Microbial Distribution In The Rhizophere And Nitrogen Removal Mechanisms Of The Ipomoea Aquatica Floating-bed System In Summer

Plant absorption and microbial nitrification and denitrification in the rhizosphere could be the two main ways of nitrogen removal in ecological floating-beds. Based on the relationship among quantity and distribution of nitrogen cycling bacteria, nitrogen removal and root morphological traits, this study investigated the nitrogen removal mechanisms of ecological floating-beds. The main results were shown as follows:1) This study discussed the relationship between quantity and distribution of nitrogen cycling bacteria and nitrogen removal in the Ipomoea aquatica floating-bed system. The number of total nitrogen cycling bacteria, ammonifying bacteria, nitrosobacteria and nitrobacteria in the Ⅰ. aquatica floating-bed system was significantly higher than that in the control (P<0.01). No significant difference was found for the number of denitrifying bacteria between the Ⅰ. aquatica floating-bed system and the control (P>0.05). The total number of ammonifying bacteria, nitrobacteria and denitrifying bacteria in the water were all significantly higher than that in the roots (P<0.01). Nitrosobacteria was not found in the water. The difference in the distribution of ammonifying bacteria was not significant among endo-rhizosphere, rhizoplane and water. Nitrosobacteria was mainly distributed in the endo-rhizosphere and rhizoplane. Nitrobacteria was mainly distributed in the rhizoplane and denitrifying bacteria was mainly distributed in the water. In the Ⅰ. aquatica floating-bed system, there was significantly or extremely significantly negative correlation between DO and the number of ammonifying bacteria, nitrosobacteria, denitrifying bacteria. There was significantly negative correlation between pH and the number of nitrosobacteria; there was significantly or extremely significantly positive correlation between ORP and the number of nitrosobacteria, denitrifying bacteria; there was significantly or extremely significantly negative correlation between water temperature and ammonifying bacteria, nitrosobacteria, nitrobacteria, and denitrifying bacteria. There was significantly or extremely significantly negative correlation between water temperature and the number of ammonifying bacteria, nitrobacteria in the control. The removal rate of NH4+-N was91.8%in the Ⅰ. aquatica floating-bed system and88.5%in the control, respectively. NH3volatilization could be the main way of NH4+-N removal in the control. Microbial nitrification could be the main way of NH4+-N removal in the Ⅰ. aquatica floating-bed system. The removal rate of total nitrogen was48.2%in the Ⅰ. aquatica floating-bed system and62.1%in the control, respectively. Due to the decay of Ⅰ. aquatica induced by insects after the19th day, the removal rate of total nitrogen in the Ⅰ. aquatica floating-bed system was lower than that in the control. Based on the correlation analysis between the number and distribution of the nitrogen cycling bacteria and the nitrogen concentration, we found that the contribution of ammonifying bacteria and nitrosobacteria in the rhizoplane is higher than that in the endo-rhizosphere and water. The contribution of nitrobacteria and denitrifying bacteria in the water is higher than that in the endo-rhizosphere and rhizoplane.2) This study also discussed the relation between root morphological traits of Ⅰ. aquatica floating-bed and nitrogen removal. During the whole experiment, the root length, total root length, root surface area, root volume, number of root tips, root fresh weight of Ⅰ. aquatica floating-bed increased continuously in the tank and river. As the experiment went on, the average root diameter decreased continuously. No significant difference was found for the root fresh weight, root length of Ⅰ. aquatica floating-bed between the tank and the river (P>0.05). The total root length, root surface area and number of root tips of Ⅰ. aquatica floating-bed in the river was extremely significantly higher than that in the tank (P<0.01). The root volume of Ⅰ. aquatica floating-bed in the river was significantly higher than that in the tank (P<0.05). But the average root diameter of Ⅰ. aquatica floating-bed in the tank was extremely significantly higher than that in the river (P<0.01). Water temperature had an extremely significant influence on roots moiphological growth of Ⅰ. aquatica floating-bed in the tank and river (P<0.01). There was extremely significantly positive correlation between water temperature and root length, total root length, root surface area, root volume, number of root tips, root fresh weight in the tank and river (P<0.01). And there was extremely significantly negative correlation between water temperature and average root diameter in the tank and river (P<0.01). In the tank, the degree of each index affected by water temperature was:number of root tips> root length> root volume> root fresh weight> root surface area> total root length> average root diameter. In the river, the degree of each index affected by water temperature was:root surface area> root volume> total root length≈number of root tips> root length> average root diameter> root fresh weight. The removal rate of NH4+-N was98.09%in the Ⅰ. aquatica floating-bed system of the tank. The changes of NH4+-N concentration was affected by the way of NH4+-N removal, roots morphological traits, the number of microorganisms and so on. The removal rate of total nitrogen was83.45%in the Ⅰ. aquatica floating-bed system of the tank. The changes of total nitrogen concentration was affected by roots morphological traits, microbial denitrification, NH3volatilization and so on. The degree of correlation between each index and NH4+-N concentration, total nitrogen concentration was:root length> number of root tips> total root length> root surface area> root fresh weight> root volume> average root diameter in the Ⅰ. aquatica floating-bed system of the tank.3) This study revealed the nitrogen removal mechanisms of ecological floating-beds. With the development of roots, the root length, number of root tips, root surface area, root volume, root fresh weight and other root morphological indexes increased continuously, and the number of corresponding bacteria in the rhizosphere changed correspondingly. In general, there was extremely significantly negative correlation between the total number of nitrogen cycling bacteria, the number of ammonifying bacteria per unit root weight and root length, total root length, number of root tips (P<0.01). There was significantly negative correlation between the total number of nitrogen cycling bacteria, the number of ammonifying bacteria per unit root weight and root surface area (P<0.05). There was extremely significantly positive correlation between the total number of nitrogen cycling bacteria, the number of ammonifying bacteria per unit root weight and average root diameter (P<0.01). There was extremely significantly negative correlation between the number of nitrosobacteria, denitrifying bacteria per unit root weight and root length, total root length, root surface area, root volume, number of root tips, and root fresh weight (P<0.01). There was not significant correlation between the number of nitrobacteria per unit root weight and all roots morphological indexes (P>0.05). The degree of correlation between the number of ammonifying bacteria in the rhizoplane and root length, total root length, root surface area, root volume, number of root tips, and root fresh weight were larger than that in the endo-rhizosphere and water. The degree of correlation between the number of nitrosobacteria in the rhizoplane and root length, total root length, root surface area, root volume, number of root tips, root fresh weight were larger than that in the endo-rhizosphere. But the degree of correlation between the number of nitrosobacteria in the endo-rhizosphere and average root diameter was larger than that in the rhizoplane. The degree of correlation between the number of nitrobacteria in the water and root length, total root length, root surface area, root volume, number of root tips, average root diameter, root fresh weight were larger than that in the rhizoplane and endo-rhizosphere. The degree of correlation between the number of denitrifying bacteria in the rhizoplane and root length, total root length, root surface area, root volume, number of root tips, and root fresh weight were larger than that in the endo-rhizosphere and water. The degree of correlation between the number of denitrifying bacteria in the water and average root diameter was larger than that in the rhizoplane and endo-rhizosphere. Compared with other environmental factors, the number of ammonifying bacteria in the rhizoplane, the number of nitrobacteria in the water, the number of denitrifying bacteria in the endo-rhizosphere and water, NO3-N concentration, NH4+-N concentration, root length, total root length, root surface area, root volume, number of root tips, root fresh weight, water temperature, DO concentration and ORP all have high influences on the changes of total nitrogen concentration.In short, this dissertation studied the relationship among quantity and distribution of nitrogen cycling bacteria in the rhizosphere, nitrogen removal and root morphological traits through the experiment of tank and river. The nitrogen removal mechanisms of rhizo-sphere micro-communities in the ecological floating-bed system was further investigated. The findings contribute to the understanding of nutrient removal of floating beds and thus have an important theoretical significance and practical value. Besides, the findings also provide a prerequisite for the large-scale application of ecological floating-beds technology in eutrophicated rivers and lakes.