| It is a common practice for heavy metals(such as Zn and Cu)and antibiotics to be added in the diet or therapeutically used on farm animals bred intensively,which would prevent and cure animal diseases,promote optimum nutrient supply and animal growth.Heavy metals and antibiotics cannot be completely absorbed by animals and were found in the manure of a high residual concentration,and,hence,in the composts obtained from that manure.The application of these contaminated composts in agricultural soil could lead to a succession of environmental problems,such as destroy the micro-ecology balance,inhibit plant growth,accumulate in food chain and bring about antibiotic resistance to indigenous microorganisms in environment,which brought potential risks to human health.In this study,we chose extensive animal manures and composts and exploring the heavy metals,antibiotics and antibiotic genes in collected samples.1)An extensive survey(212 samples)of the China market for commercial composts was carried out and acquired comprehensive information on the heavy metals distribution and arsenic species content of the local situation.Results revealed wide variations of investigated heavy metals,which is the major technical problem with agriculture application of composts.Concentration ranges of zinc(Zn),copper(Cu),chromium(Cr),nickel(Ni),lead(Pb),arsenic(As),cobalt(Co),cadmium(Cd)and mercury(Hg)were 11.8-3692,3.55-916,0.69-6603,0.68-72.7,0.05-188.5,0.37-71.7,0.11-93.9,0.012-8.72 and 0.014-1.93 mg kg-1,respectively.13.7%of the composts examined surpassed the As limits referred to Chinese standard for organic fertilizer.Further analysis on arsenic species showed that arsenate(As?,counted for 36-100%total As)was the dominant arsenic species in composts while in manures the most concentrated arsenic species were dimethyl arsenic(DMA,counted for 47-63%total As)and arsenite(As?,counted for 24-33%total As).Zn,Cu and As presented a significantly positive correlation,indicating they have the same origin(e.g.,animal feed addition).Monte Carlo simulation showed that Zn,Cu,Cd and Hg were the most cautionary elements of which the content could be doubled in 200-300 years with continuous application of composts in agricultural soil.2)We used appropriate extraction(Solid Phase Extraction.SPE)and detection(High Performance Liquid Chromatography-two stage Mass spectrometry,HPLC-MS/MS)method of antibiotic,i.e.tetracyclines,quinolones and sulfanilamides in manures and composts,which was highly repeatable and the recoveries of target compounds approached 100%.The selected antibiotics included the most usually applied antibiotics in intensive cultivation.The most commonly detected antibiotics in poultry,swine and cattle manures were tetracyclines and quinolones.In most of the samples,concentrations of the selected antibiotics were in ?g kg-1 level,however,in certain poultry manure sample,the content of antibiotic could reach mg kg-1(5550 ?g kg-1 norfloxacin and 1235?g kg-1 ofloxacin),moreover,2817?g kg-1 oxytetracycline was detected in one cattle manure.In the compost sample which collected during the whole composting period and based poultry,swine and cattle manures,lots of antibiotics were found,such as 99.5 ?g kg-1 oxytetracycline,3633 ?g kg-1 norfloxacin and 376 ?g kg-1 sulfaclozine.There was antibiotic residueed in compost product,such as 3633 ?g kg-1 norfloxacin.antibiotics in animal manures decreased,e.g.the general removal ratios were 55%-78%for poultry manure,40%-55%for swine manure and 10%-100%for cattle manure.Due to the heterogeneity of raw materials,some exceptional antibiotics did not conform the common tendency.3)We assessed type and concentrations of 295 antibiotic resistance genes(ARGs)in three stages of manure(poultry,swine,cattle)composting period by high-throughout quantitative PCR.There were abundant and diverse ARGs found in selected samples,115-146 ARGs in animal manures,77-114 ARGs in high-temperature period compost samples and 69-103 ARGs in compost products.The top 4 ARGs type presented resistance to aminoglycoside(7.6 × 108-3.5 × 1010 copies g-1),MGE(mobile genenitic element,3.8 × 108-3.0 × 1010 copies g-1),tetracycline(2.4 × 108-2.1 × 1010 copies g-1)and multidrug(2.1 × 109-1.9×1010copies g-1).The primary molecular resistance lechanism was antibiotic deactivation.All ARGs were normalized to molecularly estimate bacterial cell numbers and the result showed that there were multiple ARGs and MGEs(mobile genetic elements)in per cell.The total ARGs abundance and certain ARG type,e.g.multidrug,aminoglycoside.MLSB(macrotetralide),tetracycline were significantly correlated with MGEs,which suggested these genes had more horizontal gene transfer potentiality.During the composting process,the abundance and the diversity of ARGs were both lower. |
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