Effects Of Sludge After Leaching On The Growth Of Vegetable Seeds And Soil Chemical Properties

Municipal sewage sludges(SA、SB)collected from municipal wastewater plants in Wuhan city were studied by the oscillation leaching experiments with combination of saponin and citric acid.The goal of this study was to evaluate their organic matter,nutrient elements(N,P,K),changes of heavy metals(Cd,Cu,Ni,Pb,Zn,Cr),and the effects of sludge.This effect had been evaluated on radish,cabbage,and lettuce seeds at different ratios(5%,10%,20%,and 50%)of germination rate,germination index,root elongation before and after leaching.At the same time,the effects of sludge before and after leaching on the plant growth of cabbage were studied.The nutrients and enzymes activities(urease,phosphatase,catalase,sucrase)on heavy metals and their forms in the soil before and after the planting of cabbage under sludge application conditions were analyzed.The calculation of soil fertility index(IFI)was used to investigate the soil fertility level after application of sludge,and the Igeo was used to evaluate the pollution degree of heavy metals and provide a scientific theoretical basis for the rational agricultural use of sludge.The results showed that:The organic matter,N,P and K have different degrees of decline in sewage after leaching,Among them,the largest change in phosphorus.Compared with the original sludge,the total P in the leaching sludge decreased by 63.0%-68.0%,and the available phosphorus decreased by 80.0%-84.4%.Potassium does not have significant change.The contents of Cd,Cu,Ni,Pb,Zn,and Cr were all decreased after leaching of sludge,and the removal effect of Cu was better,which was more than 30%.The removal effect of Ni was poor,and both sludges were lower than 20%.The removal effects of heavy metals in the two sludges SA and SB are different.The removal of heavy metals in SA is Zn>Cd>Cu>Cr>Ni>Pb;the removal of heavy metals in SB is Cu>Pb>Cd>Cr>Zn> Ni.The germination rate of radish,cabbage,lettuce seeds decreased with the increase of the amount of sludge added.The germination rate of three vegetable seeds was higher than CK when the amount of sludge added was 5%;the germination rate of radish seeds was less than 50% when the amount of sludge added was 50%.The poisoning effect on the seed was reduced after the sludge leaching,the germination rates of the three vegetables were allabove 90% when the amount of leaching sludge added was 50%.Leaching sludge has a good effect on the seed germination index even at a high added amount(50%),the germination index of the radish seed when the leaching sludge amount is50% can reach 29.55,which is an original sludge 3 times.The effect of sludge on the germination index of three vegetable seeds from big to small was: radish> lettuce> cabbage.The effect of sludge on root length was greater than the germination rate and germination index,with the most significant adverse effects on the length of the cabbage.The root elongation inhibition rate of cabbage reached 40% when the amount of sludge added was5%.Compared with original sludge,root elongation inhibition rate of three vegetables increased after adding leaching sludge.The organic matter,alkaline hydrolysis N,available phosphorus,and available K in the soil all increased significantly after added the sludge.Compared with the original sludge,the nutrients in the soil with added leached sludge were lower,but still higher than CK.The nutrient content decreased after the planting of cabbage.The contents of Cd,Cu,Ni,Pb,Zn,and Cr increased in the soil by adding sludge,the content of Cd changed greatly,and the largest increase was 48.3%.The content of heavy metals was mostly within the limits of soil standards in China with added leached sludge.In the 2% sludge treatment,Ni and Cr content in cabbage exceeded the standard for heavy metal content in vegetables in China.However,the contents of heavy metals were not exceeded in cabbage with leached sludge,and the enrichment factor was not exceeded.Compared with the unleached sludge,the fraction changes of heavy metals in the soil with added leached sludge were not significant,and most of them were reduced with the decreasing reducible fraction and increased of residual fraction.The activity of urease,catalase and sucrase in soil increased by adding sludge,but the activity of phosphatase decreased.Sludge had the greatest impact on urease activity,and urease activity in soil with SB-5% was 6.4 times that of CK.Compared with the original sludge,the activity of the four enzymes was reduced in each treated soil to which added leaching sludge.The nutrients in the soil increased after the sludge was added,and the soil fertilityincreased from the poor(IFI 0.32)to the general-fertile level(IFI 1.16-1.66).The content of heavy metals in original sludge was high,among which Cd had the highest accumulation index,which was a very serious pollution level.After leaching,the accumulation index of heavy metals in the sludge decreases.In the treatment with 2% leaching sludge,the accumulation index of six heavy metals was less than 0,which was no pollution level;the Igeo value of Cd and Cu treated with 5% leaching sludge is greater than 0,but the relative risk was low.