Effects Of Cadmium Pollution And Its Combined Remediation On AM Fungi In Crops And Soil

Soil Cd contamination poses a serious threat to crop safety,soil quality and soil microbial composition,while the use of organic fertilizers and biochar can potentially mitigate the harmful environmental effects of heavy metal elements on farmland soil ecosystems.In this study,we investigated the effects of Cd contamination on AM fungal community composition and diversity in Cd-contaminated soils and soils remediated with biochar and organic fertilizers,and elucidated the changes in soil physicochemical and soil enzymatic properties under the combined remediation.The changes in the microbial biomass of maize inter-rooted soil after biochar and organic fertilizer restoration were also analysed by using PLFAs,which revealed the correlation between soil physicochemical properties,soil microbial biomass,AM fungal community composition and Alpha diversity.The main results are as follows:(1)The dominant genera of AM fungi in farmland soils in both Cd-contaminated and uncontaminated areas were Balcocystis spp.and Balcocystis spp.with the proportions ranging from 37.80%to 57.27%;the relative abundance of Balcocystis spp.AM fungi was highly significantly different between the two treatments(P<0.01).The number of observed species of AM fungal communities and the Chao1 richness index decreased by 21.97%and 22.36%,respectively,and the Shannon and Simpson indices also decreased significantly by 14.80%and 5.26%,with highly significant differences between treatments(P<0.01),compared to the non-Cd-contaminated farmland soils.Non-metric multidimensional scale(NMDS)analysis showed that Cd contamination significantly altered theβ-diversity of AM fungal communities in agricultural soils.Soil effective cadmium(P=0.004)and total phosphorus concentration(P=0.042)were the main factors driving changes in AM fungal community composition under cadmium contamination.Soil total cadmium concentration(P=0.01)was the main factor causing changes in AM fungal community diversity.(2)The above-and below-ground biomass of maize showed a continuous increase under combined biochar and organic fertiliser restoration,while high levels of biochar and organic fertiliser inhibited the accumulation of above-ground biomass during the seedling,tasselling,filling and finishing stages.The variation of cadmium content in the root system showed a dynamic decreasing trend with the advancement of the reproductive period.(3)Soil ammonium and nitrate nitrogen trends were similar under combined biochar and organic fertiliser restoration,showing a dynamic decrease in variation.Soil ammonium nitrogen content was highest in all treatments at the trumpet stage,but there was no significant difference between the treatments.The nitrate-nitrogen content of the soil was the lowest at the finishing stage,while the nitrate-nitrogen content at the filling stage was T2>T4>T1>T3,and there were highly significant differences between treatments(P<0.01).In addition,the levels of ammonium and nitrate nitrogen in the soil at the end of maturity were found to be lower than at the beginning of the experiment.(4)Under the combined remediation of biochar and organic fertilizer,there were differences in the pattern of change of soil enzyme activity with the progress of crop fertility.Soil peroxidase activity was lower in all treatments than in the seedling stage,and there were significant differences between T3 and T4 in the seedling and trumpet stages,but no significant differences between treatments in other periods.Soil urease activity was lower at the male tapping and filling stages during the trial,and was significantly different between T2 and T1 and T4 at seedling emergence;T3 and T4 were significantly different from T1 and T2 at the trumpet stage.Soil alkaline phosphatase activity was highest at the seedling stage and lowest at the finish stage.In addition,there was no significant effect of the different treatments on alkaline phosphatase activity at any other fertility period except for seedling emergence and filling stage.(5)The biomass of soil bacteria,fungi,Gram-positive bacteria and Gram-positive/negative bacteria were all highest under combined biochar and organic fertiliser remediation at the T1 treatment,but the differences between treatments were not significant.High levels of organic fertiliser had an inhibitory effect on the biomass of fungi and Gram-positive bacteria in the soil.In addition,the microbial biomass under each treatment was bacteria>Gram-negative bacteria>Gram-positive bacteria>actinomycetes>fungi>protozoa.Analysis of the biomass of soil AM fungal mycelia(PLFA)and sporangia(NLFA)revealed that high levels of biochar and fertiliser application had an inhibitory effect on AM fungal mycelia.(6)The soil AM fungal flora under combined biochar and organic fertiliser remediation belonged to one phylum,four orders,four classes,12 families and 14 genera.At the genus level,soil AM fungal species richness was dominated by the genus Coccidioides,followed by Coccidioides.In addition,there were no significant differences in soil AM fungal community richness,α-diversity and β-diversity among the groups between treatments.Redundancy analysis showed that total phosphorus was the main factor driving changes in AM fungal community composition.pH had a greater effect on AM fungal community diversity,while total cadmium and urease content were key factors driving changes in AM fungal abundance and diversity.