Nitrogen is the most important macronutrient for plant development. It is a fundamental component of chlorophyll, necessary for photosynthesis, and an integral part of amino acids, proteins, and DNA. Natural soil reserves are often insufficient for modern crop demands, making external nitrogen fertilizer necessary to maximize yields. This added nitrogen must be in a chemical form that plants can readily absorb.
The Three Primary Nitrogen Forms in Fertilizer
Commercial fertilizers rely on three primary chemical forms of nitrogen: nitrate (\(\text{NO}_3^-\)), ammonium (\(\text{NH}_4^+\)), and urea (\(\text{CO(NH}_2)_2\)). These forms have distinct properties governing their behavior and availability in the soil.
Nitrate is the most immediately available form for plant uptake because it is soluble in water and does not bind to negatively charged soil particles. This allows it to move quickly with soil water, making it fast-acting. However, this high mobility makes it highly susceptible to loss through leaching beyond the root zone.
Ammonium is a positively charged cation (\(\text{NH}_4^+\)) that binds to negatively charged clay and organic matter in the soil. This binding provides a more stable, slower-release source of nitrogen, less prone to leaching than nitrate. However, ammonium must be converted by soil microbes before it becomes the dominant form in well-aerated soils.
Urea is an organic nitrogen compound and an economical choice for fertilizer. Before plants can use it, urea must undergo hydrolysis, facilitated by the urease enzyme. This process converts urea into ammonium, which then follows the same conversion pathway as direct ammonium application.
How Plants Absorb and Utilize Nitrogen
Plants have the ability to absorb both nitrate and ammonium ions directly from the soil solution through specialized transport proteins in their root cell membranes. Although plants can absorb both, they generally prefer nitrate, especially in well-aerated soils where nitrate is the predominant form. Once inside the plant, nitrate is highly mobile and is often transported to the leaves for processing.
The plant’s utilization of the two forms involves different energy costs. Nitrate must be reduced back to ammonium before it can be incorporated into organic molecules, a process requiring energy and specific enzymes like nitrate reductase. Ammonium can be directly integrated into amino acids like glutamine and glutamate, requiring less metabolic energy for the initial step. However, an excess of ammonium can be toxic to the plant, so it is quickly processed and usually assimilated near the root zone. The resulting amino acids are then used to synthesize essential components for growth and development.
The Nitrogen Cycle in the Soil
Once nitrogen fertilizer is applied, microbial and chemical transformations dictate its fate in the soil. Urea, if applied to the surface, quickly undergoes hydrolysis, converting it into ammonium and creating a temporary zone of high alkalinity. If this conversion happens at the soil surface, a portion of the nitrogen can be lost to the atmosphere as ammonia gas (\(\text{NH}_3\)) through volatilization, especially in warm, high pH soils.
The ammonium form is then subject to nitrification, a two-step biological process carried out by specific soil bacteria. First, bacteria convert ammonium (\(\text{NH}_4^+\)) into nitrite, and then a second group of bacteria rapidly oxidizes the nitrite into the highly mobile nitrate (\(\text{NO}_3^-\)). This conversion is temperature and moisture-dependent, slowing down in cold or waterlogged conditions.
The mobile nitrate is the form most vulnerable to loss pathways. Leaching occurs when nitrate moves below the root zone, making it inaccessible to the plant. In poorly drained, water-saturated soils, another process called denitrification can occur, where anaerobic bacteria convert nitrate into nitrogen gas (\(\text{N}_2\)) or nitrous oxide (\(\text{N}_2\text{O}\)), which escapes back into the atmosphere.