Nitrogen_Cycle

Candidate No. 20854 Question 1 a), The earth's atmosphere contains 79% N2 gas, however N2 is unavailable for use by most organisms. The major conversion of N2 into ammonia (NH3), and hence proteins for plants, is achieved biologically by microorganisms in the process called nitrogen fixation, small amounts of ammonia is produced by lightning & some produced industrially by the Haber-Bosch process. Estimates of nitrogen fixed globally1. |Fixation type |N2 fixed (106 tonnes / year) | |Non-biological |  | |Industrial | 50 | |Combustion | 20 | |Lightning | 10 | |Total | 80 | |  |  | |Biological |  | |Agricultural land | 90 | |Forest / non-agricultural land | 50 | |Sea | 35 | |Total |Approx 175 | Microorganisms have a central role in almost all aspects of nitrogen availability • Some bacteria convert N2 into ammonia by nitrogen fixation; these bacteria are either free-living or form symbiotic associations with plants or other organisms (e.g. termites, protozoa) • Other bacteria transform ammonia to nitrate NO3, • Many blue-green bacteria can also "fix" atmospheric nitrogen. • Some nitrogen-fixing bacteria, such as Rhizobium, live in root nodules of leguminous plants (such as peas or beans). These processes contribute to the nitrogen cycle (See figure 1). 1) DF Bezdicek & AC Kennedy, in Microorganisms in Action. Blackwell Scientific Publications 1998. Candidate No. 20854 Question 1continued [pic] b) Human activities generally increase the levels of fixation of molecular Nitrogen into organic compounds; they can also result in industrial process, which have adverse affects. These include:- I. Intensive cropping requiring heavy application of Nitrogen based fertilisers that often leach into watercourses causing eutrophication and high Biological Oxygen Demand levels. Nitrogen enrichment can alter large areas and threaten the survival of entire ecosystems. II. Livestock ranching causes the release of large amounts of Ammonia, which again enters the Hydrological, cycle through leachate. III. Industrial processes and burning of forests and fossil fuels increase atmospheric nitrate pollution and generate Nitric Oxide, which is a significant greenhouse gas and acidifier (acid rain, deforestation and acid lakes etc). c) Heaths / Dunes are habitat to a range of birds, reptiles, invertebrates and lower plant communities which are sensitive to nutrient changes, caused by atmospheric deposition, or acid rain, resulting in:- • Soil, vegetation & habitats changing or becoming more acid • Proliferation of beetles, causing extensive defoliation of heather. • Increased nitrogen favours grass growth and invasion of non-native species such as nettles and changes the interactions and ecological balance within existing species. • Disappearance of species i.e. sea-holly, and the increase in scrub attributed to increases in nitrogen levels. 400 words