Nitrates in drainage water
On this page
- The situation
- A look at overall N loss
- The effect of rate
- Application timing and the use of inhibitors
Nitrogen (N) is the single largest component of the atmosphere and is an important building block for all living organisms. It is found in many different forms in the soil depending on the nitrogen cycle (Lamb et al., 2014). It is 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, do not require additional N inputs because they have the ability to fix N from the atmosphere in their root systems. Overall, N sourced by crops for plant growth comes from fertilizer, soil organic matter, atmospheric deposition, animal manure, and fixation (for legumes only).
Artificial drainage is not the only pathway of nitrate to surface waters, but it is the most easily seen and measured, and therefore under more scrutiny than other transport mechanisms.
Losses of nitrate, a mobile form of N, to water systems have been a concern for many years because of human health issues. Ingestion of nitrate by mammals, especially human infants less than 6 months old, 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 USEPA (http://water.epa.gov/drink/contaminants/basicinformation/nitrate.cfm). 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 surface waters in Minnesota, rather than excess nitrate, that leads to increased plant and algae growth and significant surface water quality problems. For decades, there has been no established contaminant standard for nitrate-N in class 2 (aquatic life and recreation) waters in Minnesota (Carlson et al., 2012). However, standards are currently under development and will be phased in over the next few years.
Recently, hypoxia in the Gulf of Mexico has led to increased scrutiny on nitrate contributions to surface waters from agricultural systems (Gulf Hypoxia Action Plan, USEPA, 2008). Subsurface agricultural drainage or "tile drainage" has been the primary focus of the scrutiny. Tile drainage is a highly visible pathway of water transporting 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.
The University of Minnesota established Best Management Practices (BMPs) for the application of N fertilizer in the early 1990s; they were updated in 2008 (Lamb et.al., 2008). These region-specific BMPs are detailed guidelines designed for the efficient use of N fertilizer to maximize profit, while minimizing N loss to the environment. The BMPs focus on management factors like N application timing, N fertilizer source, and the use of nitrification inhibitors, which delay the conversion of ammonium to nitrate. Additional aspects include soil nitrate testing, split applications, and the use of supplemental N under certain circumstances. Other extension bulletins, including Fertilizing Corn in Minnesota and Fertilizer recommendations for agronomic crops in Minnesota, provide N rate recommendations for most agronomic crops grown in Minnesota.
The increased attention being placed on the loss of nitrate via agricultural drainage has led many to call for significant changes in both management of N fertilizer and of agricultural drainage systems. It is essential that if improvements are to be made, there is a full understanding of nitrate fluxes from agricultural systems in Minnesota, as well as 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 is already present (https://engineering.purdue.edu/watersheds/conservationdrainage/index.html). This publication looks at the impact that management of N fertilizer inputs can have prior to loss to surface water.
A look at overall N loss
Corn is the most important crop in Minnesota in terms of total acreage and economic value (Minnesota Agricultural Statistics, USDA NASS, 2012). In addition, it is the single largest user of N fertilizer on the Minnesota landscape. Most of the corn in Minnesota is either continuous (corn following corn), or in a rotation following soybeans. Investigations on nitrate loss from cropping systems in Minnesota have looked at all aspects of a crop rotation, but have focused on corn, for the afore mentioned reasons.
Plots to collect drainage water were established at the Southern Research and Outreach Center in Waseca in 1975. The collection of data was automated in 2009.
Research data on nitrate loss from cropping systems through drainage systems is not as common as one might think. In Minnesota there are plots used to measure drainage water quantity and quality located at the University of Minnesota Research and Outreach Centers in Waseca (SROC) and Lamberton (SWROC). These were established in the early 1970s. In the nearly 40 years that these plots have been used, many nitrogen management practices have been examined. These include N rate, application timing, source, and the use of nitrification inhibitors. In addition, they have 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 nitrate concentration of the water. These numbers are used to calculate the total edge-of-field outflow of N via the drainage system. 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, Randall (2004) showed that soil nitrate storage increased in the soil profile following dry years, but was then subject to loss during wet years. For this reason, 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.
A literature review of a large number of drainage studies worldwide shows annual nitrate-N loss via tile lines varies from 0 lb/A to 124 lb/A (Randall and Goss, 2008). Plots kept devoid of vegetation (fallow) at Waseca measured an average annual loss of nearly 20 pounds of nitrate-N/A from bare ground (G.W. Randall, personal communication, 2013). The source of this nitrate loss was from N mineralized from organic matter. Corn grown without the addition of N fertilizer lost around 10 pounds of nitrate-N/A annually (Randall and Vetsch, 2011). Loss rates from soybeans (which 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 lb. nitrate-N/A (Table 1) on the low end at Waseca to 40 lb/A on the high end (Table 2) at Lamberton during four wet years. A separate project at the SROC using larger plots located approximately one mile away confirmed annual losses ranging from approximately 10 to 18 lb/A (Sands, et.al., 2008).
Over the 40+ years of drainage research at the ROCs, the only method shown to drastically reduce nitrate loss was to use perennial vegetation (as either native prairie plants or alfalfa) at the Lamberton site (Randall and Mulla, 2001). Over a four year period these plots had an annual average flow weighted nitrate concentration ranging from near zero to a high of 4 ppm. In addition, because the total drainage volume was greatly reduced, loss rates of nitrate-N averaged only 1 – 1.5 lb/A (Table 2).
Table 1. Four year nitrate-N loss from a corn-corn-soybean cropping system at Waseca from 2007 – 2010. Nitrate losses calculated for the crop underlined in the Crop Rotation column. (Randall and Vetsch, 2011)
|Nitrate-N (4-yr avg)|
|Crop rotation||N rate||N timing1||Concentration||Total|
|1SPL - Split applied; PP - Preplant application|
Table 2. Effect of cropping system on cumulative drainage volume, nitrate-N concentration and N loss in subsurface tile drainage during a 4 – year period (1990 – 1993) at Lamberton. (Randall, et. al., 1997)
|Nitrate-N (4 yr)|
|Cropping system||Total discharge (4 yr)||Concentration||Total|
|Corn - soybean||35.5||23||182|
|Soybean - corn||35.4||22||180|
The effect of rate
Figure 1. Corn grain yield and residual soil nitrate-N response as affected by fertilizer N rate on a Webster clay loam soil near Waseca, MN, averaged from 2001 - 2003. Note that the amount of residual N follows a similar curve but inverse to yield response to N (Vetsch and Randall, unpublished).
Crop response to fertilizer N rate generally follows a curve, where yield is maximized at some point and additional N inputs do not 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 examination of the response curve relationship (Figure 1) 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 shows the importance of rate, as excessive N inputs are highly likely to be lost to the environment.
Application timing and the use of inhibitors
Fall application of N fertilizer is a common practice in much of Minnesota. However, current BMPs do not recommend fall application in the southeastern part of the state, where there is very little artificial drainage (Randall et al., 2008). The use of urea as a fall fertilizer source is recommended only 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 fall application of AA difficult due to the presence of corn residue from the previous year, especially with conservation tillage. A survey conducted in 2011 showed approximately 40 percent of N fertilizer was applied in the fall in southwestern, west central and south central Minnesota (Bierman, et. al, 2011).
Research has shown, on average, that fall applications of AA with a nitrification inhibitor (where recommended) have similar nitrate-N losses as spring applications (Randall and Vetsch, 2005b). This, of course, varies from year to year based on climatic conditions. Mild falls and wet springs tend to increase nitrate loss. Randall (unpublished data) 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 likely indicator of decreased loss of N into the environment.
Table 3. Nitrate-N concentrations, losses in tile water, and corn grain yield as affected by rate and time of N application (as anhydrous ammonia) at Waseca, 2000–2003 (Randall, unpublished).
|N application||Nitrate-N lost (2000-2003)|
(4 yr avg.)
|lb N/A||mg/L||lb/A/4 cycles||bu/A|
|1FW = flow weighted|
The impact of climate and a growing crop
The volume of water moving through tile lines is determined by available water in the soil profile, evapotranspiration (plant water use and evaporation), and precipitation. Therefore, movement of water through artificial drainage can be thought of as episodic, or characterized by events. An actively growing crop also affects this as root penetration into the soil profile and water demand by the growing plant decreases available water in the soil profile, thus making saturation and therefore movement by artificial drainage less likely.
Figure 2. Relationship between monthly subsurface tile drain flow from facility B in 1987 – 2001 and 30 year normal monthly precipitation and water use (ET) by corn at Waseca, MN (Randall, 2004. Used with permission).
In Southern Minnesota, soils are typically frozen from early December until late-March. Examination of 15 years of drainage records from the SROC show that the majority of tile drainage occurs in April, May and June (Figure 2). While there can be drainage events in the later months of the growing season, they are 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 7 days annually (Randall, 2004).
The loss of N via tile drainage is not only the result of water movement, but also the presence of nitrate in the soil profile. Total N losses on a lb/A basis mirror drainage volumes when looked at on a month by month basis (Table 4). Drainage research at Waseca showed over 70 percent of all N lost through tile lines occurs in the April to June window (Randall, 2004).
Table 4. Monthly distribution of annual subsurface tile drainage and nitrate-N losses for corn in a corn/soybean crop rotation for a 15-year period at Waseca, MN, 1987-2001 (adapted from Randall, 2004).
|Month||Drain flow||Nitrate loss|
|Percent of annual total (%)|
Management to minimize nitrate loss
The well documented increase in the amount of artificial drainage in significant portions of Minnesota can be attributed to the overall profitability of this practice, as well as the increased efficiency of farmers' time (Sands, 2012). This has been accompanied by scrutiny regarding potential negative impacts including nitrate loss as one of the primary concerns. Minimizing nitrate loss via artificial drainage is in the best interests of everyone. It not only makes sense from an environmental standpoint, but also from an economic one.
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/A are lost through drainage systems annually when the soil is kept bare. This represents the soil's contribution from soil organic matter, which is typical in much of the agricultural portions of Minnesota. Corn grown with no N fertilizer inputs still loses an average of about 10 pounds of nitrate-N/A. 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.
The N cycle dictates that conversion of the various forms of organic N must occur before nitrate is present in the soil. This conversion, caused by the actions of microorganisms, is dependent on temperature and time. The subsequent movement of nitrate is dependent on the presence of water in excess of field capacity. The water demand of a growing crop lessens the likelihood of a drainage event. This timing also corresponds with the plant's need for N.
Logically, placement of N into the soil profile as a fertilizer addition would ideally be as close to the time that a plant needs the nutrient as possible to minimize the chance for loss into the environment. Best Management Practices dictate the minimum requirements to prevent excessive N loss. By further delaying application to better correspond with planting or split-applying so that some of the application occurs to a growing crop, chances of a significant leaching event are lessened. However, caution must be taken when late side dress (in-season) applications are surface-applied and not incorporated. If meaningful rainfall doesn't occur for 10 to 20 days, this N could be lost to the atmosphere. In addition it could become positionally unavailable to roots. In either case, yields will suffer due to lack of available N.
There is the potential to fine tune rates and timing to provide some nitrate reduction in surface waters, but time, climatic, and crop growth constraints set limits on how much can be achieved. Ultimately, technology and methods to mitigate nitrate in drainage and surface water may be necessary to achieve overall reduction goals from row crop systems.
Over-application of N fertilizers is another factor within the farmer's control. Nitrogen loss through tile drainage generally increases as N rate increases, especially at N rates greater than the economic optimum. As illustrated in Figure 1 above, changing the N rate from 120 lb/A to 150 lb/A 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, leaving it subject to leaching. The application of nitrogen at rates higher than the EONR represents both an economic risk associated with higher than necessary fertilizer costs and a local environmental risk associated with potential losses and should be avoided. As the departure from EONR grows, so does the risk of nitrate loss to the environment.
The USEPA has set a target for a long term reduction of nitrates in the Mississippi River of 45 percent (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, 2008). Logically, following BMPs with respect to rate, source, timing and the use of nitrification inhibitors is an important first step in reaching this goal.
Current rates of BMP adoption are not well documented. Moreover, model projections by Fabrizzi and Mulla (in Wall, 2013) suggest that only modest improvements can be achieved by further BMP adoption. Delaying applications until later in the season may achieve some reduction, but need to be evaluated and account for the farmer's ability to accomplish the application at the desired timing. These recommendations correspond with the national campaign for fertilizer applications to follow the 4Rs. The 4Rs include: the Right Fertilizer Source at the Right Rate in the Right Place at the Right Time. To learn more about the 4Rs visit www.nutrientstewardship.com.
In the end, our current cropping systems leak N and only perennial vegetation has been shown to be effective at scouring N from the soil profile. It needs to be noted, though, that while the environmental benefits of this practice are clear, an economic system to support these crops does not exist, and therefore the cost is high.
It is beyond the scope of this publication to consider a landscape-wide plan to achieve desired reductions, but many theories, suggestions, and plans for accomplishing this will be forthcoming. In the meantime, farmers and their ag advisors need to focus on making both economically and environmentally sound management decisions. These practices are easily within their control. They should also stay informed on new developments or practices that might achieve further reductions.
What about 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 (Randall, et al., 2000). You can find a detailed discussion on manure management in Manure Management in Minnesota by Hernandez and Schmitt (2012).
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Hernandez, J.A., and M.A. Schmitt. 2012. Manure Management in Minnesota. University of Minnesota Extension publication #03553. Available on-line at http://www.extension.umn.edu/agriculture/manure-management-and-air-quality/manure-application/manure-management-in-minnesota/
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