While artificial drainage offers tremendous benefits for crop production, it can also potentially transport nitrates from the soil to surface water. Here, we share strategies to help you avoid these nitrate losses, which can help protect the environment and reduce fertilizer costs.
Understanding nitrate loss
Nitrogen and its role
Nitrogen (N) is the atmosphere’s single largest component and an important building block for all living organisms. It’s found in many different forms in the soil depending on the nitrogen cycle.
It’s taken up by crops in greater quantities than any other added nutrient. Grass crops, such as corn and wheat, require the addition of N-based fertilizers to maximize productivity. Legume crops, such as soybeans and alfalfa, don’t require additional N inputs because they have the ability to fix N from the atmosphere in their root systems.
Overall, N used by crops for plant growth comes from fertilizer, soil organic matter, atmospheric deposition, animal manure and fixation (for legumes only).
Losses of nitrate, a mobile form of N, to water systems have been a concern for many years because of human health issues. When mammals—especially human infants under six months old—ingest nitrates, it interferes with the blood’s ability to carry oxygen.
Thus, a standard of 10 parts per million (ppm) of nitrate-N has been established for drinking water by the Environmental Protection Agency. For decades, the primary focus has been on groundwater because of its connection with drinking water. Less attention has been given to nitrate levels in surface water, due to decreased dependence on surface water for drinking.
In addition, phosphorus is typically the limiting nutrient in Minnesota surface waters, rather than excess nitrate, that leads to increased plant and algae growth and significant surface water quality problems.
For decades, there hasn’t been an established contaminant standard for nitrate-N in class 2 (aquatic life and recreation) waters in Minnesota. However, standards are currently under development and will be phased in over the next few years.
Scrutiny of agricultural drainage
Hypoxia in the Gulf of Mexico has led to increased scrutiny on nitrate contributions to surface waters from agricultural systems. Scrutiny has primarily focused on subsurface agricultural drainage, or tile drainage.
Tile drainage is a highly visible water pathway that transports nitrate from the landscape to surface waters. Other pathways of water movement from the landscape, such as leaching, shallow groundwater flow and surface runoff, are less visible and more difficult to sample and quantify.
Reducing nitrate in Minnesota surface waters
The increased attention on the loss of nitrate via agricultural drainage has led many to call for significant changes to both N fertilizer management and agricultural drainage systems (Figure 1).
To make improvements, it’s essential to fully understand nitrate fluxes from agricultural systems in Minnesota, and how N management can affect losses. Plans to reduce nitrate in surface waters will need to account for inputs, set reduction goals and develop management strategies on both a watershed and an individual farm level.
Several conservation technologies have been developed, which reduce nitrate from surface waters after it’s already present. On this webpage, we look at the impact of managing N fertilizer inputs before it’s lost to surface water.
Corn is the most important crop in Minnesota in terms of total acreage and economic value. In addition, it’s the single largest user of N fertilizer on the state’s landscape. Most corn in Minnesota is either continuous (corn following corn), or in a rotation following soybeans.
Investigations on nitrate loss from Minnesota cropping systems have looked at all aspects of a crop rotation, but focused on corn for the aforementioned reasons.
Research data on nitrate loss from cropping systems through drainage systems isn’t as common as you might think. In the early 1970s, the University of Minnesota Research and Outreach Centers (ROCs) in Waseca and Lamberton established plots for measuring drainage water quantity and quality (Figure 2).
Since then, they’ve examined many nitrogen management practices. These include N rate, application timing, source and the use of nitrification inhibitors. In addition, they’ve looked at various crops grown in rotation, tillage practices and mineralization of N from soil organic matter.
The drainage plots at the ROCs measure the total discharge of drainage water and the water’s nitrate concentration. Researchers use these numbers to calculate the total edge-of-field outflow of N via the drainage system.
Methods for presenting nitrate loss
Nitrate loss from tile drainage water varies greatly from year to year, primarily based on the total outflow of water from the tiles. In addition, research has shown that soil nitrate storage increased in the soil profile following dry years, but was then subject to loss during wet years.
This is why total nitrate-N loss is usually presented as either an average across years or a total amount over several years. Another method is to calculate nitrate concentration as a flow-weighted (FW) mean, which accounts for variability of total water flow from individual plots.
Annual nitrate loss
A literature review of a large number of worldwide drainage studies shows annual nitrate-N loss via tile lines varies from 0 to 124 pounds per acre. Plots kept devoid of vegetation (fallow) in Waseca measured an average annual loss of nearly 20 pounds of nitrate-N per acre from bare ground.
The source of this nitrate loss was N mineralized from organic matter. Corn grown without adding N fertilizer annually lost around 10 pounds of nitrate-N per acre. Loss rates for soybeans that received no N fertilizer were nearly identical (Table 1).
Generally, annual losses with row crops, where corn received near-optimum rates of N, ranged from 15 pounds of nitrate-N per acre (Table 1) on the low end in Waseca to 40 pounds per acre on the high end in Lamberton (Table 2) during four wet years. A separate project using larger plots at the Southern Research and Outreach Center (SROC) in Waseca located about a mile away confirmed annual losses ranging from approximately 10 to 18 pounds per acre.
The method shown to drastically reduce nitrate loss
In more than 40 years of drainage research at the ROCs, using perennial vegetation (as either native prairie plants or alfalfa) was the only method shown to drastically reduce nitrate loss at the Lamberton site.
Over a four-year period, these plots had an annual average flow-weighted nitrate concentration ranging from near zero to a high of 4 parts per million (ppm). In addition, because the total drainage volume greatly reduced, nitrate-N loss rates averaged only 1 to 1.5 pounds per acre (Table 2).
Table 1: Four-year nitrate-N loss in drainage water in Waseca
|Crop rotation||N rate||N application timing||Nitrate-N concentration (four-year average)||Nitrate-N total (four-year average)|
|Corn-soybean-corn||0 lbs. per acre||--||6.1 ppm||37.7 lbs. per acre|
|“||60+40 lbs. per acre||Split||7.8 ppm||44.8 lbs. per acre|
|“||120 lbs. per acre||Preplant||8.2 ppm||52.1 lbs. per acre|
|Soybean-corn-corn||0 lbs. per acre||--||4.6 ppm||34.0 lbs. per acre|
|“||60+80 lbs. per acre||Split||7.9 ppm||64.2 lbs. per acre|
|“||160 lbs. per acre||Preplant||8.8 ppm||62.8 lbs. per acre|
|Corn-corn-soybean||0 lbs. per acre||--||5.5 ppm||30.5 lbs. per acre|
|“||0 lbs. per acre||--||8.4 ppm||40.9 lbs. per acre|
|“||0 lbs. per acre||--||8.7 ppm||38.3 lbs. per acre|
|Cropping system||Total discharge (four-year)||Nitrate-N: Concentration (four-year)||Nitrate-N: Total (four-year)|
|Continuous corn||30.4 inches||28 ppm||194 lbs. per acre|
|Corn-soybean||35.5 inches||23 ppm||182 lbs. per acre|
|Soybean-corn||35.4 inches||22 ppm||180 lbs. per acre|
|Alfalfa||16.4 inches||1.6 ppm||6 lbs. per acre|
|Conservation Reserve Program (CRP)||25.2 inches||0.7 ppm||4 lbs. per acre|
The well-documented increase in the amount of artificial drainage in significant portions of Minnesota can be attributed to the practice’s overall profitability, as well as the increased efficiency of farmers’ time.
This has been accompanied by scrutiny about potential negative impacts, including nitrate loss. Minimizing nitrate loss via artificial drainage is in everyone’s best interests, as it makes sense from both an environmental and economic standpoint.
Crop response to fertilizer N rate generally follows a curve, where yield is maximized at some point and additional N inputs don’t increase crop yield. The point where additional N inputs no longer produce an economic return is called the Economic Optimum N Rate (EONR).
Recommendations are based on EONRs from a large number of sites and years. Further examining the response curve relationship (Figure 3) shows how applying additional fertilizer N at or above the EONR results in little or no additional yield.
This is accompanied by greater accumulation of residual soil nitrate after harvest, which is susceptible to environmental loss. This relationship follows a similar curve but is inverse to the yield response to N. It shows the importance of N rate, as excessive N inputs are highly likely to be lost to the environment.
Fall fertilizer applications
Applying N fertilizer in the fall is a common practice in much of Minnesota. However, current BMPs don’t recommend fall application in the southeastern part of the state, where there’s very little artificial drainage.
Using urea as a fall fertilizer source is only recommended in the western part of the state, where annual precipitation averages less than 26 inches. A nitrification inhibitor is recommended with fall application of anhydrous ammonia (AA) in south-central Minnesota, where annual precipitation is around 35 inches.
A recent trend toward more continuous corn has resulted in less fall application of N. Most farmers find applying AA in the fall to be difficult due to the presence of corn residue from the previous year, especially with conservation tillage. A 2011 survey showed approximately 40 percent of N fertilizer was applied in the fall in southwestern, west-central and south-central Minnesota.
Research: Fall applications of AA with a nitrification inhibitor
Research has shown, on average, that fall applications of AA with a nitrification inhibitor (where recommended) have similar nitrate-N losses as spring applications. This, of course, varies from year to year based on climatic conditions. Mild falls and wet springs tend to increase nitrate loss.
Research showed that spring applications had greater corn yields than fall applications of AA with an inhibitor (Table 3). Increased yield (although not always statistically significant) is a likely indicator of decreased N loss into the environment.
Table 3: How applying N affects nitrate-N concentrations, losses and yield
|N application: Rate||N application: Time||N application: N-Serve||Flow-weighted NO3-N concentration||Nitrate-N lost: Corn||Nitrate-N lost: Soybean||Nitrate-N lost: Total||Corn grain yield (four-year average)|
|80 lb/a||Fall||Yes||11.5 milligrams per liter (mg/L)||115 lb/a||90 lb/a||205 lb/a||144 bushels per acre|
|120 lb/a||Fall||Yes||13.2 mg/L||121 lb/a||99 lb/a||220 lb/a||166 bushels per acre|
|160 lb/a||Fall||Yes||18.1 mg/L||142 lb/a||139 lb/a||281 lb/a||172 bushels per acre|
|120 lb/a||Spring||No||13.7 mg/L||121 lb/a||98 lb/a||219 lb/a||180 bushels per acre|
The volume of water moving through tile lines is determined by:
Available water in the soil profile.
Evapotranspiration (plant water use and evaporation).
You can think of moving water through artificial drainage as episodic, or characterized by events.
An actively growing crop also affects drainage volume as its roots penetrate the soil profile. The plant’s demand for water decreases the available water in the soil profile, making saturation and ultimately movement by artificial drainage less likely.
When drainage occurs
In Southern Minnesota, soils typically are frozen from early December until late March. Examining 15 years of drainage records from the SROC show the majority of tile drainage occurs in April, May and June (Figure 4).
While the growing season’s later months can have drainage events, they’re unpredictable and tend to be shorter in duration and volume. One set of drainage plots at the SROC showed a 15-year average of 50 percent of total drainage volume occurring in just seven days annually.
The loss of N via tile drainage isn’t only the result of water movement, but also the presence of nitrate in the soil profile. Total N losses on a pounds per acre basis mirror drainage volumes when looked at on a month-by-month basis (Table 4). Drainage research in Waseca showed more than 70 percent of all N lost through tile lines occurred from April to June.
Table 4: Monthly distribution of annual subsurface tile drainage and nitrate-N losses
|Month||Drain flow||Nitrate loss|
Nitrate losses in Minnesota
Glacial till soils found in much of Minnesota are very important to agriculture because of their high organic matter content, available water-holding capacity and fertility. These soils have the potential to mineralize significant amounts of nitrogen from their organic matter.
About 20 pounds of nitrate-N per acre are lost through drainage systems each year when the soil is kept bare. This represents the soil’s contribution from soil organic matter, which is typical in much of Minnesota’s agricultural areas.
Corn grown with no N fertilizer inputs still loses an average of about 10 pounds of nitrate-N per acre. Soybeans, despite being a legume that receives no N inputs, lose about the same amount. The bottom line is that our current crop rotations involving corn and soybeans are leaky with respect to nitrogen.
Best management practices: N fertilizers
The University of Minnesota established best management practices (BMPs) for applying N fertilizer in the early 1990s, which were updated in 2008.
These detailed guidelines are designed to help producers efficiently use N fertilizer to maximize profit, while minimizing N loss to the environment:
Apply nitrogen at the right time
The N cycle dictates that conversion of the various forms of organic N must occur before nitrate becomes present in the soil. This conversion, caused by the actions of microorganisms, depends on temperature and time.
Nitrate’s subsequent movement depends on the presence of water that exceeds field capacity. A growing crop’s water demand lessens the likelihood of a drainage event. Optimum application timing also corresponds with the plant’s need for N.
Applying N fertilizer would logically and ideally be as close as possible to when a plant needs the nutrient, to minimize the chance for loss into the environment. Best management practices dictate the minimum requirements to prevent excessive N loss (Figure 5).
You can lessen the chance of a significant leaching event by further delaying application to better correspond with planting or by split-applying so some of the application occurs to a growing crop.
However, take caution when late sidedress (in-season) applications are surface-applied and not incorporated. If meaningful rainfall doesn’t occur for 10 to 20 days, you could lose this N to the atmosphere. In addition, it could become positionally unavailable to roots. In either case, yields will suffer due to lack of available N.
Over-applying N fertilizers is another factor within the farmer’s control. Generally, nitrogen loss through tile drainage increases as the N rate increases, especially at N rates greater than the economic optimum.
As illustrated in Figure 3, changing the N rate from 120 pounds per acre to 150 pounds per acre in corn following soybeans only increased yield by 4 bushels per acre. However, it increased the amount of residual N left in the soil profile by 40 percent, subjecting it to leaching.
Avoid applying nitrogen at rates higher than the EONR. It represents both an economic risk associated with higher-than-necessary fertilizer costs and a local environmental risk associated with potential losses. As the departure from EONR grows, so does the risk of nitrate loss to the environment.
A note on manure
Research conducted at the SROC found no differences in nitrate-N loss via agricultural drainage between manure and commercial fertilizer, provided recommended rates and application methods were used.
Nitrate reduction targets
The EPA has set a target for a long-term, 45 percent reduction of nitrates in the Mississippi River. Logically, following BMPs with respect to rate, source, timing and use of nitrification inhibitors is an important first step in reaching this goal.
Current rates of BMP adoption aren’t well-documented. Plus, model projections suggest further BMP adoption can only achieve modest improvements. Delaying applications until later in the season may achieve some reduction, but needs to be evaluated and account for the farmer’s ability to accomplish the application at the desired timing.
The recommendations we’ve shared here correspond with the national campaign for fertilizer applications to follow the 4Rs: The right fertilizer source, at the right rate, in the right place, at the right time.
The most effective strategy
In the end, our current cropping systems leak N and only perennial vegetation has been shown to effectively scour N from the soil profile. Note that while the environmental benefits of this practice are clear, an economic system to support these crops doesn’t exist. Therefore, the cost is high.
In the meantime, focus on making both economically and environmentally sound management decisions. These practices are easily within your control. Also, stay informed on new developments or practices that might achieve further reductions.
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Reviewed in 2021