Impact of microbial diversity and pesticide application on plant growth, litter decomposition and carbon substrate use

Microorganisms are key components of soil biodiversity and are essential for organic matter decomposition and nutrient cycling in terrestrial ecosystems. So far it is unclear whether pesticide application influences microbial communities and the contribution of microbes to soil functioning and plant growth. To address this, we manipulated soil microbial diversity and created a diversity gradient, ranging from an average of 32 fungal operational taxonomic units (OTUs) and 312 bacterial amplicon sequence variants (ASVs) to 204 OTUs and 1000 ASVs, respectively. This reduction in microbial diversity resulted in significant decreases in litter decomposition (-43.7%), carbon-substrate usage (-56.0%), acid phosphatase activity (-54.1%), and plant growth (-98.6%). Pesticide application significantly reduced fungal richness at high soil microbial diversity levels, while bacterial richness was not affected by pesticide application. Our findings also revealed that pesticides alter the role of microbial communities, especially fungi, in organic matter decomposition and plant growth. Pesticide application reduced the relative importance of fungal richness in explaining variations in plant biomass production and litter decomposition. Conversely, pesticide application increased the relative importance of bacterial richness in explaining carbon substrate utilization, particularly for carbohydrates and aminoacids. Overall, this study suggests that preserving the diversity of bacterial and fungal communities within soil mesocosms is key to support essential soil functions such as organic matter degradation, and that pesticide applications could impair the beneficial effects of fungal richness on organic matter degradation. • Soil biodiversity manipulation via sieving had a stronger impact on fungal than bacterial richness • Decreased microbial richness led to reductions in plant biomass production and organic matter degradation. • Soil biodiversity manipulation had a bigger impact on soil microbial diversity and function than pesticide application. • Pesticide application altered the relationship between microbial richness and soil functions.