| Bioretention Facility(BRF)regulates stormwater runoff and reduces pollution load mainly through natural processes such as soil infiltration,retention,storage,and purification,and is widely used in sponge city construction projects.However,the effect of conventional bioretention facilities on the removal of dissolved nitrogen(NOX-N)is unstable,and even a leaching phenomenon may occur.Therefore,it is of practical significance to study the nitrogen conversion process and the influencing factors of bioretention facilities to improve their operational efficiency.In this study,we investigated the effect of arbuscular mycorrhizal fungi(AMF)on the growth of plants commonly used in BRF(Canna indica,Cyperus alternifolius,etc.)and its infestation effect on the plants,and screened the suitable plants.Based on this study,we constructed a model of AMF bioretention device,carried out artificial rainfall simulation experiments,and studied the effects of AMF on the conversion of total nitrogen,nitrate nitrogen,ammonia nitrogen,and organic nitrogen under different carbon-to-nitrogen ratios and falling dry period conditions,analyzed the microbial community structure of soil media layer at different depths,and investigated the relationship between the denitrification efficiency of BRF and microbial community structure and the denitrification mechanism of the system.The main findings of this study are as follows:(1)The root systems of both Canna indica and Cyperus alternifolius could be infested by AMF to form a symbiotic system,and the corresponding mycorrhizal infestation rates were 34.1% and 28.1%,respectively,indicating that the mycorrhizal infestation ability of AMF on the root systems of Canna indica was better than that of Cyperus alternifolius.Inoculation of AMF had certain growth-promoting effects on both Canna indica and Cyperus alternifolius,and increased the biomass of above-and belowground parts of both plants by 33.1% and 15.9% for above-ground parts and 54.8% and51.8% for below-ground parts,respectively,and the growth-promoting effect of AMF on Canna indica was more significant.(2)With the increase of influent C/N,the removal of ammonia nitrogen,organic nitrogen,and COD by AMF-inoculated BRF and uninoculated conventional BRF showed a trend of first increase and then decrease.At C/N=13.3,the best performance of ammonia nitrogen,organic nitrogen,and COD removal was achieved by the AMF-inoculated BRF with 79.2%,82.3%,and 80.6%,respectively,which increased by 9.2%,6.4%,and 7.0%,respectively,compared with the conventional BRF.The removal of nitrate-nitrogen and total nitrogen showed an increasing trend for both facilities.At C/N=18.2,the highest removal rates for nitrate-nitrogen and total nitrogen were reached at 68.2% and 61.9,79.1%and 68.0% for the AMF-inoculated BRF and the uninoculated conventional BRF,respectively,and the removal rates of nitrate-nitrogen and total nitrogen increased by 6.3%and 11.1% for the AMF-inoculated BRF,respectively.(3)The nitrogen concentration outflow trends of the AMF bioretention system were consistent with those of the conventional bioretention system.The general trend of ammonia nitrogen effluent showed a decreasing trend followed by an increasing trend.The mean values of ammonia,nitrate,organic nitrogen,and total nitrogen concentrations in the effluent of the AMF bioretention system were all lower than those in the non-AMF bioretention system,and the mean values of the final effluent concentrations of the four indicators were reduced by 28.3%,10.4%,9.9%,and 9.6%,respectively.(4)The trends of pollutant removal rates of the BRF inoculated with AMF and the conventional BRF were approximately the same under different conditions of the falling dry period.The removal rates of ammonia nitrogen and total nitrogen were less affected by the drying period,and the removal rates fluctuated from 60% to 85%.The removal rate of organic nitrogen decreased gradually with the extension of the drying period,and the lowest removal rate was achieved at 20 d.The removal rates of organic nitrogen from AMF-inoculated BRF and conventional BRF were 61% and 55%,respectively.AMF had a certain effect on the removal of ammonia,nitrate,organic nitrogen,total nitrogen,and COD in the BRF,and the effect on nitrate was the most obvious.At 12 d,the nitrate removal rate of the AMF-inoculated BRF reached 77.2%,which increased by 15%compared with the conventional BRF.(5)AMF was able to increase the microbial abundance in the soil media layer of the bioretention system,but there was no significant difference in the species richness of microbial communities at different soil depths within the same system.The dominant phyla in both AMF-inoculated and non-inoculated BRFs were Proteobacteria,Chloroflexi,Acidobacteriota,Actinobacteria,and Firmicutes.The abundance of Phylum Proteobacteria,Phylum Chloroflexi,and Phylum Acidobacteriota in BRF inoculated with AMF increased by 10.1%,8.7%,and 8.2%,respectively,compared with the uninoculated BRF.Both P.Proteobacteria and P.Chloroflexi could provide denitrifying bacteria genera,which have important roles in biological denitrification,suggesting that AMF may change the microbial survival environment and thus the microbial community structure to improve the denitrification capacity of bioretention facilities. |
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