Nitrogen Transformations in Soil, Water, and Air
In the nitrogen (N) cycle, organic nitrogen exists in materials formed from animal, human, and plant activities that produce manures, sewage waste, compost, and decomposing roots or leaves. These organic products transform into organic soil material called humus. Inorganic nitrogen comes from minerals, and is added to soil from precipitation, or as fertilizers. Adding N to the soil is done to help living plants grow and remain healthy. However, plants cannot use organic forms of nitrogen so certain microbes living in the soil come to the rescue and convert organic forms of N into inorganic forms that plants can then use.
There are different forms of inorganic nitrogen that are available to plants.
Some of these can be stored in the soil (such as ammonium, NH4+). Other forms of nitrogen that are not held by the soil particles (such as nitrate NO3-, and nitrite NO2-) can leach out of the soil and into the groundwater, or can be transformed into nitrogen gases (N2, NO, or N2O) and ammonia gas (NH3), escape out of the soil (volatilize), and into the Earth’s atmosphere.
Luckily there are other kinds of microbes that live in the soil, and close to Earth’s surface, that can convert Nitrogen gas into inorganic forms of N that plants can then use. When plants die they contribute organic N to the soil and certain microbes, yet again, do their job of converting organic N into inorganic N that living plants can use! Below, see an illustration of the N cycle, how N changes when it is in soil, water and air, and find out what those interesting nitrogen cycle terms mean!
Definitions of Nitrogen Cycle Terms
Ammonification – nitrogen, in organic formed, is converted by microorganisms into ammonium (NH4+). Because it has a positive charge, ammonium can be adsorbed and fixated (stuck) on to the negatively charged soil particles or be taken up by plants.
Denitrification – Microbes transform nitrate or nitrite nitrogen in extremely wet soils and poorly drained soils (like in wetlands) into gases called nitrous oxide and nitrogen gas. These gases escape (volatilize) into Earth’s atmosphere and are not available for plant use.
Erosion – occurs when the soil surface is worn away by running water (runoff), wind, ice, gravity, or other geological agents. Organic and inorganic nitrogen present in the soil are carried along with the eroded soil particles. Immobilization- the opposite of mineralization. Inorganic forms of N are converted into organic forms as microbes and plants die. N that enters the soil in mineral form, not from a plant or animal source, is inorganic.
Inorganic Nitrogen – Inorganic forms of N are usually added to the soil by rain or snowfall, or as fertilizers. Microorganisms in the soil convert organic forms of N into inorganic forms, a process known as biological N fixation, that are then usable by plants .
Leaching – Like the negative end of a magnet, nitrogen in the form of nitrate is negatively charged and is not attracted to soil’s negatively charged clay and humus. Negatively charged clay repels negatively charged nitrite (NO2- ) and nitrate ( NO3- ) so they will not be absorbed by the clay and are left to move down through the soil and into the groundwater, where streams and drinking water can become contaminated.
Mineralization – when an element, such as N, is converted from an organic form to an inorganic form by microbes. Plants can then use this inorganic N to grow.
Nitrification – under certain conditions specific microbes in the soil use ammonium N in the soil for energy and in doing so “oxidize” ammonium N first into Nitrite N (NO2-) and then into Nitrate N (NO3-), which plants can then use to grow.
Organic Nitrogen – Nitrogen enters the soil in organic forms such as plant roots, leaves, and other plant materials, in addition to dead animals, insects, and microorganisms, manure, compost, and sewage sludge. As these decompose, the once recognizable plant and animal materials are transformed into soil organic matter called humus which contains organic nitrogen. Living plants cannot use these organic forms of N. This is why microbes living in the soil are so valuable, because they can convert organic N into inorganic forms of N that plants can then use.
Runoff – occurs when soil can no longer hold water so additional water moves quickly over the surface of the soil. If the soil does not have a protective vegetative cover over it, and has a steep slope, a great deal of surface soil material will be carried away (eroded) by the moving water. Organic and inorganic forms of nitrogen are removed from the soil profile along with soil particles, and eventually end up in streams, lakes, rivers and bays that cause water pollution and harm to aquatic life.
Assimilation, aka Uptake – Nitrogen and other nutrients in the soil water move into plant roots to help them to survive and grow.
Volatilization – Nitrogen in the form of a gas (such as NH3, NO, N20, or N2) is moved out of soil and into the Earth’s atmosphere.