Degradation Of Pig Manure-derived Sulfadiazine And Its Effect On Ammonia-oxidizing Microorganisms In Soil

In recent years,antibiotics have been widely used as antimicrobials especially growth promoters in livestock and poultry industry.After antibiotics enter the body of livestock and poultry,most of them are discharged in the form of original drugs and maintain biological activity.The broad-spectrum antibacterial property of antibiotics leads to a series of ecological environmental problems when they enter the soil with the excrement of livestock and poultry.Sulfadiazine?SDZ?,as an antibiotic frequently used in livestock and poultry farming,has been found to have a high concentration in pig manure and soil,causing a high ecological risk.What's more,ammonia oxidizing microorganisms?AOM?is sensitive to SDZ.Therefore,the degradation behavior of SDZ in soil and its influence on AOM are the focus of current environmental research.This paper mainly explores two problems.One is the degradation characteristics of various forms of SDZ in soil under different environmental factors and initial concentrations.The second is the effect of SDZ on AOM abundance and metabolism under different initial concentrations.The soil moisture content,temperature,light and microorganisms were selected as environmental impact factors for the bottle test,and the initial concentrations of 0,10,20,50 and 100 mg·kg^-1 were selected for the basin test.The degradation characteristics of SDZ in the soil were analyzed by degradation kinetics.What's more,the mutual transformation of SDZ forms and the influence of SDZ at different concentrations on AOM abundance and potential ammonia oxidation capacity?PAO?were analyzed.Then,the contributions of ammonia-oxidizing bacteria?AOB?,ammonia-oxidizing archeae?AOA?and Complete ammonia oxidizer?Comammox?to PAO were analyzed by multiple linear regression.Finally,the effects of different forms of SDZ degradation processes on AOM abundance under different initial concentration conditions were analyzed by linear regression,so as to link the above two problems.The results of bottle test and basin test showed that the degradation kinetics of SDZ in soil of all forms was more in line with the second-order exponential model than with the first-order kinetic fitting.When the soil moisture content is in the range of 20% MWHC ? 80% MWHC,the higher the moisture content is,the faster the degradation rate of SDZ will be,and the change of moisture content has the greatest influence on the degradation rate of extractable state of CaCl₂.When the soil temperature is within the range of 15??35?,the higher the temperature is,the faster the degradation rate of SDZ will be.The degree to which the degradation rate constants of the three forms are affected by temperature: CaCl₂ extractable state > Methanol extractable state > full amount.Light had a great influence on the degradation rate of CaCl₂ extractable state and SDZ full amount,and the influence of light and microorganism on the extractable state of Methanol was similar.The higher the initial concentration of SDZ is,the slower the CaCl₂ extractable state and SDZ full amount degradation rate is.The apparent distribution coefficient(K_app)value,DT₅₀ and DT₉₀ of SDZ can be used to determine the persistence of SDZ in different forms and the existence of SDZ in environmental media.The smaller the Kapp value is,the larger the DT₉₀ and DT₅₀ values of bioavailable forms are,indicating that residual SDZ is transformed into SDZ of bioavailable forms in the soil environment,which will increase its toxicity to microorganisms.Conversely,it indicates that SDZ is aging in the soil environment,which will reduce its biotoxicity to microorganisms.SDZ was undergoing "aging" process at the late stage of culture?after 35d?under all treatment conditions.As for the relationship between different forms of SDZ and the total amount,CaCl₂ extractable state content can represent the whole SDZ amount to a certain extent.Although the correlation between residue state and other forms is not significant,it is still the key to ecological risk control as a potential persistent pollution source.The results showed that the addition of SDZ inhibited the stimulating effect of pig manure on AOM abundance.Combined with the analysis of the correlation between the degradation process of sulfadiazine and AOM abundance,it can be seen that in general,SDZ has the strongest inhibitory effect on Comammox abundance,followed by AOA,but only under high concentration SDZ?S50 and S100?conditions will produce significant inhibition,and has little effect on the abundance of AOB.Pig manure inhibited PAO in early culture but promoted it in late culture.The effect of SDZ on PAO was as follows: low concentration of SDZ promoted PAO in a short period and high concentration of SDZ inhibited PAO in a long period.However,multiple linear regression analysis showed that only AOA abundance was positively correlated with PAO,while AOB and Comammox were not correlated with PAO?P>0.05?.