Effect Of Biochar Addition On Cadmium Transport In Red Paddy Soil And Rice Plant System

With the development of social economy and the improvement of people's living standards,land resources have been exploited and utilized unrestrainedly.A large number of heavy metals were entered into soils through many pathways,such as mining activities,organic fertilizers,and chemical fertilizers,leading to a serious problem of heavy metal contamination,most notably to south China,where the soils have been seriously contaminated by cadmium(Cd).The problem to humans that Cd not only can be retained in soil for a long time,but also will transport into the deep soil or enter into the food chain by edible crop parts and lead to serious health concerns.Therefore,soil Cd contamination has become the focus of scholars.Biochar is derived by organic residues through pyrolysis at medium temperature and limitted oxygen conditions,which has a potential effect for Cd immobilization due to its large specific area,high content of organic carbon,and porous properties.Therefore,the experiment site of this study was selected in Changsha county,Hunan Province.The experimental soil is red paddy soil.Laboratory and field experiment were combined in this study to investigated the biochar effects on Cd transport in red paddy soil and rice plant system.In this study,different amounts of biochar treatments were included:0 t ha-1(AO),10 t ha-1(0.5%,A10),20 t ha-1(1%,A20),30 t ha-1(1.5%,A30),and 40 t ha-1(1.5%,A40).Biochar was added at one time and spread onto the soil surface,and then mixed into the soil surface layer(0-17 cm)with basic fertilizer.Batch sorption and column experiments were conducted to study the adsorption isotherms of Cd and its mobility at different biochar application rate treatments.As for the field experiment,in the different rice growth stages from 2017 to 2019,samples of rice plant and soil in the 0-17 and 17-29 cm soil layers were collected to study the Cd transport in soil and rice plant system at different growth period.In rice maturing stage,undisturbed soil samples in the 0-17 cm soil layer and soil samples in the 0-5,5-10,and 10-17 soil layers were also collected.This study particularly focused on(?)effect of biochar addition on Cd transport in red paddy soil;(?)whether biochar could alter soil properties and the distribution of Cd in soil aggregates to control the Cd mobilization;(?)biochar effects on the accumulation and translocation of Cd in rice plants,and its availability in the soil surface layer and soil subsurface layer at different rice growth period.The results could provide theoretical foundations to remediate Cd in southern China.Results were observed as follows:(1)The laboratory experiments showed that biochar could control the Cd transport in soil.Batch sorption experiments showed that adsorption isotherms of Cd in biochar added soil were better fitted in the Langmuir model than in Freundlich equation.With the biochar application rate increased,the initial breakthrough time and time of curve climax increased,but the pore water velocity and dispersion coefficient decreased.Increasing biochar application rate enhanced the pH but reduced redox potential(Eh)in the effluents.Compared with A0,the Cd content retained in soil columns of A10,A20,A30,and A40 treatments increased by 19.9-80.8%.However,BCR sequential extractions showed that biochar addition increased the acid soluble fraction,which might be related to the Cd being weakly adsorbed on the surface of biochar.(2)Results of three-year field experiment showed that biochar could improve the chemical properties of soil in 0-17 and 17-29 cm soil layers.With the biochar application rate increased,the available phosphorus content,pH,and cation exchange capacity in the 0-17 and 17-29 cm soil layer showed an increasing trend.However,the content of nitrate nitrogen and ammonium nitrogen decreased as the biochar application rate increased.In terms of soil organic carbon,except the decreased trend in the soil subsurface layer of 2017,biochar could contribute to the carbon sequestration after long-term application.During different rice growth stages,the available content of Cd in 0-17 and 17-29 cm soil layers was decreased by 5.1-63.0%and 2.4-76.7%respectively after biochar amendment.Moreover,in rice maturing stage,biochar addition also reduced the available Cd content in the 0-5,5-10,10-17 cm soil layers.BCR sequential method observed that the percentage of acid soluble Cd fraction was decreased as the biochar application rate increased from 2017 to 2019.However,the percentage of oxidizable and residual Cd fractions were increased.The increased percentage of reducible Cd fractions were just shown in 2018 and 2019.And the result indicated that long-term biochar application could enhance the transformation of acid soluble Cd fraction into reducible,oxidizable,and residual fractions.Path analysis showed that soil pH,available phosphorus content and cation exchange capacity were main factors affecting Cd availability in soil surface layer and soil subsurface layer.This indicated that biochar addition could increase soil pH,available phosphorus content and cation exchange capacity in the soil surface layer and soil subsurface layer,and thus reduce the Cd availability in the corresponding soil layer.(3)In soil surface layer,increasing application rate of biochar improved the formation of soil macro-aggregates(>0.25 mm)but decreased mass percentage in micro-aggregates(<0.25 mm)from 2017 to 2019.With the amount of biochar increased,the organic carbon content in the different size of soil aggregates increased.The Cd distribution was greater in the micro-aggregates than in the macro-aggregates.Biochar addition increased the Cd distribution in the micro-aggregates by 9.4-33.5%,4.8-35.7%,and 8.6-34.0%in 2017,2018,2019,respectively.The Cd distribution of macro-aggregates in 2017,2018,and 2019 was increased by 0.9-3.0,0.5-3.6,and 1.0-3.9 times respectively after biochar amendment,reducing the risk of co-transport of Cd by soil micro-aggregates.(4)During three years,biochar could not only contribute to the increase of rice yield,but also reduce the Cd concentration in rice tissues at different rice growth stages.With biochar application rate increased,the concentration of Cd in rice roots was reduced by 2.5-47.5%,9.0-56.8%,and 6.4%-30.7%at tillering stage,heading stage,and maturing stage respectively,the Cd concentration in the aboveground was decreased by 1.8-52.3%,and 3.2-62.8%during tillering stage and heading stage,respectively.At maturing stage,biochar could reduce the concentration of Cd in shoots and rice by 8.9-48.4%and 16.3-52.1%,respectively.In terms of Cd translocation factors(TF),from 2017 to 2019,there was no significant difference in TFaboveground part/root during the tillering stage and heading stage and TFshoot/root and TF rice/root at maturing stage.And the values of TFaboveground part/root,TFshoot/root,and TF rice/root were lower than 1,these may be related to the metabolism in the body of rice plants.In conclusion,biochar addition can not only improve the soil chemical properties in soil surface and subsurface layer and increase rice yield,but also reduce the availability of Cd in soil surface and subsurface layer,Cd accumulation in rice plant tissues at different growth stages,and enhance the transformation of acid soluble Cd into the relatively stable forms.Meanwhile,soil pH,available phosphorus content and cation exchange capacity are the main factors affecting Cd availability in soil.At soil aggregate scale,biochar addition can increase the percentage of Cd distribution in the macro-aggregates,reducing the risk of co-transport of Cd by soil micro-aggregates.Therefore,biochar addition has a positive effect on improving soil physical-chemical properties,increasing rice yield,and reducing mobilization of Cd in red paddy soil and rice plant tissues.The results will play an important role in studying the mechanisms of biochar-Cd interactions in soils and protecting the ecological environment for Hunan area.