A gradient of fertilization reveals variable arbuscular mycorrhizal plasticity in crops

The arbuscular mycorrhizal (AM) symbiosis may become redundant or detrimental to plant growth when soil fertility is high. To harness AM fungi for sustainable crop production, we must understand how crop species vary in their ability to down-regulate unbeneficial colonization by AM fungi (i.e., AM plasticity). We tested variation in AM plasticity among six crop species - two legumes and four cereals - in a field experiment, by inspecting mycorrhization (AM colonization and root PLFA concentration) and AM fungal community structure (18S rRNA) in plant roots under three levels of nitrogen fertilization. Crop species mycorrhization differed in their mycorrhization response to fertilization. Among the legumes pea and faba bean, mycorrhization remained similar irrespective of fertilizer level, suggesting low AM fungal plasticity, whereas among the cereals spring wheat and spring barley, mycorrhization was plastic, decreasing as fertilization level increased. AM fungal B-diversity did not covary with mycorrhization between fertilizer levels for any crop species, suggesting that host plants regulate AM colonization in a quantitative rather than taxonomically selective manner. Winter wheat differed from other crops by systematically associating more with non-ruderal AM fungi. Understanding how fertilizer adjustments affect crop species mycorrhization is an important step towards efficient use of AM symbiosis for sustainable agriculture. The study was carried out at four Centre of Estonian Rural Research and Knowledge trial stations, Jogeva, Kuusiku, Viljandi and Voru, in different parts of Estonia in 2024.