Effects Of Biochar Amendment On The Transport Behaviours Of Pollutants And Elemental Silicon,and Pollution Mitigation Mechanism In Soil-Rice System

Biochar,a carbon-rich solid material,is produced by the pyrolysis of biomass at relatively low temperatures under oxygen limited conditions.Due to the diverse structure and properties of biochar,it has great attention for soil improvement,carbon sequestration and pollution mitigation,as well as restoration.A large number of studies have focused on organic carbon component of biochar for soil amendment,but research related to inorganic components such as silicon in biochar are poorly studied.Crops such as rice,wheat and corn have high silicon contents,and thus crop harvest take a large amount of silicon from the soil,causing soil silicon deficiency.The scanty silicon in soil causes poor rice growth,and reduces the efficacy of pollution control in rice.The biochar produced by wood chips commonly has low silicon content.The effect of biochars with different silicon content on soil silicon balance,along with interaction mechanism of siliceous migration in soil-rice system,is rarely reported.The control effects and mechanisms between the coupling of pollutants and silicon in soil-rice system have received little attention.The thesis focused on the effects of biochar on soil silicon dissolution,silicon and pollutants uptake in rice,and their interaction mechanisms.The dissolution of silicon and its influence on soil silicon balance by preparing Si-rich(silicon-rich)and Si-deficient(silicon-deficient)biochars was studied.They were applied to high available soil(HSS)and low available soil(LSS)for pot experiment,and rice was selected as a typical silicon-rich plant.The effects of biochars on the yield of rice,the silicon uptake in rice with different tissues(grains,straw and root),silicon distribution,and soil silicon cycling in soil-plant systems were studied.The surface structure changes of biochar before and after rice harvesting was examined by SFM-EDX.A pot experiment was further applied to cadmium-contaminated HSS and LSS to demonstrate the effects of biochars with different silicon content on the mechanism of cadmium and silicon uptake and their interaction.Finally,the control and mechanism of biochar on different pollutant types were studied by rice culture experiment of different pollution types(single cadmium,dichlorophenol(2,4-DCP),and their combined pollution).The main conclusions and innovations of the thesis are as follows:(1)The effect of biochar on the balance of silicon and their mechanisms were illustrated.Si-rich biochars can be considered as a slow release silicon source,and Si-deficient biochars can be used as an additional silicon sink for agricultural paddy soil.The silicon dissolution of Si-rich biochars was relatively high in comparison with Si-deficient biochars,and increased with the pyrolysis temperature.It was found that the silicon release mechanism is controlled by the protective carbon-silicon mutual protection,and the silicon dissolution is accompanied by carbon release.Once mixed with soil,the addition of Si-rich biochars improves the dissolution of soil silicon.The total accumulated soil silicon dissolution is lower than the theoretical value due to the adsorption of silicic acid by biochar.Therefore,the addition of Si-deficient biochar results in a decrease in cumulative silicon dissolution.The adsorption of silicic acid in the biochar and the protection of soil dissolved Fe make biochar a reservoir of soil silicon.(2)The effects of biochar on the migration,distribution and accumulation of silicon in soil-rice system were elucidated.Biochars can be considered as reservoirs of soil Si,which is a slow release source of available Si,to impact the speed of biogeochemical cycling of soil Si in agricultural paddy soil.The addition of biochar increased the yield of rice grains and straw without any effect on root yield,and Si-rich biochars increased the yield of rice grains,with high response to LSS.Si-rich biochar increases the silicon content of rice grains and roots,but had no effect on straw.The addition of Si-deficient biochar in HSS has little effect on the silicon content of rice,and the addition of Si-deficient biochar amended LSS has a great influence on the silicon content of rice by reducing the silicon content of rice.Si-rich biochar increases the total silicon uptake of rice,while the addition of Si-deficient biochar amended LSS reduces total silicon uptake.It is illustrated that the minerals of iron and Si were found on the surface of biochar.Biochars can fix the dissolved Si on its surface as temporary storage to prevent Si loss.(3)From three levels of concentration,morphology and rice root channel,the mechanism of Si-rich biochars release and controlled uptake of cadmium was revealed.The content of Cd for rice grains,straw,and root all corresponding treatments in HSS was lower than in LSS.The addition of biochar can reduce the concentration of cadmium in rice grains by 25.23-53.30%in HSS and by 24.53-59.45%in LSS.The treatment of Si-rich biochar is lower than that of Si-deficient biochar.Si-rich biochars was better than Si-deficient biochars to reduce Cd uptake,and accumulation in rice.The surface complexation of Cd with carboxyl groups and silicon from biochar led to enhance the Cd sequestration.High/low silicon content(soils or biochars)means relative lower/higher the value of bioaccumulation factor(BCF)and translocation factor(TF),respectively.Negative correlations between the concentrations of rice Cd and Si were observed,and it was related to lower expression of Si transporter genes(Lsil and Lsi3)by Si-rich biochar amendment in rice.These finding suggests that the released Si from biochar can reduce the gene expression of Si transport channel of rice roots and inhibit the transport channel of Si,and thus inhibit Cd uptake probably due to the utilization of the same channel for Cd and silicon Transmission and silicon ingested by rice may form a conj ugate with cadmium in plants to prevent cadmium from transferring into the grain.(4)It is proposed that silicon and carbon in biochar could control the uptake of cadmium and 2,4-DCP in rice.The combined pollution increased the uptake of cadmium and 2,4-DCP in the aboveground and underground parts of rice,compared to single pollution(cadmium or 2,4-DCP).The addition of silicon and Si-rich biochar could significantly reduce the cadmium content in aboveground and underground parts of rice,and the addition of biochar with high temperature could reduce the uptake of 2,4-DCP in rice.The cadmium and 2,4-DCP in water were fixed by the component of silicon and cabon in biochar,which reduced the content of cadmium and 2,4-DCP in the solution.At the same time,the dissolved silicon of Si-rich biochars promoted the uptake of silicon in rice,thickened the root cell wall of rice,reduced the enrichment coefficient of pollutants,and prevented the uptake of cadmium and 2,4-DCP in rice.