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Nitrogen behaviour in the environment

Plants need many different nutrients, notably nitrogen, phosphorus and potassium (“NPK”). This article outlines some basic information about nitrogen and its interactions in the environment from the point of view of manure handling and plant growth.

Nitrogen

Nitrogen (N) is a vital element found in all living things. Plants require nitrogen in relatively large amounts, making it the nutrient most often deficient in agricultural and horticultural production systems.

Managing nitrogen inputs to achieve a balance between profitable plant production and minimizing nitrogen loss should be every manager’s goal. The behaviour of nitrogen in the soil system is complex, yet understanding the basic processes can lead to more efficient nitrogen management.

The Nitrogen cycle

The chemical form of nitrogen changes continually as nitrogen moves through plants, animals, soil, water and the atmosphere. This movement and transformation of nitrogen in the environment is known as the “nitrogen cycle” (Figure 1).

Figure 1. The nitrogen cycle. (Source: US Livestock and Poultry Environmental Stewardship Curriculum)

Figure 1 shows that nitrogen is lost from the plant–soil environment through the processes of volatilisation, denitrification and leaching, in addition to being removed in plant products, such as grain and hay, and in livestock.

The following critical processes in the nitrogen cycle are pertinent to manure handling and plant growth:

Mineralization – the conversion of organic nitrogen (the nitrogen bound up in organic molecules) in soil organic matter, crop residues and manure to inorganic nitrogen (including ammonium, NH₄⁺, and nitrate, NO₃^–). Soil microorganisms break down organic nitrogen and release ammonium. Formation of ammonium increases as microbial activity increases, and microbial growth is directly related to soil temperature and water content.

Nitrification – the conversion of ammonium through nitrite (NO₂^–) to nitrate (NO₃^–). Nitrification is a biological process carried out by certain species of bacteria. It proceeds rapidly in warm, moist, well-aerated soils. It slows when the soil temperature drops.

Immobilisation – the conversion of inorganic nitrogen to organic nitrogen. Microorganisms that decompose high-carbon / low-nitrogen residues, such as maize stalks or cereal straw, need more nitrogen to break down the residue than is present in the residue. So the growing microbes take up nitrate, ammonium or both from the soil to build their proteins, and thus immobilise soil nitrogen. This leads to a temporary reduction in the amount of available nitrogen.

Volatilisation – the release of ammonia gas (NH₃) to the atmosphere. Volatilisation can cause significant losses from some surface-applied nitrogen fertilisers. Ammonia is an intermediate form of nitrogen during the process in which urea is transformed to NH₄⁺. Volatilisation rates are greatest when the soil pH is higher than 7.3 and the air temperature is high.

Denitrification – the process by which bacteria convert nitrate to nitrogen gas (N₂), which is lost to the atmosphere. Denitrifying bacteria use nitrate instead of oxygen in their metabolic processes when the soil lacks oxygen. Denitrification occurs in waterlogged soil with ample organic matter, so it is generally limited to topsoil. Denitrification can proceed rapidly when soils are warm and saturated for 2 to 3 days.

Leaching – the downward movement of a chemical through the soil profile with soil water. In contrast to the biological transformations described above, loss of nitrate by leaching is a purely physical event due to water percolation through the soil. Nitrate is highly soluble and moves with excess soil water below the root zone, where it is no longer available to plants and can enter groundwater or surface water through tile drains.

Plant nitrogen uptake

Although nitrogen can be added to soil in either organic or inorganic forms, plants take up only inorganic nitrogen (both NO₃^– and NH₄⁺). The organic nitrogen components of any fertiliser, including manure, must be mineralised first before they are available to plants. Commercial N fertilisers, legumes, manures and crop residues all are sources of NO₃^– and NH₄⁺.

Nitrogen interactions with soil

Soil consists of many negatively charged mineral and organic particles. A measure of the total negative charge in soil is the cation exchange capacity, or CEC. Most soils have enough CEC to hold onto all positively charged particles or nutrients.

Although the soil’s negatively charged surfaces can hold the positively charged ammonium ions (NH₄⁺), they repel the negatively charged nitrate ions (NO₃^–), causing them to remain in the soil solution. Consequently, the soil will retain most applied nutrients, but nitrate, being extremely soluble, keeps moving down through the soil.

Coarse-textured soils (such as those with a high sand content) have a lower water-holding capacity than fine-textured soils (such as those with a high clay content). Therefore, they have a higher potential to lose nitrate. Some sandy soils, for instance, may retain only 4% water by volume, whereas some silty loam or clay loam soils may retain up to 20% water. But even in finer-textured soils, nitrate can be leached out if excess rainfall or irrigation saturates the soil and causes water to move below the root zone.

Ammonium nitrogen has properties that are of practical importance for nitrogen management. Because of its positive charge, the soil particles hold onto it, so it resists downward movement. However, ammonium that plants do not take up is quickly nitrified to nitrate within days of ammonium application. Solutions include split applications to apply only as much as plants immediately need, and the use of nitrification inhibitors to restrict nitrogen loss and improve plant uptake. These inhibitors can be added to nitrogen fertilisers, including manure, and work by inhibiting the growth of the nitrifying bacteria.

It is not possible to totally prevent the loss of nitrogen from the soil, but sound management practices can reduce losses. Options include:

• timing applications to coincide with plant needs
• placing fertilisers near roots
• incorporating fertilisers at rooting depth
• storing and handling fertilisers to minimise losses.

This article is adapted from a module of the US Livestock and Poultry Environmental Stewardship Curriculum (http://www.lpes.org/).