Changes Of Heavy Metals During The Vermicomposting Of Sewage Sludge And Its Effect On Soil Nitrification

Vermicomposting has been considered as an effective technology for the management of municipal sewage sludge where heavy metals are the main limiting factors.Organic fertilizers containing high concent of heavy metals could have a negative impact on soil microbes,which may pose a serious threat to ecosystem stability.It is well known that nitrification is the core of the nitrogen cycle,among which the ammonia oxidation stage is the rate limiting step.Previous investigations showed that the response of soil nitrification activity and ammonia oxidation microbes to exogenous heavy metals could be studied by molecular biology technique,which can help us understand the microbial effects polluted by heavy metal pollution,and provide the basis for selecting the microbes which can evaluate the soil quality.Therefore,the objective of this paper was to study the changes of heavy metals during vermicomposting and explore the effect of exogenous heavy metals on soil microbial activities.The main results of this study were as follows:(1)The pH value,DOC,organic matter,water content,ammonia nitrogen,nitrite nitrogen,total nitrogen and total phosphorus of sewage sludge were significantly reduced after vermicomposting,while the nitrate nitrogen increased.The activities of urease,protease,invertase,asparaginase and glutaminase decreased in varying degrees while there was no significant change in the activity of alkaline phosphatase before and after vermicomposting.(2)The decrement of metals after vermicomposting(compared to initial sludge)was in the order:Pb(66.05%)>Cr(28.18%)>Zn(24.90%)>Ni(19.90%)>Cu(10.77%),The content of Fe and Mn sharply increased while Cd had no obvious change.It was indicated that Cu,Pb and Fe exists in the form of state with organic combination,while Ni and Mn in reduction state.Experimental data proved that there was no significant influence on the reduction fraction through ermicomposting.However,the bioavailable content of Cu,Zn,Ni,Pb,Cd and Cr were decreased.These results indicated that vermicomposting played a positive role in stabilizing heavy metals in the treatment of sewage sludge.(3)Soil PNR(potential nitrification rate)decreased rapidly with the increase of concentration of Cu and Zn under a low concentration,while slowly under a high metals concentration.There was a very significantly negative correlations between the content of Cu and soil PNR(P<0.01),while a significantly positive correlation(P<0.05)between PNR and the content of Zn.The response of PNR to Zn was earlier to change than that of Cu.PNR was more suitable for Zn than Cu which has been widely used as an indicator in assessing heavy metals toxicity.(4)Soil bacterial 16SrRNA copies was stimulated slightly by Cu at lower concentrations(Cu<600 mg·kg-1),while inhibited at a higher concentration of Cu(Cu>900 mg·kg-1)and inhibited slightly by Zn.AOA and AOB were influenced by exogenous heavy metals among which AOA was the dominant ammonia oxidizing microorganism in the soil.Cu and Zn at a low concentration were significant inhibitors to amoA gene copies of AOA while no significant change was observed at higher concentration of metals(Cu of 300-2000 mg·kg-1;Zn of 150-1500 mg·kg-1).AOA was more sensitive to Zn than Cu.Heavy metals of the low concentration(Cu of 600?900 mg·kg-1;Zn of 150-900 mg·kg-1)had a positive effect on AOB amoA while higher concentrations of Cu or Zn(Cu>1200 mg·kg-1;Zn>1200 mg·kg-1)had significant inhibitory effect.No significant correlation could be found between Cu and Zn content and the abundance of amoA.(5)Results indicated that heavy metals showed impact on the soil enzymes.There was a very significant negative correlation between soil invertase activity and Cu content(P<0.01),and soil protease activity and Zn content showed the same correlation(P<0.01),and the correlation between soil urease activity and Cu and Zn content was negative(P<0.05).Results showed that influence of single Cu and Pb on single enzyme activity were different,the responses of single enzyme to heavy metals were also various.