Effects of Neonicotinoid Insecticides on Soil Microbial Communities and Their Interactions with Selected Soil Properties

Abstract

Neonicotinoids are globally dominant systemic insecticides, yet their effects on belowground microbial communities remain incompletely characterised. Soil microorganisms regulate nutrient cycling, organic matter decomposition, and agroecosystem resilience, making their responses to pesticide exposure central to understanding long-term soil health. This thesis investigated how neonicotinoid seed treatments shape soil bacterial and fungal communities across multiple timescales in Luvisol and Vertisol soils with contrasting textures. The research comprised a systematic review, a 10-day experiment with imidacloprid, thiamethoxam, and clothianidin, and a 28-day experiment examining imidacloprid effects across loamy sand, sandy loam, and clay soils. The review identified highly variable outcomes and major methodological gaps, including limited field studies, poor representation of fungi, and insufficient integration of soil physicochemical data. These gaps informed experiments integrating bacterial and fungal responses with soil properties. Microbial communities were characterised using 16S rRNA gene and ITS amplicon sequencing to assess diversity, taxonomic composition, co-occurrence networks, and predicted functional profiles. Short-term experiments showed selective bacterial compositional changes without a consistent overall diversity shift. Transient increases in bacterial diversity occurred under thiamethoxam and clothianidin, whereas imidacloprid reduced diversity by Day 10. Taxonomic responses were compound-specific, with Mesorhizobium consistently enriched across all neonicotinoids, while nitrifying and other bacterial taxa responded differentially among treatments. Fungal diversity remained largely stable despite temporally variable community composition. Clothianidin produced the greatest number of discriminatory fungal taxa, increased saprotrophs, and reduced symbiotrophs. The 28-day experiment demonstrated strong mediation by soil physicochemical context. Clay soil exhibited reduced evenness, less cohesive co-occurrence networks, and broad predicted functional suppression. Loamy sand showed intermediate responses, with increased network integration despite reduced diversity. Sandy loam maintained stable richness, evenness, and phylogenetic diversity and developed denser, more modular networks, indicating reorganisation rather than collapse. Functional profiles showed downregulation of carbohydrate and nucleotide metabolism pathways in fine-textured soils, suggesting constraints on carbon turnover and microbial energy flow under neonicotinoid stress. These contrasts may partly reflect inherent soil structure. The Vertisol clay, with shrink-swell behaviour and a more dynamic, compactible pore structure, may enhance variation in pore connectivity and chemical-microbe contact, whereas the more rigid Luvisol structures may moderate exposure and support greater microbial resilience. These differences likely contributed to contrasting community stability, network organisation, and stronger functional suppression in the fine-textured Vertisol. These findings were synthesised into a fate-effect-function conceptual model linking neonicotinoid persistence, microbial exposure, taxonomic restructuring, and functional consequences. The model provides a mechanistic framework for interpreting implications for biogeochemical cycling, biodegradation potential, and soil ecosystem services. Overall, this thesis demonstrates that neonicotinoid effects on soil microbial communities are dynamic, compound-specific, and mediated by soil texture. By linking microbial structural and functional shifts to agroecosystem processes, it advances the evidence base for neonicotinoid risk assessment and supports management strategies aimed at safeguarding soil health and agricultural sustainability.