Nitrogen Indicator

The Nitrogen Indicator (official name: Indicator of the Risk of Water Contamination by Nitrogen) evaluates the risk of water contamination by nitrogen across agricultural areas in Canada. This indicator uses the Residual Soil Nitrogen Indicator, which calculates the amount of nitrogen left in the soil after harvest; as well as subsequent climatic factors to determine the risk of nitrogen reaching surface and ground water. It is important to note that the indicator examines risk of nitrate leaching—that is soluble nitrogen that has passed through the soil profile into ground water or into tile drainage water—rather than losses of nitrogen in surface runoff, which are considered to be minor. The indicator has tracked nitrogen risk associated with Canadian agricultural activities from 1981 to 2011.

Overall state and trend

The majority of farmland in Canada presented a very low risk of water contamination by nitrogen in 2011. However, over the past 30 years, the risk of annual nitrogen loss through leaching (percolating into the soil) has increased by 36% and the nitrogen concentration in leached water has increased.

The Residual Soil Nitrogen Indicator

The Residual Soil Nitrogen Indicator estimates how efficiently nitrogen is used in soils. It is estimated as the difference between total nitrogen inputs to agricultural soils (fertilizer and manure, fixation by leguminous plants, wet and dry atmospheric deposition) and total nitrogen outputs, which consist of harvested crops and gaseous losses including ammonia, nitrous oxide and nitrogen gas (N2). Surplus nitrogen, mostly in the form of water-soluble nitrate, may remain in the soil over the winter and be used by the next crop. Alternatively, it may be lost to the environment by leaching—a process in which water percolates through the soil, carrying nitrates into ground water or into drainage water from tile-drained fields. These nitrogen leaching losses are not included in the Residual Soil Nitrogen Indicator as the majority of these losses occur in the period between growing seasons. For these reasons, the Indicator of the Risk of Water Contamination by Nitrogen has been developed to estimate the leaching losses of nitrate from agricultural soils.


Current status of the Residual Soil Nitrogen Indicator

Residual soil nitrogen levels on farmland in Canada were in the "Moderate" risk class in 2011 (see performance index below), but showed an increasing trend over time due to increases in nitrogen inputs such as fertilizer, particularly since the mid-1990s.

Figure 1: Residual Soil Nitrogen Performance Index
Description of this image follows.
Description - Figure 1
Year Index Value
1981 85
1986 85
1991 80
1996 74
2001 56
2006 68
2011 56

The performance index tends to aggregate and generalize trends and so should be viewed as a policy tool to give a general overview of state and trend over time.

How performance indices are calculated

More results and information on the Residual Soil Nitrogen Indicator can be found in the publication entitled Environmental Sustainability of Canadian Agriculture, Agri-Environmental Indicator Report Series - Report #4 .

Use the interactive map below to zoom in and explore different regions. Note that risk of nitrogen contamination is considered to be very low on a national level, due to the amount of agricultural land in the Prairies, which have a lower amount of precipitation leading to a lower risk of drainage from agricultural soils. Higher risk classes can be observed in Central and Atlantic Canada where precipitation rates are higher. Risk is also closely tied to nitrogen source; nitrogen levels have been increasing in the soil as a result of mineral fertilizer use.

In addition to exploring the 2011 values, click the play button to view changes over time. From 1981 to 2011 there has been a steady increase in risk many regions of Canada, particularly in the Maritimes and in parts of Quebec, Manitoba and western Alberta.

Figure 2: Risk of contamination of surface water by nitrogen in Canada in 2011

Legend: legend


You can also explore the change in contamination risk from nitrogen in the interactive map in Figure 3. This map uses a colour scheme to illustrate negative changes (reds and oranges) and positive changes (greens) between 1981 and 2011. It is apparent that the increase in risk is occurring across Canada, but is particularly evident in parts of Manitoba and the Maritimes.

Figure 3: Change in nitrogen risk, 1981 to 2011

Legend: legend


Nitrogen performance index

The state and trend of the Nitrogen Indicator can also be seen in the performance index below.

Figure 4: Risk of water contamination by nitrogen index
Description of this image follows.
Description - Figure 4
Year Index Value
1981 93
1986 92
1991 91
1996 90
2001 89
2006 89
2011 89

As illustrated by the performance index above, in 2011 the risk of water contamination by nitrogen on farmland in Canada was very low, which corresponds to the "Desired" category as indicated by a value of 89. The index illustrates a fairly stable, yet slightly declining, trend from a high index value of 93 in 1981 to 89 in 2001, which subsequently plateaued and then remained stable until 2011. While this indicator has remained in the 'Desired' class, more and more farmland has been moving from lower risk categories to higher risk categories, and some pockets of farmland are now in the high risk and very high risk classes.

The index tends to aggregate and generalize trends and so should be viewed as a policy tool to give a general overview of state and trend over time.

How performance indices are calculated

Specific trends

Nitrogen build-up in soils across Canada increases risk to ground water

Nitrogen is applied to crops most often during or shortly after planting, although crop nitrogen requirements are highest during the later stages of plant development—often weeks after application. Once the growing season is over, any unused nitrogen remains in the soil as residual soil nitrogen until it is either used by the next year's crop or is removed from the soil via drainage water following precipitation. The residual soil nitrogen can accumulate in the soil over time, as more nitrogen is applied than is removed.

Figure 5 (below) shows the nitrogen inputs and outputs as well as residual soil nitrogen levels from 1981 to 2011. On a national basis, average nitrogen inputs have almost doubled over the past 30 years (from 44 kilograms per hectare in 1981 to 80.8 kilograms per hectare in 2011), whereas average nitrogen outputs increased by 63% (from 35 kilograms per hectare in 1981 to 57.2 kilograms per hectare in 2011) over the same time period. The greater increase in nitrogen inputs compared to outputs over time has resulted in an increase of residual soil nitrogen values by more than 150% (from 9.4 kilograms of nitrogen per hectare in 1981 to 23.6 kilograms per hectare in 2011). The residual soil nitrogen value of 25.3 kilograms per hectare calculated for 2001 was primarily attributable to the low level of nitrogen outputs that year caused by the reduction in yields and crop nitrogen uptake associated with droughts in many regions in Canada. Because uptake and removal of nitrogen by crops accounts for 95% of the Canadian nitrogen output during the growing season, any variations in yield can dramatically affect the amount of nitrogen left in the soil. The elevated levels of residual nitrogen in agricultural soils has increased the likelihood of higher concentrations of nitrates in drainage water and consequently increased the risk of water contamination in many parts of the country.

Figure 5: The estimated nitrogen (N) input, nitrogen (N) output and residual soil nitrogen (RSN) in Canadian soils, between 1981 and 2011, measured in kilograms per hectare (kg N ha-1)
Description of this image follows.
Description - Figure 5
The estimated nitrogen input, nitrogen output and residual soil nitrogen in Canadian soils, between 1981 and 2011, measured in kilograms per hectare
Year N input N output Residual soil nitrogen
1981 44.44 35.02 9.43
1982 44.65 35.26 9.4
1983 44.91 34.5 10.41
1984 48.35 35.2 13.15
1985 49.59 36.17 13.41
1986 48.4 39.07 9.33
1987 47.57 38.26 9.31
1988 50.27 34.78 15.49
1989 49.27 37.06 12.2
1990 49.64 39.71 9.94
1991 49.78 38.17 11.61
1992 51.81 38.86 12.94
1993 53.71 41.57 12.14
1994 56.84 43.03 13.8
1995 58.64 43.59 15.05
1996 61.5 46.23 15.27
1997 63.85 43.95 19.9
1998 64.88 45.93 18.95
1999 65.07 48.58 16.49
2000 67.65 47.54 20.11
2001 67.52 42.24 25.27
2002 69.05 41.63 27.42
2003 68.87 46.37 22.51
2004 70.96 50.9 20.06
2005 68.82 52.18 16.64
2006 69.86 51.43 18.43
2007 74.05 49.82 24.23
2008 77.97 55.58 22.39
2009 77.51 55.3 22.21
2010 78.88 56.63 22.25

Why this indicator matters

Nitrogen is an essential nutrient for all plants and animals. It is applied to soils in the form of fertilizers and manures in order to maintain crop yields. Incomplete nitrogen uptake by crops inevitably results in some inorganic nitrogen remaining in the soil at the end of the growing season (see Residual Soil Nitrogen). The environmental risk is greater when large surpluses of nitrogen are present in the soil, especially between cropping seasons in regions that receive considerable precipitation. Most of the residual inorganic nitrogen, which is in the form of nitrate, is water soluble and can readily leach through the soil into ground water or can move from tile drains into ditches, streams and lakes. High nitrate levels in surface waters contribute to algae growth and eutrophication and have been linked to human health impacts.

Agriculture has the potential to mitigate risk from nitrogen by implementing beneficial management practices (BMPs) that reduce application rates or that prevent nitrogen from reaching water bodies.

Beneficial management practices

Strategies for reducing the risk of water contamination by nitrogen include managing nitrogen inputs, avoiding build-up of nitrogen in the soil and reducing the risk of transport.

Split application of nitrogen—whereby producers make two or more fertilizer applications during the growing season rather than providing all of the crop’s nitrogen requirements with a single treatment prior to, or at, planting—is a method that can be used to reduce mineralization in the soil and losses during the growing season. Side-banding is a practice used by many producers to ensure a slower release of nitrogen when the crop needs it. The timing of fertilizer and manure application is also critical. Applying fertilizer as close as possible to the period of rapid crop uptake will minimize nitrogen losses from the field and will ensure adequate nitrogen availability to the crop during critical growth periods. Analyses of plant nitrogen status can also help producers choose more economical application rates. Using urease or nitrification inhibitors to slow fertilizer conversion to nitrate is an option that can reduce ammonia volatilization and nitrous oxide losses and improve nitrogen uptake by crops. Non-leguminous cover crops may also provide a solution as they can capture residual nitrogen from the soil, convert it to organic nitrogen in their tissues and then make it available to the next crop as their tissues decompose over the following spring and summer.

Management practices that enhance soil quality (for example, those practices that increase soil organic matter levels and improve soil structure) can be implemented to increase the water holding capacity of the soil and allow excess water to drain from fields during high rainfall events.

Lastly, a number of end-of-pipe solutions are available for producers to control or mitigate the surface water impacts of nitrate leaching from tile-drained fields. These include controlled tile drainage systems to manage the water table and retain the nitrate in the fields, where it can be used by growing crops; constructed or natural wetlands to trap nitrates, and reactive biofilters to reduce the amount of nitrate in drainage before it gets to local surface waters. Some of these methods may enhance denitrification losses from soils, and could result in (as yet unquantified) pollution-swapping trade-offs (for example, nitrous oxide emissions or phosphorus losses in surface runoff); and as well tend to have much higher installation and operating expenses, therefore reducing nitrogen inputs via nutrient management is the desired practice.

How performance indices are calculated

The agri-environmental performance index shows environmental performance state and trends over time, based on weighting the percentage of agricultural land in each indicator class, such that the index ranges from 0 (all land in the most undesirable category) to 100 (all land in the most desirable category). The equation is simply "(% in poor class multiplied by .25) plus (% in moderate class multiplied by .5) plus (% in good class multiplied by .75) plus (% in desired class)." As the percentage of land in the "at risk" class is multiplied by zero, it is not included in the algorithm.

The table below shows the index classes. The index uses the same five-colour scheme as the indicator maps whereby dark green represents a desirable or healthy state and red represents least desirable or least healthy.

The index classes
Scale Colour scheme Class
80-100 Dark green Desired
60-79 Light green Good
40-59 Yellow Moderate
20-39 Orange Poor
0-19 Red At risk

The index tends to aggregate and generalize trends and so should be viewed as a policy tool to give a general overview of state and trend over time.

Related indicators

Additional resources and downloads