Summary
- Nitrogen is an essential crop nutrient. However, excess soil nitrogen from fertilizer and manure can reach waterways, kill aquatic life and pose a risk to human health.
- The Residual Soil Nitrogen Indicator estimates the amount of nitrogen remaining in the soil at the end of the growing season. This is essential to assess the risk of nitrogen loss from agricultural soils and how this risk is changing over time.
- Overall, residual soil nitrogen on Canada’s farmland has increased over time. This is mainly because nitrogen additions to soil have increased faster than nitrogen removal by crops from the soil.
- Residual Soil Nitrogen showed a larger than normal increase between 2016 and 2021. This was because drought conditions across the country reduced crop production. Lower crop yields meant less nitrogen was used by the plants. The result was an increase in nitrogen left in the soil at the end of the growing season, residual nitrogen.
- Residual soil nitrogen can be decreased by reducing nitrogen additions to soil and reducing nitrogen losses from soil. Some options include applying nutrients where (for example, in the root zone) and when (spring, in-season), at the right rate (for example, determined through soil testing and estimated crop needs), and with the right source. Growing cover crops can also help manage residual soil nitrogen.
Residual soil nitrogen in Canada: why does it matter?
Nitrogen is an essential crop nutrient that is important for crop growth and development, yield and quality. Farmers often add fertilizers or manure to add to the nitrogen already in the soil. Residual soil nitrogen — excess nitrogen that remains in soil after the growing season — can negatively impact the environment and human health and puts pressures on the producer’s time and money.
Residual soil nitrogen can escape into the atmosphere as nitrous oxide and ammonia, gases which can contribute to climate change, lead to ozone layer depletion, reduce air quality and contribute to smog production. It can also reach waterways through 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 high algae growth, which can reduce oxygen levels in water, thereby impairing or killing aquatic life.
Description of the image above
The nitrogen cycle infographic illustrates how nitrogen cycles through the environment.
Soil nitrogen sources: The image shows 3 arrows leading from an atmospheric nitrogen gas bubble (or N2) to be eventually deposited to the soil as ammonium (or positive NH4+ ions) through different processes. One of the arrows leads to biological nitrogen fixation by bacteria and by legume symbiosis and from there to a soil ammonium bubble. Another arrow leads to atmospheric fixation where the ammonium can be deposited in rain or dust particles. The third arrow from atmospheric nitrogen leads to a factory where nitrogen gas is transformed through an industrial process into synthetic fertilizers. The infographic shows a tractor applying fertilizer in a field with an arrow leading the soil ammonium bubble. There is also an image of a cow dropping manure with an arrow leading to the soil ammonium bubble.
Soil nitrogen cycle: The image has an arrow from the soil ammonium bubble through a bubble representing soil microbes and ending at a soil nitrite bubble (or negative NO2- ions). From the nitrite bubble, there is an arrow through another bubble representing soil microbes to end at a soil nitrate bubble (or negative NO3- ions). There are a number of arrows leading from the soil nitrate bubble, illustrating various environmental fates. One arrow leads to a field with crops and plants that assimilate the nitrate. Two arrows show how nitrate can contaminate water bodies, either over the surface as runoff or below the surface through leaching. A sub-arrow from the leaching arrow shows that nitrate can percolate lower in the soil, below the rootzone.
Soil nitrogen pathways back to the atmosphere: The image shows how nitrogen applied to soil can re-enter the atmosphere. From the tractor applying fertilizer, there is an arrow showing that ammonia (or NH3 gas) can escape at the time of application as ammonia volatilization. There is also an arrow from the livestock manure showing the same thing. From the soil nitrate bubble, there is an arrow through a bubble representing soil microbes labelled as denitrification that then leads to nitrous oxide gas (a greenhouse gas) in the air. There are an additional 2 arrows leading to the nitrous oxide, one from the tractor and the other from the fertilizer factory. A final arrow leads from the nitrous oxide bubble to the nitrogen gas bubble, completing the nitrogen cycle.
High nitrogen concentrations in drinking water have been linked to methemoglobinemia (a blood disorder), reproductive dysfunction, thyroid disease and cancer. Many of Canada’s land animals use farmland waterways at some stage in their life cycles (Species at risk public registry). Species at Risk are species that are in danger of disappearing from Canada. 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 the Species at Risk Act) might also require protection of these species or their habitats. (See the Nitrogen Indicator for more information on the risk of nitrogen contamination of water in Canada).
Proper nitrogen management benefits producers and the environment. Appropriate fertilizer management can save producers time, resources and money, and reduce environmental impacts. The Government of Canada also must report on residual soil nitrogen on farmland. This helps inform the Canadian public, and other countries, if Canada’s farmlands are healthy, and identify where improvements to farming practices need to be made.
What determines residual soil nitrogen?
Small amounts of nitrogen are added to soil naturally from rain, lightning, snow and small airborne particles. Through a process called nitrogen fixation, some crops like legumes, with the help of specialized soil bacteria, can add nitrogen to the soil by extracting it from the air. However, nitrogen is often the most limiting nutrient for plant growth in agricultural soils. Therefore, fertilizer and manure are applied to add most of the nitrogen needed to optimize crop growth and yield.
Ideally, the amount of nitrogen applied through fertilizer and manure would closely match the amount taken up by crops. This would lead to most of the added nitrogen being removed from fields after crops are harvested. However, this rarely occurs for many reasons including weather and soil limiting factors. Residual soil nitrogen can be high when too much fertilizer is applied or nitrogen uptake by plants is low, or both. Uptake by plants can be naturally low due to inefficient root systems (for example, potatoes), or when crop growth is low because of unseasonably low or high temperatures, drought, flood, pests, pathogens or weeds. Climate change and climate variability are expected to increase nitrogen losses from soil if excess rain or drought conditions interfere with crop nitrogen uptake.
Residual Soil Nitrogen Indicator
The Residual Soil Nitrogen Indicator assesses the risk of nitrogen loss from agricultural soils. It does this by estimating the amount of nitrogen remaining in the soil at the end of each growing season.
To assess this, we need to first estimate the amount of nitrogen that is being added to farmland soil. This is calculated as the amount of fertilizer applied to crops, the amount of manure produced by livestock, the amount added by legumes through nitrogen fixation, plus the amount that is deposited onto soil from the air. Second, we need to estimate the amount of nitrogen that is removed from farmland soil. This is calculated as the amount removed through crop harvest and pasture, plus the amount lost to the air as gas. Residual soil nitrogen is calculated as the difference between the amount of nitrogen added and the amount of nitrogen removed. Residual soil nitrogen amounts are then categorized as very low (0 to 9.9 kg nitrogen per hectare), low (10 to 19.9 kg nitrogen per hectare), moderate (20 to 29.9 kg nitrogen per hectare), high (30 to 39.9 kg per nitrogen hectare), or very high (≥ 40.0 kg nitrogen per hectare).
The Government of Canada calculates and reports the Residual Soil Nitrogen Indicator every 5 years. It helps the Government determine how the risk of residual soil nitrogen on farmland is changing over time and identifies where changes to farming practices are needed.
Residual soil nitrogen in Canada: current state and change over time
In 2021, residual soil nitrogen varied across Canada. Residual soil nitrogen was mostly low or very low in southeastern Alberta and mostly moderate or high in southern Saskatchewan. Residual soil nitrogen was high or very high in most of British Columbia, northeastern Alberta, northeastern Saskatchewan, Manitoba, Ontario (except for a large area of moderate risk from eastern Georgian Bay to Ottawa) and the Atlantic provinces.
Residual soil nitrogen has increased since 1981 from an average of 9.1 kg N per hectare to 22.0 kg N per ha in 2016. Compared to 2016, residual soil nitrogen doubled to 44.2 kg per ha in 2021. This large increase was because of drought conditions across many of Canada’s agricultural regions. Crops took up much less nitrogen because the average yield and biomass production were reduced.
Areas of high concern include Atlantic Canada (more than 85% of land was in the very high risk class in each province), Quebec (93% of land was in the very high risk class), British Columbia (88% of land was in the 2 highest-risk classes) and Manitoba (92% of land was in the 2 highest-risk classes).
Across Canada, residual soil nitrogen more than quadrupled between 1981 and 2021. This corresponded with a decrease in most of the land in the very low-risk class (-87), and increases in moderate (+38%), high (+183%) and very high (+600%) risk classes.
Increases in residual soil nitrogen occurred because more nitrogen was added to the soil than was removed from the soil. Overall, almost twice as much nitrogen was added to soils in 2021 as in 1981 (increasing from 56.3 to 107.2 kilograms of nitrogen per hectare). Over this period, nitrogen inputs increased in all provinces. This was because of the use of new high-yielding crop varieties (with greater nitrogen requirements), shifts to higher-nitrogen fertilizer types, reduced summer fallow in Western Canada (resulting in greater overall use of fertilizer), and increased pulse production (which adds nitrogen to soils).
However, the amount of nitrogen removed from the soil increased less from 1981 to 2016 and 2021 than that added to the soil during this period, with removal amount of 47.3 in 1981, 80.8 in 2016 and 60.3 in 2021, in kilograms of nitrogen per hectare), leading to an overall net increase in residual soil nitrogen. Nitrogen removal increased because of increased crop yields and improved farming practices (for example, the introduction of higher-yield crop varieties, changes in crop types and distribution, and reduced summer fallow in the Prairies).
Residual soil nitrogen 2021
Changes in residual soil nitrogen between 1981 and 2021 varied across regions. Residual soil nitrogen increased greatly in northern Alberta, northern Saskatchewan, most of Manitoba, southern Quebec, New Brunswick and PEI. Residual soil nitrogen was similar but slightly higher in 2021 than in 1981 in southern Alberta, southern Saskatchewan, northern agricultural areas of Ontario, most of Nova Scotia and in the St. Lawrence lowlands. Residual soil nitrogen decreased considerably between years in southern Ontario, in part because of higher yields in 2021 stemming from favourable weather in the region.
Change in residual soil nitrogen between 1981 and 2021
How can residual soil nitrogen be reduced?
Residual soil nitrogen levels have been increasing in many areas of Canada. Some strategies for reducing residual soil nitrogen include:
Reducing nitrogen inputs:
- Implement a nutrient stewardship plan to ensure efficient nitrogen fertilizer application: the right source of fertilizer, applied at the right rate and time, and in the right place.
- Test soils in spring to determine the optimal fertilizer rates. This can help account for nitrogen additions from past legume crops and manure applications.
- Adjust the amount of nitrogen applied to crops during the growing season by using techniques such as split nitrogen application and side-dress nitrogen application. Use in-season crop sensors and test plant tissue at important crop growth stages to determine when and how much to apply additional fertilizer to match crop needs and adjust for growing conditions.
Reducing nitrogen losses from soil:
- Incorporate manure and organic amendments when applying to minimize nitrogen losses to runoff, erosion and ammonia off-gassing.
- Use cover crops to take up residual soil nitrogen at the end of the growing season. This can prevent overwinter and early spring nitrogen losses during snow melt and rain.
- Use improved crop rotations and conservation tillage practices (such as minimum or zero-tillage) to increase soil organic carbon. This will improve soil structure and water-holding capacity and allow plants to be more resilient in adverse weather conditions.
Description of the image above
An infographic showing an agricultural landscape with crops, a tractor, soil and grazing livestock adjacent to a natural landscape with a watercourse, forest and wild animals. Info boxes are placed to show to which element of the landscape each agricultural sustainability indicator pertains. Arrows connect some of the info boxes to show interrelationships. One info box is present for each of the following indicators: Soil cover, particulate matter, soil organic matter, soil erosion, soil salinization, nitrogen, pesticides, phosphorus, ammonia, greenhouse gases, coliforms and wildlife habitat.
Agriculture and Agri-Food Canada's agri-environmental indicators (AEIs) 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), 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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