The Responses Of Plant And Characteristics Of Pollutant Removal In Subsurface Flow Wetland Under Acid-Base Stress

Acid-base stress is one of the main reasons for the inhibition of plant root development,which directly affects root growth,water and nutrient absorption.At the same time,acid-base stress will destroy the surface structure of microorganisms,affect the charge of polar nutrients,and hinder their metabolic process.Under acid-base stress,wetland plants and rhizosphere microbial communities all show different response characteristics,which further affects the removal effect of pollutants in constructed wetlands.However,the interaction mechanism between wetland plants and rhizosphere microorganisms in this habitat has not been fully understood.In addition,it remains to be studied whether the tolerance characteristics of plants to acid and alkali will change positively due to the substantial increase of root biomass after the shallow-bed constructed wetland,that is,the redundant development of plant roots.In this paper,two kinds of subsurface constructed wetlands（shallow bed:0.1m,conventional depth:0.6m）and five kinds of p H values（strong acid-base stress:p H=3,11;Weak acid-base stress:p H=5,9;Control group:under the condition of p H=7.6）,the stress characteristics,the change characteristics of rhizosphere microenvironment and the response mechanism of rhizosphere microbial community structure and enzyme activity of two wetland plants（Acorus calamus and Canna canna）were studied,which provided a new basis for the treatment of acid-base wastewater and the improvement of the operation stability of constructed wetland when acid-base conditions changed.The main findings and conclusions are as follows:（1）Under weak acid-base stress,the content of malondialdehyde（MDA）in Acorus calamus leaves first increased and then decreased,and the peak value was 1.30～1.58times of the initial value.The content of MDA in Canna leaves accumulated to a certain extent,and the maximum value was 1.62～2.44 times of the initial value.The MDA content of Acorus calamus leaves remained high at the late stage of strong acid-base stress,which was 2.68～3.08 times of the initial value.However,the content of MDA in canna leaves with irreversible damage peaked in the middle and late stage of stress and then fell back,and the peak content was 3.20～3.32 times of the initial value.This is because the total antioxidant capacity（T-AOC）of Acorus calamus is stronger than that of Canna under acid-base stress.The maximum T-AOC content of Acorus calamus leaves is 3.79～4.08 times of the initial value,and the peak T-AOC content of Canna leaves is 2.63～2.97 times of the initial value.Although Acorus calamus is more acid and alkali tolerant than Canna,the MDA content in leaves of Acorus calamus remains high at the late stage of strong acid and alkali stress,which indicates that the influent p H=3and 11 exceeds the tolerance threshold of Acorus calamus.（2）The chlorophyll synthesis and photosynthetic system of the two plants were inhibited and damaged to varying degrees by acid-base stress.The reason for the strong adaptability of Acorus calamus to acid-base environment is that its antioxidant system has a certain protective effect on photosynthetic system,but the soil and plant analyzer development（SPAD）of leaves still decreased under strong acid-base stress,and the lowest was 79～83%of that of the control group,but the damage of Acorus calamus leaves was reversible.However,the damage of canna leaves was irreversible,and SPAD was 47～63%of that of the control group.SPAD is closely related to the radial oxygen loss（ROL）and root exudate（RE）of plants.The ROL rate and DOC secretion of Acorus calamus under strong acid-base stress are 64.17%and 52.54%of those of the control group.The ROL rate of canna was 24.70%of that of the control group.Combined with the growth state of the root system,it was inferred that the DOC of the root system was mainly passively exudation,and its secretion was 53.30%of that of the control group.（3）Acid-base stress caused obvious changes in the proportion of various substances in Acorus calamus RE.Alkaline environment can promote the organic acids in plant roots,while acidic environment can inhibit the secretion of organic acids.The proportion of organic acids in each gradient is 1.63%,4.65%,5.91%,14.13%and 20.35%（corresponding to influent p H values of 3,5,7.6,9 and 11,respectively）.Under weak acid and alkali conditions,the change characteristics of RE composition of Canna are consistent with that of Acorus calamus,while under strong acid and alkali stress,the change of RE composition is mainly passive exudation,and the proportion of organic acids in each gradient is 2.38%,5.78%,8.10%,12.24%and 2.38%.The acid-base regulation ability of Acorus calamus root system is better than canna.In addition,it also promotes the secretion of glycerol under alkaline conditions,the highest proportion of which can reach 10.31%,and glycerol attached to the root system surface in alkaline environment can protect the root system.The average concentration of RE in shallow-bed wetland is about 6 times that of conventional wetland,and RE contains substances with acid-base regulation ability,such as esters and organic acids.Therefore,compared with conventional deep-bed wetland,shallow-bed wetland has stronger acid-base buffering ability,which makes wetland plants in shallow-bed constructed wetland suffer less acid-base stress.（4）Due to the differences in acid and alkali tolerance characteristics between calamus and canna,acid and alkali stress affects the physiological response of the two plants,changing the ROL rate,RE amount,and proportion of various substances,resulting in differences in the structure and activity characteristics of the rhizosphere microbial community between the two plants.Under strong acidic and alkaline conditions,specific colonies that can adapt to acidic and alkaline environments have been formed in both calamus and canna wetlands;In addition,Thauera only became the dominant genus in the calamus wetland,which to some extent indicates that the calamus root system has a greater protective effect on rhizosphere microorganisms.However,under acid-base stress,the Chao1 index of the rhizosphere microbial communities of both plants decreased.The Chao1 index of the calamus wetland was 3678.74,4601.68,5978.08,5526.11,and 3563.65,respectively（corresponding to influent p H values of 3,5,7.6,9,and 11,respectively）.The Chao1 index of the canna wetland was 3094.46,4379.90,6172.04,4286.63,and 3529.58,respectively;The microbial community index of Shannon and Simpson also showed the same trend.Therefore,the structural complexity and diversity of rhizosphere microbial community in acorus calamus and canna wetlands decreased under acid and alkali stress,especially in canna wetlands.In addition,compared with shallow bed wetland,the structural diversity of rhizosphere microbial community in conventional deep wetland decreased more.（5）Under acid-base stress,the self-regulation ability of Acorus calamus was superior to that of Canna sativa,and its ROL rate and root DOC secretion were significantly higher than those of Canna sativa（p<0.05）.Therefore,compared to the canna wetland,the calamus wetland has a stronger acid-base buffering capacity,and its rhizosphere microbial enzyme activity is significantly higher than that of the canna wetland.The rhizosphere microbial enzyme activity is positively correlated with the wetland pollutant removal rate;Correspondingly,under strong acid-base stress,the removal rates of COD,TN,NH₄⁺-N,and TP in the calamus wetland were significantly higher than those in the canna wetland（p<0.05）;After the redundant growth of plant roots in wetlands,the ROL rate and RE concentration were significantly increased,which enhanced the removal of pollutants.