| Vegetable production systems are one of the intensive agricultural production systems with high rates of nitrogen (N) fertilizer application and irrigation, and thus have a high potential for nitrate (NO3-) leaching and nitrous oxide (N2O) emissions. The high rates of N fertilizer applied may also affect soil microbial community structure and functions impacting on the long-term sustainability of the production systems.In this thesis, lysimeter and field trials were conducted to study the effects of N fertilizer use on soil properties, groundwater quality and vegetable quality in vegetable production systems in Wuhan suburbs, Central China. The amount of N leaching losses affected by N fertilizer application was quantified, and the N transformation processes in the soil was also studied. Based on these results, effective methods to reduce nitrate leaching, while at the same time, increasing farmers' income were proposed for sustainable vegetable production.The main research results of this thesis are as follows:1. Lysimeters were used to study the effects of different N fertilizer application rates on nitrate content in vegetables, in soil and in leachate. Results showed that, both N leaching losses and NO-3--N content in leachate were significantly correlated with N fertilizer application rate. High rates of N fertilizer application increased the NO3--N concentration in the leachate, exceeding the drinking water standard. With the increase of N fertilizer application rates, NO3--N concentration in leachate, and total N content, available N and NO3--N in soil increased significantly. Soil organic matter content was increased at first, then, reduced by the N fertilizer over-application. Soil pH values were significantly reduced. It is clear that excessive amounts of N fertilizer application have many harmful impacts on the soil and groundwater.2. In a long-term monitoring trial, NO3--N accumulation in the soil profile and in the soil solution had a direct relationship with N fertilizer application rate. The amount and rate of NO3--N leaching in soil profile were significantly influenced by the physical properties of soil and irrigation amount. With increasing N application rates, NO3-content in vegetables and leachate, amount of N leaching losses increased significantly. There were positive correlations between N fertilizer application rate (X) and NO3--N leaching losses (Y1), and average NO3--N concentration in leachate (Y2)(Y1=0.2882X+33.66(R2=0.9940) and Y2=0.0114X+1.24(R2=0.9928)). Thus, large amounts of N fertilizer application not only result in wasting of the fertilizer resources but also adversely affect the quality of vegetables and ground water.3. There was a significant difference in the amount of NO3--N leached and the rate between different soil profiles with different crop varieties, root systems, and growth periods of vegetables. Leaf vegetable planted could improve nitrogen utilization efficiency, but it could not absorb the N in the deeper soil profile. Solanaceous and root vegetable could absorb N which was leached to the deeper soil profile, but they also led to large amounts of NO3--N leaching losses, because little N was required at the early growth periods. In the vegetable ecosystem, the amounts of N losses and N utilization efficiency had a significant negative correlation in different vegetable rotation modes. The results indicate that the varieties of vegetables could affect the N utilization efficiency.4. Along with the increase of N fertilization application, soil urease activity increased gradually; protease and nitrate reductase activities were increased at first, then decreased with the amount of N application. The results showed that appropriate nitrogen application rates could improve the enzyme activity, which were related with the transformation of N in soil, and was beneficial for increasing vegetable yield and N content. Bacteria numbers were more than those of actinomyces and ray fungi. Along with the increase of nitrogen application rates, bacteria and actinomyces numbers increased first, and then decreased sharply, and fungi numbers were reduced gradually. According to the bacteria and fungi numbers, the appropriate nitrogen application would improve the qualities of soil. However, excessive fertilizer would lead to the lower soil qualities. At the same time, the results using the BIOLOG to study the soil microbial communities showed that soil metabolic activity was the same in different nitrogen fertilization conditions, and the microbial diversity index had a little change. However, there were differences of carbon utilization and soil microorganism types between different nitrogen application rates. It means that soil microorganism diversities were significantly influenced by nitrogen application.5. In the long-term study of nitrogen fertilizer use, the soil denitrifying community functional genes, such as narG, nirK, nirS, nosZ, were divided into3or4parts. N fertilizer application could change the soil microbial community composition. The index analysis showed that, the adequate application amounts of N fertilizer would increase the diversity of the gene narG, nirS and nosZ, but excessive amounts of N fertilizer would reduce the diversity of narG, nirK, nirS and nosZ. The copy numbers of functional genes narG, nirK, nirS, qnorB and nosZ were increased gradually with the increase of N fertilizer application, and the numbers of soil denitrification functional genes changed significantly by the N fertilizer application. The number of16S rDNA increased with the application amounts of N fertilizer, but its difference was not significant than the copy numbers of soil denitrification functional genes in different N rates. The sequence of the copy numbers of soil denitrification functional genes was:narG>(nirK+nirS)> qnorB> nosZ. Results showed that the N fertilizer application could affect denitrification functional genes and the microbial community structure, and denitrifying functional genes are more sensitive to N fertilization rates.6. The nitrification inhibitor DCD significantly influenced yields, qualities of vegetable and NO3--N concentrations in soil and ground water. The application of DCD significantly increased the vegetable yields by6.2-43.96%compared with control, and there was a positive correlation between the increased production and growth and a negative correlation between increased production and temperature. DCD application significantly decreased the NO3--N concentrations in vegetables by5.3-39.7%. DCD also increased N content in vegetables, the N offtake was increased by12.9%-61.23%, and N use efficiency was increased significantly.NH4+-N concentrations in soil profile were increased significantly, and NO3--N concentrations were decreased by the application of DCD. N leaching losses, NO3--N concentrations in the leachate and N2O emissions were significantly decreased by the application of DCD. It means that the application of DCD not only increased the fertilizer utilization efficiency, but also improved the quality of soil and ground water.7. Basal application of trace element fertiliazers (B, Zn and Mo) would increase the yields, qualities and N content of vegetable and N utilization efficiency, and it would decrease NO3--N concentrations in soil solutions and soil profiles. The results showed that, the vegetable yields were increased by15.9%-42.4%and4.4%-16.5%, NO3-concentrations in vegetable were decreased by21.6%-31.2%and10.7%-44.3%, N content in vegetables was increased by9.6%-19.3%and0.4%-25.9%, N offtake was increased by27.1%-83.7%and4.6%-9.6%, by the basal application of the trace element fertilizers, respectively. At the same time, the NO3--N content in soil profile and soil solution were decreased by the application of trace element fertiliazers B, Zn and Mo. Thus, trace elements not only increased the economic effectiveness in vegetable production system, but also improved the quality of soil and the environment.
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