| As nitrogen (N) is the largest yield limiting nutrient in rice cropping systems worldwide, the application of chemical N fertilizer (CF-N) becomes the most common and effective way in rice production. For a higher yield and acceptable economic rewards, a large amount of N fertilizers were applied in China by farmers over the past few decades even the amount was far beyond normal requirements. As a result, approximately 30% of the world's nitrogen fertilizer is consumed in China. While, the mean apparent plant recovery of CF-N applied in rice production was only 30% to 35%, much lower than other major rice growing countries. Besides the huge waste of resources, over input of CF-N also led to an unanticipated result, a series of environmental problems including eutrophication of surface waters, nitrate pollution of groundwater, acid rain, soil acidification, greenhouse gas emissions, and other forms of air pollution, which is not only faced by China, but also the whole world. Therefore, the development of a more efficient and environmental friendly practice for CF-N application is of great importance and urgence in today's China, and even for the entire world.In this study, a sophisticated soil microcosms and 15N trace technique were used to investigate the effectiveness of an optimal management of nitrogen fertilizer aiming at reducing N leaching loss so as to improve nitrogen use efficiency in a double rice cropping system; besides, an approach based on 16S PCR-DGGE and BIOLOG analysis was used to determine the effectiveness of the nitrification inhibitor, DCD, in improving nitrogen uptake and to explore its microbial mechanism. The main results were as follows:1. A sophisticated soil microcosm was designed to collect soil solutions that leached from the double rice cropping system. The 15N trace technique was used to analyze the balance, uptake and use efficiency of CF-15N in addition to the retention and use efficiency of 15N-labeled urea of the second harvest season. Topsoil (0 cm-20 cm) and subsoil (20 cm-50 cm) samples were collected from a traditional double rice cropping field in the Jiangxi Province, China, and these soil samples were derived from Quaternary red clay. Treatments were randomly assigned with two irrigation regimes and three N application rates (no application control,80% traditional rate and 100% traditional rate noted as NO, N1 and N2, respectively). The levels of 15N recovery of plants, 15N and N remaining in soil were determined. Moreover, the N dynamic of soil solution from different layers of the soil profile was surveyed. The results showed that the effects of irrigation management and N application rate varied in different rice growing seasons. Irrigation regimes had remarkable effects on grain yield and chemical 15N fertilizers (CF-15N) uptake. When compared to flood irrigation (FI), the shallow water depth with wetting and drying (WD) increased grain yield up to 5.7%-20%. Although the highest grain yield was obtained with reduced N application level, both N apparent recovery (NAR) and 15N use efficiency (the percentage of plant N uptake derived from applied N,%Ndfan) significantly decreased with increasing N inputs. However, the interaction between irrigation management and N application rate on grain yield and N use efficiency (NUE) of CF-15N were not significant.2. A survey of soil solutions every 5 d indicated that NH4+-N was the main residual form of N, and high NH4+-N leaching was observed. When compared to FI, WD decreased vertical NH4+-N and TN leaching, especially at 10-50 cm depths of soil profile in the second season. NH4+-N was the main N residual form in the soil profile. Therefore, in this study, the WD irrigation regime and reduced rate (N1) was the optimal irrigation and fertilizer management strategy to increase the NUE of CF-N, increase the after effects of CF-15N, decrease leaching loss of CF-15N and minimize the shallow groundwater pollution risk, which were all beneficial for the ecological environment.3. In order to further clarify the relationship between the form of nitrogen leaching and soil nitrification, a self-made root box was conducted to determine the effectiveness of the nitrification inhibitor, DCD, in inhibiting nitrification in two different paddy soils and to explore relationships between DCD and ammonia oxygen microbial diversity in the rhizosphere and non-rhizosphere. The results showed that nitrification strength in paddy soil of Hubei, in which the main N leaching form was NO3--N, was stronger than that of Jiangxi, in which the main form was NH4+-N. DCD was found to be highly effective in decreasing the nitrification strength especially in Hubei paddy soil, which increased its N apparent recovery up to 17.6%.4. The results of DGGE and BIOLOG analysis indicated that Urea, DCD, soil type and location of sampling points were all factors that could highly affect both the metabolism of soil microbial community and structural diversity. In the case of non-fertilizer treatment, a higher soil microbial community diversity and structural diversity were found in the rhizosphere soil compared with non-rhizosphere soil. The application of urea promoted the microbial metabolic activities, while significantly reduced the community structure diversity. However, the application of DCD significantly reduced both microbial metabolic diversity and community structure diversity. DGGE analysis of ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) also revealed that the genetic diversity of AOA in Jiangxi acidic paddy soil was significantly higher than that in Hubei alkaline paddy soil. In Hubei paddy soil, the NO3--N leaching loss and nitrification rate were both significantly related to DGGE band numbers of AOB, other than AOA, indicating that AOB dominants the nitrification in Hubei paddy soil, which differed from the interaction of AOA and AOB in Jiangxi paddy soil.
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