Drainage class and soil phosphorus availability shape belowground microbial communities in grasslands

Intensification of grass production systems is common practice in agriculture and is usually characterized by high nutrient loading including phosphorus (P) and land drainage. Despite this widespread practice, only a paucity of studies have investigated microbial adaptations to these intensification measures in grasslands. We hypothesized that intensively managed grasslands will sustain distinctive belowground microbial communities when compared to extensive grasslands. Soil drainage and soil P content effects in bacteria, fungi and Arbuscular Mycorrhizal Fungi (AMF) were evaluated in twenty Irish grasslands in the following subgroups: Well drained with High P (WHP) and Low P (WLP) and poorly drained with High P (PHP) and Low P (PLP). In addition, soil chemical properties, phosphatase activities and phoD gene abundance were evaluated. Both drainage and soil P levels affected microbial communities. Poorly drained soils (PLP & PHP) showed higher relative abundances of the Acidobacteria. Distinct fungal communities were found in PLP soils with higher relative abundances of Trichoderma and the Glomerocmycota and the genus Trichoderma. Drainage class affected bacterial abundance with Actinobacteria and Firmicutes presenting higher abundances in well drained soils. High P soils showed greater abundances of the fungal Ascomycota and Zygomycota and the fungal genera Mortierella and Fusarium increased in relative abundance with the soil's P status. Changes in soil bacteria populations were mainly driven by soil pH whereas soil P availability and phoD abundance best related with changes in the fungal communities. Symbiotic microbes appeared to be favored in low P soils. hHowever 16S amplicons associated withknown P solubilizing microbes where also highly abundant in high P soils. Our findings suggest that microbial symbiosis can be beneficial for grassland productivity and that P solubilizing microbes can be detrimental in P mining scenarios. This study demonstrates that while soil drainage and P status affect fungal and bacterial communities differently, Low P and poor drainage soils harbor a distinct microbiome.