|MadSci Network: General Biology|
Hi Shourya! Great questions... I love plants and the nitrogen cycle. OK, the first point of confusion that needs to be cleared up concerns the form of nitrogen assimilated by plants. Plants absorb nitrogen from the soil in two inorganic forms: ammonium (NH4+) AND nitrate (NO3-). Ammonium is the preferred form because, as you note, plants use the ammonium ions directly for the synthesis of amino acids and proteins. However, when plants absorb nitrate, they must convert the nitrate into ammonium within the plant cell before it can be used to synthesis amino acids (nitrate is converted into nitrite within the cytoplasm, and then the nitrite enters the plastid where it is converted into ammonium, which is then assimilated to amino acids). So, plants can live happily with either ammonium or nitrate as their sole source of nitrogen. However, plants compete for ammonium with certain abundant soil bacteria (Nitrosomonas and Nitrobacter) that use ammonium in their own metabolism. These bacteria are called nitrifying bacteria because they convert ammonium to nitrites (Nitrosomonas) and nitrates (Nitrobacter). This means that, in the soil, there is often more nitrate than ammonium available for plants. The nitrification link, I think, is where most of your confusion rests... plants are not needlessly converting ammonium to nitrate; rather, they have adapted to be able to utilize the “waste” by-products of ubiquitous soil bacteria to meet their nitrogen demands. As I said, the plant itself will only convert nitrates into ammonium ions within the plant body, not within the soil... they will not convert ammonium into nitrate! So, it seems like there is probably some misleading information on this website!! Plants can absorb nitrates, but it needs to be converted to ammonium to be used in amino acids and protein synthesis. Regarding nitrogen turnover in the soil, nitrogen will be returned to the soil through the decomposing activities of bacteria AND FUNGI. Fungi are actually chiefly responsible for the decomposition of most plant material, breaking down tough plant cell walls and releasing the stored nitrogenous components back into the soil (which is then absorbed by plants, fungi and bacteria, or transformed by nitrifying bacteria). Certain types of fungi (the white-rot fungi) are the only organisms on earth that can degrade lignin, the polymer that makes wood hard. Lastly, yes, ammonium ions can cause nitrogen toxicity in soils. In agriculture, ammonium fertilizer is supplied to plants in the form of urea, and too much urea can be toxic to many organisms. I hope this helps resolve some of your confusion!
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