Pharms on farms: Variable impacts of veterinary pharmaceuticals on an arbuscular mycorrhizal symbiosis and soil microorganisms

Circular agriculture promotes the reuse of organic waste, such as animal manures, to reduce reliance on synthetic fertilisers. However, manures can introduce veterinary pharmaceuticals into soils. Arbuscular mycorrhizal (AM) fungi are critical for plant nutrition, yet their responses to manure-associated pharmaceuticals remain poorly understood. We investigated the effects of three commonly used veterinary pharmaceuticals—the antibiotics doxycycline (DOX) and sulfamethoxazole (SMX) and the anthelmintic flubendazole (FLUB)—applied at environmentally relevant concentrations, on AM structure and function and wider microbial communities.

Using dwarf runner bean, we quantified AM-mediated transfer of phosphorus (³³P) and nitrogen (¹⁵N) to plants and plant carbon (C) allocation to AM fungi. Bacterial (16S) and fungal (partial 18S and total ITS) community compositions were characterised using DNA amplicon sequencing.

DOX exposure increased AM abundance and AM-mediated ³³P transfer to plants, and reduced soil bacterial diversity, while plant biomass and N acquisition were unaffected. SMX had no effects on these metrics, despite increasing shoot N content. FLUB reduced AM root colonisation and plant C allocation to AM fungi but did not alter fungal ³³P or ¹⁵N transfer or microbial diversity.

Overall, our results demonstrate compound-specific effects of veterinary pharmaceuticals on AM fungal symbioses and soil microbial communities, with DOX positively impacting AM fungi, while FLUB negatively impacted the symbiosis and SMX did not affect AM fungi. DOX reduced soil bacterial diversity. These findings emphasise the importance of considering microbial functional responses when assessing the sustainability and feasibility of manure reuse in agricultural systems.