Elevated CO₂ concentration modifies the effects of organic fertilizer substitution on rice yield and soil ARGs

Antibiotic resistance and rising CO₂ levels are considered among the most significant challenges we will face in terms of global development over the following decades. However, the impact of elevated CO₂ on soil antibiotic resistance has rarely been investigated. We used a free-air CO₂ enrichment system to investigate the potential risks posed by applying mineral and organic fertilizers to paddy soil at current CO₂ concentration (370 ppm) and future elevated CO₂ (eCO₂, 570 ppm predicted for 2100). Organic fertilizer substitution (substituting the mineral fertilizer by 50% N) alone increased the plant uptake and soil residue of sulfamethazine, and enriched sulfonamide resistance genes (sul1, sul2), tetracycline resistance genes (tetG, tetM) and class 1 integron (intl1). But it decreased the rice grain yield (by 7.6%). Comparatively, eCO₂ decreased the sul2, tetG and intl1 gene abundances by organic fertilizer substitution, and meanwhile increased grain yield (by 8.4%). Proteobacteria and Nitrospirae were potential hosts of antibiotic resistance genes (ARGs). Horizontal gene transfer via intl1 may play an important role in ARGs spread under eCO₂. Results indicated that future elevated CO₂ concentration could modify the effects of organic fertilizer substitution on rice yield and soil ARGs, with unknown implications for future medicine and human health.