Risk of Water Contamination by Nitrogen

Summary

• Nitrogen is an essential crop nutrient. However, excess soil nitrate can leave agricultural land and reach waterways, adversely impact aquatic life and pose a risk to human health.
• The Risk of Water Contamination by Nitrogen Indicator measures the risk of nitrate from agricultural sources reaching waterways, and examines how this risk changes over time.
• Overall, the risk level has increased over time.
• The Prairies contain the most agricultural land in Canada. The risk of nitrate loss in this region is relatively low and stable over time, because of low rain and snowfall when compared to other areas of Canada.
• However, the risk has increased in some other areas that have greater precipitation. For example, in 2021, the average nitrogen concentration in 7 provinces reached or exceeded the 10 milligrams of nitrogen per litre of drinking water guidelines. This was partly because of drought conditions which limited the amount of added nitrogen that the crop could remove. This led to increased soil nitrate losses after harvest.
• The risk of water contamination by nitrogen can be decreased by increasing crop nitrogen uptake and/or reducing residual soil nitrogen at harvest. Options include improved nutrient management planning, conservation tillage and planting cover crops.

Why do nitrate losses in drainage water matter?

Nitrogen is an essential crop nutrient. It is important for crop growth and development, yields and crop quality. Small amounts of nitrogen are added to the soil through natural processes, including rain, lightning, snow and small airborne particles. Through a process called nitrogen fixation, some crops, such as legumes, can also capture nitrogen in the air, convert it into compounds of nitrogen and store them in the soil. However, a majority of nitrogen is added to farmland soils through fertilizer and manure.

Excess nitrogen that is not used by crops during the growing season can reach waterways through surface runoff, tile drainage and leaching through the soil. Nitrogen that reaches waterways can put aquatic systems and human health at risk.

High nitrogen concentrations in freshwater can be toxic to aquatic life (including fish, amphibians and invertebrates). It can also lead to excessive algae growth, which can reduce oxygen levels in water, thereby impairing or killing aquatic life. High nitrogen concentrations in drinking water have been linked to methemoglobinemia (a blood disorder), reproductive problems, thyroid disease and cancer. Many of Canada’s Species at Risk (Species at Risk are species that have already disappeared or are in danger of disappearing from Canada) and animals use farmland waterways during some stages in their life cycles. To help protect these species, it is important to understand how farming impacts their health and habitat. In some cases, provincial or federal regulations (such as provincial wildlife acts or the federal Species at Risk Act) might also require the protection of these species or their habitats.

Good nitrogen management benefits producers and the environment. Appropriate use of fertilizers can be financially beneficial to farmers and can reduce environmental impacts. The Government of Canada must report on the risk of water contamination by nitrogen on farmland. This helps the public and other countries know if Canada’s farmlands are impacting the environment and whether or not improvements to farming practices need to be made.

Factors influencing nitrate contamination in water

Ideally, the amount of nitrogen applied through fertilizer should closely match the amount taken up by crops. This would lead to most nitrogen being removed from fields when crops are harvested. However, this is challenging, especially since weather impacts crop growth and nutrient losses.

When nitrogen added to the soil exceeds the amount removed by crops, this results in residual soil nitrogen and the excess nitrate in the soil could be lost with runoff and drainage water. High levels of residual soil nitrogen can occur when excess fertilizer is applied, or when crop stress (drought, floods, pests, diseases) reduces nitrogen uptake by plants, or both.

Precipitation (rain, snow) or snowmelt can also impact nitrate losses by controlling water movement from cropland to water bodies. High precipitation or snow melt can increase nitrogen transport to waterways through surface runoff, tile drainage or leaching below the crop root zone. Regions with high precipitation levels generally have a higher risk of contamination of waterways by nitrogen.

Climate change is expected to add to the risk of water contamination by nitrogen, as it influences precipitation amounts, timing and frequency. This can in turn affect crop growth and nitrogen uptake. For instance, excess rain can increase the transport of nitrate to waterways because of increased drainage and runoff, while drought can suppress crop nitrogen uptake and increase residual soil nitrogen.

The Nitrogen Indicator

The Indicator of the Risk of Water Contamination by Nitrate-N (IROWC-N) estimates the proportion of residual soil nitrogen that can be lost from the agricultural land into the waterways. The historical and current risk of loss is considered.

The indicator first determines the total amount of nitrogen that is being added to farmland soil from all sources (nitrogen-containing fertilizers, manure produced by livestock, the amount added to soil through nitrogen fixation by legume crops, and the amount of nitrogen in both wet and dry deposition from the air). Secondly, the indicator estimates the amount of nitrogen removed from farmland soil. This includes the amount removed through crop harvest and pasture grazing, plus the amount lost to the air. The residual soil nitrogen is the amount of inorganic nitrogen that is left in the soil after the crop is harvested. The IROWC-N indicator then calculates how much of the residual soil nitrogen could reach waterways through hydrological processes, based on characteristics of the soil, climate and landscape. The overall risk is scored as very low, low, moderate, high or very high.

The Risk of Water Contamination by Nitrogen Indicator is assessed every year and reported every 5 years. It helps the Government of Canada know the status of the risk of water contamination by nitrogen on farmland, and how it is changing over time. This helps to identify where changes to farming practices are needed to ensure a sustainable environment.

How the Nitrogen Indicator has changed over time

Nitrogen removal by crops in Canada was 25% higher in 2021 (60.3 kilograms of nitrogen per hectare) than in 1981 (47.3 kilograms of nitrogen per hectare). This was because of increased crop yields and improved farming practices leading to increased nitrogen uptake, for example by the introduction of higher-yielding crop varieties, changes in crop types and distribution, and reduced summer fallow in the Prairies.

However, the amount of nitrogen added to the soil almost doubled over the 40 years (increased from 56.3 to 107.2 kilograms of nitrogen per hectare from 1981 to 2021). This was because of changes in crop types and area, the introduction of higher-yielding crop varieties which have higher nitrogen requirements, reduced summer fallow in the Prairies (resulting in greater overall use of fertilizer), and increased pulse production (which adds nitrogen to soils). In fact, the average drainage water nitrogen concentration reached or exceeded the drinking water guideline of 10 milligrams of nitrogen per litre of drinking water in 7 provinces in 2021.

In 2021, because more nitrogen was added than was removed, combined with drought conditions that limited crop growth and nitrogen uptake, residual soil nitrogen levels were high that year (44.2 kg N per hectare) – over 4 times greater than it was in 1981 (9.2 kg N per hectare). Overall, this means that the risk of nitrate loss in Canada was much greater in 2021 than in 1981. The amount of farmland in the very low-risk class decreased from 69% to 13%, and the amount of farmland in the low, moderate, high and very high risk classes increased considerably. The amount of farmland in the very high risk class more than tripled from 6% to 20%.

Risk of contamination of surface water by nitrogen in Canada in 2021

Most of Canada’s farmland is in the Prairies. Therefore, the estimation of Canada’s overall risk level is significantly influenced by the risk levels in the Prairie provinces. In 1981, the majority of farmland in Canada was in the very low-risk class, mostly because of conditions in the Prairie provinces and in northern Ontario. In 2021, farmland in the very low-risk class decreased overall in the Prairies and in northern Ontario. However, there remained large areas in the very low-risk class in southern Alberta and southern Saskatchewan in 2021. There was a noticeable increase in the high and very high risk classes in BC, Manitoba and the Atlantic provinces. The risk of nitrate loss in water was generally highest where both residual soil nitrogen and precipitation were high.

Change in nitrogen risk, 1981 to 2021

Regional trends in the Nitrogen Indicator

British Columbia

In British Columbia, the amount of farmland in the high and very high risk classes increased between 1981 and 2021 (from 78% to 84%). This was largely caused by one or two risk class increases in agricultural areas in northern British Columbia. In these areas, nitrogen loss from farmland increased because of greater nitrogen inputs and greater annual drainage in 2021.

The Prairies

Overall, the risk level was lower in the Prairie provinces compared to other regions of Canada. This is because lower precipitation results in lower drainage volumes and nitrate losses from fields. The risk remained stable over time in southern Alberta and southern Saskatchewan. However, it increased by one or two risk classes in northern Alberta, northern Saskatchewan and most of Manitoba. There was a noticeable increase in the proportion of farmland in the high and very high risk classes in Manitoba in 2021 (increasing from 0% to 46%). Increased risk was mainly caused by increased nitrogen inputs and increased drainage in 2021.

Ontario

Unlike most other regions of Canada, risk decreased in Ontario over time. Between 1981 and 2021, the proportion of farmland in the very low and low risk classes increased from 8% to 26%. At the same time, the proportion of farmland in the high and very high risk classes decreased from 84% to 64%.

Like many other regions of Canada, Ontario showed different trends in the northern and southern parts of the province. Risk increased in northern Ontario because of the nitrogen inputs increased at a faster rate than the outputs in crop removal. Whereas in southern Ontario, the risk level was generally lower in 2021 than in 1981 because of greater nitrogen uptake by higher-yielding crops as a result of the favourable weather conditions.

Quebec

In Quebec, the proportion of farmland in the very high risk class increased considerably from 5% in 1981 to 69% in 2021. These increases were mainly due to increased nitrogen inputs.

The Atlantic Provinces

In general, the risk was higher in the Atlantic provinces than in most other areas of Canada. Higher nitrogen inputs and high precipitation lead to higher drainage and nitrate losses from fields.

In 2021, no farmland in the Atlantic provinces had very low risk. Between 1981 and 2021, the proportion of farmland in high and very high risk classes increased greatly in New Brunswick (from 10% to 88%), Nova Scotia (from 35% to 83%) and Prince Edward Island (from 0% to 100%) and decreased in Newfoundland and Labrador (from 59% to 48%).

How to reduce nitrate contamination in water

Many areas of Canada show very high risk of water contamination by nitrogen. However, results from Ontario suggest that reducing risk is possible. Some strategies for reducing risk include reducing residual soil nitrogen levels by adopting sustainable nutrient management practices.

Reducing residual soil nitrogen:

• Implement a “4R” nutrient stewardship approach to ensure efficient nitrogen fertilizer application: the right source of fertilizer, applied at the right rate and time, and in the right place.
• Use soil testing and/or in-season crop sensors to adjust the fertilizer rates. This can help account for nitrogen additions from past legume crops and manure applications.
• Adjust the amount of nitrogen applied to crops by using techniques such as split nitrogen application, incorporating nitrogen into the soil, and using enhanced efficiency fertilizers that contain both urease and nitrification inhibitors.

Reducing nitrogen losses from soil:

• Incorporate manure and organic amendments to minimize nitrogen losses through runoff, erosion and ammonia off-gassing.
• Plant cover crops after harvest to take up residual soil nitrogen left over at the end of the growing season. This can prevent overwinter and early spring nitrogen losses.
• Use improved crop rotations and conservation tillage practices (such as zone tillage) to increase soil organic carbon and reduce nutrient runoff. This will improve the structure and water-holding capacity of the soil.
• Manage drainage water. This can be achieved in irrigation systems or using controlled drainage systems on tile-drained land.
• Use nature-based solutions, such as preserving or constructing wetlands and planting vegetative filter strips, to capture edge-of-field losses.

Agriculture and Agri-Food Canada's agri-environmental indicators (AEI) provide a science-based snapshot of the current state and trend of Canada’s agri-environmental performance in terms of soil quality (soil organic matter, soil erosion, soil salinization), water quality (nitrogen, pesticides, phosphorus, coliforms contamination), air quality (particulate matter, ammonia, greenhouse gas emissions) and farmland management (agricultural land use, soil cover, wildlife habitat). While indicator results are presented individually, agro-ecosystems are complex, so many of the indicators are interrelated. This means that changes in one indicator may be associated with changes in other indicators as well.

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