Poultry manure-derived microorganisms as a reservoir and source of antibiotic resistance genes transferred to soil autochthonous microorganisms

Animal husbandry is increasing yearly due to the growing demand for meat and livestock products, among other reasons. To meet these demands, prophylactic antibiotics are used in the livestock industry (i.e., poultry farming) to promote health and stimulate animal growth. However, antibiotics are not fully metabolized by animals, and they are evacuated to the environment with excreta. Animal manure is used as fertilizer to reduce the volume of waste generated in the livestock sector. However, manure often contains microorganisms harboring antibiotic resistance genes (ARGs). Then, the microbiome of manure applicate to the soil may contribute to the spread of antibiotic resistance in the environment, including autochthonous soil-dwelling microorganisms. The present study was conducted during the crops growing season in Poland (May to September 2019) to determine the influence of poultry manure as well as poultry manure supplemented with selected antibiotics on the diversity of the soil microbiome in treatments that had not been previously fertilized with manure and the ability of antibiotic-resistant bacteria to transfer ARGs to other soil bacteria. Antibiotic concentrations were elevated at the beginning of the study and decreased over time. Poultry manure induced significant changes in the structure of microbial communities in soil; the diversity of the soil microbiome decreased, and the abundance of bacterial genera Bradyrhizobium, Streptomyces, and Pseudomonas, which are characteristic of the analyzed manure, increased. Over time, soil microbial diversity was restored to the state observed before the application of manure. Genes conferring resistance to multiple drugs as well as genes encoding resistance to bacitracin and aminoglycosides were the most frequently identified ARGs in the analyzed bacteria, including on mobile genetic elements. Multidrug resistance was observed in 17 bacterial taxa, whereas ARGs were identified in 32 bacterial taxa identified in the soil microbiome. The results of the study conclude that the application of poultry manure supplemented with antibiotics initially affects soil microbiome and resistome diversity but finally, the soil shows resilience and returns to its original state after time, with most antibiotic resistance genes disappearing. This phenomenon is of great importance in sustainable soil health after manure application.

Toxicity assessment of molybdenum slag as a mineral fertilizer: A case study with pakchoi (Brassica chinensis L.)

Large quantities of molybdenum (Mo) slag are generated as a by-product during mining and smelting, which not only occupy huge stretches of arable land and natural habitats but also threaten the local ecosystem and environment. How to recycle this Mo slag is becoming an urgent issue. Here, we reported the toxicity assessment of Mo slag as a mineral fertilizer for slag recycling in agricultural practices. The results showed the following: (1) Lower rates of slag (1.0%, 2.5%, and 5.0%) fertilization, especially 5.0% slag, increased the activities of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase), the contents of chlorophyll, and both the maximum quantum yield and quantum efficiency of photosystem II; decreased the content of malondialdehyde and the non-photochemical quenching of photosystem II; and eventually increased the height, leaf area, and biomass of pakchoi seedlings; (2) Higher rates (7.5% and 10.0%) of Mo slag application resulted in a reduction in the aforementioned physiological and morphological parameters (except for peroxidase activity) of pakchoi seedlings; and (3) Although fertilization with 5.0% slag increased the accumulation of the non-essential elements arsenic (As), lead (Pb), and cadmium (Cd) in pakchoi seedlings, their contents were still lower than the maximum levels of the Codex Alimentarius Commission, European Union, and standards of China. From the perspectives of plant nutrition and food safety, our results showed that Mo slag fertilization at rates lower than 5.0% can be applied as a mineral fertilizer for pakchoi grown on calcareous soils.

PAHs content of sewage sludge in Europe and its use as soil fertilizer

The European Commission has been planning limits for organic pollutants in sewage sludge for 14years; however no legislation has been implemented. This is mainly due to lack of data on sewage sludge contamination by organic pollutants, and possible negative effects to the environment. However, waste management has become an acute problem in many countries. Management options require extensive waste characterization, since many of them may contain compounds which could be harmful to the ecosystem, such as heavy metals, organic pollutants. The present study aims to show the true European position, regarding the polycyclic aromatic hydrocarbons (PAHs) content of sewage sludge, by comparing the Italian PAHs content with European Union countries, and at assessing the suitability of sewage sludge as soil fertilizer. The FOCUS Pearl model was used to estimate the concentration of benzo [a] pyrene (B(a)Pyr), the most toxic PAH in soil, and its exposure to organisms was then evaluated. The simulated B(a)Pyr and PAHs, expressed as B(a)Pyr, concentrations in soil were much lower than the B(a)Pyr's most conservative lowest observable effect concentration (LOEC) for soil organisms. Furthermore, the results obtained indicate that it is more appropriate to apply 5tha(-1) sewage sludge annually than 15tha(-1) triennially. Results suggest, the EU maximum recommended limit of 6mgkg(-)(1) PAHs in sewage sludge, should be conservative enough to avoid groundwater contamination and negative effects on soil organisms.