Ammonia, a compound of nitrogen and hydrogen, is a component of Earth’s systems produced through various biological and geological processes. Its presence is part of the nutrient cycling that sustains life, playing a role in ecosystems from the soil to the atmosphere.
Natural Production of Ammonia
Ammonia’s primary natural origin is the decomposition of organic material. When plants and animals die, microorganisms like bacteria and fungi break down the nitrogen-containing molecules in their tissues. This process, known as ammonification, releases nitrogen back into the environment as ammonia.
Animal wastes are another source of natural ammonia. As animals metabolize proteins, they excrete excess nitrogen as urea or uric acid. Bacteria in the environment then convert these compounds into ammonia, acting as a natural fertilizer.
Geological and atmospheric events also generate ammonia. Volcanic eruptions can release ammonia gas into the atmosphere. The energy from lightning strikes can also break apart atmospheric nitrogen, allowing it to react with hydrogen and form ammonia that reaches the ground through rainfall.
The Nitrogen Cycle and Ammonia
Ammonia is a central molecule in the nitrogen cycle. The cycle begins with nitrogen fixation, where certain bacteria convert unusable atmospheric nitrogen gas into ammonia. These bacteria include species like Rhizobium, which live on legume roots, and free-living soil bacteria like Azotobacter.
Once ammonia is present in the soil, it can undergo nitrification. Distinct groups of nitrifying bacteria convert ammonia or its ion form, ammonium, first into nitrite and then into nitrate. Nitrate is a form of nitrogen that is readily absorbed by plants.
The cycle continues as plants absorb ammonia and nitrates from the soil in a process called assimilation, using the nitrogen to build proteins. Animals acquire nitrogen by consuming plants. When organisms die, decomposition (ammonification) returns ammonia to the soil, completing the loop.
Ecological Importance of Ammonia
Ammonia is a direct nitrogen source for many plants and microorganisms, supporting ecosystem productivity. Some plants prefer absorbing ammonium ions over nitrates for growth. This nitrogen uptake is necessary for building proteins and nucleic acids like DNA.
Ammonia also influences soil chemistry. In some soil and water systems, it acts as a pH buffer, helping to neutralize acidic conditions. This interaction helps maintain a stable environment for soil microbes and plant roots.
Ammonia Across Natural Environments
In soils, ammonia exists in a balance between its gaseous form (NH3) and the ammonium ion (NH4+), influenced by soil pH and temperature. Ammonium ions adhere to soil particles, which prevents the nitrogen from being washed away and keeps it available for plants. Microbial communities in the soil constantly influence the concentration and transformation of ammonia.
A similar equilibrium exists in aquatic systems. At higher pH levels, more nitrogen is present as ammonia gas, which can be toxic to fish. At a lower pH, the less toxic ammonium ion is more common. Ammonia also serves as a nitrogen source for phytoplankton, the base of most aquatic food webs.
The atmosphere contains trace ammonia from sources like volatilization from soils and animal waste. In the air, ammonia neutralizes acidic compounds, influencing the formation of aerosols and clouds. This atmospheric ammonia eventually returns to Earth through deposition in rain or snow, redistributing the nutrient.