lämmastiktsüklis

The nitrogen cycle, known in Estonian as lämmastikutsüklis, describes the biogeochemical cycling of nitrogen atoms through the Earth's biosphere, atmosphere, and lithosphere. Nitrogen is an essential element for all living organisms, forming a key component of amino acids, nucleic acids, and proteins. However, atmospheric nitrogen (N2), which makes up about 78% of the air, is largely unavailable to most life forms due to the strong triple bond holding the two nitrogen atoms together.

The cycle begins with nitrogen fixation, the process by which atmospheric N2 is converted into usable nitrogen compounds like ammonia (NH3). This is primarily carried out by nitrogen-fixing bacteria, often in symbiosis with plants, or through abiotic processes like lightning. Ammonia can then be converted into nitrites (NO2-) and nitrates (NO3-) by nitrifying bacteria in a process called nitrification. Plants absorb nitrates and ammonia from the soil, incorporating them into their tissues.

When organisms die, decomposers, including bacteria and fungi, break down organic matter, releasing nitrogen back into the soil as ammonia in a process called ammonification. Denitrifying bacteria then convert nitrates back into atmospheric nitrogen, completing the cycle through denitrification, which returns nitrogen to the atmosphere. Human activities, such as the use of fertilizers and the combustion of fossil fuels, significantly impact the natural nitrogen cycle, leading to increased nitrogen levels in ecosystems and potential environmental problems like eutrophication.