| At present, carbon sequestration in a paddy soil is a front-line issue under the background of globle climate changing and of demanding for decreasing green-house-gas emission in our country’s agriculture. The interaction among soil, rice and microbe involved in the process of carbon sequestration is maybe a special issue of carbon cycle in paddy soil. So, the study of microbial community structures and functional activities is needed which can encrich and develop the theory of soil carbon cycle. Both of rice cultivation and different fertilizations can largely affect the intensity of carbon sequestration in rice paddy where there was a higher potential of carbon sequestration than that in dryland. In this paper, two subjects are invesgated for understanding the changing of the interaction among soil, rice and microbe under rice cultivation and different fertilizations. One is what are changes of microbial floras and activities with soil organic carbon (SOC) in rice paddy cultivated from natural wetland. The other is what are changes of functional microbes under different fertilizations in a mature rice paddy. Probing the two problems can help us to understand the changing law of microbes in a paddy soil and can provide the scientific data for the law’s application in bio-geo-process.In the first study, the topsoil samples were collected from natural wetlands and rice paddies in Jingjiang region, Hunbei, China. SOC and microbial biomass carbon (SMB-C) were examined with sulfate digestion method with potassium dichromate and chloroform fumigation combining extraction with lemery, respectely. Microbial flora numbers and community structure diversities were measured with plate counting method and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), respectely. Microbial activity was determined with soil enzyme assays as well. In the second study, the topsoil samples were collected from a23-year fertilizing paddy field in the Tai Lake region, Jiangsu, China where the fertilization treatments were designed as follows:no fertilizer application (NF), chemical fertilizers only (CF), chemical fertilizers plus pig manure (CFM), and chemical fertilizers plus rice straw return (CFS). The methods of plating and PCR-DGGE combining gene cloning and sequencing were adopted for investing the soil microbial communities of nitrifying, denitrifying, azotobacter and CO2-fixing under four treatments with amoA and Arch-amoA, nirK, nifH and cbbLR gene as the indicators of ammonia oxidizing bacteria (AOB) and archaeal (AOA), denitrifing bacteria (NRB) and azotobacter and CO2-fixing bacteria, respectively. Normalized method was using for charactering overall soil enzyme activities and soil nutrients richness, which is help for discussion the relativity changes of SOC, microbe and functional activity. The main results were as follows:1. Changes of soil microbial community and activity in a short time in a rice paddy cultivated from natural wetland.The SOC content increased by55.42%in rice soils compared to wetland. SMB-C was found as180%higher in paddy soil than in wetlands. There was no significant difference in abundance of the bacterial, fungi, actionmycetes and autotrophic azotobacter, and diversity of both bacterial and fungi between wetlands and paddy field. However, the observed microbial enzyme activities of invertase, urease and alkaline phosphatase enhanced by89%,70%and72%, respectively, in paddy field over wetlands. Statistical analysis revealed a significant correlation of SMB-C and normalized overall enzyme activity with soil organic carbon contents, respectively. These observations supported that the size of soil microbial biomass and microbial functional activity with enzymes were enhanced as SOC storage increased due to rice cultivation for several decades, which indicated that microbes promote SOC accumulation through accelerating nutrients turnover in a paddy’ development.2. Changes of microbe communities involved in N-cycle in a rice paddy under long-term fertilization treatmentsThe long-term fertilization treatments had changed community structure of Azotobacter. The clony forming unite (CFU) of Azotobacter under CF treatment, CFM treatment and CFS treatment were found as higher0.58,0.66and1.06times, respectively compared with that in NF treatment, the abundances of nifH as well as were found as higher2.13,10.79and3.44times. Statistical analysis revealed there was a significant correlation of the CFU of Azotobacter with SOC (R2=0.9378, P=0.0319), as well as the abundance of nifH with SNRN of SOC, T-N and T-P (R2=0.9707, P=0.0147). The soil nitrogen fixation activities under CFM and CFS treatments were found as higher204%and196%, respectively compared with that in NF treatment. However, there was not a significant correlation of the soil nitrogen fixation activity with the CFU of Azotobacter or the abundance of nifH gene.The long-term fertilization treatments had changed community structures of AOB AOA and denitrifier significantly. Compared with the microbial abundance in NF treatment, the abundances of amoA and nirK gene in CF, CFM and CFS treatment were as higher1.36,39.57and29.98times and0.51,2.81and0.80times, respectively, as well the abundances of Arch-amoA gene under CFM and CFS treatments were as higher446.97and88.19times, respectively. The abundance of amoA is two to four orders higher than the one of Arch-amoA though both are the same change trend across the four treatments. There were significant correlations of abundance of amoA and nirK with soil nutrient richness normalized (SNRN) of soil organic carbon (SOC) and total nitrogen (T-N)(R2=0.9101, P=0.0460) and of SOC, T-N and total phosphorus (T-P)(R2=0.9101, P=0.0460), respetively, which indicated that SOC accululation and nutrient richness under manure treatments can increase the abundances of AOB, AOA and denitrifier. There was higher soil nitrification potential under CFS treatment and a positive correlation between SOC and soil nitrification potential (R2=0.9110, P=0.0470). There was a highest denitrification potential in CFM treatment while a lowest one in CFS treatment, which indicated a large reducing potential of N2O emission under pig manure treatment. However, the abundances of amoA/Arch-amoA and nirK were decoupling with the soil nitrification potential and denitrification potential across four treatments, respectively.3. Changes of functional microbe involved in carbon cycle——CO2-fixing bacteria community in a paddy soil under long-term fertilization treatmentsThe long-term fertilization treatments had changed community structure of microbe involved in carbon cycle. The abundances of cbbLR under CF treatment, CFM treatment and CFS treatment were found as higher2.13,10.79and3.42times, respectively compared with that in NF treatment. Statistical analysis revealed there was a significant correlation of the abundance of cbbLR with SNRN of SOC, T-N and T-P (R2=0.9920, P=0.0020).In summary, not only the changes of both soil microbial biomass and bio-chemical activities in a rice paddy reclaimed from wetland, but also th changes of both the abundances and structures of microbe involved carbon and nitrogen cycle under long-term fertilization response the changes of SOC and nutrients. Moreover, there was positive relationship between the above microbial charisteres and SOC content, which showed that there was a substantially increaseing of abundance and diversity along with SOC increasing. The above results indicated the SOC accumulation changed the living condition of microbe so that the relationship among SOC, microbe and enzyme activity was evolutionarily buding up, which enhanced soil bio-function and crop production and favored the rice paddy ecosystem stable. In addition, significantly coorelation between functional microbial abundances and activities involved in N transform under different fertilization treatments were not observed, which indicated that SOC accumulation increased the redundancy rate of functional genes. This is, therefore, probably inner mechanism of the stability of rice ecosystem which deserved the further study. In this study, the result showed that a decreasing denitrification activity but increasing avtivities of nitrification and nitrogen fixation under the CFM treatment in a rice paddy as well. |
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