Simulation Study On The Interfacial Process Of Bensulfuron Methyl In Wetland Under Salt Stress

Agricultural non-point pollution and wetlands degradation for lack of water are twodifficult problems in ecological environment. An effective way to solve these problems is torestore wetlands with the water resources of farmland drainage and to purify drainage byengineering wetland. However, the effect of herbicide residues from drainage on wetlandsystem and its dissipation process are still not clear. Understanding the behavior of herbicideand its environment risk, which is important to the ecological conservation, restoration andutilization of wetland, requires an assessment of the processes influencing its fate, transport,bioactivity and persistence in wetland. Investigation of the ecological impact andenvironment behavior of herbicide in wetland and the controlling key factors involved arethus necessary and effective.This dissertation is aimed to revealthe ecological impacts of bensulfuron methyl (BSM), inreed wetland under salt stress, understand the interface behavior of BSM in the soil-watersystem, determining the possibility to supply the dry wetlands by farmland drainage withBSM residues, identifying the potential contributions of soil componts and its colloids toBSM retention, evaluating the dissipation of BSM in the rhizosphere and the correspondingmicrobiological and biochemical resopnses, and analysing the influence of iron plaque androot exudates, to develop strengthening wetland treatment techniques to decrease BSMcontamination. The information derived from this work will contribute to a betterunderstanding the interactions and their impacts on the environmental behavior of organicand inorganic combined contaminants in the flooded soil-plant system. The main experimentand conclusions are as follows:It was proved by field survey and sampling investigation that, the application ofsulfonylurea herbicides in saline and alkali paddy field can cause the problem of herbicidesresidues in gricultural drainage, and the BSM in gricultural drainage was up to1.68³.77μg/L in Yueliangpao irrigation district of Jilin Province. The in-situ test revealed that thepurification function of saline reed wetland to BSM was obvious, and the removal rate of BSM (0.05mg/kg⁰.50mg/kg) by saline reed wetland (Na+25mmol/l) after90days canachieveï¹¥65%and can be more efficient when the concentration of BSM was lower.Information of the resistance of reed wetland to BSM from drainage under salt stress wasgained by in-situ and laboratory simulated experiments with sexual and asexual reproductionof reeds. The results indicated that the response of reed seeds were closely related to themembrane permeability of seed under stress, the seed germination rate, germination vigor,germination index and vigor index all were inhibited by salt stress (Na+50mmol/l),meanwhile the BSM (0.05mg/l⁰.50mg/l) can alleviate this inhibition, but the inhibitionfunction weakened gradually along with the increasing BSM concentration; the influence onreed plumule was little, and BSM at low concentration can also alleviate the inhibition reedheight and biomass under salt stress. At the same time, it was discovered that the soilmicrobial biomass carbon and nitrogen were inhibited by BSM, and the inhibition rate wasabove20%, but BSM can improve the invertase activity though it was not significant at0.05level. In general, the entire reed wetland ecosystem showed tolerance to BSM under saltstress, to some extent.The adsorption and desorption of BSM on reed wetland soil and different colloids of puresoil minerals and humic acids were measured to obtained the potential contributions of thesematters to BSM retention in wetland under salt stress.Above70%BSM(0.10～20.0mg/l)canbe adsorbed by reed wetland soil, the residual rate still could maintainï¹¥60%after singledesorption, and the residual rate increased along with the BSM concentration and tended tobe stable, which was the combined result of the increasing adsorption rate and decreasingdesorption rate. The adsorption and desorption isotherms of BSM by reed wetland soil,predicted accurately by the dual-mode model (DMM), showed good linearity at BSM>1.0mg/l, but the misalignment characteristic was more obvious at the low concentration(BSM≤1.0mg/l). In dynamics process, the adsorption and desorption quantity of BSM allincreased sharply in a short time, but both process can be basically balanced after120min. Byremoving organic carbon from the wetland soils with H₂O₂, the adsorption decreased anddesorption increased. The colloid improved adsorption rate by>25%, which extracted fromthe original soil, played an important role in the process of BSM adsorption. The adsorptionaffinity for BSM by different colloids was in the order: humic acid colloid,montmorillonite-ca²⁺colloid>goethite+humic acid compound colloid> goethite colloid>montmorillonite-K+colloid> kaolinite colloid, but the desorption affinity was completelyopposite. It was also discovered that the salt stress inhibited adsorption and promoted desorption of BSM on all soil components and all kinds of soil colloids, especially themontmorillonite-ca²⁺colloid. The irreversible process of adsorption-desorption of BSM inreed wetland soil was strongly indicated by hysteresis index (HI), which also inhibited by saltstress, and hysteresis index decreased with the increasing balanced concentration.The rhizophere effects on the dissipation of BSM was investigated with a speciallydesigned rhizobox where reeds were planted for90days in the soil with BSM at differentconcentration, the soil in the box was divided into six separate compartments at variousdistances from the root surface. Obtained results indicated that the dissipation of BSM wasvaried along with the distance from the reed root, the dissipation gradient of BSM followedthe order: near-rhizosphere> root compartment> far-rhizosphere soil zones, and the largestand most rapid loss of BSM in soil was observed at about2mm from the root zone other thanat the root zone. The increase in soil microbial biomass and the activity of soil invertase wererelated to the enhanced dissipation of BSM. Further investigation was conduced usingphospholipid fatty acids (PLFAs) profiles to follow the millimeter spatial response of the soilmicrobial community, the results showed52PLFAs with carbon chain length from C12toC20were identified under the combined exposure of salt and BSM, the microbial communitywas affected by BSM significantly, and some unique microbial markers were induced by thecombined exposure of salt (0.130mg/l) and BSM (50mmol/l), such as ISO17:1G,19:0cycloc11-12,18:2w6c,20:4w6c,18:1w9t Alcohol,16:1w7c, ANTEISO17:1AT9,19:1(w8?)Alcohol and so on.The results drawn from a hydroponics experiment to investigate the response of rootexudates of reed to BSM under salt stress showed that, the pH in environment was notaffected significantly by the root exudates with the induction by BSM (2.5mg/l,5.0mg/l,10.0mg/l,50mmol/l) under salt stress (50mmol/l), but the induction strengthened thesecretion of total organic carbon (TOC), total sugar and the amino acid from reed root, andthe induction function enhanced gradually along with the increasing BSM concentration.Simultaneously, the induction of BSM and salt were antagonistic in the total sugar and aminoacid of root exudates, especially in total sugar significantly.A special hydroponics experiment was conducted to elucidate the effects of iron plaquefrom root surface on the response of reed to BSM under salt stress. The Fe²⁺at lowerconcentration (0-20mg/l) can promote the growth of reed, however Fe²⁺at higherconcentration (>20mg/l) inhibited the growth, and the inhibition of BSM and salt to the rootgrowth of reed presented synergistic effects significantly. The mass of iron plaque on reedroot surface was greatly affected by the Fe²⁺concentration in environment, and the iron plaque mass increased significantly with Fe²⁺concentration increasing rather than the reedbiomass. The root exudates of reed induced by salt stress might stimulate the formation ofiron plaque on root surface. The iron plaque mass substantially influenced the adsorption ofBSM on root surface, and both presented a significantly positive correlation. The effect ofsalt stress on the adsorption of BSM by iron plaque on root surface was reflected in theimprovement of adsorption efficiency, because of the inhibition of root biomass.