Long-term soil nutrient deficiencies reshape connections between arbuscular mycorrhizal fungi and plant communities in a managed grassland

Many agroecosystems experience soil nitrogen (N), phosphorus (P), or potassium (K) deficiencies due to imbalanced fertilization or insufficient replenishment of nutrients withdrawn by biomass harvest. Nutrient deficiencies affect both arbuscular mycorrhizal fungi (AMF) and their associated plant hosts. Since AMF and plant communities are interconnected by their symbiosis, soil nutrient deficiencies can indirectly influence AMF communities through changes in plant community composition and vice versa. Furthermore, changes in soil nutrient availability alter soil physicochemical properties, thereby affecting both AMF and plant communities.This study examined the coupled responses of AMF and belowground plant communities to long-term soil N, P and K deficiencies in a managed grassland. Additionally, we evaluated the associations between AMF and belowground plant communities, particularly at the genus and functional guild levels, while controlling for edaphic factors.The study was conducted in a long-term managed grassland experiment in Admont (Styria, Austria), where N, P, and K, as well as lime and organic fertilizers, have been applied in different combinations for more than 70 years. Aboveground vegetation was harvested three times annually for over seven decades, resulting in long-term nutrient depletion of specific nutrients in non-fertilized plots. AMF communities in soil and roots were characterized using RNA- and DNA-based amplicon sequencing of the 18S rRNA gene, respectively. Belowground plant community composition was evaluated by amplicon sequencing of the chloroplast rbcL (RuBisCo large subunit) gene region from mixed root samples.Our analysis shows that AMF and belowground plant community compositions differed significantly between plots receiving lime and organic fertilizers, and those fertilized with inorganic treatments. N, P, and K deficiencies affected both soil AMF and plant community compositions, whereas root-associated AMF community compositions responded significantly to only K deficiency. Since pH exerted the strongest influence on soil and root AMF as well as belowground plant community compositions, we performed Partial Mantel tests controlling for pH to examine associations between AMF and plant communities. Both soil and root AMF communities were significantly correlated with belowground plant community composition, with comparable correlation strengths for soil AMF (r=0.20, p