of 51%, although variance was again high reflecting site specific factors.
While these chemical inhibitors have passed ecotoxicology regulatory screens, and are generally considered non-toxic, they can alter soil microbial communities and their full effects on ecosystems remain to be assessed. In addition, chemical inhibitors can have relatively high costs per unit of GWP reduction. A US assessment estimated that the cost of nitrification inhibitors even for cases where they have high effectiveness lies in the range of $60 (best case for maize) to $421+ (worst case for sorghum) per ton of CO?-equivalent emissions (4). Most carbon avoidance strategies aspire to ~$100 per avoided emission ton, so only some use cases make sense at current performance.
In response to concerns about chemical inhibitors, there has been increased interest in finding biological NI's. As reviewed by Pinxterhuis et al. (2024), adding species of ribwort plantain (a common grassland weed) to pasture can reduce nitrification from livestock pasture hotspots by ~25%, through an unknown mechanism.
An alternative biological NI, brachialactone - a compound identified in the root secretions of a tropical grass - can suppress up to 80% of nitrification activity at very low concentrations in vitro and has been shown to inhibit both the main nitrification pathway as well as the hydrozylamine side reaction. Other biological NI's with promising in vitro results include limonene, MHPP and MHPA.
Denitrification Inhibitors (DI) target the denitrification pathway. I have been unable to find any commercial chemical denitrification inhibitors, although a candidate targeting the enzyme of denitrification's second-step - copper nitrite reductase (catalyzes NO?? -> NO) - has been identified by a Japanese-led research group (5). Some researchers have investigated the denitrification inhibition effects of substances produced naturally from plant roots. For example, procyanidins - a common tannin - have been shown to decrease denitrification activity substantially, without affecting microbial populations. These remain at laboratory stage.
For now, biochar appears to be the only DI demonstrating wide effectiveness. It has been shown to reduce N?O emissions by ~50% in the first year after application, although not in acidic soils.
N?O Abatement Startups
There does not appear to be any startups developing novel N?O inhibition technologies or protocols to allow N?O abatement to participate in carbon-credit schemes. This is perhaps because the commercial case for nitrous oxide reductions is completely dependent on agricultural policy. And policy development has been poor.
From a first level search, there have been some non-profit and university sponsored initiatives in these areas, but nothing appears to have momentum. First Climate, a well-established Swiss firm, offers a carbon credit program for N?O inhibitor usage on Swiss farms.
Conclusion
If you've made it this far, you should have the takeaway that nitrous oxide abatement is a complex area. So it's not surprising that no startups have yet found a way to innovate in the space. This may change as other climate techs such as Enhanced Rock Weathering or Soil Health/Continuous Cover that can increase N?O emissions gain traction, and increase interest in N?O mitigation technologies.
Other Articles in this ClimateTech Series
3: Introduction to Enhanced Geothermal Energy (Fracked Geothermal)
领英推荐
The Climate Change Impacts Series
References
(1) Butterbach-Bahl, K., Baggs, E.M., Dannenmann, M., Kiese, R. and Zechmeister-Boltenstern, S., 2013. Nitrous oxide emissions from soils: how well do we understand the processes and their controls?.?Philosophical Transactions of the Royal Society B: Biological Sciences,?368(1621), p.20130122.
(2) Selbie, D.R., Lanigan, G.J., Laughlin, R.J., Di, H.J., Moir, J.L., Cameron, K.C., Clough, T.J., Watson, C.J., Grant, J., Somers, C. and Richards, K.G., 2015. Confirmation of co-denitrification in grazed grassland.?Scientific Reports,?5(1), p.17361.
(4) ICF International, 2013. Greenhouse Gas Mitigation Options and Costs for Agricultural Land and Animal Production within the United States. USDA, Washington DC.
(5) Kumar, A., Matsuoka, M., Matsuyama, A., Yoshida, M. and Zhang, K.Y., 2023. Identification of fungal and bacterial denitrification inhibitors targeting copper nitrite reductase. Journal of Agricultural and Food Chemistry, 71(13), pp.5172-5184.
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