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Doctor of Philosophy




A study of N mineralization in soil columns (soil-sand mixture) incubated at 15, 25, or 35(DEGREES)C with intermittent leaching (to remove the mineral N produced) with 0.01 M CaCl(,2) for a total period of 30 weeks showed that the cumulative amounts of N mineralized in field-moist and air-dried soils treated with, or not treated with, minus-N nutrient solution after leaching were generally linearly related with time of incubation at 15 and 25(DEGREES)C in the 10 soils studied. This relationship was not consistent at 35(DEGREES)C; it was linear for most soils, and curvilinear for others. The rate of N mineralization increased as the incubation temperature increased and was not affected by addition of minus-N nutrient solution to soil column after every leaching. Expressed as percentage of organic N, the cumulative N mineralized in the 10 soils studied was 1-2% at 15(DEGREES)C, 2-5% at 25(DEGREES)C, and 6-10% at 35(DEGREES)C. Although there was no general trend in Q(,10) values for N mineralization in air-dried and field-moist soils at 25 and 35(DEGREES)C, Q(,10) values were always higher at 25(DEGREES)C (> 2) than at 35(DEGREES)C (2 or < 2);The rates of N mineralization under aerobic conditions in air-dried soils at 35(DEGREES)C were significantly correlated with amidase, glutaminase, and asparaginase activities but not with protease activity;The amounts of N mineralized under waterlogged conditions during 14 days at 30(DEGREES)C were highly significantly correlated with amidase, glutaminase and asparaginase activities in soils and also with N mineralization under aerobic conditions, especially at 25 and 35(DEGREES)C;A simple method was developed for assay of nitrate reductase in soils. It involves determination of the NO(,2)('-)-N produced when soil, 2,4-dinitrophenol (DNP), and KNO(,3) are incubated under waterlogged conditions. At a certain concentration, depending on soil type, DNP inhibited nitrite reductase but not nitrate reductase. Maximum amount of NO(,2)('-)-N was produced at 5 mM NO(,3)('-)-N, and nitrate reductase was inhibited by high concentrations of NO(,3)('-) (substrate). The relationship between time of incubation and amount of NO(,2)('-)-N produced showed lag phases of 10 hours at the early stages of NO(,2)('-)-N production with some soils, but generally, after the lag phase, the relationship was about linear for a certain length of incubation time, which varied among the soils used;The lag phases obtained were not completely eliminated by either preincubation or N(,2) bubbling to deplete the oxygen in the soil-water mixture before addition of NO(,3)('-). Nitrate reductase is soils was inactivated at 40(DEGREES)C and was completely destroyed on autoclaving (121(DEGREES)C for 2 hours). The relationship between the amount of soil used and NO(,2)('-)-N produced was linear unless the substrate became limiting to the reaction rate. Kinetic studies showed that, for 2 of the 3 soils studied, the K(,m) values were 2.9 and 3.7 mM and the V(,max) values were 122 and 126 (mu)g NO(,2)('-)-N produced/g soil/24 hours. The third soil, however, showed significantly higher K(,m) (33.3 mM) and V(,max) (962 (mu)g NO(,2)('-)-N produced/g soil/24 hours) values.



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Hago Mohammed Abdelmagid



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189 pages