The case was brought by Greenpeace, which said the government was not doing enough to lower illegally high levels of nitrogen oxide emissions caused by intensive farming and heavy use of fertilizers, as well as traffic and construction in the densely-populated Netherlands.

The court in The Hague said the government had clearly failed to comply with European regulations to preserve vulnerable nature reserves and cut excessive emissions of nitrogen oxides and ammonia, which hurt biodiversity and damage the quality of water.

It ordered the government to meet its target of reducing the emissions to legally allowed levels in 50 per cent of all affected nature reserves by 2030, and ruled that it should be fined 10 million euros (C$15 million) if the goal was not met – a sum unlikely on its own to provide a major impetus for change.

Agriculture minister Femke Wiersma said she was disappointed by the court ruling and was considering an appeal.

“We take the nitrogen problem very seriously,” she said in a post on X. “But we can’t ask the impossible of people and companies.”

The problem is seen as a potential stumbling block for the already fragile government coalition, which includes far right leader Geert Wilders’ Freedom Party and a farmers party that was created to protest against nitrogen measures.

The nitrogen problem has plagued the Netherlands for years following rulings in 2018 by the European Court of Justice and in 2019 by the Netherlands’ Council of State that Dutch policies were failing to address it.

Efforts to reduce emissions by buying out livestock farmers triggered large protests, while courts have routinely blocked major construction projects until the problem is solved.

The country’s previous government in 2022 laid out targets for reducing nitrogen pollution in some areas by up to 70 per cent by 2030, but policies to reach that goal have largely been scrapped by the current government as farmers argued they were poorly conceived and unfair.

“Measures were already largely insufficient to reach the 2030 goal and there is no improvement in sight,” the court said, adding that the government’s lack of action was unlawful.

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“Ambitious action to reduce nitrous oxide emissions could move the world closer to meeting a wide range of global climate, ozone and other environmental and human health goals,” said the assessment, published by the Climate and Clean Air Coalition of over 180 governments, NGOs, and international organizations.

Nitrous oxide is the third most prevalent greenhouse gas and the worst ozone-depleting gas. Emissions, driven primarily by the agricultural use of synthetic fertilizers and manure, have increased globally by 40 per cent since 1980, and are on pace to rise 30 per cent over 2020 levels by 2050, the report said.

The Global Nitrous Oxide Assessment (N2O) report is similar to the 2021 Global Methane Assessment, which showed that human-caused methane emissions can be reduced by up to 45 per cent this decade and laid the groundwork for 150 countries to commit to the Global Methane Pledge to curb those emissions by 30 per cent by 2030.

Taking global action to reduce emissions of nitrous oxide (N2O) could avoid the equivalent of up to 235 billion metric tons of carbon dioxide emissions by 2100, it said.

A U.S. State Department official told Reuters earlier this year that slashing N2O emissions from production of fertilizers or the production of materials like nylon is cheap, costing as little as $10 per metric ton (C$13.90) through projects using the voluntary carbon offset market.

The U.S and China are the biggest emitters of the greenhouse gas.

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What the research says

The primary driver of nitrous oxide emissions from agricultural soils is denitrification, which is promoted by wet soils, ample available mineral nitrogen (e.g., nitrate), the presence of carbon sources, and freeze-thaw cycles. Cover crops influence each of these conditions. A 2014 meta-analysis found that cover crops decreased N2O emissions in 40 per cent of studies and increased them in the other 60 per cent².

The following factors determined cover crop effects on emissions:

• Species grown: Legumes increased emissions while non-legumes had a neutral effect.
• Time of year: Cover crops reduced emissions while they grew (by lowering soil nitrate and decreasing soil moisture), but increased emissions after termination (by contributing a flush of available carbon and nitrogen to the soil).
• Termination method: Incorporation by tillage increased N2O emissions.

Ontario studies

New research conducted at the University of Guelph is providing a more complete picture of cover crop impacts on N2O.

A recent study compared a three-crop rotation with cover crops to a two-crop rotation without cover crops³. The researchers, led by Dr. Claudia Wagner-Riddle, uncovered that a four-way cover crop mixture (cereal rye, crimson clover, oats, and daikon radish) seeded after winter wheat lowered emissions during the non-growing season relative to soybean stubble (Figure 1 below). The cover crop lowered soil nitrate in the fall and better insulated soil against freeze-thaw events over winter. However, N2O emissions in corn the next growing season were higher – likely driven by carbon and nitrogen release from decomposing residues. Total emissions were greater with cover crops present.

Another study from Dr. Wagner-Riddle’s group found that even a low-biomass cover crop can elevate N2O emissions⁴. An annual ryegrass-crimson clover mix interseeded into corn (Figure 2 at top), which achieved less than 200 pounds of biomass per acre, increased emissions during spring thaw. Winterkilled crimson clover residues were the suspected culprit, fueling higher emissions with elevated soil nitrate levels. In this trial, the cover crop was not thick enough to insulate the soil over winter and lessen emissions from freeze-thaw cycles.   

Management practices to lower nitrous oxide emissions

There is not a cut-and-dried answer on cover crops and nitrous oxide emissions from soil. What is clear is that management matters and makes the difference between increased or decreased emissions. The following practices are more likely to lower emissions:

1. Seeding grass cover crops (e.g., cereal rye) that scavenge left-over soil nitrate and lower soil moisture levels.
   • Select grass cover crop species ahead of soybeans or edible beans. Before corn, a multi-species mix with grasses and legumes can be a good compromise. Adjusting the commercial fertilizer rate by crediting the nitrogen added by legumes is also important.
2. Leaving cover crops undisturbed by tillage maximizes the insulating effect of surface residues and slows residue decomposition^1,4.
   • Avoid or delay tillage of cover crop, if practical, until spring to lower risk of N2O losses.
3. Using a dual nitrification and urease inhibitor (e.g., TribuneTM, Excelis Maxx ®) shows promise in lowering emissions for nitrogen applications to corn following cover crops. In the previously mentioned cropping system study, a dual inhibitor reduced N2O emissions by 22 per cent in the corn growing season after a four-way cover crop mix and brought overall emissions in line with those of the two-crop rotation with no cover crop³.
   • Consider a dual inhibitor or a nitrification inhibitor if it makes agronomic sense for your cropping system.

The bottom line

Cover crops can provide both environmental and agronomic benefits. Their impact on greenhouse gas emissions depends on how they are managed.  

Keep these latest findings in mind when making cover crop management decisions. Consider ways to lower risk of nitrous oxide losses in a way that makes agronomic and economic sense for your production system.

References

• Greenhouse Gas Emissions and Agriculture
• Do cover crops increase or decrease nitrous oxide emissions? A meta-analysis (2014). A.D. Basche, F.E. Miguez, T.C. Kaspar, and M.J. Castellano. Journal of Soil and Water Conservation. 69: 471-482.  
• Increased N2O emissions by cover crops in a diverse crop rotation can be mediated with dual nitrification and urease inhibitors (2024). A. Tariq, N. Menheere, Y. Gao, S. Brown, L.L. Van Eerd, J.D. Lauzon, S. Bruun, and C. Wagner-Riddle. Agriculture, Ecosystems & Environment 374: 109178
• Nongrowing season soil nitrous oxide emissions as influenced by cover crops and fall tillage termination (2023). Y. Gao, K.A. Borden, S.E. Brown, and C. Wagner-Riddle. Canadian Journal of Soil Science. 103: 527–537.

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The current inventory of nitrous oxide from plant residues relies solely on the amount of nitrogen in the residues, while crucial factors such as degradability are not included. According to researchers, this leads to misleading inventories, which also misrepresents possible mitigation measures.

Crop residues are an important resource in agriculture. They contribute carbon to the soil, increase soil fertility and play an important role in the agricultural ecosystem, but they also play a role in relation to energy supply and recycling of nutrients.

Crop residues can be diverse and have widely different composition and properties. They can be cover crops, grass, grass-clover, vegetables, straw, etc. They may consist of residues from roots or from aboveground crop parts.

Professor Jørgen E. Olesen from the department of Agroecology at Aarhus University is heading a new scientific study highlighting the differences in crop residues and how they affect nitrous oxide emissions from agricultural fields in different ways.

The Intergovernmental Panel on Climate Change prepares guidelines on how to make national inventories of greenhouse gas emissions, including nitrous oxide emissions, when crop residues are returned to the fields.

The IPCC is an international organization established in 1988 by the World Meteorology Organization and the United Nations Environment Programme. Its purpose is to assess scientific knowledge about climate change, its causes, impacts and possible adaptation and mitigation strategies.

The panel plays a key role in gathering and assessing the latest scientific literature on climate change and preparing reports that inform politicians and decision-makers worldwide. These reports are used as the basis for international climate negotiations and policy development.

The IPCC’s inventory method includes nitrous oxide from crop residues such as annual cereal and seed crops, root crops, vegetables, fodder crops and grassland renewal.

”Non-harvestable crops such as cover crops are not taken into account,” says Olesen, who adds that the biochemical properties of crop residues and their degradability of carbon and nitrogen are also not included in the accounting of emissions.

”The current method only considers the nitrogen content in the plant residues, but our studies show that the degradability of carbon in plant residues actually is more important. There is therefore a need for an improved accounting method so that inventories are accurate and mitigation measures can be effective.”

According to researchers, there may be large differences in how much nitrous oxide the crop residues emit. Olesen says many factors come into play.

“A very important factor is the concentration of degradable carbon and nitrogen. When the concentration is high, the potential for producing nitrous oxide also increases.”

A high concentration of both easily degradable carbon and nitrogen in immature crop residues, such as cover crops, grass, legumes and vegetables is often seen, while mature crop residues such as straw do not have such high concentrations.

“A distinction between mature and immature crop residues could help to ensure a more accurate estimation of the short-term effects of crop residues on nitrous oxide emissions. For the more long-term effects, i.e., years and decades, we should account for the residual effects on soil quality and nitrogen content. They are affected by local climatic conditions, just as the soil conditions are of importance,” says Olesen.

Nine per cent of agricultural emissions of nitrous oxide stem from the input of crop residues to the fields. A distinction between mature and immature crop residues may be an approach that, according to researchers, could improve the accuracy of the inventories.

It will also improve the possibilities of finding appropriate mitigation strategies.

“Reviewing how we calculate emissions from specific crop residues and determine the right time and place to use them requires more research. There are also important questions in the research into emissions from crop residues that we still need to answer,” says Olesen.

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Researchers drew on massive amounts of data, from weather patterns to soil conditions to land use and agricultural management practices, to feed the model and quantify changes in nitrous oxide emissions from U.S. soils. 

The research breaks soil emissions down by ecosystem types and major crops and found that the expansion of land devoted to agriculture since 1900 and intensive fertilizer inputs have predominantly driven an overall increase in N2O emissions.

The use of such ecosystem models to assess the sources of N2O emissions could help guide policymakers as they enact conservation plans and responses to climate change, said Chaoqun Lu, associate professor of ecology, evolution and organismal biology. 

“The model we are using is a process-based ecosystem model,” Lu said. “It’s similar to mimicking the patterns and processes of an ecosystem in our computer. We divide land into thousands of pixels at a uniform size and run algorithms that simulate how ecological processes respond to changes in climate, air composition and human activities.”

The study found N2O emissions from U.S. soil has more than tripled since 1900, from 133 million tonnes of carbon dioxide equivalent (MMT CO2 eq) per year at the beginning of the 20th century to 404 MMT CO2 eq per year in the 2010s. Nearly three-quarters of that rise in emissions originates from agricultural soils with corn and soybean production driving more than 90 per cent of the ag-related emissions increase, according to the study.

“Our study suggests a large [nitrous oxide] mitigation potential in cropland and the importance of exploring crop-specific mitigation strategies and prioritizing management alternatives for targeted crop types,” the study authors wrote in their paper.

The rise in emissions corresponds to an expansion of cropland in the U.S, Lu said. The computer models found land devoted to agricultural production emits more N2O than natural landscapes. That’s largely due to the widespread application of nitrogen fertilizers to agricultural land and legume crop production, Lu said. The added nitrogen is partially used by crops, and the remainder either stays in soils or is lost to the environment. 

Better understanding the dynamics of which crops lead to the greatest emissions can help shape climate mitigation policy, Lu said. Because more nitrogen fertilizer is applied in corn production on average than other crops, the study found soils where corn is grown tend to emit more N2O per unit of fertilizer used, Lu said.

The researchers designed mathematical models that mimic ecological processes. The models rely on mountains of data gathered and developed over the course of years, Lu said. 

The researchers compiled government data on crops, land use, weather and other variables. They also factored in historic and survey data from farmers and other landowners and compared the results from their model with real-world data to validate results.

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