| Animal manure,a high-quality organic fertilizer,has been traditionally recommendedto be land applied for a long time.For the intensive productionof livestockand poultry,some feed additives,such as trace elements and veterinary antibiotics,are used to proventdiseases and promote growth.Due to the low utilization efficiencies of trace elements and antibiotics for food animals,most of them are excreted via urine and faeces,leading toaccumulation of heavy metals and antibiotics in the manures.Field application of animal manure might inadvertently result in potential risk of environmental health and food safety.Litchi(Litchi chinensis Sonn.)is an important tropical and subtropical fruit in C hina.Meanwhile,animal manureis widely used in litchi production.In the present study,a typicallitchi plantation soil from Guangdong,C hina and pig manure containing metals and tetracycline antibiotics(chlortetracycline and doxycycline)from a intensive pig farm were used to conduct a soil incubation experiment for 360 d.Theincubationtrial included four treatments,namely four uselevels of pig manure(0,1%,2% and4%)(w/w)with four repetitions,with the objectives to investigate the effect of pig manure on soil nutrient content,enzyme activity anddegradation of the both antibiotics.Meanwhile,a litchi culture experiment was carried out using the same materials and treatment as the soil incubation.The litchi seedlings were harvested at 153 and 210d after transplating respectively,with the aims to examine the uptake of metals and antibiotics bylitchi plants at different growth times and the accumulation of both metals and antibiotics in soils.In addition,a field experiment was conducted in a typical litchi plantation at Zengcheng,Guangdong as well,with the targets to study the growth of litchi,fruit quality and accumulation of metals and antibiotics inthe fruits as affected by various rates of pig manure(0,20,40,80 kg/plant).The results are shown as below:(1)The contents of soil alkali-hydrolyzable nitrogen,available phosphorus and available potassium were significantly enhancedby application of pig manure.A high level of soil nitrogen supply was observed while pig manure was applied at the levels of 1% and 2% of pig manurewithin 100 d.However,soil nitrogen supplyin the4% treatment waseven less than the 1%treatment during 40-150 d due tothe competitive nitrogen capture by soil microbes.From 150 to 360 d,the contents of alkali-hydrolyzable nitrogenin the three manure application treatments gradually decreased and became close,but werestill significantly higher than those of the control(P<0.05).The contents of available phosphorus and available potassium significantly increased with the increasing pig manurerate,and theirvariation in the same treatment was relatively small.The concentrations of available Ca and Mgwere significantly increased by increased manure rate(P<0.05).Soil available Fe,Mn,Cu,Zn was significantly increased by application of pig manure(P<0.05),and the available contents of Mn,C u and Zn significantly increased with the increas ing pig manure rate,with the exception of soil soil availableFe in the 4% treatment,which was similar to those in the 2% treatment.The contents of soil available Fe,Mn,Cu and Zn reached to abundant while pig manure was used at the level of 1%.(2)After incubation for 360 d,soil chlortetracycline and doxycycline were degraded by 88.3±0.6%~95.6±0.9% and76.6±5.6%~88.9%±1.8%,respectively.The average half-lives of the two antibiotics were 26.84 and 58.89 d,respectively.Although he half-lives of antibiotics in the 4% treatment were significantly shorter than those in the 1% treatment,,the residualsof antibioticsin the former were still highertat the end of the incubation.Soil chlortetracycline and doxycycline remained at the ranges of 0.1±0.0~0.3±0.0 mg/kgand 0.2±0.1~0.5±0.1mg/kg in the three manure application treatments,respectively.(3)The activity of soil catalaseincreased with theincreasing rate of pig manurewithin 100 d.From 100 to 360 d,thecatalase activitiesin the 1% and 2%treatments were similar,however,catalase activity in with the 4% treatment was significantly higher than those of the two low-rate treatments.For urease activity,the 1% and 2% treatments were significantly higher than the controlandno obvious differencewere observed between them.The urease activity in the 4% treatment was significantly higher than those in thetwo low-ratetreatments.Application of pig manure increased the activity of soil acid phosphatase,but the effect was not closely related to the application rate of pig manure.The correlation analysis showed that,the activities of the three enzymes were positively correlated to the concentrations of soil chlortetracycline and doxycycline while pig manure was used at the rates of both 1% and 2%(P <0.05).Soil urease activity was not related to soilchlortetracycline and doxycycline,andsoil acid phosphatase activity was not influenced by soil doxycycline but still positively correlated withsoil chlortetracycline while pig maure was used at the rate of 4%.(4)Application of pig manure inhibitedthe growth of litchi seedlings in the initial stage,however,significantly promotedtheir biomass as time went on.Generally,the contents ofFe,Mn,C u and Zn in the overground partsand the roots of litchi plants were increased by increased manure rate.Litchi plants can absorb chlortetracycline and doxycycline from pig manure.With the growth time prolonged,plant chlortetracycline was significantly promoted,but not related to the use level of pig manure.And,plant doxycyclinesignificantly increased with the increasingmanure level,but diluted by litchi biomass.C hlortetracycline was strongly translocated upward withthe transfer factersof 0.595~0.789 in litchi plants,however,doxycycline was not easily transported with very low transfer factorof 0.01.(5)Concentrationsof total and available Mn,C u and Zn significantly increased with the increasing manure level,with the exceptionoftotal Fe.Totally,the residuals of bothantibiotics in soilswereenhanced by increased manure rate as well.(6)Litchi field experiment showed that application of pig manure reduced the transpiration rate and stomatal conductance of litchi leaves at fruit swellingstage,but did not considerabley affect the intercellular carbon dioxide concentration.Although there was no significant difference for the net photosynthetic rateamong treatments,manure application did show a trend to reducethe net photosynthetic rate of litchileaves.The soluble solids,the soluble sugar,the titratable acid,the total free amino acids and the flavor amino acids of litchi fruits were not considerably affected by manure application,however,both the pulp pH and the ratio of soluble sugar overtitratable acid showed a tendency to decrease with the increasing manurerate.Although pig manure contained high levels of four metals and bothantibiotics,manure application had no effect on the concentrations of four metals in the edible litchi fruits.Moreover,both antibiotics could not be detected in the fruitsin all treatments.Conclusively,pig manure is a valablesource of crop nutrients(N,P,K,Ca,Mg,Fe,Mn,C u,Zn),however,soil C u and Zn contamination might occur if manure was land applied heavily for long term due to its high contents of Cu and Zn.The half-lives of tetracycline antibiotics from pig manure were short in the soil,but theywould accumulate in the soilas well under long-term manure application.Since litchi plants are prone to take in Zn(with the enrichment factor of 0.54~0.78)and tetracycline antibiotics,the potential contamination risks of metals and antibiotics(especially chlortetracycline)for litchi fruitneed to be objectivelyevaluated by long-term field experiment even litchi fruit quality was not affected by manure application for one year in the present work. |
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