Understanding the effects of multi-species grassland mixtures on the composition and function of the soil microbiome in temperate grasslands.

This thesis investigated the effects of multi species grassland mixtures on the composition and function of the soil microbiome in temperate grasslands. Three major gaps in knowledge were identified: (1) What effects do multi species swards have on the composition, diversity and nitrogen cycling capabilities of the soil microbiome? (2) What effects do multi species swards have on the resistance and resilience of the soil microbiome to drought, in terms of composition, diversity and nitrogen cycling potential? (3) What effects do deep rooting plants in multi species swards have on the composition, diversity and carbon cycling capabilities of the soil microbiome? Plant identity was a major determinant of microbial diversity, abundance and function in the soil, while plant interspecific species interactions generally had no effect. Soil microbial community abundance, composition and nitrogen cycling functions were generally resistant and resilient to drought, regardless of plant identity. However, where vulnerability to drought was observed, drought resistance and resilience was in many cases mediated by plant identity. In contrast, plant interspecific species interactions generally had no influence on soil microbial drought resistance and resilience. 300N L. perenne monocultures were less resistant and resilient to drought, suggesting over fertilisation can make grasslands more vulnerable to climatic stress. Soil microbial abundance and carbon cycling function decreased, and community composition changed, along the soil depth gradient, regardless of plant identity. Plant identity significantly influenced 16S microbial community composition in the top 15cm of soil and fungal community composition between 15-30 cm in soil. Carbon cycling enzymes were significantly more active at depth when deep rooting plant species were present. This work will contribute significantly to the knowledge of microbial diversity and function in species diverse grasslands, and help provide evidence-based advice for maintaining productive and sustainable agricultural grassland systems in the face of changing global climates