Nitrification and Soil Conditions that affect Nitrification

Nitrification

Ammonium ions in the soil may be enzymatically oxidized by certain soil bacteria, yielding first nitrites and then nitrates. These bacteria are classes as autotrophs because they obtain their energy from oxidizing  the ammonium ions rather than organic matter. The process termed nitrification consists of two main sequential steps. The first step results in the conversion of ammonium to nitrite by a specific group of autotrophic bacteria (Nitrosomonas). The nitrite so formed is then immediately acted upon by a second group of autotrophs, Nitrobacter. The enzymatic oxidation releases energy and may be represented very simply as follows:

Nitrification process in soil

So long as conditions are favorable for both reactions, the second transformation is thought to follow the first closely enough to prevent accumulation of nitrite. This is fortunate, because even at lower concentrations of just a few parts per million, nitrite is quite toxic to most plants and to mammals.
Regardless of the source of ammonium, nitrification will significantly increase soil acidity by producing hydrogen ions, as shown in the preceding reaction. Such acidification from the oxidation of both nitrogen and sulfur compounds results in calcium and magnesium ions being replaced from the exchange from the soil solution.

Microbes that take part in nitrification - soil science

Soil Conditions Affecting Nitrification

The nitrifying bacteria are much more sensitive to environmental conditions than are broad group of heterotrophic organisms responsible for the release of ammonium from the organic nitrogen compounds. Nitrification requires a supply of ammonium ions, but excess ammonium ion is toxic to Nitrobacter and must be avoided. The nitrifying organisms, being aerobic, require oxygen to make nitrogen dioxide and nitrate ions and are therefore favored in well-drained soils. the optimum moisture for these organisms is about the same as that for most land plants. Since they are autotrophs, their carbon sources are bicarbonates and carbon dioxide. They perform best if the temperature is kept between 20 to 30 degree Celsius and perform very slowly if the soil is cold (< 5 degree Celcius).

Nitrification proceeds most rapidly where there is an abundance of exchangeably metallic cations such as calcium ions and magnesium ions and where the nutrient levels are optimum for the growth of higher plants. Nitrification is often constrained in soils high in smectite or allophane clays. These clays hold certain nitrogen containing organic compounds in their intermicelle pores, thereby protecting them from microbial attack, including nitrification.

Nitrifying organisms are quite sensitive to some pesticides, being constrained if these pesticides are applied at high rates. Most studies suggest, however that at ordinary field rates, the majority of the pesticides have only a minimal effect on nitrification.
In recent years, chemicals have been found that can inhibit or slow down the nitrification process, thereby reducing the nitrate leaching potential.

Nitratification is such a rapid process that nitrate is generally the predominant mineral form of nitrogen in most soils. Irrigation of an initially dry arid region soil, the first rains after a long dry season in the tropics, the thawing and rapid warming of frozen soils in spring, and sudden aeration by tillage are examples of environmental fluctuations that typically cause a flush of soil nitrate production.