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28.08.15 The debate over nitrous oxide emissions from cover crops

A few weeks ago (back when the sun was shining and there was hope that we would have a nice hot summer!) I was getting ready for presenting at the National Organic Combinable Crops event and sorting out information to accompany the carbon footprint that we did of the host farm. One of the issues that I was trying to get my head around to explain was the GHG emissions associated with cover crops.

Indeed on the host farm leguminous green manures (mainly red clover) contributed 17% of emissions through nitrous oxide released. In organic systems (as the host farm was) legumes are often seen as the highest risk of nitrous oxide emissions. When the legume is incorporated and the soil bugs mineralise the Nitrogen, it is then available. It is this flush of Nitrogen that needs to be properly managed to be fully utilised by the following crop to avoid losing the Nitrogen that has been biologically fixed.

During researching my talk I found that the area of nitrous oxide emissions from using cover crops was something which needed further research and conflicting results had been found. As this month is nitrous oxide month, it was something that I was keen to come back to and explore further.

The rest of this blog is based on information from a research paper that was published at the end of 2014 (and can be accessed here). This paper has performed a meta-analysis on existing research to try and draw some conclusions as to the dynamics of nitrous oxide emissions from cover crops and how we can maximise the positive benefits that arise from using these crops and minimise the potential losses.

The well-known benefits of cover crops

There are many environmental benefits to incorporating cover crops into rotations including their potential to reduce soil erosion, reduce nitrate leaching losses, build soil organic matter levels (which leads to an increase in soil carbon stocks), reduce pest and weed pressures and provide a biologically fixed source of Nitrogen to the following crop.

How do they impact on Nitrous oxide emissions?

The net impact of cover crops on nitrous oxide emissions is not well understood. Nitrous oxide is emitted as a natural part of soil ecosystem function; however the extent to which it is emitted depends on various factors. These include the available mineral Nitrogen content, the soil water content, the availability of carbon and the physical structure of the soil. Cover crops can affect these factors and as such impact on the amount of nitrous oxide that is emitted.

For example when a cover crop is growing, it can suck up soil mineral Nitrogen (thus minimising the risk of ‘excess’ nitrogen in the soil to turn into N2O), as well as decreasing soil water through transpiration. Once the cover crop is cut, the mulching effect of residues on the soil surface can increase soil water (and thus the potential for the anaerobic conditions needed for denitrification and N2O emissions), depending on weather conditions. Also these residues decomposing on the soil surface can temporarily immobilise soil Nitrogen, and then increase carbon (from the residues) and Nitrogen which will affect emissions as well (it’s all very complicated!).

This research looked at three different suggestions as to the dynamics of cover crops and nitrous oxide emissions. These were:

That the type of cover crop would affect the nitrous oxide emissions (whether it was a nitrogen fixing legume or not). Legumes logically should have a greater potential to increase nitrous oxide emissions due to the fact that they are biologically fixing Nitrogen from the atmosphere.

Rainfall and the residues themselves would have an impact – because the denitrification process (which emits N2O) requires anaerobic conditions, if the soil is waterlogged then oxygen will be lacking, and a carbon source (provided by the residues).

Timing of measurements of N2O would be influential on what the different research papers had concluded. As mentioned earlier, minimising N2O emissions from cover crops has a lot to do with achieving synchrony of Nitrogen and managing the ‘flush’ of nitrogen that comes when the cover crop is incorporated – if measurements are taken during this flush then the emissions will be higher than if they are taken earlier in the growing season.

What did the research find?

After analysing lots of results, there were some conclusions that we can use to help us in understanding the dynamics of nitrous oxide emissions.

Denitrification – this process (which produces N2O) requires a Carbon and a Nitrogen source

Types of cover crop – was found to have an impact on nitrous oxide emissions. Because cover crops take up Nitrogen that might otherwise be lost to leaching, or because legumes fix nitrogen, cover crops may increase soil Nitrogen availability during decomposition and may increase the amount of nitrate available for the denitrification process (and thus nitrous oxide emissions from the field).More research is needed in this area.

When you measure it – as suggested earlier, depending on when you measure emissions the amount will change. Interestingly when looking at longer term experiments this study suggested that there is a net neutral effect of a cover crop on Nitrous oxide emissions, even if particular periods of the year see larger impacts. Again more research needed.The analysis revealed that the highest emissions occurred during the cover crop decomposition period, as the additional carbon added from the residues, with the available mineral Nitrogen led to high emissions of N2O. Emissions from the cover crop growth period were found to be the lowest of all the measurement periods. This suggests that the nitrogen uptake by cover crops holding that nitrogen over the risk period creates a larger sink than a source of nitrous oxide emissions or nitrate leaching.

To incorporate or not? The N2O emissions from studies that ploughed in cover crops rather than leaving them on the surface were significantly higher.Incorporation of cover crop residues contributes to an increase in Nitrous oxide emissions.

The effect of rainfall – Cover crops may alter the soil water status and thus the potential for anaerobic conditions (and denitrification) which normally follow intense rainfall events. Studies looked at had different responses to rainfall, however this report concluded that regardless of the cover crop type, above a certain threshold of rainfall a field with a cover crop is more susceptible to nitrous oxide emissions than one without.

Soil Organic carbon – The study found inconclusive evidence that cover crop biomass was an important factor controlling N2O emissions.

The effect of cover crops on global warming potential – Nitrate lost through leaching from fields is subject to denitrification and Nitrous oxide emissions off-site. Therefore given the ability of cover crops to reduce nitrate leaching losses, cover crops may contribute to an overall decrease in net global warming potential.

What did they conclude?

This is one of the most comprehensive analyses of research to date, but in some ways it throws up more questions than it answers. This highlights the fact that this subject needs more research done into the impact of different cover crops within rotations and the effect on emissions from management of these crops. It also shows that there isn't ‘one size fits all’ answer to these questions, due to the diversity of agricultural systems, crop rotations, soil types, weather (and all the other factors) each system is unique.

This study did conclude that from the literature analysed cover crops increased N2O emissions from the soil surface in 60% of published observations while cover crops decreased nitrous oxide emissions from the soil surface in 40% of studies.

Legume cover crops had higher relative N2O emissions at low N rates and lower emissions at high N rates whereas nitrous oxide emissions from non-legume cover crops increased as N rate increased.

Cover crops on average only lead to a small or negligible increase in N2O emissions when measured for time periods of one year or greater.

More research is needed!

Source: Basche et al (2014), Do cover crops increase or decrease nitrous oxide emissions? A meta - analysis, Journal of Soil and Water Conservation

07.08.15 Sustainable crop and animal production to help mitigate nitrous oxide emissions

The information in the blog below comes from a paper published last year which examines the different management options that are available to farmers to improve nitrogen use efficiency within farm production systems. To read the full paper please click here.

A common fact that we are presented with, which frames this debate is the need to feed more people with more food using less resources. Nitrogen plays a big part in this, as nitrogen is a key component of crop growth and increased yield, which will be needed to feed more people. In the past, increased food production has been made possible by the production and use of commercial Nitrogen fertiliser (with associated increases in emissions).

In order to address the current need for sustainable intensification, nitrogen use efficiency is a key component. How we increase crop productivity at the same time as protecting natural resources and the environment must be intrinsically linked with managing nutrients more efficiently and minimising losses.

Mitigation challenges

Improving Nitrogen use by crops

When crops are grown under laboratory conditions, the amount of nitrogen which is taken up by the plant ranges from 45-65%, while on-farm the plant nitrogen uptake (as a percentage of applied nitrogen) is often below 40%. This demonstrates that there are opportunities to alter management practices to more efficiently use Nitrogen inputs to reduce N losses that affect direct and indirect nitrous oxide emissions.

The issue is not as simple as just reducing the amount of N fertiliser that is applied, as this would jeopardise sustainable food production.

The grand challenge is how to improve Nitrogen use efficiency that leads to reduced nitrous oxide emissions while also achieving greater N effectiveness in crop and livestock production (i.e. more food output per unit of N input).

Other challenges exist with improving efficiency in that there is no single management change that can bring about both increased crop productivity and reduced nitrous oxide emissions equally well across different soil and climatic conditions.

Another compounding issue is that direct nitrous oxide losses are equivalent on average to about 1% of the nitrogen applied (so not a massive economic loss). To gain greater interest and be more likely to achieve significant reduction in both direct and indirect nitrous oxide emissions, it will be necessary to focus more broadly on practices which lead simultaneously to greater nitrogen use efficiency and effectiveness.

Nitrogen losses that occur through volatilisation of ammonia (from spreading slurry and manure on warm days, or application of urea), and nitrate run off (when Nitrogen is applied and the crop is not actively taking it up), leaching and drainage pathways may receive more attention as these losses represent a greater economic loss to the farmer than direct nitrous oxide emissions.

Nutrient management

Research shows that ‘mismatched timing of Nitrogen availability with crop need is probably the single greatest contributor to excess N loss in annual cropping systems.’ There is a global initiative that details the ‘4R Nutrient Stewardship initiative, which is based on the principle of using the right nutrient source, at the right rate, right time and in the right place to achieve the basic economic, social and environmental elements of sustainability.

It is also important to remember that adapting management to mitigate losses of nitrous oxide must be balanced with other considerations.One of my commonly used adages is that we work within complex biological systems, and as such it is important to not simply ‘swap pollutants’ between leaching and gaseous losses.

Use of precision farming technology

An increasing number of farmers and crop advisors around the world have access to GPS resources and GIS. This along with advances in technologies may make it increasingly possible to better match N rates and times of application with are sensitive to in season crop N demands.

The use of Nitrogen sensing technology aims to better match crop nitrogen needs with in-season sensitivity that leads to improved Nitrogen use efficiency, and greater farm profitability.

American studies have shown that using Nitrogen sensors across 16 trial sites could potentially save farmers 10-50kg of nitrogen per hectare on maize.

The uptake of these technologies by farmers is now well underway, indeed this paper states that the Yara N-Sensor was being used on more than 1.2 million hectares of the total 104 million cropland hectares in the EU-27.

Not synchronising nitrogen applications with crop needs has been cited as potentially the single greatest contributor to excess nitrogen loss in annual cropping systems. However achieving synchrony in nitrogen supply may not always reduce cumulative nitrous oxide emissions, instead, management practices that influence the rate of nitrification and soil nitrite accumulation, may be most likely to reduce nitrous oxide emissions.

Cover crops

Winter cover crops help provide soil cover and thus minimise erosion, help to build organic matter levels (and soil carbon), and help in the capture and retention of excess inorganic nitrogen. However there are some studies which suggest that in some soils cover crops stimulate and increase nitrous oxide emissions because of the release of carbon and nitrogen from crop residues (I will be writing another blog on this subject in a couple of weeks). Site variables including soil type, irrigation, variety and mix of cover crop, soil structural stability and organic matter Nitrogen mineralisation will all impact on the level of nitrous oxide that is emitted.

Management of livestock

One of the most promising ways for many livestock growers to enhance nitrogen use efficiency is to more optimally manage the protein content of the diet. In ruminants, the bulk of excreted Nitrogen is in the urine, while in pigs it is present in urine and faeces. Nutritionists can aim to make adjustments in crude protein levels in the diet to match animal nutritional requirements and significantly reduce ammonia and nitrous oxide emissions. Taking full account of manure nitrogen content, and maintaining optimum stocking rates, as well as the inclusion of clover within pastures can potentially raise the whole farm N use efficiency from 30% to nearer 65% which reduces Nitrogen losses and improves farm profits.

Management of manure

Incorporating manures greatly reduces ammonia emissions, with leaves a more ‘potent’ product, which is then more susceptible to losses of nitrous oxide. However when the manure is incorporated, and ammonia losses reduced, effectively a smaller amount of that manure is needed to provide the crop’s nutrient requirements and therefore the potential for nitrous oxide production is reduced.

This trade-off is often seen, and reducing losses of ammonia is important (since these losses indirectly emit nitrous oxide).

Improving nutrient use efficiency in livestock production systems will require site specific and targeted combinations of genetic improvements, feed planning and rationing for utilisation, and improved storage, handling and application of manure.

An emphasis on increased crop or animal outputs per unit of Nitrogen input (more efficient use of inputs) will help nitrous oxide emissions and also safeguard natural resources and improve profits. As with all agricultural issues though, we can’t look at nitrogen in isolation, research, policies and advice need to consider how to improve other essential nutrients, water and production as they all impact on nitrogen use efficiency.

Source: Agriculture: sustainable crop and animal production to help mitigate nitrous oxide emisisons

06.08.15 Field Effectiveness of nitrification inhibitors

Enhanced efficiency fertilisers (including nitrification inhibitors and urease inhibitors and slow release fertilisers have been developed to increase the efficiency of fertiliser use by crops. Currently Nitrogen use efficiency is fairly low from what is applied to what the crop takes up, as such research has been targeted to try and increase the efficiency percentage and minimise the risk of losses of nitrogen (either through nitrate leaching or nitrous oxide emissions).

Different compounds

Nitrification inhibitors are compounds which delay the bacterial oxidation of NH4+ (ammonium) by depressing the activities of nitrifiers in the soil.

Urease inhibitors are compounds that delay the hydrolysis of urea.

Slow release fertilisers show the rate of nutrient release through coating or chemical modification of the fertiliser itself.

What do they do?

Science has studied these compounds intensively and findings indicate that they can be effective in increasing nitrogen use efficiency and have other benefits such as reducing labour and fuel costs and reducing the incidence of nitrate leaching. The information below comes from a review paper which looks at different experiments on these compounds and tries to draw some conclusions as to their effectiveness in the field.

The IPCC 3rd Assessment report confirmed that management of Nitrogen Fertiliser by the use of Nitrification inhibitors, slow release fertiliser and organic manure could tentatively cut nitrous oxide emissions from nitrogen fertiliser use by 30% on a global scale.  The next (4th ) Assessment report looking at nutrient management including the technologies described above in more detail and concluded that the mean mitigation potential of nitrous oxide through nutrient management was 0.07tonnes of carbon dioxide equivalent per hectare per year.

What has the research looked at

Nitrification inhibitors have been the most widely studied as a mitigation option for nitrous oxide emission from agricultural soils. There have been some studies that have looked at the use of polymer coated, and sulphur coated fertilisers and a few using urease inhibitors.

Despite all these published field experiments, it is difficult to draw general conclusions because the performance varies depending on soil and climatic conditions and field management strategies. The authors of the paper looked at different studies and categorised results using different land use and type of fertiliser applied. This study also only included data from field experiments (rather than ones in a lab that were grown in pots).

What did they find out?

On average nitrification inhibitors significantly reduced nitrous oxide emissions compared with conventional fertilisers. The effect of nitrification inhibitors on nitrous oxide emissions also varied with land use type, with grassland having the best average reduction in N2O emissions of 60%. The coated fertiliser also significantly reduced N2O emissions compared with conventional fertilisers. When the study looked at urease inhibitors, the effect on emissions was not significantly different from the control treatments.

How do nitrification inhibitors work?

Both nitrification and denitrification are important pathways for nitrous oxide production in soil. They work by inhibiting ammonium monooxygenase, thereby blocking the first reaction or ammonium to nitrite. By minimising the rate of nitrification until the primary crop is in its log phase of growth. Nitrification inhibitors can give the crop a better opportunity to absorb nitrate and increase nitrogen use efficiency. By suppressing nitrification, inhibitors potentially reduce subsequent denitrification and nitrate leaching, thus reducing N2O emissions. The effectiveness of nitrification inhibitors was found to be more consistent compared with coated fertilisers.

What affected the field experiments?

Environmental factors such as temperature, alter the effectiveness of nitrification inhibitors in the field. They also differ in terms of how water soluble they are and their volatility. One product used in America found that using it:

·         Increased crop yield by 7%

·         Improved soil nitrogen retention by 28%

·         Reduced nitrate leaching by 16%

·         Reduced nitrous oxide and methane emissions by 51% (compared with conventional N fertiliser) 

Practical considerations with Nitrification Inhibitors

In general higher nitrous oxide emissions were observed from grassland than the other land uses in the studies. Nitrification inhibitors were more effective in reducing nitrous oxide emissions from grassland compared with their effect on other crop types.

Another advantage of nitrification inhibitors is that they can be used with both chemical and organic fertilisers, whereas coatings can only be used with bagged Nitrogen. Nitrification Inhibitors are effective in reducing nitrous oxide emissions from chemical and organic fertilisers and the consistent effect indicates that they are a potent mitigation option for future emissions.

Coated fertilisers (PCF)

These work by releasing nutrients by diffusion through a semi-permeable polymer membrane and the release rate can be controlled by varying the composition and thickness of the coating. PCFs can be effective in increasing nitrogen use efficiency and can substitute for split applications thus reducing the requirement for multiple field operations and in turn reducing labour and fuel costs.

When nitrogen release from PCFs is well synchronised with plant demand, PCFs have the potential to reduce nitrogen losses to the environment, such as nitrate leaching and nitrous oxide emissions.  In contrast, nitrogen use efficiency can be reduced significantly and environmental losses increased when nitrogen released from PCF doesn’t match plant demand. The effect of PCFs on nitrous oxide mitigation showed varying results depending on land use, crop and soil type.

Urease inhibitors

These slow the conversion of urea to ammonium and hence reduce the concentration of ammonium present in the soil solution (and the potential for that ammonium to be volatilised as ammonia). Together with uptake by plants, a lower concentration of ammonium in the soil can result in less nitrogen potentially undergoing subsequent nitrification and denitrification. One of the drawbacks of urease inhibitors is that they only delay the hydrolysis of the urea and the urea will eventually be hydrolysed and become ammonia. These compounds have not been as widely tested as the other two groups of inhibitors, and as such more studies are needed.

Take up by the industry

Although these products have been well researched, there is limited uptake of them in the field. At the moment, the uptake of the nitrification inhibitor technology is relatively slow, but that will change in the next 10 or 20 years as policies are developed that try to manage the nitrogen losses that are occurring and improve nitrogen use in the field. As farmers taking account of application timing, source of nitrogen being applied, application method, soil texture, and tillage are all factors that should be evaluated to determine how efficiently Nitrogen is being used in the system.

Source: Akiyama, H et al, (2009) Evaluation of effectiveness of enhanced efficient fertilisers as mitigation options for N2O and NO emissions from agricultural soils; meta - analysis

31.07.15 4 for 1000: A new program for carbon sequestration in agriculture

Source: French Food in the US

Agriculture can and must be part of the solution to climate change. The French Minister of Agriculture, Stéphane Le Foll, and Ambassador for Paris Climate 2015, Laurence Tubiana, emphasized this imperative at a conference that took place in Paris on April 27, 2015, during which they introduced the carbon sequestration program for agriculture, named “4 per 1000.”

What is the programme about?

This program aims to adapt agricultural practices with the goal of storing carbon more efficiently in the soil. According to Jean-François Soussana, Scientific Director for Environment of the French National Institute for Agronomical Research (INRA), an annual increase of “4 per thousand” (0.4%) each year of organic matter in soil would be enough to compensate for the global emissions of greenhouse gases. Indeed, soil is a veritable reservoir for carbon; it contains 2.6 times more carbon than the atmosphere thanks to plants that siphon carbon from the air and deposit it into the soil once dead. But through most agricultural practices, the soil lets its stock of carbon escape into the air. On average, cultivated soils around the world have lost 50 to 70% of their initial carbon stock, according to Jean-François Soussana. But certain agricultural practices can reverse this trend, fostering carbon-rich soils that will in turn be better suited for production. According to Stephan Le Foll, this program will “reconcile food security and climate change.”

Stephan Le Foll and Laurence Tubiana presented a work schedule for the researchers participating in this international program, which details the actions to be taken leading up to the climate conference in December 2015 in Paris.

Practical ways to build carbon

The following agricultural practices are recommended by the French National Institute for Agronomical Research (INRA) for fighting global warming:

Reducing the prevalence of chemical fertilizers by best management practices as well as by more accurately predicting crop yields and planning nutrient applications. This would reduce the emissions of nitrogen oxide in particular.

Using legumes during crop rotations. Legumes are able to harness nitrogen from the air and use it in the soil for plant growth. They act like a natural fertilizer for the subsequent crop in the rotation, which will then require fewer chemical fertilizers.

Developing no-till cultural practices. If not tilled, the soil retains its structure and stores carbon more efficiently. Furthermore, this practice saves fuel.

Planting more cover crops. It is preferable to plant crops instead of leaving the soil bare. This can help limit the emissions of nitrogen oxide.

Developing Agro-forestry: planting trees is a good way to utilize carbon from the air, and offers many beneficial effects to the crops.

Improving the management of grasslands by prolonging the pasture season, reducing fertilization, among other strategies.

Reducing the emissions of methane and nitrogen by changing the diet of cattle.

Retaining the methane by utilising anaerobic digestion.

Reducing the use of fossil fuels on the farm by improving insulation and ventilation in livestock buildings and greenhouses, for instance.

For more information on soil carbon why not check out the dedicated section on the FCCT pages? 

29.07.15 Inventories and scenarios of nitrous oxide emissions

As the third most important anthropogenic greenhouse gas, as well as the largest remaining anthropogenic stratospheric ozone depleting substance currently emitted, nitrous oxide (N2O) is one of the most important forms of nitrogen pollution.

How we mitigate and manage N2O emissions requires an understanding of where the sources of N2O are and how they may increase this century. One of the issues with N2O is that it is a by-product of several fundamental (and critical) reactions of the N cycle. As agricultural land has expanded and land management has advanced (including the use of legumes as N fixers), the N cycle has been altered. Another big impact has been the development of the Haber Bosch process, which plays a central role in feeding the world’s rapidly increasing population.

This growth in man-made fixed N has led to an unintended increase in global N pollution, including N2O emissions driven largely by the fact that mismatch between crop N demand and soil N supply frequently leads to N losses. It is impossible to completely eliminate global N pollution particularly from agriculture, its largest source.

This blog comes from a paper which tries to quantify the amount of N2O emitted from various sources, looks at how those emissions are calculated, what the common ways of measuring N2O are, and tries to project what will happen to the levels of N2O emissions under different management scenarios in the future. To read the full paper please click here.

Natural emissions sources

There are a range of measuring options used to assess the levels of emissions from different areas. ‘Bottom up estimation uses field based measurements and ‘top down’ estimations are based on measurements from the atmosphere and models.

There are lots of caveats and supposition to the figures as well as uncertainties, however both accounting approaches (whether you are basing measurements from the field or the atmosphere) suggest that natural emissions were and probably still are between 10-12 Tg of N2O per year.

Anthropogenic emissions (since 1850)

Difficulties in accounting strategies

Again top down and bottom up approaches can be applied to N2O emissions derived from human activities. Protocol has been developed by the IPCC for counties to estimate their N2O emissions by emissions factors (bottom up approach which calculates the amount of N2O emitted per unit of activity). Within agriculture, emissions factors are used to calculate the direct emissions from soils, amounts of fertiliser N that is leached into watercourses and volatilised as ammonia or N2O, as well as the indirect N2O emissions from downstream (which can be substantial). For example emissions from coastal, estuarine or riverine waters are estimated to be ~9% of total anthropogenic sources, although the original source of most of this N is from agricultural applications in the field.

This method (Tier 1, IPCC) has a lot of inaccuracies though and is difficult to apply across diverse systems. As with biological processes, relationships between variables are not always simple and studies have discovered that it is more likely to be non-linear than linear relationships. The non-linear relationship is likely the result of large increases in N2O emissions once N application rates are in excess of plant demands.

Advances are underway in terms of developing a more robust accounting method. Countries that have sufficient data are permitted (under IPCC Tier 2) to calculate more specific emissions and developments of validated biogeochemical models look towards Tier 3. There are new bits of kit (including laser technology) which can measure fluxes of N2O and these will all help with emission factors but it will always be a challenge due to the large spatial and temporal variation.

Providing an average for the UK doesn’t take into account the diversity of farming systems that exist, let alone the unique relationships between soil type, climate management and enterprise which makes every farm an individual entity (and probably incomparable).

Improvements in the quality of activity data for each county, including fertiliser application rates, livestock production and manure handling procedures will aid in the accuracy of estimates.

Emissions from agriculture (some headline figures)

Picture source: California Department of Food and Agriculture, Crop N uptake and partitioning 

Agriculture is the largest source of anthropogenic N2O emissions, responsible for 66% of total.

Emissions estimates include direct soil emissions from N fert and manure applications and indirect emissions from downstream or downwind waterbodies and soils after nitrate leaches away from fields and after N emitted from fields as ammonia or Nitrogen gas is deposited back.

Also included are N2O emissions resulting from crop residues, manure management, cultivation of organic soils and crop biological N fixation.

The central factor responsible for agricultural N2O emissions is a lack of synchronisation between crop N demand and soil N supply with an average 50% of N applied to soils not being taken up by the crop.  This is something that needs addressing.

Other sector contributions

Industry and fossil fuel combustion (responsible for about 15% of total anthropogenic N2O emissions.

Biomass burning (~11% of total gross N2O emissions)

Waste water, aquaculture and other sources

Projections for future emissions

So as you can imagine, as we can’t reliably assess what’s going on now, it becomes incredibly difficult to predict future scenarios. Especially when there are lots of things that could change including population growth, rates of food waste, nutrient use efficiency, land use change, climate change and other variables.

Four sets of published N2O emission scenarios have been looked at to characterise the range of future anthropogenic emissions.

Scenarios looked at from research

Business as usual – if we don’t mitigate any emissions and continue ‘business as usual’ then emissions are set to increase by ~83% based on 2005 levels.

Moderate mitigation scenarios – if moderate mitigation methods were achieved an increase of 26% compared to 2005 would be possible.

Concerted mitigation – would lead to emissions reductions on average per year of 1.8Tg N2O to 2020, with a reduction of 57% by 2050.

It is important to remember however that these projections are based on 2005 figures and since then significant differences have been seen. So far (up to 2013 when the study used the data from) actual global N2O emissions have been closer to Business as usual trajectories rather than the mitigation scenario which research was expecting.

What does this all mean for agriculture?

Agriculture currently accounts for 56-81% of gross anthropogenic N2O emissions. Some N2O emissions associated with food production are inevitable, but future N2O emissions from agriculture will be determined by several factors including population, dietary habits, and agricultural management to improve N use efficiency.

Another rising issue is the growing demand for biofuels on future N2O emissions which is uncertain depending on types of plants grown, their nutrient management and the land resource needed. More research will help to full these current knowledge gaps. Accounting methodologies are not fully developed and further research is needed, however what we can do as farmers is try and look at N balances and flows in crop rotations and try and ensure synchrony between N supply and crop demand as much as possible.

Source: Davidson, E. and Kanter, D. Inventories and scenarios of nitrous oxide emissions,  Environ. Res. Lett. 9 (2014) 105012

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