| Nowadays,the ecological threat caused by the rice arsenic(As)accumulation in arsenic-contaminated soil has become one of the important concerns of food security in the world.Therefore,green and economically feasible remediation measures are urgently needed to alleviate the negative impacts of As on its health risk to humans.Taking the As-contaminated soil as the research target,the pot experiments were conducted to determine the effects of rice husk biochar(DBC),rice straw biochar(SBC),wheat straw biochar(XBC),corn straw biochar(YBC),cotton straw biochar(MBC),bamboo biochar(ZBC),smooth cordgrass(Spartina alterniflora)biochar(HBC)and silicate fertilizer(Si)on As accumulation in rice and As mobility in soil.In our study,we monitored As concentrations in different rice tissues(including brown rice,rice straw and roots),porewater and extractable As fractions in soil during the rice growth period.Furthermore,the data selected from pot experiments was used to evaluate the actual effects of biochar,silicate fertilizer and the coapplication of the two on rice As accumulation.Associated with pot experiments,the controlled microcosm experiments were conducted to quantify the impacts of different doses of biochars(0.5%~5%,w/w)on As mobility in soil and explore the mechanisms under anaerobic conditions.The study was aimed to provide scientific basis and important guidance for the remediation of rice As contamination.Through this study,the main conclusions were as follows:(1)In pot experiment 1(i.e.,rice cultivation in different kinds of biochar amended soils),the As concentrations of rice grain(brown rice),straw and roots were ranged from 0.23~0.30mg/kg,7.1~13.1mg/kg and 75.5~160.6mg/kg,respectively.Under the application of low doses of biochar(0.5%,w/w),As levels in brown rice were decreased by 6.9%~20% compared with the control treatment(0.3mg/kg).In addition,the effects of seven kinds of biochar amendments on decreasing the accumulation of As in rice are quite different: the rice straw biochar(SBC)revealed the best performance in reducing brown rice As levels.The result showed that the effect in decreasing As accumulation from high to low as follows:SBC> YBC> HBC>XBC=MBC>DBC>ZBC;It indicated that the application of low dose of biochar could reduce grain As levels but the decreasing effects were influenced by biochar characteristics.(2)Although biochar at low rate significantly decreased grain(brown rice)As concentrations,the overall As uptake by brown rice and straw was slightly decreased by 1.1%~4.1%.It has also been found that grain As levels in amended treatments were still above the safety possibly posing a threat to human health.Our results showed that the low application rate of biochar(i.e.,0.5%,w/w)had limited effects on decreasing As accumulation in rice plant,which were primarily due to the small changes of As mobility/phytoavailability in amended-soil.During the rice cultivation(in rice tillering,jointing,heading and harvesting stage),porewater As levels,phosphate/oxalate extractable As fractions(specially absorbed As and Fe/Mn bound As fractions)in soil in the all amended treatment groups generally showed no significant differences relative to the control treatment.(3)The microcosm experiments(i.e.,soil incubation with different doses of biochar under anoxic conditions)illustrated that soil solution As levels in biochar treatment groups both increased substantially with the increasing doses of biochar,particularly at relatively higher doses(1%–5% w/w).Among the 7 kinds of biochar,DBC performed best.As levels increased by ~10.8 times following 3%(w/w)rice husk biochar.For As in soil solution,the sequence is:DBC> HBC>YBC>SBC> XBC>ZBC>MBC.In our study,5%(w/w)doses of DBC and HBC increased the number of Fe reducing bacteria in soil by 1.6~3.2 folds by MPN method,further demonstrating that the underlying mechanism of As release with biochar amendment may be mainly due to biochar stimulated microbial-mediated dissolution of Fe(oxyhydr)oxides.(4)In pot experiments 2(i.e.,rice cultivation in silicate fertilizer amended soils),the concentrations of As in grain(brown rice),straw and roots decreased by 7.1%~14.5%,9.8%~20.3% and 5.7%~23.2%,respectively,in amended treatments compared to the control treatment.However,As accumulation was slightly decreased following silicate amendments.Consequently,biochar and silicate fertilizer in our study could have a slight effect on inhibiting As accumulation in rice.The results might be attributed to the inadequate amounts of plant-available Si provided by biochar and silicate fertilizer in our pot experiments.For example,biochar and silicate fertilizer didn't lead to the significantly higher porewater Si and sodium acetate-extractable As in soil.(5)The coapplication of silicate fertilizer and biochar(DBC and HBC)could decrease As accumulation in rice by 7.9%~8.3% relative to the reduction achieved with single application of silicate fertilizer.It indicated that the coapplication of silicate fertilizer and biochar had great application peospect and thus its mechanism and field experiments are worthy of further research. |
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