lämmastiktsükli

lämmastiktsükli, also known as the nitrogen cycle, is the biogeochemical process by which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. Nitrogen is an essential element for all living organisms, being a key component of amino acids, proteins, and nucleic acids.

The cycle begins with atmospheric nitrogen (N2), which is abundant but largely unusable by most life forms. Nitrogen fixation, the conversion of N2 into ammonia (NH3) or ammonium (NH4+), is primarily carried out by nitrogen-fixing bacteria, often found in the soil or in symbiotic relationships with plants. Some atmospheric fixation also occurs through lightning.

Once ammonia or ammonium is formed, it can be assimilated by plants. In the soil, other bacteria convert ammonia/ammonium into nitrites (NO2-) and then into nitrates (NO3-) through a process called nitrification. Nitrates are readily absorbed by plants.

Denitrification is the process by which nitrates are converted back into atmospheric nitrogen by denitrifying bacteria, completing the cycle. This process typically occurs in anaerobic conditions, such as in waterlogged soils. Ammonification is the decomposition of organic nitrogen compounds in dead organisms and waste products back into ammonia, which can then re-enter the cycle.

Human activities, such as the use of fertilizers and the burning of fossil fuels, significantly impact the nitrogen cycle, often leading to increased nitrogen pollution in ecosystems.