Studies On The Responses Of Plant And Characteristics Of Pollutant Removal In Subsurface Flow Constructed Wetland Under Water Stress

In order to ensure the sustainable,efficient and stable operation of the constructed wetland system,the internal matrix needs appropriate"rest"to release the accumulation of wetland load.Secondly,the wetland system will also face the occurrence of non-rainfall period,outage maintenance and other conditions.This leads to the inevitable water stress（intermittent drying cycle）in the operation of constructed wetlands.However,the intermittent drying cycle has not been defined in the domestic and foreign standards,and the guide related to the maintenance of sponge sponge in sponge city also lacks detailed hydration maintenance instructions.Therefore,four different intermittent operating conditions were set up according to the intensity of water stress in this study.Physiological response mechanism and stress tolerance of Canna and Iris in 0.6m and0.1m bed deep subsurface flow constructed wetlands were studied.The relationship between the abnormal metabolic behavior of plant roots and the response characteristics of rhizosphere function under water stress and the change rule of microbial community structure and pollutant removal characteristics was analyzed.The necessity of intermittent drying period for wetland stable operation was evaluated.The main results and conclusions were summarized as follows:（1）By adjusting the reasonable proportion of plant biomass,water stress can make the growth of the above-ground part stagnant,reduce the transpiration loss of stems and leaves,promote the synthesis of substances in the underground part,and improve the water use efficiency of plants to a certain extent.The root-shoot ratio of plants under mild and moderate drought stress was significantly increased（P<0.05）,and the increase of root-shoot ratio was beneficial to water absorption and storage.The water absorption capacity of fine roots was enhanced by the increase of specific root length,and the specific root length of Iris and Canna was 3.93 and 3.24 times of that of control group,respectively.At the late stage of severe drought stress,plant leaves withered,the ratio of root length and root-shoot ratio decreased,and the water content of leaves decreased to0.25 and 0.40 times of the control group,which inhibited the water transfer between root and stem and leaf,and affected the synthesis and accumulation of biomass.The root/shoot ratio of plants in 0.1m bed depth wetland was 3 times that of 0.6m bed depth wetland.Root space limiting stress induced excessive root growth,and the change of root architecture could enhance plant stress resistance.（2）By stimulating the initiation of plant antioxidant system and osmotic regulation mechanism,water stress can remove excessive reactive oxygen species in the body,thus reducing the degree of membrane oxidation.The total antioxidant capacity of plants under mild drought stress was 4.58 and 5.40 times that of control group,respectively.Meanwhile,a large amount of proline was produced to maintain osmotic pressure and promote water absorption.Under severe drought stress,plant cell membrane was severely attacked,and its permeability,contents and relative conductivity increased.The content of malondialdehyde was significantly increased（P<0.05）,but decreased sharply in the later period.Severe water deficit aggravated the degree of membrane lipid peroxidation,and finally caused membrane system damage and lost response to stress factors.In addition,under root-space restricted stress,excess roots can enhance the antioxidant capacity of plants and protect the structural and functional integrity of cell membranes to a certain extent.（3）Compared with normal waterlogging conditions,water stress can promote the absorption of nitrogen and phosphorus,enhance photosynthesis and prevent water loss in cells.Under light to moderate drought stress,nitrogen and phosphorus uptake of stems and leaves was higher than that of roots.Sufficient nitrogen was beneficial to chlorophyll synthesis of stems and leaves,and a large amount of phosphorus could improve water retention of primary colloid in cells,promote cell metabolism,and facilitate plant proliferation and removal of wetland pollutants nitrogen and phosphorus.Under severe drought stress,stomatal contraction of stems and leaves restricts CO₂supply,inhibits chlorophyll synthesis,affects the growth and development of various organs,and weakens the assimilation of nitrogen and phosphorus in plants.The intermittent drying period of 2 to 5 days can promote the assimilation and absorption of nitrogen and phosphorus,which is beneficial to the growth and development of plants.However,the 9d drying period will affect the synthesis of plant materials,and thus reduce the absorption and utilization of nitrogen and phosphorus.（4）Water stress can enhance the metabolism of plant roots and induce the formation of a large number of cavities in root cortex,thus improving the ability of plant roots to secrete oxygen.Mild drought stress promoted the release of ethylene by enhancing the activity of hormone synthetase,and the ethylene content of Iris was 3.71of CK.Secondly,the root cortex cells dissolved and formed a large number of pores,which enhanced the oxygen delivery and secretion capacity of plants,and thus improved the oxygen environment of the wetland.The aerenchyma content of Iris was2.59 times that of CK.Under severe drought stress,root cell respiration was weakened,the root activity of Iris and Canna was only 0.18 and 0.45 times of the initial value,and oxygen secretion decreased by 74.15%.The intermittent drying period of 9d inhibited the stability of root metabolism,reduced the rate of oxygen secretion,and then affected the oxygen content of wetland rhizosphere.Excessive root system can enhance rhizosphere effect and promote oxygen secretion under rhizosphere restricted stress.（5）Water stress affects rhizosphere physiological response to change the composition of secretions,and then changes the microbial community structure.The composition and proportion of plant root exudates were different due to the intensity of water stress.However,Proteobacteria,Bacteroidetes,Firmicutes and Actinobacteria were the dominant rhizosphere bacteria.Mild drought stress promoted the secretion of organic compounds such as esters and acids,and increased the relative abundance of functional bacteria such as Proteobacteria.Severe drought stress damaged root physiological function,inhibited root secretion behavior,hindered root material circulation,and reduced the abundance ratio and diversity index of dominant bacteria.Rhizospatial restriction stress could strengthen the physiological function of roots,promote the increase of secretions,and further enhance the activity of functional bacteria such as Proteobacteria.Secondly,the abundance proportion of Bacteroidetes in0.6m bed depth wetland increased,which might be because of the stronger tolerance of Bacteroidetes than other bacteria.（6）Compared with flooding condition,mild to moderate drought stress can increase DO concentration and microbial community richness in rhizosphere,and promote COD,N and P removal.TP removal rate in two depth wetland beds was 1.47and 1.21 times that of CK,and TN removal rate was increased by 7.56%and 3.59%.The positive physiological response of plants effectively improved the rhizosphere microsystem,and thus enhanced the removal performance of wetland pollutants.The decreased root vitality under severe drought stress will inhibit the oxygen secretion of roots,reduce the abundance and vitality of functional bacteria.The intermittent drying period of 2 to 5 days can improve the oxygen content of wetland,promote the enrichment of functional bacteria,and thus enhance the stable operation of wetland and the removal efficiency of pollutants.However,the intermittent drying period of 9 days destroys the growth habitat of rhizosphere microorganisms,which is not conducive to the efficient removal of pollutants,and thus affects the stable operation of wetland system.In addition,a large number of roots could enhance oxygen secretion in the rhizosphere and accelerate the nitrification process of denitrifying bacteria under rhizosphere restriction stress.