Seelig, B.D. and Nowatzki, J.

North Dakota State University, Fargo, ND 58105

Our activities as human beings have changed the balance of nitrogen on the planet. Burning fossil fuels for energy, intensive use of land to grow food, and disposal of organic wastes have an effect on the nitrogen cycle. Studying the influence of our activities on the nitrogen cycle helps us understand the consequences of changing the balance of nitrogen in the environment. Some of the consequences are positive such as improved crop yields, but other consequences are negative such as water resource deterioration. When attempting to manage nitrogen, all of the consequences need to be considered. Groundwater contamination by nitrogen is commonly related to the conditions and activities around water wells. Wells that are most frequently contaminated with nitrogen are shallow and have large diameters. Location of barns, barnyards, septic systems, feedlots, silos, buried waste, and fertilizer storage and handling sites have all been implicated in contributing to well contamination. However, groundwater contamination due to agricultural field activities cannot be discounted and should be considered when assessing for sources of nitrogen in a given area. Surface water contamination by nitrogen has been shown to be more prevalent in agricultural areas compared to other landuses. However, contamination of streams with ammonia from municipal sewage treatment is commonly seen downstream from urban areas.

There is less nitrogen available to be leached to groundwater under permanent vegetation compared to cultivated areas. Evidence from groundwater investigations often shows higher concentrations of nitrate in groundwater under tilled fields compared to less intensive landuse. Compared to other crops corn, potatoes, and shallow rooted vegetables have greater potential to allow nitrate movement beyond the rooting zone. The combination of crops grown in a rotation also affects the potential for groundwater contamination by nitrogen. For example, less nitrogen moves beyond the rooting zone with a corn-soybean rotation compared to continuous corn. Rotations of deep-rooted crops such as sunflowers, safflower, alfalfa and sweet clover will scavenge some of the nitrogen leached from crops with shallow root zones.

There is a strong connection between runoff, erosion, and nutrient losses. Because the type of crop grown influences runoff, nutrient losses vary with different crops. In general soils under row crops such as corn, beans, potatoes, and sunflower have greater erosion compared to soils under small grain crops. However, crop effects on nutrient losses can be complicated when combined with other factors that influence runoff such as annual precipitation, land slope and drainage type. For example, in one watershed study more runoff and erosion occurred in fields planted to row crops compared to small grains in years of average precipitation. But in years with above average precipitation, more runoff and erosion occurred in areas where small grains predominated, because small grains were grown in areas of steeper slopes compared to row crops.

Crop nutrient requirements and crop use efficiency influence the amount of nutrient losses from fields. Higher losses of nitrates can be expected from corn and potatoes compared to small grains and hay crops due to higher nutrient requirements and lower water and nutrient use efficiency.

Tillage tends to increase the amount of nitrates in soil because it enhances mineralization. Most cultivated land has some degree of tillage. Comparative studies of different land uses show that groundwater underlying cultivated soils generally contains higher levels of nitrates compared to natural uncultivated soils.

The type of tillage also influences nitrogen movement through the soil profile. Tillage causes disruption of macropores. In recent years innovations in equipment and chemicals have led to reduced levels of tillage in many areas. This trend seems to be a benefit for groundwater protection in some areas because of reduced mineralization and increased immobilization. However, in many areas improvement of soil structure in no-tilled soils has created a network of macropores that conduct water and nutrients through the soil profile. The end result is increased flow of water and its dissolved load to groundwater. It is apparent that tillage has both advantages and disadvantages with respect to groundwater that vary with local conditions.

• Conservation tillage may cause greater immobilization of nitrogen so its availability to leaching would be reduced. However, it may also increase nitrogen leaching by increasing water infiltration and percolation. Information from studies on local soils should be used to determine if conservation tillage will have positive or negative impacts on groundwater.

Tillage practices that leave the soil surface unprotected greatly increase the potential for runoff, erosion, and nutrient losses from fields. The major input of nitrogen to surface water is generally associated with sediment eroded from land surrounding streams or lakes. Fields with reduced tillage or no tillage contribute much less total nitrogen to surface water than conventionally tilled fields. However, nitrate nitrogen has often been observed to be greater from fields with reduced tillage compared to conventionally tilled fields.

Minimum tillage tends to increase percolation and reduce runoff. Unless tile drainage is installed in a field, this reduction in runoff decreases potential surface water contamination from nitrogen. Studies also indicate reduced tillage promotes smaller quantities of residual soil nitrogen after harvest, decreasing the potential for nitrogen losses from erosion.

The effects of tillage systems on nitrogen losses from fields depends on surface texture, bulk density, aggregate stability and surface soil chemistry. For example, little difference has been demonstrated in runoff and soil loss between conservation and conventional tillage in heavy-textured soils.

Soil conservation practices used to protect tilled fields from erosion will not effectively protect surface water unless they are applied to critical areas and are designed to reduce soluble loads of nutrients.

• Conservation tillage will help protect surface water from nitrogen, but not to the same degree on all soils. The advantages of increased stored soil moisture under conservation tillage are greatest for drier soil types
• When possible tillage should follow the contour or allign transverse to the direction of the slope. This will reduce the erosive power of water flowing down the hillslope. Combining tillage on the contour with alternating strips of grass or hay crops provides even further protection of the soil.
• In areas of uniform slopes tillage along the contour may be combined with field terraces as an additional method to reduce the erosive power of hillslope runoff.
• Grassed waterways may be installed to protect areas of concentrated runoff (drainageways) from gully erosion.
• Low lying areas of wet soils along drainageways and adjacent to streams and wetlands should be protected from tillage to the greatest extent possible. In the their natural state, these areas function as environmental filters and help to protect surface water from activities further upslope.

Summer fallow tends to reduce crop-water-use efficiency due to deep percolation of water below the rooting zone. The fallowed soil not only loses a portion of the precipitation that infiltrates the surface, but also a portion of the solutes present in the soil, such as nitrates released by mineralization. Summer fallow generally increases the potential for groundwater contamination by nitrogen. Although nitrate leaching from summer fallowed fields with coarse textured soils is more likely to occur, significant leaching may also occur in finer textured soils.

The effect of summer fallow in the northern plains on surface water is related to soil erosion. The higher content of available nitrogen in summer fallowed soils coupled with greater potential for water and wind erosion increases the potential for nitrogen losses from these fields. With respect to surface water protection, chemical fallow has much less negative impact compared to "black" fallow. This is because crop residue protects the surface and erosion losses are similar to fields with conservation tillage.

Management recommendations

• Black summer fallow (tilled for weed control) should be either eliminated from the crop rotation or reduced to a small percentage of the total farmed acres.
• Use herbicides to control weeds and maintain crop residue during the fallow period. This alternative provides similar protection from erosion compared to conservation tillage.

The nitrogen-use-efficiency of fertilizer applications of most cropping systems rarely averages greater than 50% and generally decreases with increased amounts of nitrogen applied. Improving the nitrogen-use-efficiency is a critical factor in reducing environmental impacts of nitrogen. Nitrogen fertilizer inputs match plant nutrient requirement during the growing season will generally improve nitrogen-use-efficiency. Despite the low efficiency of use for nitrogen fertilizer, groundwater contamination is rarely a direct result of fertilizer application.

Application of nitrogen fertilizer does not necessarily increase the potential for groundwater contamination. Improved plant growth due to the application of recommended rates of nitrogen fertilizer can actually result in lower nitrate leaching losses. Fertilized healthy plants will extract more nitrogen from the soil as compared to less vigorous unfertilized plants.

In northern prairie soils the lack of rainfall has the greatest influence on nitrogen leaching to groundwater compared to crops with low nitrogen-use-efficiency. In most soils of the Northern Great Plains, nitrate leaching beyond the root zone only occurs in years of above average precipitation. Regular soil testing is useful to account for residual nitrate nitrogen left in the soil after the growing season, because it will be available for the crop in the following year. This is important to both economic and environmental management, because if the residual nitrogen is not taken into account, fertilizer applications will be in excess of crop needs. Nitrogen leaching beyond the rooting zone can and does occur in the region, but is generally limited to years of excessive rainfall and on sandy or gravelly soils.

Several factors need to be considered when using organic wastes for fertilizer so that efficient utilization occurs. Nitrogen not mineralized in the first year after application of manure generally becomes part of the soil organic matter and releases nitrogen relatively slowly. Because manures from various sources decompose at different rates, it is necessary to determine the decomposition rates for proper fertilization. Constant annual manure applications that supply enough nitrogen for crop demands will ultimately cause excessive fertilization; therefore decreasing amounts need to be applied each year to lower the potential for nitrate leaching.

The method of application or form of organic waste may make a measurable difference with respect to leaching losses. Liquid manure application generally results in more leaching than solid manure. Land application of sewage sludge should be aged, dewatered, and applied to the soil surface for best protection of groundwater. Excessive production of nitrates from nitrification of land-applied sludge may be managed by the addition of organic carbon.

Fertilizer recommendations for groundwater protection have been demonstrated to be effective under many circumstances in lowering the potential for nitrogen contamination. Sometimes these recommendations have little or no impact on yield or a profitable return. However, this is not always true. Economic efficiency may require above-optimum fertilizer applications. Under these circumstances, if fertilizer rates are reduced for environmental protection crop yields will also be reduced. Agronomic recommendations should always be tested for local conditions to determine the balance between economic returns and resource protection.

• The level of residual soil nitrate should be determined by analysis of soil samples taken from each cropped field. Fertilizer applications should be based on soil analyses results and selection of a reasonable yield goal.
• Nitrogen applications should be managed very carefully on sandy or gravelly soils due to the high potential for leaching losses. Fall applications are not recommended on these types of soils. Split applications of nitrogen should be considered to ensure that adequate nitrogen is available during critical stages of crop growth. Slow -release fertilizer or nitrification inhibitors are recommended to reduce the potential for build-up of nitrate and sudden loss due to rapid leaching from intense rainfall.
• If anhydrous ammonia or urea is applied in the fall on finer textured soils, soil temperature should be 45° F or less. At these temperatures conversion to nitrate is slow, so the potential for nitrate leaching losses is reduced.
• Storage of nitrogen fertilizer should be located in an area protected from excessive surface runoff or water infiltration. A fertilizer storage area should have an impermeable surface from which runoff is diverted. In the case of commercial fertilizers, they should be stored in areas where the integrity of packaging can be maintained and where spills or leaks can be easily detected and managed.
• When animal manure is used as fertilizer, it should be tested to determine its nutrient value. Application of manure must account for the rate of decay or mineralization so that the proper amount of nitrogen is available for crop requirements. Nitrate release from manure occurs over a period of many years so applications in previous years must be accounted for each growing season.
• Injection of liquid manure is not recommended into sandy or gravelly soils that overlay shallow groundwater. Nitrogen and other contaminants have been shown to have greater mobility in a liquid slurry compared to dry manure.

There is a positive correlation between amount of agricultural land in a watershed and the concentration of nitrogen in watershed streams. In some areas excessive applications of nitrogen is strongly linked to high concentrations of nitrogen in streams. However, under most circumstances nitrogen levels in streams cannot be directly linked to a single source such as fertilizer applications. In fact, it has been shown that proper fertilizer application according to plant growth needs results in decreased nitrogen losses from fields. Fertilizer applications improve plant growth and increase crop residue which contributes to reduction of runoff and erosion.

Land application of organic wastes has potential to result in nitrogen contamination of surface water if not managed correctly. The potential for transport of land-applied organic wastes applied to fields is subject to many different factors, which makes it variable within watersheds and among watersheds. However, greater losses of nitrogen occur from fields fertilized with animal wastes as compared to inorganic fertilizer. Generally, application of organic fertilizer to more erodible soils will result in greater potential for contamination of surface water resources. Applying higher rates of organic fertilizers also generally adds to the potential for surface water contamination. Incorporation of animal wastes into the soil after application reduces the potential for movement off the land by surface water.

It is important to base animal waste applications on a balance between nitrogen and phosphorus. Animal waste applications based solely on nitrogen recommendations can result in high phosphorus levels that contribute to surface water problems.

• The first five fertilizer management recommendations for groundwater protection also apply to surface water protection, because they are designed to reduce excess nitrate in the soil that can be lost to water resources.
• Manure applied to the soil surface should be immediately incorporated or injected on soils with slopes greater than 6%.
• Manure applications should be avoided on frozen ground or during excessively wet periods of time. Manure should never be applied any closer than 25 to 30 feet from a stream or lake or within 200 feet if the soil is frozen.
• Manure applications must balance plant requirements for both nitrogen and phosphorus. Applications based only on nitrogen requirements of plants will eventually result in excessive levels of soil phosphorus.

Drainage from septic systems has been identified as one of the sources for elevated nitrates in groundwater. The main form of nitrogen that exits a septic drainage system is ammonium (NH₄), but is quickly changed to nitrate and subsequently leached. Leaching occurs within a few feet of the drainage field, so there is little opportunity for dilution or plant uptake. The amount of nitrogen added to a septic drainage field from a family of four is approximately 200 X the amount mineralized from soil organic matter plus the amount deposited from the atmosphere. This means that under normal circumstances high concentrations of nitrate leach from most septic drainage fields. Nitrate contamination of groundwater from septic drainage fields is most likely to be a problem in areas of low rainfall and high development density.

Nitrogen contamination of wells has been associated with the proximity of livestock yards and animal waste. However, reliable relationships that predict the levels of nitrogen contamination using the distance between wells and livestock yards do not exist. Significant leaching of nitrates from livestock feedlots is most likely to occur on sandy or gravelly soils, or when compacted conditions are not maintained due to low stocking rates, frequent disturbance of compacted layers, or lot abandonment.

• Water wells should be constructed according to modern standards as outlined in Article 33-18 of the North Dakota Century Code to prevent surface water infiltration through seams, cracks or holes in the casing. They should not be located in depressional areas or low landscape positions that receive surface water runoff. Wells should be located at least 50 feet from privy pits, cesspools, septic tanks, and sewage filtration fields; 100 feet from barnyards or feedlots; and 250 feet from livestock manure storage areas. If these precautions are used, the potential for well contamination with nitrogen from organic wastes will be reduced.
• Abandoned wells in the vicinity of livestock yards or manure storage should be sealed according to appropriate methods as outlined in NDSU Extension publication AE-996.
• The surface of an animal yard or feedlot should be maintained by allowing the compacted layer of manure immediately above the soil surface to remain undisturbed. This 3 to 4-inch layer serves as a seal with respect to nitrate leaching. Only when the area is no longer used for animal production, should the compacted layer of manure be removed.
• Recommendations for organic waste applications that protect groundwater are listed in the Fertilizer Applications section.

The management of organic wastes and runoff from areas in close proximity to surface water has a significant effect on the amount of nitrogen that is mobilized and transported to surface water resources. Proper design of storage facilities, regular maintenance of animal yards, and diversion of runoff water will reduce the potential for contamination of surface water from livestock facilities. Proper septic system installation and maintenance are needed to ensure that human septage does not run directly into surface water resources.

• Septic system installation should conform with standard design, siting, and construction requirements as outlined in NDSU Extension Service publication AE-892. This will ensure that waste receives proper treatment.
• Septic systems should be maintained through regular inspections and avoiding excessive amounts of grease, oil, or caustic chemicals that will plug or damage the system. Plugged septic drainage fields allow septage to flow to the surface and contaminate water resources.
• Manure (liquid or dry) should be stored in properly designed facilities that are protected from excessive runoff, flooding, or overflow conditions that would allow contamination of surface water. Proper design and location of animal waste facilities may be determined from the Midwest Planning Service Bulletin 18.
• Recommendations for organic waste applications that protect surface water are listed in the Fertilizer Applications section.

Irrigation should not be linked to nitrate contamination of groundwater without consideration of other factors that influence nitrogen fate in soils and the geologic materials below. Irrigation often occurs where groundwater is shallow and soils have high sand content. These two factors alone increase the potential for groundwater contamination. Many crops that are irrigated, such as potatoes, are quite intolerant of low soil water contents, so maintaining an optimal agronomic environment results in greater potential for downward water movement. When conditions that maximize vertical water movement through the soil profile are coupled with the presence of a mobile chemical such as nitrate, the potential for groundwater contamination increases. Many high-valued crops, such as potatoes and vegetables, have large nutrient requirements and shallow rooting depths, which further increase the potential for groundwater contamination.

Irrigation water management appears to be the most important factor in reducing potential for nitrogen leaching. The method of irrigation water application influences the leaching process. Generally the potential for leaching is smallest for drip irrigation and highest for furrow irrigation. Deficit water scheduling that depletes the soil of water in the fall substantially reduces nitrate leaching from irrigated fields.

• Schedule irrigation appropriately by monitoring soil water and crop water use. Regular measurement of soil water is an accurate way of determining when to irrigate. An indirect method used to estimate soil-water balance, commonly called the "checkbook method," is based on knowledge of the soil water holding capacity, daily crop water use, and daily precipitation measurements. Soil water content determined using the checkbook method should be verified occasionally with field measurements. It is critical that the water budget is determined systematically and accurately so that applications of water meet the needs of the crop but do not result in over-application.

• Time water applications to avoid water movement beyond the rooting zone. Weather patterns should be assessed prior to each irrigation. Deficit irrigation techniques that leave room in the rooting zone for additional water from rainfall have been demonstrated to conserve water without yield reductions. Irrigation should not fill the soil to field capacity and the soil profile should never be used to store irrigation water through the winter. To the contrary, irrigation water should be managed so that stored soil water is at a minimum in the fall.

• Adjust water application amounts to meet varying crop demands at different growth stages. Irrigation has the potential to meet these variable demands more readily than dryland agriculture, thus maintaining a stable environment for plant growth. Large amounts of unused available N are not likely to be left in the soil, if management results in maintenance of vigorous plant growth throughout the year. The potential for NO₃ leaching and groundwater contamination is diminished if this practice is followed.

• Irrigation water must be applied uniformly and accurately. A functional flow meter and accurate pressure gauge, either at the pump or on the pipeline near the point of discharge, are essential for accurate application of irrigation water and N fertilizer. Uniform application rates can only be accomplished if irrigation equipment functions properly; therefore, sprinklers, nozzles, pipes, etc. must be checked regularly. Placing catch cans under the system to measure actual amounts of water delivered to the soil surface can check uniformity of application.

• When N is injected into an irrigation system, chemigation equipment that protects the water supply must be used. State regulations regarding the proper chemigation equipment required to protect the water source from back-siphonage must be followed. Chemigation provides an opportunity to ensure that adequate N is supplied to the crop during critical growth stages. Application of nitrogen through the irrigation system can be accomplished at later growth stages when other methods of delivery are not possible. In addition, applying nitrogen through the irrigation system helps to split applications and avoids applying all the nitrogen in a single application, which has both economic and environmental benefits.

• The chemigation unit must be calibrated with each use to ensure accurate application of N. An accurate way of measuring the amount of chemical being injected into the irrigation system is essential to good irrigation management. Accurate measurement of the amount of applied N not only optimizes chemical usage but also ensures a uniform application over the entire irrigated field if the system is designed and operating correctly.

• Secondary containment should be provided where N fertilizer is stored near the irrigation well when chemigation is practiced. Secondary containment, made of impermeable material, reduces the risk of contamination in the case of a leak or spill.
• Recommendations related to groundwater protection under the Cropping System, Tillage, Fertilizer Applications, and Organic Waste sections are also appropriate to irrigated fields.

The processes that affect nitrogen availability, mobility, and translocation to surface water under irrigated agricultural systems are the same as those for non-irrigated agriculture. Fields consistently maintained at field capacity are more likely to generate runoff during rain events compared to similar soils allowed to dry down to lower water contents. If the application of irrigation water causes surface runoff or subsurface drainage that outlets to surface water, the potential for nitrogen contamination exists.

Many irrigated soils are susceptible to wind erosion. Topsoil removed from irrigated fields and deposited in ditches, streams, and lakes is a source of water pollution. Conservation practices that reduce wind erosion are particularly important in irrigated fields with low residue crops such as potatoes.

• The first four recommendations for groundwater protection under irrigated fields are also appropriate for surface water protection. These practices promote efficient water input, which helps reduce over-application of water and saturated conditions that may lead to runoff. In addition good irrigation scheduling should help avoid excessively dry conditions that may lead to wind erosion.
• Recommendations related to surface water protection under the Cropping System, Tillage, Fertilizer Applications, and Organic Waste sections are also appropriate to irrigated fields.

For information also related to nitrogen and water quality refer to:

???# "Water quality and nitrogen"?

???# "How to assess for nitrogen problems in water resources".

EB-64 "Managing nitrogen fertilizer to prevent groundwater contamination".