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Competency Area 2: Basic Concepts of Soil Fertility

PO 13. Describe how the following affect the fate of N in soil: the Nitrogen Cycle.

  1. Fixation by clay
  2. Ammonification and mineralization: R-NH2 → NH3 → NH4+  (organic N & ammonia & ammonium)
  3. Nitrification: NH4+ (ammonium) → NO2- (nitrite) → NO3- (nitrate)
  4. Volatilization: CO(NH2)2 (urea) → NH4+ (ammonium) → NH3 (ammonia)
  5. Denitrification: NO3- (nitrate) → NO2- (nitrite) → NO (nitric oxide gas) → N2O (nitrous oxide gas) → N2 (dinitrogen gas)
  6. Immobilization: NH4+ (ammonium) and NO3- (nitrate) → R-NH2 (organic N)
  7. Leaching
  8. Plant uptake
  9. Symbiotic fixation: N2 → NH3 → R-NH2 → amino acids → proteins

Nitrogen is an essential and often growth-limiting plant nutrient. Crops take up and release N through a series of processes known as the Nitrogen Cycle.  N availability limits the productivity of most cropping systems in the US, and a deficiency in nitrogen leads to yield declines or even complete crop failure.  Excessive applications however may contribute to acid rain, destruction of the ozone layer in the stratosphere, the greenhouse effect, eutrophication of surface waters, contamination of ground water, and fish and other marine life kills, as well as blue baby syndrome in infants and amphibian mortality and deformations. The nitrate concentration in ground and surface waters is an important water-quality index; the U.S. Environmental Protection Agency (EPA) has set the Federal Standard for the maximum permitted amount of nitrate N in drinking water at 10 mg N/L or 43 mg NO3-/L.

It is important from both an economic and an environmental standpoint to manage N optimally. Thus, the two primary objectives of N management are:

  1. To have adequate inorganic N available during the growing season
  2. To minimize the availability of inorganic N during the fall, winter, or early spring, when N may be transported to surface and groundwater.

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Fixation by clay refers to association of nitrogen with the soil. Since the soil has a negative charge, the ammonium ion (NH4+) can be bound to the soil particle. Depending on the type of clay, this ion can be trapped in the actual structure of the clay mineral and become unavailable to plant uptake.

Ammonification and Mineralization is a process that converts organic N in manure, crop residues and soil organic matter to ammonia and ammonium. Annual mineralization rates vary, though in general about 1.5-3.5% of the organic nitrogen in the soil will be mineralized each year.  The exact rates depend on soil temperature, moisture and aeration status; most rapid mineralization occurs in hot climates (68-95°F), well-aerated soils, and moist soils. If large amounts of N-rich organic materials with narrow C:N ratios (<15-20) are added, significant levels of NH4+ can be produced. This will then be converted to nitrate via nitrification, absorbed by plants, fixed or held by the soil, or converted to ammonia and lost to the air via volatilization. In NY, about 60-80 lbs N/acre is mineralized from soil OM each year.

Nitrification is the process by which microbes use enzymes to convert ammonium (NH4+) to nitrate (NO3-) to obtain energy. It is a two step process, with a different species of bacteria performing each step. Nitrate is most readily available to plants and is the preferred N form. Nitrification is most rapid when soil is warm (67-86°F), moist, and well aerated (late May, June). However, it will not occur when soil temperature drops below 41°F or goes above 122°F. Because the process releases H+ ions, nitrification lowers soil pH.

Volatilization is the production and loss of ammonia gas from ammonium. Ammonia volatilization increases with soil pH, as the high H+ concentration promotes the conversion of nitrate to ammonium. Volatilization losses may be high for unincorporated urea fertilizer or manure (urine). The high level of evaporation assists this loss. Incorporation of manure and fertilizers can reduce ammonia losses by 25-75%.

Denitrification occurs when NO3- is converted into gaseous forms of N. The process is common in poorly drained (anaerobic) soils, even those that are tile-drained, and in warm conditions.

Immobilization is the reverse of mineralization. Microbes compete with crops for NH4+ and NO3- for their own survival; when nitrogen is scarce the microbes convert inorganic N forms into their own organic forms, preventing plants from taking the N up. This commonly occurs in aerated soils (as opposed to denitrification, which occurs in anaerobic soils), particularly with high carbon-to-nitrogen (C:N) ratio.  This happens when materials like straw, sawdust, etc. are incorporated. Immobilization ties up available N in microbial tissue, which must be "re-mineralized" to become available to plants again.

Leaching is the loss of NO3- from the soil with water movement. Since nitrate is an anion, it does not attach to soil particles and thus easily leaches from the soil. Total losses are determined by water movement and nitrate contents of the soil.

Plant uptake occurs when nitrate is available and conditions are aerobic (i.e. not wet or flooded).

Symbiotic fixation is the conversion of N2 from the atmosphere to plant protein. Atmospheric N is fixed in a symbiotic process carried out by microorganisms, the Rhizobium bacteria which form root nodules in legumes. This nitrogen becomes available when N fixers die. The process requires energy and the enzyme nitrogenase (Fe, Mo, P, S), so if a plant-available N source is present, the crop will use that instead of fixing N from the air.