土壤中微量抗生素对氧化亚氮排放的影响及机理研究

A number of recent studies have shown that antibiotics in the mg/kg level can result in delayed or inhibited denitrification rates and may have direct consequences for non-point source N2O or NO3- pollution. While the aforementioned concentrations are typical immediately following the application of antibiotic-bearing manure or sewage sludge, the concentration of bioavailable antibiotics can be significantly reduced by sorption, biodegradation and mobilization. How and to what magnitude these residual concentrations of antibiotics impact the structure and function of soil microbial communities has rarely been considered. Preliminary research in the PI’s lab strongly suggests that exposure to ultralow levels (ng/L or ng/kg) of antibiotics has the effect of stimulating or inhibiting microbial activity, and the potential to increase microbial production of N2O, a powerful greenhouse gas. A more thorough investigation into the impacts of low and residual antibiotic concentrations on nitrification and denitrification processes in soils, especially the mechanisms and pathways of N2O production is thereby warranted. The objectives of the proposed project are to: (1) quantify the adsorption coefficients and bioavailable concentrations of selected antibiotics in different types of soils; (2) quantify the effects of ultralow concentrations of select antibiotics on nitrogen transformation rates, N2O flux rate, and the size of the NH4+, NO2-, and NO3- pools in soil microcosms using a combination of standard incubation techniques and stable 15N isotope tracers under both single-dose exposure and long-term, repeat exposure conditions; and (3) characterize structural and functional changes to the soil microbial community resulting from short- and long-term exposure to ultralow levels of selected antibiotics.

最近的一些研究表明 mg/kg量级的抗生素会对土壤氮循环产生影响。但土壤中的抗生素浓度经过吸附、迁移、降解等过程后会降至µg/kg或更低。如此低浓度的残留抗生素对土壤氮循环，尤其是N2O(强温室气体)排放的研究还属于空白。我们的初步研究表明, 暴露于超低水平的抗生素(ng/L或ng/kg)会刺激或抑制微生物活性，并且有可能增加N2O排放通量。本课题将利用先进的同位素示踪及功能基因丰度检测的方法对低浓度抗生素对土壤硝化和反硝化过程的影响， 尤其是N2O生产的途径和机制进行深入研究。

此前我们对µg/kg量级(典型土壤残留浓度)的抗生素对土壤氮循环尤其是N2O排放的影响知之甚少。因此本项目旨在系统评估痕量抗生素（0.001-10 µg/kg）对土壤氮循环的影响。我们选取了3种常用抗生素（Ciprofloxacin, Oxytetracycline, Sulfamethazine）及4种典型农用土壤（黑土、黄绵土、紫色土、红壤），完成了大量的短期暴露硝化及反硝批实验（共计5000余个样品），并对其中有代表性的样品进行了宏基因组测序工作。结果表明痕量抗生素短期暴露对硝化及反硝化均会产生较为普遍的影响（约50%的硝化实验样品以及约38%的反硝化实验样品与空白有显著差别），但无统一的影响趋势，促进与抑制均有可能。对属水平的物种相对丰度进行主坐标分析表明四种土壤具有不同的微生物组成。不同种类的痕量抗生素对不同土壤属水平的物种种类（包括与氮循环相关的物种）以及相对丰度会产生一定影响，但难以跟硝化及反硝化过程中的痕量抗生素的促进或抑制作用直接挂钩。