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(P), or potassium (K) deficiencies due to imbalanced fertilization or insufficient replenishment of nutrients withdrawn by biomass harvest.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<0.01) and root AMF (r=0.21, p<0.01). Partial correlation (controlling for pH) analyses between plant and AMF genera showed that more correlations were observed between root-associated AMF and plant genera than between soil AMF and plant genera. Additionally, all AMF genera showing correlations with plant genera belonged to the rhizophilic functional guild, which is characterized by a higher proportion of intraradical relative to extraradical hyphae.

Our findings suggest that long-term soil nutrient depletion influences AMF and plant community composition both directly and indirectly, through shifts in soil parameters and plant–AMF associations. They further indicate that rhizophilic AMF play a central role in mediating plant–AMF associations. These findings highlight that integrating the ecology of subsurface AMF communities—often overlooked beyond monoculture frameworks—can substantially enhance our understanding of plant community responses in a changing environment.