Irrigation Chapter 5 - Nitrate Movement and Loss Under Irrigated Crop Production
Why Nitrate Loss is a Problem
Loss of nitrate-nitrogen because of leaching (washing) from the bottom of the crop root zone should concern farmers for at least two reasons:
• It contaminates groundwater that is used for individual and municipal water supplies. • It represents a loss of money to the producer, and may result in reduced yields or a need to apply more nitrogen fertilizer.
In this chapter we will look at the nitrate leaching problem, including the possible amount and timing of nitrate loss, and see how management can influence both when and how much nitrogen loss occurs.
Today, many cities, small towns and rural residents have to deal with excess nitrate concentration in their water supplies. In Nebraska, much (but certainly not all) of the groundwater nitrate is non-point source contamination from intensive production of irrigated corn. Non-point source means that there is no identifiable single source of the contamination. Rather, it comes from a wide area like a field, farm or an entire farming area (Figure 5.1).
The U.S. Environmental Protection Agency has set a maximum contaminant level (MCL) of 10 parts per million (ppm) of nitrate-nitrogen in public water supplies. The need to find new supplies or to treat water to meet these standards is proving to be both difficult and costly for many small towns and villages. In some cases local governments have established well-head protection zones around municipal wells. This provides a legal means with the potential to control the management of agriculture within the zone to greatly limit the loss of nitrate from the crop root zone. A major issue facing farmers and town residents is how to meet the need for low nitrate drinking water while meeting the farmer’s need to manage production to assure a good yield and a reasonable profit. There are no simple answers.
Time Required for Nitrate Contamination to Develop in Aquifers
Nitrate contamination of groundwater has been a problem for many years in some of Nebraska’s river valleys where the subsoil is sandy and the water table is shallow (10 to 30 feet). Any nitrate leaving the crop root zone will reach the water table in a matter of weeks or at most a few months (Figure 5.2). While contamination problems may appear and then develop relatively rapidly, better management of water and nitrogen may begin to improve groundwater quality in a matter of two to three years.
|Figure 5.2. For shallow aquifers overlain by sand, the time required for nitrate to move from the root zone to the water table may be less than a year.|
More recently nitrate is also beginning to be found in groundwater where the water table is much deeper and is overlain by 80 to 100 feet of fine textured material. Under such conditions there is a long delay between nitrogen loss from the root zone and arrival at the water table. Travel time from root zone to water table may be 20 to 30 years or more (Figure 5.3). Unfortunately, this means that management improvements on the surface may take many years to appear as an improvement in groundwater quality.
It is important for Natural Resource Districts and other agencies involved in natural resource management to closely monitor changes in groundwater quality and advise producers of increasing nitrate concentrations well before concentrations reach the 10 ppm MCL. A groundwater contamination problem may develop gradually for several years before it becomes apparent. Part of the difficulty in seeing the problem early is the result of how water is usually sampled. Nitrate-nitrogen moving from the root zone to the groundwater arrives at the top of the water table and then very slowly mixes with the water below. However, as shown in Figure 5.4, a water sample taken from a pumping irrigation well is a mixture of water entering the well from many depths in the aquifer. There can be substantial accumulation of nitrate in the upper groundwater, while the sample from the pumped well shows a much lower value that tends to represent an average over the aquifer depth. The thicker the aquifer, the longer it takes for the concentration in the irrigation water to arrive at the 10 ppm MCL.
Why Nitrate Loss Occurs
In the production of many crops some nitrogen loss is almost unavoidable due to a combination of factors, two of which are described below:
- In the root zone, most of the applied nitrogen fertilizer is converted eventually to the nitrate form through the action of soil microbes. Nitrate is readily dissolved in soil water and will move with water.
- The root zone is relatively porous and leaky. When irrigation or rainfall temporarily increases the water content of the root zone above field capacity, the excess will drain downward, carrying nitrate with it. In most cases, that nitrate will eventually reach the water table.
Even under ideal conditions some nitrate-nitrogen will be leached from the root zone of most cultivated crop land each year. With very careful management of nitrogen and water, nitrate leaching during the growing season may be quite limited (in absence of excessive rainfall); however, some part of the residual nitrate from both fertilizer nitrogen and that mineralized from organic matter will be subject to leaching by off-season precipitation. Significant losses also will occur during the growing season if rainfall or irrigation is excessive or if nitrogen applications are excessive or made long before crop uptake.
Recharge From Precipitation and Irrigation
Irrigation pumping draws large volumes of water from Nebraska’s aquifers each year. If the groundwater were not periodically recharged, the water table would fall several feet per year in heavily pumped aquifers, with no recovery. Recharge occurs when precipitation is more than required to refill the water storage capacity of the root zone. With a few exceptions, almost all the water which recharges the aquifers enters the soil surface, passes through the crop root zone, and then slowly percolates to the groundwater. In some river valleys or in some locations near rivers and streams, part of the recharge water may move from the stream into the aquifer. Even so, much of the recharge may still come from root zone drainage. In those parts of Nebraska where the water table is declining, recharge continues to occur during extended wet periods. If not, the water table would decline even faster. In all of these locations, when root zone drainage occurs, part of the residual nitrate-nitrogen in the root zone is leached out and started on its way to the groundwater.
The net irrigation requirement represents the amount of water an irrigator would need to apply if the application were 100 percent efficient. For example, in south central Nebraska the average net irrigation requirement is about 10 inches. Even if a producer were able to irrigate with 100 percent efficiency, excess precipitation could still occur and result in drainage through the root zone. A good part of that drainage would occur during late winter and early spring when there is significant rainfall and the evapotranspiration rate is minimal. All of this water draining through the root zone has the potential to transport nitrate-nitrogen to the water table. If the irrigation were 70 percent efficient, the producer would pump 14 inches during the growing season instead of 10. Assuming that the additional water soaks into the field, the extra 4 inches of water would also percolate through the root zone, carrying nitrate-nitrogen with it. The lesser the irrigation efficiency, the greater the amount of excess root zone drainage that will occur.
Corn production is recognized as a major contributor to nitrate contamination of groundwater under irrigated land. Under continuous corn the soil tends to store organic nitrogen from crop residues. Some of this organic nitrogen is readily mineralizable. During the late summer and early fall, when the soil is warm, mineralization can add substantial amounts of mineral nitrogen to the soil after most crop uptake is completed. This nitrogen is rapidly converted to nitrate and is then subject to off-season leaching.
Soybean is recognized as a scavenger crop that will use residual nitrate-nitrogen in the soil before it fixes its own nitrogen. This reduces the amount of nitrate available for leaching during the growing season. Studies have shown that in many instances a corn-soybean rotation can cut leaching losses as compared to continuous corn. Nebraska research has also shown, however, that under some conditions a corn-soybean rotation can have as much nitrogen loss as a field in continuous corn production. If the legume credit is underestimated (or not considered at all) in calculating the nitrogen requirement for corn in the rotation, or for some other reason the corn is over-fertilized, leaching losses under the rotation can be substantial.
A good stand of alfalfa is an excellent scavenger of nitrate-nitrogen. It takes up nitrate that is found in irrigation water or produced through mineralization. In general, the only way that any significant amount of nitrate can be pushed through the alfalfa root zone during the growing season is by excessive irrigation with high nitrate water. If the stand is allowed to thin excessively, excess spring snow melt and rainfall may move some nitrate toward the groundwater. The greatest loss from alfalfa comes after the crop is plowed up or chemically killed and then the field is planted to a non-legume crop such as corn. Rapid mineralization of nitrogen that had been stored in the alfalfa root system may provide a flush of nitrate greater than the needs of the succeeding crop. It may continue to deliver a large amount of leachable nitrate to the root zone after nitrogen uptake by the corn is completed. Additional loss may occur if the nitrogen contribution from alfalfa is underestimated the following year.
In parts of the Platte River Valley, intensive alfalfa production was practiced until the leap in fuel prices in the late 1970s. There were no serious groundwater nitrate problems in these areas until the switch was made to continuous corn. Some of those same areas are experiencing a growing nitrate problem today.
Calculating the Amount of Nitrate-Nitrogen in Soil and Water
One of the first requirements for developing a management plan to limit nitrate leaching loss is to estimate the amount of nitrate-nitrogen in the irrigation water and the soil. If there is nitrate in the water, it can substitute for nitrogen fertilizer if irrigation is applied before or during a period that the crop needs nitrogen. Sampling the soil for residual nitrate at the beginning of the growing season can tell the producer how much may be available for crop use, enabling a reduction in the amount of fertilizer to be applied.
When samples of water or soil are sent to the laboratory for analysis, results usually are stated in units of ppm (parts per million). These values need to be converted to pounds of nitrate-nitrogen per inch of water applied or pounds per acre in the case of soil samples. The calculation for water is very straightforward.
Nitrate-Nitrogen in Irrigation Water
The amount of nitrate-nitrogen applied per acre during irrigation is calculated as:
Equation 5.1 lb/acre = 0.23 x Cw x Di
Cw = concentration of nitrate-nitrogen in the irrigation water (ppm) Di = depth of irrigation water applied (inches).
For example, in one week 2 inches of water containing 20 ppm of nitrate-nitrogen is applied by sprinkler irrigation The amount of nitrogen applied is calculated as: lb/acre = 0.23 x 20 x 2 = 9.2. It is likely that all of the nitrate is held in the upper portion of the root zone.
Suppose that 4 inches of water containing 30 ppm of nitrate-nitrogen is applied by furrow irrigation. The total amount of applied nitrogen = 0.23 x 30 x 4 = 27.6 lb/acre. Whether all the nitrate in the irrigation water stays in the root zone depends on the soil water deficit before irrigation and how uniformly the water is distributed across the field. If the soil water deficit is only 3 inches at the time of irrigation, it is highly likely that part of the excess water and the nitrogen it contains will drain through the root zone and be lost.
Nitrate-Nitrogen in the Soil
The calculation for the amount of nitrate-nitrogen in the soil is a bit more complicated and requires knowledge of or an assumption about the bulk density (Bd) of the soil. If the laboratory sends back both the ppm and lb/acre of nitrate-nitrogen, the value for lb/acre is based on some average value of Bd, which may or may not reflect the actual field situation. For soil, the nitrate-nitrogen concentration in ppm represents pounds of nitrate-nitrogen per million pounds of soil for the soil depth from which the sample was taken. The lb/acre of nitrate-nitrogen can be calculated using the following equation:
Equation 5.2 lb/acre = 0.23 x Cs x Bd x Dl
Cs = concentration of nitrate-nitrogen on a soil basis (ppm), Bd = bulk density of the soil (known or estimated), Dl = depth increment from which sample was taken (inches).
For example, a 40-acre field of silt loam soil was sampled in eight locations, from the soil surface to 3 feet. The samples were mixed and a sub-sample was sent to the lab. Results showed a concentration in the soil of 10 ppm. The average amount of nitrate in the 0-3 foot increment can be calculated as:
lb/acre = 0.23 x 10 x Bd x (3 x 12) = 82.8 x Bd.
If an average Bd value of 1.32 is assumed for a silt loam, then there would be approximately 109 lb/acre of nitrate-nitrogen in the 3-foot profile.
When soils laboratories report lb/acre they almost always assume the average Bd value for a silt loam. If the soil is compacted or if it is sandy, assuming an average Bd value for silt loam will underestimate the amount of nitrogen in the soil. Sandy soils usually have a Bd of 1.45 to 1.55 or more. The lab value for ppm is fine but the lb/acre has to be adjusted to get a correct value of nitrogen in the profile. For Bd of 1.5, the amount of nitrate-nitrogen in the previous example would be calculated from Equation 5.2 as 0.23 x 10 x 1.5 x 36 = 124 lb/acre, or about 14 percent more. We are also finding many silt loams across the state with Bd values greater than the assumed 1.32. Very fine textured soils such as silty clays or clay loams may have a lower Bd. In such cases the lb/acre of nitrogen reported by the laboratory may be greater than the actual field situation.
The Leaching Process
The movement of soil-water and the nitrate-nitrogen it holds in solution is a complex process which is difficult to describe or predict in detail. For most purposes, however, it is sufficient to have a general idea of what to expect and roughly how far nitrate might move during periods of excess rainfall or irrigation. An example and some figures will help to show qualitatively what happens. Figure 5.5A is a graph that shows the average concentration of nitrate-nitrogen (soil basis) in the top 6 inches of a silt loam soil, where 100 lb/acre of urea ammonium-nitrate (UAN) have been floated on and disked in. The fertilizer has been in the soil long enough for the urea and ammonium to be converted to nitrate. We will consider only the nitrate from the applied fertilizer and ignore other nitrate in the soil.
Now, suppose that 3 inches of rain fall on the field and it all soaks in. (This is clearly an idealized example.) If water moved uniformly at the same rate through all soil pores, the movement of the nitrate layer would be like that shown in Figure 5.5B. The distance the nitrate layer moved could be estimated by dividing the inches of water entering the soil by the volumetric water content of the soil at field capacity. If the field capacity water content were 35 percent by volume, the movement of the layer would be approximately 3/0.35 = 8.6 inches. After the rain the graph of concentration would look like Figure 5.5B. The nitrate layer would retain the same thickness and concentration in the soil as before, but would be displaced downward.
In the field the flow of water and solute is more complicated than described by Figure 5.5. The space between soil particles forms many different pore sizes, ranging from those that are microscopically tiny to pores so large they cannot retain water against the pull of gravity. Just as a larger pipe can convey more water than a smaller one (under a given set of conditions), the large and medium sized pores will convey more water than the smaller ones. Water will flow faster through the larger pores. Water held in the small soil pores moves very slowly or may be trapped in “dead end” pores and not move at all.
Since the nitrate is held in the soil water, when the water moves, the nitrate moves with it. The nitrate does not move as shown in Figure 5.5b, however, because of the different velocities in the different sized pores. The wide range of flow velocities causes layers with greater concentration (like shown in Fig. 5.5a) to spread out, resulting in a reduced concentration extending over a larger soil volume.
After 3 inches of rain, the concentration profile in this example would look more like Figure 5.6. The center of the layer may move down by roughly the distance calculated by the piston flow model (a model that neglects dispersion of water in the profile and is named because the water moves through the soil like a piston); however, there is a substantial amount of nitrate left in the small pores in the top of the soil profile, and some nitrate that moves down well beyond the bottom of the band of Figure 5.5B. The nitrate that moved very deep in the profile of Figure 5.6 was carried by water that flowed more rapidly through the larger pores.
If water actually moved uniformly in all pores, adding a volume of water equal to the field capacity water content would flush all the nitrate-nitrogen from the soil. For example, if field capacity were 35 percent by volume, the amount of water required to “flush” a three-foot soil profile “clean” would theoretically be 0.35 x (3 x 12) = 12.6 inches. In reality, it takes several times this much water to move almost all of the nitrate out of the soil profile. In fact, after repeated “flushings” a small amount would almost always remain trapped in the smallest pores and in “dead end” pores.
Water Application Intensity Affects Leaching
The way water is applied to the soil affects how much water is required to move a given quantity of nitrate from one depth to another in the root zone.
Situation I: When the soil water content is at field capacity or less, most of the water and its load of nitrate will be in the medium and small pores. Under well designed sprinkler irrigation or a gentle rain, the largest pores do not fill with water. That is, while the surface may be very wet, it never becomes completely saturated. Under this condition, water will enter the soil and flow only through the medium and smaller sized pores, not the largest ones. Much of the water flow will be through the sizes of pores where most of the nitrate has been held. A sprinkler irrigation or gentle rain that does not saturate the soil surface will be relatively efficient in moving nitrate downward.
Situation II: Under an intense rainfall, furrow irrigation or any other situation where the surface is completely saturated, water can enter and flow through the large soil pores and the macro-pores. (Macro-pores are the long, continuous openings left by old plant roots, worm holes, cracks, etc.) This allows part of the water to bypass the nitrate held in the medium and smaller pores. In this situation, there is less nitrate loss per inch of deep percolation as compared to very slow, gentle water application. More root zone drainage is required to remove a given amount of nitrate. Excessive root zone drainage does occur under furrow irrigation or excessive rainfall, resulting in a greater amount of nitrate being leached through the soil profile.
Examples and Summary of Leaching
Here are some examples to illustrate these points. Figure 5.7 shows a band of nitrate that has formed from the nitrification of an anhydrous ammonia band. A wetting front is moving down from sprinkler irrigation. When the wetting front reaches the band we would expect the band to spread downward. Nitrate stored in medium sized pores moves down more rapidly because of the relatively larger volume and faster velocity of water flow in those pores as compared to the small pores where water will move much more slowly. Some may not move at all. The net result is that the nitrate band tends to spread mostly downward and not very laterally, with some nitrate left in its original position (Figure 5.8).
|Figure 5.7. Wetting front under sprinkler irrigation approaches nitrate band.|
Under furrow irrigation there is usually a deeper, more uneven wetting of the soil profile (Figure 5.9). As previously indicated, water will flow into the larger pores and the macro-pores, moving down more quickly through the soil as compared to sprinkler irrigation. Even though the amount of nitrate moved per inch of water is not as great as under sprinklers or gentle rain, the larger volume of water applied under furrow irrigation moves more nitrate in total. Under furrow irrigation, a nitrate band like that of Figure 5.7 would tend to spread both vertically and horizontally (Figure 5.10). Excess irrigation would spread the nitrate more and move it deeper.
When urea-ammonium nitrate (UAN) is applied by fertigation through a sprinkler system, the nitrate and most of the urea will move with the wetting front. It spreads through the soil to the depth of penetration of the applied water (Figure 5.11). The ammonium will remain bound to the clay and organic matter near the soil surface until it is converted to nitrate in a few days. The result is that about three-fourths of the applied nitrogen is spread through the upper portion of the soil profile, while one-fourth remains at the surface until it is converted to nitrate. Fertigation with a one-inch water application will usually leave all the applied nitrogen in the top foot of medium and fine textured soils. Penetration can be deeper in sands, depending on the soil water content at the time of irrigation.
If fertilizer materials such as urea-ammonium nitrate (UAN) are applied directly in the water under furrow irrigation, the part of the fertilizer that is mobile will flow through the larger pores and the macro-pores, penetrating more deeply and less uniformly, like the water pattern of Figure 5.9. On a fine textured soil with a reduced intake rate, water may infiltrate to a shallower depth, with less probability of being lost to deep drainage; however, there is still likely be some deep penetration because of macro-pore flow.
Summary Comments on the Leaching Process
Nitrate can be leached from the root zone under crops produced with furrow irrigation, with sprinkler irrigation, or with no irrigation at all. Any water moving through the soil, whether from irrigation or rainfall, also will transport nitrate downward in the root zone. If the irrigation and/or rainfall amount is more than is needed to bring the root zone to field capacity, water will drain from the root zone and take at least some nitrate with it. The more water in excess, the greater the nitrate loss. Soils with lesser infiltration rates such as clays, silty clays, and some clay loams will not on the average suffer as much leaching as lighter textured soils, since there is more opportunity for excess water to run off rather than percolate through the soil.
Field Measurements of Leaching Loss of Nitrate-Nitrogen in Nebraska
Two long-term studies have evaluated nitrogen loss under continuous corn production on medium textured soils in Nebraska. Results indicate how much nitrogen leaching loss might be expected where careful water management is practiced and nitrogen fertilizer amounts are within the normal range.
Sprinkler Irrigated Corn at North Platte
Research was conducted on sprinkler-irrigated corn at North Platte over a six-year period. The soil was a Cozad silt loam. Nitrogen applications averaged 182 lb/acre during the study. Nitrogen was applied as a sidedress application of ammonium-nitrate at the six- to eight-leaf growth stage. Irrigations were scheduled according to soil water deficit, with room left in the soil to hold an inch of rain if rainfall came just after irrigation.
Results showed an average annual loss of 53 lb/acre of nitrate-nitrogen during the six-year study. This is an average over 12 months, not just for the growing season. Losses ranged from 18 lb/acre in a dry year to 83 lb/acre during a year with above average precipitation.
The average root zone drainage was 9.3 inches per year, ranging from 3.6 inches to 15.1 inches, depending on amount and timing of precipitation. About half the losses occurred from mid-June to mid-September, and half during the rest of the year. Losses during the growing season occurred in spite of the strict irrigation scheduling. Both excess rainfall and rain immediately following irrigation contributed to the losses.
The average concentration of the root zone drainage water was 25 ppm of nitrate-nitrogen. This concentration in 9.3 inches of drainage gives an average nitrogen loss of 5.7 lb per inch of drainage. (53 lb/9.3 inch = 5.7 lb/inch). This compares with values of 8-10 lb per inch of drainage found in some other studies in Nebraska.
Furrow Irrigated Corn at Clay Center
At Clay Center, a detailed evaluation of nitrate leaching loss was made on a field that had been in continuous corn under furrow irrigation for 15 years. The tillage system was conventional disk-plant on a Hastings silt loam. This soil has a moderate to slow intake rate and is well adapted for furrow irrigation. The average nitrogen application was 202 lb/acre, applied preplant as anhydrous ammonia. Irrigations had been carefully scheduled using the water balance “checkbook” method.
To estimate nitrate loss, continuous core samples were taken from the soil surface to a depth of 59 feet. Each foot of the samples was analyzed for nitrate and water content. Other measurements showed that little if any of the nitrate loss had leached past the 59-foot depth. The results provided a clear picture of what had happened under this management program.
Over the 15-year experiment the average annual nitrogen loss ranged from 65 to 70 lb/acre. This resulted in a total leaching loss of 1200 lb/acre of nitrate-nitrogen over the 15-year period. At the time of sampling, all of this nitrate was in transit from the bottom of the root zone to the water table. When mixed in the groundwater, it would be enough nitrate-nitrogen to increase the concentration in a typical 100-foot thick aquifer by about 20 ppm.
The average rate of movement of nitrate below the root zone was about 3 feet/year at the upper end of the field and 2.5 feet/year at the lower end. The difference was the result of greater infiltration and deep percolation at the upper end during furrow irrigation. At this rate of travel, the transit time from root zone to a water table 75 feet below the surface would be 25-30 years. This is the rate of movement that would result from around 9-11 inches of root zone drainage at the upper end and about 2 inches less at the lower end. If 10 inches is used as an average, then the nitrogen loss would be about 6.5 to 7 lb. per inch of root zone drainage.
Leaching loss of nitrate-nitrogen under ridge-till in the same field was less than 45 percent of that under conventional tillage, even though corn production under both systems received the same amount of nitrogen fertilizer. There were two apparent reasons for the difference. The annual disking resulted in more mineralization of mineral nitrogen from organic matter. In addition, the ridge-till system had a moderately greater average yield, indicating greater use of nitrogen by the crop, leaving less residual. Even with the reduced nitrogen loss rate, results showed an average of 530 lb/acre of nitrate-nitrogen in transit from the root zone to the water table.
Summary of Nitrogen Loss Findings
Based on these findings, it appears that careful water management and currently recommended nitrogen fertilizer practices may result in average losses of 50-70 lb/acre of nitrogen from irrigated corn, depending on the soil and irrigation system. Nitrogen losses on the order of 5-8 lb per inch of root zone drainage may be expected. Substantially greater losses will occur if less care is taken with water management or if nitrogen applications are excessive.
Sprinkler irrigation has the advantage that limited amounts of water can be applied and may cause less in-season loss on medium to lighter textured soils. If nitrogen losses are to be further limited, however, new and innovative nitrogen management practices will need to be widely implemented.