Grazing does not increase soil antibiotic resistome in two types of grasslands in Inner Mongolia, China

Long-term livestock grazing often comes with inputs of manure and urine of animals to grassland soils and can influence the microbial communities. However, the effects of grazing on the diversity and abundance of soil antibiotic resistance genes (ARGs) are still obscure. Here we profiled soil resistome using high-throughput quantitative PCR analysis from desert and typical grasslands (Siziwang and Xilingol steppe, respectively) in Inner Mongolia, each with four grazing intensities (no grazing, light grazing, moderate grazing, and heavy grazing). Totally, 54 and 39 ARGs were detected in the Siziwang steppe and the Xilingol steppe, respectively. The relative abundance of ARGs significantly decreased in the Siziwang steppe or remained unchanged in the Xilingol steppe along the gradient of increasing grazing intensity. Mobile genetic elements (MGEs) were significantly and positively correlated with ARGs in the Siziwang steppe but not in the Xilingol steppe. Structural equation model revealed that grazing indirectly affected ARGs through its impact on MGEs, bacterial diversity, and bacterial community composition. These findings suggest that livestock grazing can impact soil resistome via regulating microbiome in grassland soils. However, grazing intensity has not resulted in a significant impact on the soil resistomes in these desert and typical grasslands.