Objectives Due to the intensive production of plants and animals, the organic fertilizer derived from the wastes contains a certain level of heavy metals and antibiotic contaminants inevitably. We studied the agronomic and environmental effects of compost from new composting of high-temperature pretreatment combined with biochar addition, in order to decrease or eliminate the potential organic and inorganic impacts on crops brought by compost application.

Methods Pig manure and wheat straw were used as raw materials of the composting processes, and three composts were prepared for the pot experiment, conventional compost (TC), high-temperature pretreatment compost (HTC), and high-temperature pretreatment combined with biochar (10%, w/w) compost (HTBC). At 50 days of growth of the test crop lettuce (Lactuca sativa), the pot experiment was stopped for sampling. Lettuce plants were divided into leaves and roots, the rhizosphere soils were collected by shaking the roots slightly. The biomass of fresh and dry weight of lettuce leaves and roots were weighed, and quality of lettuce were measured. The contents of 6 heavy metals and 11 antibiotics, and the absolute abundance of 6 antibiotic resistance genes (ARGs) in leaves and rhizosphere soil were determined, and the bioconcentration factors of lettuce on heavy metals and antibiotics were calculated.

Results Compared with TC, the application of HTC and HTBC increased the fresh weight of lettuce leaves by 9.2% and 13.1%, the soluble protein content by 14.2% and 8.3%, and decreased the nitrite content by 19.9% and 19.4%, respectively. Both HTC and HTBC application significantly reduced the contents of heavy metals in lettuce leaves by 6.0%−38.9% and 13.8%−65.6%, respectively, 22.4%−64.3% and 33.0%−100.0% for antibiotics, and 41.6%−47.6% and 59.2%−76.2% for the absolute abundance of ARGs. The bioconcentration factors of heavy metals (except Cd) in HTC and HTBC decreased by 8.7%−43.8% and 8.0%−68.8%, respectively, compared with TC treatment. In HTC treatment, the bioconcentration factors of sulfonamide and tetracycline antibiotics reduced by 14.5%−66.2% and 4.5%−37.9%, respectively, and the reduced rates were 33.4%−100.0% and 34.2%−38.8% in HTBC treatment. Among the 11 antibiotics tested, HTBC could significantly reduce the content of 9 antibiotics in lettuce rhizosphere soil, while HTC could only reduce the content of 5 antibiotics. Among the 6 ARGs subtypes tested, HTBC could significantly reduce the absolute abundance of 5 ARGs, while HTC could only reduce 2 ARGs. HTBC was generally more effective in reducing hazardous substances than HTC. The correlation analysis results showed that there was a significant positive correlation between heavy metals content, antibiotics content, and absolute abundance of ARGs in lettuce leaves and the corresponding pollutants in rhizosphere soil.

Conclusions The organic fertilizer produced by high-temperature pretreatment combined with biochar composting process not only improved the yield and quality of lettuce, but also effectively reduced heavy metals, antibiotics and ARGs in rhizosphere soil. The decrease in bioavailability of heavy metals, antibiotics content and absolute abundance of ARGs in rhizosphere soil, was the main influential factor for reducing the accumulation of hazardous substances in lettuce leaves through the application of high-temperature pretreatment combined with biochar compost.