Microbial diversity and function with soil depth is differentially affected by grassland species

Agricultural grassland ecosystems are a major store of terrestrial carbon (C), yet little is known about the capacity of these systems to cycle and store C in deeper soil horizons. Further, it is unclear how the plant community composition and species identity within agricultural grasslands influences the microbial community composition and C cycling capacities along the soil depth gradient. This study investigated whether the aboveground community composition in intensively managed agricultural grassland communities influenced the belowground microbial community composition, abundance and function with depth. Microbial diversity and functional analyses were assessed in four soil horizons: A (0-15 cm), B (15-30 cm), C (30-60 cm) and D (60-90cm) in monocultures of six grassland species, and a mixture of all six. Microbial functional capacity was measured through enzymatic and substrate induced respiration assays. Microbial abundance was quantified using quantitative polymerase chain reaction (qPCR) and diversity was measured through sequencing of 16S (prokaryotic) and ITS (fungal) genes. Soil microbial phylogenetic gene abundance and enzymes related to C cycling decreased, and community composition changed, along the soil depth gradient, regardless of plant community. Microbial abundance was not significantly influenced by plant community type across the entire soil depth profile; however, 16S microbial community composition was significantly influenced by plant community in the top 15 cm of soil and fungal community composition was significantly influenced by plant community type between 15-30 cm in soil. Plant community types mediated the rate at which microbial C cycling, as measured through enzymatic assays, decreased along the soil depth gradient and selected C cycling enzymes were significantly more active at depth when Chicorium intybus, a deep rooting species, was present. This study provides greater understanding of how plant communities affect the soil microbiome along the soil depth gradient in agricultural grasslands, with potential implications for management of these systems towards enhanced soil function.