Nitrogen addition weakens the biodiversity-multifunctionality relationships across soil profiles in a grassland assemblage

The dynamics of multiple ecosystem functions (that is, multifunctionality) are closely associated with biodiversity, often show positive relationships, but the dynamics of the biodiversity-multifunctionality relationship along soil profiles remain unclear. Eutrophication significantly affects biodiversity and soil functions, whereas the mechanisms underlying the impact of nitrogen addition on the biodiversity-multifunctionality relationship are poorly understood. Here, we conducted a common garden experiment manipulating plant diversity and nitrogen addition to quantify the effects of biodiversity on soil multifunctionality in the topsoil and subsoil under different nitrogen addition levels. We showed that nitrogen addition had a minor impact on soil multifunctionality but weakened the positive biodiversity-multifunctionality relationship. We also found that the positive plant diversity effect on soil multifunctionality decreased with soil depths, resulting in a weaker biodiversity-multifunctionality relationship in the subsoil. The weak biodiversity-multifunctionality relationship in the subsoil was caused by weakening the relationship between aboveground biomass and soil multifunctionality; nitrogen addition weakened the biodiversity-multifunctionality relationship through the attenuation of the association of aboveground biomass and soil microbes with multifunctionality. Our study demonstrates that nitrogen addition undermines the biodiversity-soil multifunctionality relationship and the relationship is weakened along the soil profiles, suggesting that Earth system models must represent these heterogeneous soil dynamics to accurately predict future feedbacks to global changes.