Effect Of Different Soil Amendments On Cadmium Bioavailability In Tobacco-growing Soils In Chongqing

Cadmium(Cd)is an extremely toxic heavy metal and is highly hazardous to environment even with small amounts.Cd could easily move from soil to plants through the root absorption and accumulate in plant tissues,resulting in the serious impact on physiological and biochemical processes of plants.Moreover,it could also severely affect human health since it could enter and accumulate in human body through food chain for a long term.The Cd content in plants is closely related to its content and existing forms in soil,and hence understanding the Cd content and existing forms in agricultural soil is the central premise for the remediation of Cd-polluted soil.Here we focused on typical tobacco planting zones in Chongqing and investigated their pollution situations in soil with heavy metal,including the Cd.In view of Cd-migration under acid condition,we did the correlation analysis on the soil acidification and Cd-availability,and aim to provide a basis for controlling the Cd pollution in tobacco planting soils in Chongqing.And on this basis we created series of field trials and pot experiments to reveal the effect of different soil amendments on soil pH and available contents of Cd.We also evaluated whether it could reduce the Cd accumulation in different parts of tobacco plants and its impact on yield and quality of tobacco.(1)Contents of heavy metals in typical tobacco planting zones in ChongqingWe collected total 1056 soil samples from typical tobacco planting zones in Chongqing(including the Wulong,Pengshui,Qianjiang,Fengdu and Shizhu)and investigated their contents of heavy metals including the Arsenic(As),Cd,Cupper(Cu),Mercury(hg),Lead(Pb)and Zinc(Zn).The average contents of As,Cd,Cu,Hg,Pb and Zn in Chongqing tobacco zones are 18.39?0.22?25.6?0.13?36.94?53.42 mg/kg respectively.Based on the Nemerow comprehensive pollution index,contents of Hg,Pb and Zn in Chongqing tobacco zones are in a secure level at present,and contents of As and Cu are under potential pollution risk.The content of Cd is in a slight pollution status and the Cd pollution distributes mainly in west Wulong and Pengshui.The soil acidification is common in tobacco planting zones in Chongqing and the pH values of about 49.7% of tobacco planting fields in Chongqing are under 5.5.It increases significantly about 72% than that in 2002.The available Cd in soil increased with the decrease of pH,and the soil acidification should promote the Cd activation.(2)Effect of different soil amendments on available Cd and pH adjustment with pot experimentsWe selected four kinds of soil amendments(including the lime,Si-Ca fertilizer,alkaline organic conditioner(active shell powder)and biochar)to adjust the soil pH with different dosage(0.05%,0.1%,0.2% and 0.1%)and evaluated its effect on available Cd and p H adjustment with pot experiments.With the soil amendments,contents of available Cd were significantly decreased in all treatments.Among these four kinds of soil amendments,active shell powder exhibited best ability to decrease the contents of available Cd,while the biochar exhibited the worst ability.The biochar with 0.4% dosage only reduced 40.66% of available Cd in soil,while shell powder,lime and Si-Ca fertilizer with 0.1% dosage could reduce the available Cd in soil at 55.17%,50.57% and 40.23% respectively.The available Cd in soil decreased extremely significantly with the pH increasing.These soil amendments could improve the soil p H and hence decreased the available Cd contents in soil.(3)Effect of different soil amendments on Cd contents in tobacco and tobacco yield and quality with field trialsWe investigated the Cd contents in different tobacco parts and evaluated the effect of these soil amendments on available Cd contents in soil.The Cd could easily accumulate in tobacco plants and the Cd contents in different tobacco parts decreased in proper order according to lower leaves,middle leaves,upper leaves and stem.The Cd contents in lower leaves,middle leaves,upper leaves and stem at maturation stage were 4.57,2.51,1.27,0.61 mg/Kg respectively.The Cd contents in tobacco leaves were obviously higher than that in soli,suggesting that the tobacco plants have the strong enrichment ability on Cd.With the soil amendments,contents of available Cd were significantly decreased in all treatments and the Cd contents in upper leaves and lower leaves were decreased with 8.55%-16.07% and 4.47-9.28% respectively.Different soil amendments exhibited different decrease ability on Cd content.The lime exhibited best ability to decrease the contents of available Cd at spherical plant stage,while active shell powder exhibited best ability at vigorous growth stage,and Si-Ca fertilizer exhibited best ability at maturation stage.The available Cd in soil decreased extremely significantly with the pH increasing.These soil amendments could improve the soil pH and hence decreased the available Cd contents in soil.Treated with soil amendments,no obvious difference was observed on yield of leaves numbers and it could improve the leaves area.Moreover,soil amendments could improve the chemical quality of tobacco leaves and improve the percentage of superior product of tobacco leaves.Among these soil amendments,the Si-Ca fertilizer exhibited best ability to improve the quality of tobacco leaves.All taken together,our results demonstrated that application of these soil amendments could significantly improve the soil pH and hence it could decrease the available Cd contents in tobacco leaves and tobacco planting soils.Among them,Si-Ca fertilizer(2250 kg/hm2)exhibited best potential for remediation Cd polluted soil and improved the quality of tobacco leaves.