| Based on the sediments under different hydrological loading, water depth and with or without floating vegetation (Vegetation present/absent, VP/VA) in a riverine constructed wetland adjacent to Nanfei River, our study tried to explore the influence of hydrological loading, water depth and vegetation on the potential rates and functional gene abundances of nitrification and denitrification and their relationships between each other by using chemical analysis, fluorescence quantitative PCR method, liner mixed-effects model, Tukey-HSD test, Principle component analysis (PCA), Redundancy Analysis (RDA) and Person correlation analysis. Our result showed that:(1) Variance analysis revealed that the physico-chemical properties of overlying water and sediment were mainly influenced by CW hydrological loading. Surface water dissolve oxygen (DO) concentration was significant influence by hydrological loading, water depth and vegetation and the orders from high to low were 110cm/d> 80cm/d> 50cm/d,20cm> 40cm> 60cm and VA> VP. Higher hydrological loading could increase sediments total nitrogen (TN), ammonium nitrogen (NH4+), organic matter (OM) content and C/N ratio (C/N) while decrease sediments pH value and nitrate nitrogen (NO3-) content. In contrast to hydrological loading, sediment TN, OM content and C/N increased with water depth drawdown. Except overlying water DO, the present of vegetation also led to a significant reducing of sediment NH4+ content. In addition, the interaction between vegetation and hydrological loading or water depth indicated that this significant decrease in surface water DO concentration with vegetation present was more pronounced under the highest hydrological loading and the highest water depth. However, there is no significant effect of hydrological loading, water depth and vegetation on sediment nitrite nitrogen (NO2) content.(2) Method of chemical analysis was carried out to measure the potential nitrification (PNR) and denitrification (PDR) rates of CW sediments under different hydrological loading, water depth and with or without floating vegetation. The result of variance analysis showed that the potential rates of nitrification and denitrification were significant impacted by hydrological loading and water depth. Sediments PDR was significant influenced by hydrological loading change and the order from high to low was 50cm/d>80cm/d>110cm/d, meanwhile, the interaction between hydrological loading and water depth revealed that the highest PDR is observed in the shallowest ditches but this notable phenomenon was only significant under highest hydrological loading. On the contrary, PNR was significantly higher in the deepest ditches and this significant effect was more pronounced when hydrological loading was 110cm/d while the significant decrease in sediments PNR with vegetation present was only pronounced under the highest hydrological loading.(3) Realtime fluorescence quantitative PCR was carried out for the quantitative research of the functional gene abundances of nitrification and denitrification under different hydrological loading, water depth and with or without floating vegetation. Our result showed that the abundances of target functional genes were significant impacted by hydrological loading and vegetation. The highest abundance of ammonia-oxidizing archaea (AOA) was detected when hydrological loading was lowest and this significant change was more pronounced in ditches with vegetation while the notable decrease in sediments AOA abundance with vegetation present was only pronounced under highest hydrological loading. Conversely, ammonia-oxidizing bacteria (AOB) exhibited a higher abundance under highest hydrological loading than lowest hydrological loading. Compare to nitrifizers, hydrological loading, water depth and vegetation showed weaker influence on the abundance of denitrifizers. While hydrological loading, water depth and vegetation exhibited no significant effects on sediments nirS gene abundance, nirK gene abundance was only significant influenced by vegetation.(4) The result of PCA and correlation analysis showed that oxygen availability, sediments OM, NH4+ and NO3- content were the most important environmental factors controlling the potential rate and functional gene abundances of nitrification and denitrification. Sediment oxygen availability could influence the potential rate and functional gene abundances of nitrification and denitrification in CW sediments, directly. Sediment OM could stimulate the rate of denitrification as the organic carbon source of heterotrophic microorganism or the electron donor of denitrification. Meanwhile, OM could also indirectly influence nitrifizers and denitrifizers in sediments by varying sediment oxygen availability caused by the consumption of O2 due to the growth of heterotrophic microbe. As the substrate of nitrificantion and denitrification, sediment NH4+ and NO3- content directly stimulated the growth or activities of nitrifizers and denitrifizers. Our result revealed that hydrological loading, water depth and vegetation can alter these environmental characteristics, influencing the potential rate and functional gene abundances of nitrification and denitrification, indirectly. And the lack of relationship between PNR/PDR and abundances of functional genes indicated that sediment environmental conditions rather than functional abundances drive PNR and PDR after changes in hydrological loading, water depth and vegetation. |
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