Nitrification Activity And Composition Of Nitrifying Bacteria Of Soils In China

Nitrification, which is performed by the ammonia-oxidizing bacteria (AOB) converting NH4+ to NO2- and then by the nitrite-oxidizing bacteria (NOB) converting the latter to NO3-, is a crucial process of the global N cycle. Soil nitrification rate is very different among soil types due to their differences in physical and chemical properties. This different nitrification activity affects environment greatly, but far little is known about the composition of nitrifying bacteria community.In this investigation, three soils in China were used to study soil nitrification activities and composition of nitrifying bacteria communities through molecular approaches and conventional culture methods. The results obtained were stated as follow. A 28-days long-term soil incubation showed that the maximum nitrification potential was found in the Fluvo-aquic soil with almost 100% of inorganic N present as NCV-N, while the minimum nitrification potential was very little in Red earth with only a 4.9% of conversion rate. The short-term liquid incubation of soils showed that the maximum nitrification intensity was found in the Fluvo-aquic soil, and the minimum nitrification intensity was in Red earth. The short-term liquid incubation of nitrifying bacteria also showed that the maximum nitrification intensity was found in the nitrifying bacteria selected from the Fluvo-aquic soil, while the minimum nitrification intensity was in the nitrifying bacteria of Red earth. There was no relationship between nitrification activity and numbers of nitrifiers of soil. Conventional most probable number (MPN)-Griess method counted ammonia oxidizers, but failed in enumerating nitrite oxidizers. Therefore we used an MPN-PCR procedure, which gave a convincible result of nitrite oxidizer counting instead of MPN. The denaturing gradient gel electrophoresis (DGGE) of DNA extracted from soil and amplified using a primer specific for the 16S rRNA gene and the amok gene showed the amok gene PCR gave a more detailed information of soil ammonia oxidizer community than PCR of 16S rDNA gene. The DGGE columns of the three soils were different from each other. There were 2 similar bands present in DGGE columns of the Fluvo-aquic soil and the Permeablepaddy soil, but no similar band was found in DGGE columns of the Red earth. The further study on the sequence of amoA indicated that all ammonia oxidizers in these soils were grouped in Nitrosospira cluster 1 and 3, and each soil had common band similar to the other soils and special band different from the other soils. The results revealed the similarity and the difference of ammonia oxidizer composition in three soils. Extracting DNA from the soils is the first step to process molecular biological experiments, so we compared the different methods of extracting DNA from soils. The results of different methods to extract DNA from soils indicated that method 1 (cell were cracked by frozing and thawing, and DNA extracted using phenol) was a useful method for the DNA extraction, and DNA extracted by this method was better than the other methods.The thesis revealed that it is the composition of ammonia oxidizers community and the identity of soils that might be associated with soil nitrification activity. It is necessary to combine molecular methods with traditional methods to further understand nitrification activity and nitrifying bacteria in soils with different ecological charicteristcs.