Multispecies Pasture Reshapes Subsoil Bacterial and Fungal Communities Compared to Ryegrass Monoculture

Abstract

Perennial ryegrass (Lolium perenne L.) underpins the pasture-based dairy industry in Australia, yet its shallow root system limits adaptation to drought conditions. Multispecies pastures, consisting of grasses, legumes and herbs, are increasingly adopted to improve ecosystem resilience, but their interactions with soil microorganisms, particularly in deeper soil layers, remain poorly understood. This study investigated bacterial and fungal communities in both topsoils and subsoils across monoculture (perennial ryegrass) and multispecies (ryegrass, chicory, clovers and other plants) pastures at eight dairy farms in Victoria, Australia, using quantitative PCR and amplicon sequencing. We found that microbial abundance and diversity declined with soil depth in both pastures, and were broadly comparable between monoculture and multispecies pasture systems. Microbial community compositions differed significantly between monoculture and multispecies pasture systems in both the topsoil and subsoil, across all sampling sites. Co-occurrence network analysis indicated that at three farm sites, the subsoil bacterial and fungal network of multispecies pastures exhibited a reduced number in nodes, edges and average degree, compared to monoculture ryegrass. This indicated multispecies pasture has a stronger interference to the subsoil microbiome than that of the monoculture ryegrass. Our results demonstrate distinct subsoil microbial communities between multispecies pastures and ryegrass monocultures, which indicate that pasture composition influences microbial community assembly at depth. This suggests that subsoil microbial interactions should be considered when evaluating the ecological impact of multispecies pasture selection.