| As an important component of global ecosystem function,soil ecosystems play an essential role in elemental cycling.Soil ecosystem stability is defined as the inherent ability of a soil ecosystem to resist change and to recover after disturbance.Soil ecosystem stability can be measured using resistance and resilience indexes.Soil resistance and resilience are not only important indicators of soil health and soil quality,but also an important criterion for assessing the vulnerability of soil ecosystem.Understanding these key factors can be helpful to explain the mechanisms underlying soil resistance and resilience,and even more important to predict and to improve them in relation to environmental disturbance and soil degradation.There has been a long debate on the effects of biodiversity on the functional stability of an ecosystem,or named diversity-stability relationship.The existing studies demonstrated conflicting results,including positive relationship and no relationship.In addition to soil microbial diversity,soil physicochemical properties such as soil pH,clay and organic matters can largely affect the pollutants toxicity and the activity of microorganisms,therefore may influence soil resistance and resilience.Nevertheless,the effects of soil physicochemical properties other than organic matter content on soil resistance and resilience were seldom studied.In conclusion,the key factors that drive soil resistance and resilience were far from being clear.We therefore collect a total of 24 soil samples across China.These soils were different in microbial diversity and soil physicochemical properties.The resistance and resilience of glucose induced respiration rate as a microbial function was determined following copper(Cu2+)perturbation.We aimed to find out the key factors which drive soil resistance and resilience among soil physicochemical properties along with soil microbial community diversity and abundance,and to set up models which quantify the relationship between the key factors and soil resistance and resilience.This study clearly showed that soil resistance and resilience to Cu2+ perturbation was determined by soil pH and sand proportion,respectively.The exponential model with Eq.Rt = 1.0673-1.1335e-0.2224pH is optimal for the relationship between soil pH and soil resistance,while the polynomial model with Eq.Rl=-3.1493S2+1.81145+0.7116 is optimal for the relationship between soil sand proportion and soil resilience.Soil microbial community diversity,abundance and substrate induced respiration rate has no significant effect on the soil resistance and resilience.Nevertheless,the factors that drive soil resistance and resilience vary with perturbation types,and our results might apply to heavy metal perturbations.Soil microbial functional resilience refers to the ability of a function to recover from disturbance.Due to the lack of reliable methods of continuous online monitoring for soil microbial functions,the dynamic variation of functions is hardly detected.Microbial fuel cells(MFCs)are devices in which bacteria create electrical power by oxidizing organic matter.It has been reported that the current or voltage can be a signal that monitor the activity of electrogenic bacteria and the toxicity of pollutants.In this project,the MFCs are used for soil to generate current to monitor soil microbial functional resilience.Results show that increases in soil Cu2+ concentrations reduced voltage and postponed start-up.The quantity of generated electrons decreased and ST increased significantly with increasing Cu2+concentrations.The results are in agreement with SIR.We propose that electrical signals may be used to evaluate the toxic effect of Cu2+ on soil microorganisms.Free Cu2+ activity decreased with the recovery of soil microbial function.And copA/16S rRNA gene copies ratio of the contaminated soil are higher than the control without perturbation.These can explain soil microbial functional resilience.In addition,electrogenic bacterium were isolated from the anode biofilm of microbial fuel cells(MFCs)inoculated with soil perturbed with 100mg kg-1 Cu.Identify the strain based on the morphology,physiology and biochemistry,and 16S rRNA gene sequencing analysis.Inoculate the strain in the dual chamber microbial fuel cells with LB medium and potassium ferricyanide as anolyte and catholyte respectively,for the characterization of its electrogenic ability.The mechanism of the extracellular electron transfer through cyclic voltammetry.Observe the biofilm on the anode surface using scanning electron microscope.This study isolated two eletrogenic bacterium named as R3 and R6.Phylogenetic analysis based on 16S rRNA gene sequence indicated that strain R3 belonged to Clostridium.While the sequence of the 16S rRNA gene of the strain R6 was 100%phylogenetically related to Clostridium sporogenes.Their morphology and physiological and biochemical characteristics were also identical.The optimum pH level of R3 and R6 was 7.The optimum temperature of R3 and R6 was 30℃,while the optimum NaCl concentration range of two strains was 0.3%to 1.5%.The maximum power density of the MFCs inoculated with R3 and R6 were 96.3mW m’2 and 97.6 mW m-2,respectively.CVs indicated the existence of the electrochemical active substance by direct extracellular electron transfer for both two strains.Strain R3 and R6 can utilize a wide range of substrates as carbon source for electricity generation in MFCs,including glycerol,citrate,glucose and acetate,but with different power generation abilities.The LB medium produced the highest maximum current density of 116.67 mA m-2 for R3 and 100 mA m-2 for R6,respectively.The minimum inhibitory concentration for Cu2+ of two strains was 25mg kg-1.And the voltage decreased below 50mV when the Cu2+ concentrations exceeds 25mg kg-1. |
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