Key Takeaways & Executive Findings
- •• • Sludge fermentation product application increased total ARG abundance by 25.47% in rhizosphere soil and 73.08% in phyllosphere of pakchoi compared to unfertilized control, indicating significant resistance enrichment in edible plant parts. • • Soil antibiotic content rose by 12.70% in sludge-treated plots versus only ~3% with chemical fertilizer, demonstrating that sludge-derived exogenous antibiotics are a dominant contributor to resistance selection pressure. • • Chemical fertilizer application reduced ARG abundance by 53.40% in rhizosphere soil and 13.50% in phyllosphere, likely due to decreased microbial community abundance and diversity, which weakens ARG hosts and dissemination vectors. • • Specific microbial taxa, Sphaerobacter thermophilus and Aggregatilinea lenta, showed strong positive correlations (r≈0.95–1.00) with multiple ARGs, while Solirubrobacter sp. CPCC_204708 exhibited negative correlation (r≈−0.91), identifying potential microbial indicators for ARG monitoring.
Abstract
To investigate the effects of applying sewage sludge aerobic fermentation products on antibiotic resistance genes (ARGs) in the rhizosphere soil and phyllosphere of pakchoi (Brassica chinensis L.), field experiments were conducted with three treatments: sludge product (sludge group), chemical fertilizer (fertilizer group), and no fertilizer (control). Antibiotic residues, abundances of ARGs and mobile genetic elements (MGEs) were measured in rhizosphere soil and phyllosphere, and microbial community composition and virulence factor (VF) contributions were annotated via metagenomics. Results showed that antibiotic concentrations in rhizosphere soil were generally higher than in phyllosphere. Compared with control, sludge application increased soil antibiotic content by 12.70%, whereas fertilizer increased it by only ~3%, indicating a more significant exogenous input from sludge. At the resistance level, total ARG abundances in rhizosphere soil and phyllosphere of the sludge group increased by 25.47% and 73.08%, respectively, relative to control, with concurrent increases in beta-lactam resistance genes and MGEs such as integron intI1. Sludge application may enhance integron-mediated gene capture and horizontal transfer potential, driving resistance risk accumulation in both phyllosphere and rhizosphere soil. Conversely, fertilizer application reduced ARG abundances by 53.40% in rhizosphere soil and 13.50% in phyllosphere compared with control, consistent with decreased microbial community abundance and diversity, suggesting that reduction of host bacteria and dissemination vectors was a key reason. In community structure, Proteobacteria dominated the phyllosphere, while Chloroflexi dominated rhizosphere soil. Correlation networks identified Sphaerobacter thermophilus and Aggregatilinea lenta positively correlated with multiple ARGs (r≈0.95–1.00), whereas Solirubrobacter sp. CPCC_204708 was negatively correlated (r≈−0.91). Virulence factor contributions followed trends similar to ARGs. Sludge fermentation products simultaneously increased ARG prevalence and related risk indicators in both rhizosphere soil and phyllosphere of pakchoi, providing a reference for risk identification and safe application of sludge fermentation products in agriculture.
1. Introduction
The escalating global production of sewage sludge, projected to reach 1.1×10^8 tonnes (80% moisture) by 2030 in China alone, presents a dual challenge of resource recovery and contaminant management. Sludge concentrates over 50% of wastewater-borne pollutants, including antibiotic resistance genes (ARGs), antibiotic-resistant bacteria, and heavy metals. While aerobic fermentation followed by land application is a mainstream treatment route, the fate of ARGs during fermentation is complex: thermophilic phases can reduce some ARGs, but residual antibiotics and mobile genetic elements (MGEs) may facilitate horizontal gene transfer (HGT) post-application, potentially amplifying resistance in soil-plant systems. Existing studies often lack comparative controls with chemical fertilizers and overlook the phyllosphere—the edible above-ground parts of leafy vegetables—where ARGs can directly enter the food chain.
This study addresses these gaps by conducting a field trial with pakchoi (Brassica chinensis L.), a short-cycle leafy vegetable, under three treatments: sludge fermentation product, chemical fertilizer, and no fertilizer. By measuring antibiotic residues, ARG and MGE abundances in both rhizosphere soil and phyllosphere, and integrating metagenomic analysis of microbial communities and virulence factors, the research quantifies the additional ARG input and enrichment risk from sludge application relative to conventional fertilization. The findings provide critical evidence for risk assessment and safe utilization of sludge-derived fertilizers in agriculture.
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MA Shijin, HE Dahai, WANG Bin, HE Ruoxue, ZHOU Litao, LI Jiang, JU, HAI Lipeng (2026). Effects of Field Application of Sewage Sludge Aerobic Fermentation Products on Antibiotic Resistance Gene Prevalence in Pakchoi (Brassica chinensis L.). Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511057
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Frequently Asked Questions
What is the magnitude of ARG enrichment in edible plant parts when sludge fermentation products are applied, and how does it compare to chemical fertilizer?
Sludge application increased total ARG abundance by 73.08% in the phyllosphere and 25.47% in rhizosphere soil relative to unfertilized control. In contrast, chemical fertilizer reduced ARG abundance by 13.50% in phyllosphere and 53.40% in rhizosphere soil. This indicates that sludge-derived ARGs can significantly accumulate in edible tissues, posing a direct dietary exposure risk.
What are the key microbial taxa associated with ARG proliferation or suppression in this system?
Sphaerobacter thermophilus and Aggregatilinea lenta showed strong positive correlations (r≈0.95–1.00) with multiple ARGs, suggesting they may serve as potential hosts or vectors. Conversely, Solirubrobacter sp. CPCC_204708 exhibited negative correlation (r≈−0.91) with ARGs, indicating a possible suppressive role. These taxa could be targeted for monitoring or biocontrol strategies.
How does sludge application influence the horizontal transfer potential of ARGs via mobile genetic elements?
Sludge application increased the abundance of integrons such as intI1, which are key MGEs mediating gene capture and horizontal transfer. This suggests that sludge-derived MGEs may enhance the potential for ARG dissemination among soil and phyllosphere microbial communities, amplifying resistance risks beyond simple input.
What is the contribution of antibiotic residues to the observed ARG enrichment?
Sludge application increased soil antibiotic content by 12.70% compared to control, whereas chemical fertilizer increased it by only ~3%. This indicates that sludge-derived antibiotics exert selective pressure that can drive ARG proliferation, although other factors like co-selection with heavy metals and microbial community shifts also play roles.
Are there differences in microbial community structure between rhizosphere soil and phyllosphere that affect ARG distribution?
Yes, Proteobacteria dominated the phyllosphere, while Chloroflexi dominated rhizosphere soil. These distinct community compositions likely influence the abundance and diversity of ARG hosts and MGEs, contributing to the higher ARG enrichment observed in the phyllosphere (73.08%) compared to rhizosphere soil (25.47%) under sludge treatment.
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