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Understanding nitrogen in soils
What you should know
- Numerous nitrogen (N) sources exist. Consider these when evaluating the N budget.
- Soil type and climate greatly affect nitrogen loss from the soil system.
- Because Minnesota has such diverse soils and climate, N cycle interpretations should be site-specific.
Here, we’ll cover how nitrogen behaves in Minnesota soil systems and how to manage it for more profitable and environmentally friendly crop production.
Basics of nitrogen
Environmental and economic issues have increased the need to better understand the role and fate of nitrogen (N) in crop production systems. Nitrogen is the nutrient most often deficient for crop production in Minnesota, and its use can result in substantial economic return for farmers.
However, when N inputs to the soil system exceed crop needs, there’s a possibility that excessive amounts of nitrate (NO3--N) may enter either ground or surface water.
Managing N inputs to achieve a balance between profitable crop production and environmentally tolerable levels of NO3--N in water supplies should be every grower’s goal. Nitrogen’s behavior in the soil system is complex, yet understanding these basic processes is essential for a more efficient N management program.
Nitrogen exists in the soil system in many forms, and changes (transforms) very easily from one form to another. The route N follows in and out of the soil system is collectively called the nitrogen cycle (Figure 1).
The nitrogen cycle is biologically influenced. Biological processes, in turn, are influenced by prevailing climatic conditions along with a particular soil’s physical and chemical properties. Both climate and soils vary greatly across Minnesota and affect N transformations for the different areas.
Inputs of N for plant growth
Atmospheric N is the major reservoir for N in the N cycle (air is 79 percent N2 gas).
Although unavailable to most plants, leguminous plants can use large amounts of N2 via biological N fixation. In this biological process, nodule-forming Rhizobium bacteria inhabit the roots of leguminous plants and, through a symbiotic relationship, convert atmospheric N2 to a form the plant can use.
Legumes can fix substantial amounts of N2 into usable N. An alfalfa crop, for example, has the potential to fix several hundred pounds of N per acre per year. Any legume crop that’s left after harvest, including roots and nodules, can supply N to the soil system when the plant material is decomposed.
Several nonsymbiotic organisms fix N, but N additions from these organisms are quite low (1 to 5 pounds per acre per year). In addition, precipitation adds small amounts of N to the soil. In Minnesota, precipitation supplies an average of 5 to 10 pounds of N per acre per year.
Commercial N fertilizers are also derived from the atmospheric N pool. The major step is to combine N2 with hydrogen (H2) to form ammonia (NH3). Anhydrous ammonia is then used as a starting point in the manufacture of other nitrogen fertilizers.
Anhydrous ammonia or other N products derived from NH3 can then supplement other N sources for crop nutrition.
Nitrogen can also become available for plant use from organic N sources. Before these organic sources are available to plants, they must be converted to inorganic forms. Nitrogen is available to plants as either ammonium (NH4+-N) or nitrate (NO3--N).
Animal manures and other organic wastes can be important sources of N for plant growth. The amount of N supplied by manure will vary with the type of livestock, handling, rate applied and method of application. Because the N form and content of manures widely varies, a manure analysis is recommended to improve N management.
Crop residues from non-leguminous plants also contain N, but in relatively small amounts compared to legumes. Nitrogen exists in crop residues in complex organic forms and the residue must decay – a process that can take several years – before N becomes available for plant use.
Soil organic matter
Soil organic matter is also a major source of N used by crops. Organic matter is primarily composed of rather stable material called humus that has collected over a long period of time.
Easily decomposed portions of organic material disappear relatively quickly, leaving behind residues more resistant to decay. Soils contain approximately 2,000 pounds of N in organic forms for each percent of organic matter. This portion of organic matter decomposes at a rather slow rate and releases about 20 pounds of N per acre per year for each percent of organic matter.
Nitrogen, present or added to the soil, is subject to several changes, or transformations. These dictate the availability of N to plants and influence the potential movement of NO3--N to water supplies.
Organic N that’s present in soil organic matter, crop residues and manure is converted to inorganic N through the mineralization process. In this process, bacteria digest organic material and release NH4+-N.
Formation of NH4+-N increases as microbial activity increases. Bacterial growth is directly related to soil temperature and water content. The NH4+-N supplied from fertilizer is the same as the NH4+-N supplied from organic matter.
Ammonium-N has properties of practical importance for N management. Plants can absorb NH4+-N. Also, because ammonium has a positive charge, it’s attracted or held by negatively charged soil and soil organic matter. This means that NH4+-N doesn’t move downward in soils.
Nitrogen in the NH4+-N form that isn’t taken up by plants is subject to other changes in the soil system. Nitrification is the conversion of NH4+-N to NO3--N.
Nitrification is a biological process. It rapidly proceeds in warm, moist, well-aerated soils, and slows at soil temperatures below 50 degrees Fahrenheit.
Nitrate-N is a negatively charged ion and isn’t attracted to soil particles or soil organic matter like NH4+-N. Nitrate-N is water-soluble and can move below the crop rooting zone under certain conditions.
In denitrification, bacteria convert NO3--N to N gases that are lost to the atmosphere. Denitrifying bacteria use NO3--N instead of oxygen in the metabolic processes. The process takes place in waterlogged soil and with ample organic matter to provide energy for bacteria.
For these reasons, denitrification is generally limited to topsoil. Denitrification can rapidly proceed when soils are warm and become saturated for two or three days.
Immobilization, or the tie up of soil N, can temporarily reduce the amount of plant-available N. Bacteria that decompose high-carbon, low-N residues, such as corn stalks or small grain straw, need more N to digest the material than is present in the residue.
Immobilization occurs when the growing microbes use NO3--N and/or NH4+-N present in the soil to build proteins. The actively growing bacteria that immobilize some soil N also break down soil organic matter to release available N during the growing season.
There’s often a net gain of N during the growing season because the additional N in the residue will be the net gain after immobilization-mineralization processes.
Nitrogen loss from the soil system
When developing N programs and evaluating environmental effects, consider nitrogen’s mobility factor in the soil. Sandy soils may lose N through leaching, while heavy, poorly drained soils may lose N through denitrification.
Unlike the previously described biological transformations, loss of nitrate by leaching is a physical event.
Leaching is the loss of soluble NO3--N as it moves with soil water, generally excess water, below the root zone. Nitrate-N that moves below the root zone has the potential to enter groundwater or surface water through tile drainage systems.
Coarse-textured soils have a lower water-holding capacity and, therefore, more potential to lose nitrate from leaching compared to fine-textured soils. Some sandy soils, for instance, may retain only a half inch of water per foot of soil while some silt loam or clay loam soils may retain up to 2 inches of water per foot.
Nitrate-N can be leached from any soil if rainfall or irrigation moves water through the root zone.
Denitrification can be a major loss mechanism of NO3--N when soils are saturated with water for two or three days. Nitrogen in the NH4+-N form isn’t subject to this loss. Management alternatives are available if denitrification losses are a potential problem.
Significant losses from some surface-applied N sources can occur through the process of volatilization. In this process, N is lost as ammonia (NH3) gas.
Manure and fertilizer products containing urea can cause nitrogen to be lost this way. Ammonia is an intermediate form of N during the process that transforms urea to NH4+-N. Incorporating these N sources will virtually eliminate volatilization losses.
Nitrogen loss from volatilization is greater when:
- Soil pH is higher than 7.3.
- Air temperature is high.
- The soil surface is moist.
- There’s a lot of residue on the soil.
Substantial amounts of N are lost from the soil system through crop removal. A 250-bushel-per-acre corn crop, for example, removes approximately 175 pounds of N with the grain. Crop removal accounts for a majority of the N that leaves the soil system.
Nitrogen can be lost from agricultural lands through soil erosion and runoff. Losses through these events normally don’t account for a large portion of the soil N budget, but should be considered for surface water quality issues.
Incorporating or injecting manure and fertilizer can help protect against N loss through erosion or runoff. Where soils are highly erodible, conservation tillage can reduce soil erosion and runoff, resulting in less surface loss of N.
Key points for crop producers
Considering the many transformations and reactions of N in soils, there are some major points to keep in mind:
- Although you can add either organic or inorganic N forms to soil, plants only take up inorganic N (that is, NO3--N and NH4+-N).
- One form isn’t more important than the other and all N sources can be converted to NO3--N. Commercial N fertilizers, legumes, manures and crop residues are all initial sources of NO3--N and NH4+-N.
- Once it’s in the plant or water supply, it’s impossible to identify the initial source.
- Nitrate is always present in the soil solution and will move with the soil water.
- Inhibiting the conversion of NH4+-N to NO3--N can result in less N loss and more plant uptake. While it’s not possible to totally prevent the movement of some NO3--N to water supplies, sound management practices can keep losses within acceptable limits.
Reviewed in 2018