Denitrifying Bacteria And Nitrate Reducing Bacteria Physiological Groups And Restore Characteristics Of Flooded Soils

34 Abstract Soils used in this paper were selected based on their contrasting potentials for dissimilatory nitrate reduction to ammonium (DNRA). Yangzhou soil, as indicated by the name, was sampled from the experimental farm of Yangzhou University, Jiangsu Province, China. Griffith soil was sampled from Griffith, New South Wales, Australia. Different soil management practices were employed in the two soils. Rice-straw mulching-fallow has been practiced in Griffith soil for at least sever years (high carbon input) while rice-wheat-straw removal has been practiced for many years in Yangzhou soil (low carbon input). The populations of nitrate reducers and denitrifiers in Yangzhou .and Griffith soil were enumerated. The ratio of denitrifiers to total nitrate reducers in Yangzhou soil was 13.6%, but 1.8% in Griffith soil, indicating that the denitrifiers were not the dominant flora among the bacteria that are capable of reducing nitrate in submerged soils. All nitrate reducers from the two soils were purified and studied. The data showed that the composition of nitrate reducers in the two soils differed significantly. In Yangzhou soil, 43.7% of nitrate reducers isolated were denitrifiers while only 6.25% was DNRA bacteria. On the contrary, in Griffith soil, more than half of nitrate reducers were DNRA bacteria, while only 2.9% were denitrifiers. This data situation corresponded to the DNRA ability of the two soils. All isolated bacteria were primarily identified. Most of them were spore-forming groups, which constituted 56.25% and 80% of the total nitrate reducers isolated in Yangzhou soil and Griffith soil respectively. The dominant genus responsible for DNRA in Griffith soil was Bacillus sps0 The amount of labile organic carbon in these two soils was examined. With soil incubation method, it was 216.2 mg C02-C kg?and 93.2 mg C02-C kg? respectively in Griffith soil and Yangzhou soil. But with potassium dichromate oxidation method, there was no notable difference between these two soils. Incubation method is based on the microbial decomposition and is therefore thought to be more appropriate for labile carbon measurement than the chemical 35 oxidation method. The amount of soil labile organic carbon measured by incubation method was closely correlated with DNRA potentials of the soils. Because of carbon input affects soil labile carbon turnover, it was reasoned that soil management affected soil labile carbon content and soil labile carbon content in turn affected DNRA process. Seven isolated bacteria were selected for further study to see their characters of nitrate reduction. Cell yield of two denitrifiers (C2 and A19(2)), as indicated by optical density, was proportional to the amount of nitrate added to media. But cell yield of two DNRA bacteria (C8 and C12) was not. DNRA bacteria converted nitrate into ammonium during their logarithmic growth phase. Ammonium produced as such was in excess of cell growth and cell death during this phase was minimal. These data suggested that nitrate assimilation and re-mineralization of cell materials were not important processes contributing to the ammonium formed. On the contrast, denitrifiers and a nitrite accumula...