Physicochemical And And Biological Characterization And Diagnostic Index Of Cadmium Pollution In Vegetable Rotation Ecosystems

Three representative soils(Red yellowish soil,silt loamy soil and purplish clayey soil), which are widely distributed in Yangtze River Delta of China,were collected from Deqing County,Xiasha District of Hangzhou City,and Jiaxin county,Zhejiang province.Laboratory experiments were conducted to investigate the chemical and biological characteristics of cadmium(Cd)in soils and microbial response to external Cd loading.Greenhouse experiments with three vegetable crops in rotation were conducted using the two soils,i.e.red yellowish soil(RYS)and silt loamy soil(SLS)to study Cd accumulation in pakchoi(Brassica chinensis L.),tomato(Lycopersicon esculentum)and radish(Raphanus sativus L.).Root morphology and physiological characteristics of pakchoi were also studied using both soil and hydroponic culture.The effects of increasing Cd loadings on microbial biomass,microbial activity,and microbial community structure were examined and the influences of rotation system and root activity on microbe-Cd interactions were evaluated.Based on these experiments,critical Cd concentrations were established for RYS and SLS by using safety food health standards and microbiological indices.The safety food health standards of Cd criteria are compatible with those obtained from soil microbial response and appear more reliable for assessing Cd pollution in soil.The major findings from this study are summarized as follows:1.Adsorption of Cd²⁺(added as Cd(NO₃)₂)in the three soils was well described by a simple Langmuir equation or a Freundlich model.Behavior of Cd²⁺adsorption-desorption was related to soil properties and the adsorption capacity of Cd²⁺varied among the three soils in the order:PCS＞SLS＞RYS.The maximum adsorption values(X_m)of Cd²⁺obtained from the simple Langmuir model were 6098,4065 and 3534 mg kg^-1,respectively,for PCS,SLS and RYS.The adsorption of Cd²⁺caused a significant decrease in equilibrium solution pH.The extent of pH decrease depended on soil types and the amount of adsorbed Cd²⁺.Most of the adsorbed Cd²⁺couldn't be released from soils.The behavior of Cd²⁺desorption differed considerably among soil types.After three successive extractions with NaNO₃,only 0 to 15% of the total adsorbed Cd²⁺in the RYS soil was desorbed and the corresponding values were 0 to 9.1%and 0-6.8%,respectively for the SLS and PCS soil,indicating that PCS has a higher affinity for Cd²⁺than RYS or SLS. 2.The accumulation of Cd in vegetables and the change of extractable Cd in soils after vegetable planted are affected by soil type,vegetable species,and external Cd loadings.For pakchoi and tomato,Cd was mainly accumulated in roots,with a portion of absorbed Cd being transported to the aerial parts and only a small proportion of Cd being accumulated in fruit.Cadmium concentrations in both shoots and roots(pakchoi and tomato)increased with increasing Cd loading levels,but root Cd concentration increased faster than shoot Cd. However,contrasting results were obtained with radish.At the same Cd loading rate,Cd concentrations were higher in shoots than roots.Large differences in tissue Cd concentrations were also observed among the three species.Tomato contained higher Cd in roots and shoots than pakchoi and radish,but Cd concentrations in tomato fruits were low.Analysis of variance revealed that soil type significantly affected Cd concentrations in vegetable tissues. Cadmium concentrations in vegetables grown in RYS were significantly greater than those in SLS at the same Cd loading rate.The results also indicated that ammonium acetate-and water-extractable Cd increased with total soil Cd after regardless of vegetable species planted. Ammonium acetate extracted a larger amount of soil Cd than water.Extractable Cd in soil, especially by water,was infuenced by vegetable species planted.Plant availability of Cd was generally higher in RYS than in SLS.3.Shoot growth of vegetables was not inhibited or even stimulated by light Cd pollution (0-7.00 mg kg^-1).Increased Cd²⁺concentration up to 0.25 mg L^-1inhibited the growth of pakchoi in hydroponic culture.The root growth of pakchoi was promoted by Cd addition at the concentrations＜3.50 mg kg^-1in soil culture and 0.05 mg L^-1in hydroponic culture.High Cd concentration(＞0.05 mg L^-1)resulted in the inhibition of root development,as evidenced by reduced root total length,root average diameter,and root surface area.The chlorophyll content of pakchoi increased by low Cd addition,but was reduced by increasing Cd concentration up to 0.25 mg L^-1.Photosynthesis was inhibited at high Cd concentrations (＞3.50 mg kg^-1in soil or＞0.25 mg L^-1in culture solution).The activity of peroxidase(POD) increased initially,but decreased with increasing concentration of Cd from 0.50 to 8.00 mg kg^-1in soil culture or from 0.05 to 0.50 mg L^-1in hydroponic culture.The content of malondialdehyde(MDA)increased when Cd concentration was 0.70 mg kg^-1in soil or 0.05 mg L^-1in culture solution,whereas the content of dissociative proline(Pro)fluctuated with increasing Cd in soil or culture solution. 4.The response of microbial biomass to Cd input is related to external Cd loading rate and soil types.RYS and PCS had higher microbial biomass carbon(MBC)than SLS,and the responses of MBC to Cd were different among soil types.Soil microbial biomass in RYS and SLS was enhanced by 4-8 mg Cd kg^-1soil,and the stimulating effect was less and less with increasing Cd rate up to16 mg kg^-1.The MBC in SLS was stimulated by lower Cd loading rate than RYS,as SLS had lower organic matter and clay content,which resulted in higher Cd bioavailability in the SLS.Soil respiration was enhanced at low Cd loading(＜8 mg kg^-1),but was restrained at higher Cd levels,but still larger than the control,especially in the medium-term of incubation.Although the response of microbial metabolic quotient(MMQ) to Cd was different among soil types or incubation periods,it was generally greater at low Cd loading(4-16 mg kg^-1)for different incubated time in the three soils.Low concentrations of Cd(0-16 mg kg^-1)slightly increased microbial quotient,whereas higher concentrations(＞16 mg kg^-1)of Cd²⁺had inhibitory effects.Urease activities in RYS and SLS were promoted when Cd loading was less than 16 mg kg^-1after being incubated for 2 or 4 weeks,while considerable restricting effect was observed when Cd loading was higher than 32 mg kg^-1 with prolonged incubation time.5.In the rotation system of three vegetables,soil microbial biomass and MQ were enhanced at low Cd levels(＜1.00 mg kg^-1),but was inhibited consistently with increasing Cd rate though the MQ value was still higher than that in control at the Cd rate of 7.00 mg kg^-1. Soil respiration and microbial metabolism quotient(MMQ)were restrained at low Cd loading (＜1 mg kg^-1),and enhanced at higher Cd levels.Microbial activities in three soils were affected by pakchoi-tomato-radish rotation system.The values of basal respiration rate and MMQ in tomato-growing soil were higher than those in pakchoi- or radish-growing soils. Microbial biomass and MQ in tomato-growing soils were lower than those in pakchoi- or radish-growing soil.Root activity of vegetables had a positive effect on soil microbes,the values of basal respiration,microbial biomass carbon,MMQ and MQ in rhizospheric soil were all greater than those in bulk soil.The rhizosphere effect varied between RYS and SLS, and among pakchoi,tomato and radish.Basal respiration rate and microbial biomass were greater in RYS than those in SLS,while SLS had higher values of MMQ and MQ.6.External Cd loading and vegetable species affected microbial community structure in pakchoi-tomato-radish rotation system.Microbial ability of metabolizing different carbon substrates was decreased with increasing Cd loading rate after pakchoi planted,while increased after radish harvest.With same Cd treatment,the ability of microbes in metabolizing carbon substrate in rhizospheric soil was greater than that in bulk soil.Microbial community structure was apt to be more stable after rotation.The responses of microbial community richness and Shannon index to Cd stress were different among soil types or vegetables,and were not suggested as available index to assess Cd pollution in RYS and SLS. The phospholipids fatty acid(PLFA)in soil microbe was sensitive to increased Cd concentrations.The relative content of general PLFAs in fungi,Gram-positive bacteria or arbuscular mycorrhizal fungi(AMF)was all positively correlated with available Cd in soil, while that in bacteria,actinomycetes or Gram-negative was all negatively correlated with available Cd in soil.Cadmium and root activity increased the amount of AMF.Both microbial functional diversity and PLFAs profiles differed between RYS and SLS soil,and between the rhizosphere and nonrhizosphere environment.Microbial community in rhizospheric soil suffered less Cd stress,and was more stable.7.Evaluation indices system for soil Cd pollution was established based on soil-plant system and soil-microbe system.The values of Cd thresholds based on soil-microbe system were much higher than those accoding to soil-plant system,and thus the microbial indices didn't fit to assess Cd pollution in soil.Cadmium thresholds for potential dietary toxicity should be considered in establishing evaluation indices system for soil Cd pollution.The species of vegetables,soil types and Cd availability should be all taken into consideration for establishing Cd thresholds in soil.Ammonium acetate extractable Cd was suggested to assess Cd thresholds for potential dietary toxicity in pakchoi,tomato and radish.The corresponding values were 0.066 and 0.166 mg kg^-1,0.089 and 0.092 mg kg^-1,0.051 and 0.045 mg kg^-1in RYS and SLS,respectively in pakchoi,tomato and radish-growing soil.