Effect Of Adding Different Feedstock Biochar On Green Roof Substrate Characteristics And Runoff Quality

As urban construction continues to accelerate and increase urban stormwater hazards,the construction of green roofs has become a low environmental impact urban stormwater management approach and is widely used.However,nutrient loss from green roof substrates can impact the environmental quality of urban water bodies.Improving green roofs’ stormwater management capacity and reducing runoff pollution has become one of the hot spots in urban green space ecology research.Green roof substrate characteristics are one of the main factors affecting green roof runoff quantity,quality and stormwater management capacity.Biochar has received increasing attention and research in green roof substrate material improvement because of its excellent water and fertilizer retention function,but there is still a lack of research on the effects of biochar addition on green roof substrate characteristics and runoff quality.In this study,we first assessed the effect of biochar on the ecological functions of green roofs using Meta-analysis,followed by a field experiment to evaluate the effect of biochar made from maize straw(MSB)and rice husk(RHB)mixed with soil,perlite,and vermiculite at volumetric ratios(10%,15%,and 20%)on the physicochemical properties,biological properties,runoff quantity,and runoff quality of the substrate,to provide a scientific basis for the use of biochar in green roof substrates.The main results were as follows:(1)The addition of biochar significantly improved the physicochemical properties of the substrate.The addition of RHB and MSB significantly improved the substrate’s water retention capacity and aerated porosity.It reduced the capacity and saturation weight of the substrate with the increase of the additional amount.In contrast,the effect of MSB was slightly better than RHB,but there was no significant difference.The total nitrogen,total carbon,total phosphorus,and available phosphorus contents of the substrate increased significantly with increasing biochar addition rate,but the opposite was found for nitrate nitrogen,and the effect of adding MSB was more significant than that of RHB.The effect of adding two types of biochar on substrate p H was insignificant.(2)The addition of biochar significantly affected the diversity and community structure of bacteria in the substrate.The addition of RHB increased the Chao 1 index and Shannon index of the substrate bacterial community compared with the control,but the effect of adding MSB was insignificant.The addition of biochar had no significant effect on the relative abundance of dominant bacterial groups in the substrate,but significantly increased the relative abundance of Desulfobacterota,MBNT15,Nitrospirota,and hydrogenedentes in the substrate.Lefse multilevel species difference discriminant analysis The results showed that the marker groups in the substrate were Actinobacteriota,Chloroflexi,Bacteroidota,Gemmatimonadota,Patescibacteria,Proteobacteria,the addition of biochar resulted in a decrease in the number of marker flora of bacteria in the substrate.(3)Fungal diversity and community structure in the substrate did not respond significantly to the addition of biochar.Only the Chao 1 index of RH20 was significantly higher than that of the control.The dominant fungal communities in the substrate were Ascomycota,Mortierellomycota,and unclassified＿k＿Fungi,and the addition of biochar had no significant effect on the composition of the fungal communities in the substrate.The results showed that Ascomycota was enriched in all treatments,and the addition of MSB reduced the number of marker groups in the substrate,while the addition of RHB increased the number of marker groups in the substrate.(4)Addition of biochar significantly affected runoff quality but not runoff quantity and runoff p H.The addition of both types of biochar reduced the concentration of total nitrogen in the runoff,and the effect of MSB was more significant than that of RHB.The effect of different additions on the concentration of total nitrogen in runoff was insignificant.Total phosphorus concentration increased significantly with the addition of biochar,and the runoff P concentration was significantly higher in the treatment with MSB than in the treatment with RHB.Regression analysis indicated that biochar affected runoff N and P concentrations mainly by reducing the substrate’s effective state N and P.The effect of biochar addition on runoff DOC concentration decreased and then increased with time.Still,the total amount of DOC leaching decreased significantly with the increased biochar addition rate throughout the experimental period.The total amount of DOC leaching in the treatment with MSB was significantly lower than that in the treatment with RHB.In conclusion,the addition of biochar can effectively improve the physicochemical properties of the substrate,improve its bacterial community structure,and reduce the DOC and TN concentrations in runoff and increase the runoff p H,but it will increase the runoff TP concentration,considering that we suggest that adding 10%-15% MSB by volume to the green roof substrate can effectively improve the substrate characteristics and enhance the runoff quality.