Phosphorus availability modifies the responses of Glomeromycotina and Mucoromycotina to nitrogen addition

Grassland fertilisation modifies the relationship between mycorrhizal fungi and their plant hosts. The composition and diversity of the fungal community depend on the background nutrient availability and changes in nutrient availability caused by fertilisation. We determined the effects of nitrogen and phosphorus addition at 12 sites with different background nutrient availabilities using 5 species of bait seedlings planted in the field. We studied two groups of mycorrhizal fungi – arbuscular mycorrhizal fungi (Glomeromycotina, G-AMF) and fine root endophytes (Mucoromycotina, M-FRE) – using molecular methods and light microscopy. Both fungal communities showed large differences among forb and grass host species and among individual sites. G-AMF community composition varied more along background nutrient availability gradients than with short-term fertilisation. Phosphorus availability affected the composition of both fungal communities, but nitrogen availability only affected G-AMF community composition. Nitrogen fertilisation affected both fungal communities more than phosphorus fertilisation, and the effects of nitrogen addition varied with the local phosphorus availability. Our study provides an understanding of the long-term adaptation of mycorrhizal symbiosis to nutrient availability and how it interacts with nutrient addition.