| The hydraulic conditions of constructed wetlands are directly related to the morphological and physiological metabolism of plants in wetlands,the activity and community structure of microorganisms,the physical and chemical properties of water bodies and the removal efficiency of pollutants in water.The current research lacks a mechanistic explanation of the complex process that different hydraulic conditions cause different states of plants,microorganisms and water quality in wetlands.In this paper,by revealing different hydraulic loading（0.1m3/（m2·d）、0.7m3/（m2·d）、1.3 m3/（m2·d））,flow direction（horizontal flow,downward flow,upward flow）and bed depth（0.1 m,0.6m）and other different hydraulic conditions,the response mechanism of the life activities of different plants（Phragmites australis,Juncus effusus L.）and their rhizosphere microorganisms after the sewage is introduced into the subsurface flow,and the pollutant removal characteristics of the corresponding wetland,in order to provide reference for the design and operation of the subsurface flow wetland.The results show that:（1）When the hydraulic load is 0.1m3/（m2·d）（the hydraulic retention time of 0.1 m bed depth wetland is 1 d,and the hydraulic retention time of 0.6 m is 6 d）,the wetland has the best removal effect on pollutants.Longer hydraulic retention time can significantly improve the removal rate of pollutants.The TN removal performance of the up-flow wetland is better.Due to the oxygen secretion of the plant roots in the wetland bed and the bottom water,it is easy to form an alternating aerobic and anaerobic environment in the wetland bed,which is more conducive to the transformation of N.Compared with the wetland with a bed depth of 0.6 m,the wetland with a bed depth of0.1 m has a higher average concentration of dissolved oxygen（DO）,and the treatment efficiency of pollutants such as COD and NH4+-N per unit volume is higher.（2）The total antioxidant capacity（T-AOC）and membrane lipid peroxidation of reed were consistent in the change behavior,that is,when the content of malondialdehyde（MDA）in reed increased,the activity of T-AOC decreased.It shows that the adverse effects of reed mainly come from membrane lipid peroxidation.There was no significant change in T-AOC enzyme activity during the whole experiment.The resistance of Phragmites australis and Juncus effusus L.was enhanced,and the total antioxidant capacity and membrane lipid peroxidation level tended to be stable after a large change from the initial to the middle stage.（3）The root zone thickness of all wetlands with hydraulic load of 1.3 m3/（m2·d）was significantly different from that of 0.1m3/（m2·d）（p<0.05）.Combined with the root morphological indexes of plants,the greater the hydraulic load,the greater the biomass of plant roots,the more developed the roots.The roots of plants in the up-flow wetland are more developed,which may be related to the mass transfer of nutrients.The root biomass of the wetland with 0.1 m bed depth was significantly higher than that of the wetland with 0.6 m bed depth,which was due to the spatial stress imposed on the plant roots by the shallower bed.（4）The root activity,radial oxygen loss（ROL）rate and ethylene release of plants in the up-flow wetland with a bed depth of 0.1 m and a hydraulic load of 1.3 m3/（m2·d）were higher than those in other wetlands,and there were significant differences（p<0.01）.It can be considered that the two hydraulic conditions of 1.3 m3/（m2·d）hydraulic load and upward flow have obvious effects on enhancing the metabolic capacity of plants.The spatial stress of 0.1 m bed depth induced the redundancy of plant roots（the thickness of root zone,the number of first-order lateral roots,root fresh weight and root dry weight of0.1 m wetland were 1.07,1.10,1.14 and 1.19 times of those of 0.6 m bed depth wetland,respectively）,which strengthened the metabolic capacity of plant roots.The hydraulic load of 1.3 m3/（m2·d）indicates that more nutrients enter the system per unit time,sometimes causing water potential stress on plants,which in turn causes plants to secrete ethylene.Ethylene can promote the formation of aerenchyma in plant roots.The water inflow mode of the up-flow wetland makes the water distribution inside the system more uniform and the distribution of nutrients more uniform.Compared with the other two types of wetlands,the overall physiological indexes of plants in the wetland are better.Secondly,due to the poor reoxygenation ability of the up-flow wetland,there is oxygen source stress,which can in turn promote the ROL effect to a certain extent.（5）When the hydraulic load rate is 0.1 m3/（m2·d）and the flow is downward,the dehydrogenase activity of reed wetlands is stronger.Perhaps at this time,the DO level in the wetland is relatively high and the hydraulic retention time is longer.When the hydraulic load rate is 0.7 m3/（m2·d）,the dehydrogenase activity in the rhizosphere of rush grass is the lowest in the horizontal flow wetland.It is speculated that the uneven distribution of nutrients in the flow direction is the cause.The hydraulic load rate is0.1 m3/（m2·d）,and the water flow direction is downward.The urease content in the rush grass wetland is the highest,which may be related to root exudates.（6）Defluviicoccus,SC-I-84 and Nocardioides were the dominant genera in rhizosphere microorganisms.Defluviicoccus and SC-I-84 belonged to Proteobacteria,and Nocardioides belonged to Actinobacteria.The wetland with a bed depth of 0.1 m and the up-flow wetland had strong ability to enrich the dominant bacteria Defluviicoccus and SC-I-84.The wetland with a hydraulic load of 1.3 m3/（m2·d）has an advantage in enriching SC-I-84.The possible reason is that the ROL rate of wetland plants with a hydraulic load of 1.3 m3/（m2·d）is greater,providing an oxygen source for the metabolism of aerobic microorganisms.Under this condition,more alkanes in root exudates provided high-quality carbon sources for wetland microorganisms.（7）On the whole,when the hydraulic load is 1.3 m3/（m2·d）,although the removal rate of pollutants in the wetland is slightly lower than that of the other two hydraulic loads,the physiological state reflected by the physiological indexes of plants under this condition is better than that of other wetlands.The oxygen secretion of plant roots in the up-flow wetland has been enhanced,and the root exudates are mostly alkanes that are easy to decompose,which makes the up-flow wetland more capable of enriching the dominant flora,and the ability of the wetland to treat pollutants has also been strengthened. |
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