Kitaran nitrogen ialah proses yang melibatkan nitrogen ditukarkan antara pelbagai bentuk kimianya. Transformasi ini boleh dijalankan melalui kedua-dua proses biologi dan fizikal. Proses-proses penting dalam kitaran nitrogen termasuk pengikatan , pengammoniaan , nitrifikasi , dan denitrifikasi. Walau bagaimanapun, nitrogen atmosfera mempunyai ketersediaan terhad untuk kegunaan biologi, membawa kepada kekurangan nitrogen boleh guna dalam banyak jenis ekosistem. Kitaran nitrogen merupakan minat tertentu kepada ahli ekologi kerana ketersediaan nitrogen boleh menjejaskan kadar proses ekosistem kunci, termasuk pengeluaran utama dan penguraian. Kegiatan manusia seperti pembakaran minyak fosil, penggunaan baja nitrogen buatan, dan pengeluaran di dalam air buangan telah secara dramatik mengubah kitaran nitrogen global.
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Nitrogen is both the most abundant element in the atmosphere and, as a building block of proteins and nucleic acids such as DNA, a crucially important component of all biological life.
The nitrogen cycle is a complex biogeochemical cycle in which nitrogen is converted from its inert atmospheric molecular form N2 into a form that is useful in biological processes. Nitrogen fixation Atmospheric nitrogen occurs primarily in an inert form N2 that few organisms can use; therefore it must be converted to an organic — or fixed — form in a process called nitrogen fixation.
First, nitrogen is deposited from the atmosphere into soils and surface waters, mainly through precipitation. This is done by microorganisms that fall into three broad categories: bacteria living in symbiotic relationships with certain plants, free anaerobic bacteria, and algae.
Crops, such as alfalfa and beans, are often planted in order to remedy the nitrogen-depletion in soils, and nitrogen-fixing bacteria employ an enzyme, known as nitrogenase, to split atmospheric nitrogen molecules into individual atoms for combination into other compounds.
Nitrogen can also be fixed through man-made processes, primarily industrial processes that create ammonia and nitrogen-rich fertilizers. Nitrification While ammonia can be used by some plants, most of the nitrogen taken up by plants is converted by bacteria from ammonia — which is highly toxic to many organisms — into nitrite NO2- , and then into nitrate NO This process is called nitrification, and these bacteria are known as nitrifying bacteria.
Assimilation Nitrogen compounds in various forms, such as nitrate, nitrite, ammonia, and ammonium are taken up from soils by plants which are then used in the formation of plant and animal proteins. Ammonification When plants and animals die, or when animals emit wastes, the nitrogen in the organic matter reenters the soil where it is broken down by other microorganisms, known as decomposers. This decomposition produces ammonia which is then available for other biological processes. Denitrification Nitrogen makes its way back into the atmosphere through a process called denitrification, in which nitrate NO3- is converted back to gaseous nitrogen N2.
Denitrification occurs primarily in wet soils where the water makes it difficult for microorganisms to get oxygen. Under these conditions, certain organisms — known as denitrifiying bacteria — will process nitrate to gain oxygen, leaving free nitrogen gas as a byproduct.
Also included is an extensive glossary of terms relating to nitrogen and the nitrogen cycle. The material covered includes biogeochemical cycles, properties of nitrogen, an illustration of the nitrogen cycle, and the effect of human activity on the cycle. Nitrogen Cycle Dr. Terry Cooper from the University of Minnesota provides a good basic overview of the nitrogen cycle.
Nutrient Overload: Unbalancing the Global Nitrogen Cycle This article by the World Resource Institute discusses potential impacts of the overabundance of usable forms of nitrogen which can affect both terrestrial and aquatic ecosystems.
The Nitrogen Cycle This lesson from Science in the Real World: Microbes in Action Program has students measure the nitrogen levels in a classroom aquarium over the period of a few weeks.
The experiment explains the nitrogen cycle, and contains detailed worksheets for students and instructions for the teacher. Traveling Nitrogen Middle school students can get involved in this UCAR Windows to the Universe interactive classroom exercise by playing the role of nitrogen atoms as they move to different?
The Environmental Literacy Council. Nitrogen Cycle.