Study On Application Of Biochar Amended Soil Aiming At Green Roof In Nanning

In developed countries,light-weight artificial substrate is commonly used in green roof,due to poor floor-slab bearing capacity of old buildings.In China,however,it is entirely feasible to utilize natural soil with low cost and environmental friendliness as green roof substrate.Considering shortage of cultivated land resources in China,local non-agricultural soils with amendment are recommended for green roof substrate.Biochar is a kind of soil amendment,which is alkaline,eco-friendly and low-cost.Therefore,biochar is used as soil amendment for local expansive soil from Nanning,Guangxi in the current study.Aiming at green roof,engineering properties of biochar amended soils are explored.Main works and conclusions of the study are provided as follows:(1)Engineering properties(water holding,permeability,thermal insulation,heat storage and crack development,etc.)of unplanted soils were evaluated.Considering short-term water storage and long-term water retention,5% biochar amended soil is the best among four kinds of substrates.Under different conditions(dry density and degree of saturation),5% biochar amended soil always conducts water faster than natural soil.Soil heat insulation is enhanced by biochar through reducing soil thermal conductivity.However,effects of biochar amendment on soil heat storage are negligible.Under drying-wetting cycles,generation and propagation of cracks are mitigated by biochar.The effects of biochar amendment are quantitatively evaluated,and the feasibility and advantages of biochar application in green roof are analysed.(2)Microstructure and chemical composition of biochar and soils were analysed.More chemical bonds and cementitious agents are observed in biochar amended soils,which contributes to mitigating cracks.According to thermogravimetric analysis,biochar and natural soil have satisfactory thermal stabilities.Also,aging of biochar is observed in thermogravimetric analysis.Moreover,main pores on biochar surface are mesopores and micropores,while mesopores and macropores are dominant on soil surface.The specific surface area(SSA)of biochar is 3 times of SSA of expansive soil.The physical and chemical mechanisms of biochar amendment are revealed.(3)During the vegetation experiment,meteorological parameters,evapotranspiration and plant growth condition were recorded.After vegetation experiment,hydraulic conductivities of soils from different depths were measured.Further,numerical simulations were conducted based on measured results.It is found that evapotranspiration is dominated by air humidity and light intensity.All indexes show that 5% biochar amended soil is the best substrate for plant growth,while excessive biochar would inhibit plant growth.Moreover,direct effects of biochar on soil permeability are negligible,as compared to indirect way(i.e.,affecting root growth).Permeability of 5% biochar amended soil is acceptable,while deeper layer of natural soil is low permeable.According to simulation results,runoff peak volume for most rainfall events could be reduced effectively by 5% biochar amended soil,and runoff peak is also delayed.The vegetation-biochar-soil interaction is revealed,which contributes to popularizing green roof with biochar amendment in Nanning.