What does colonisation tell us? Revisiting the functional outcomes of root colonisation by arbuscular mycorrhizal fungi

In this study, I revisit the function of colonisation metrics using a simple but structured experiment. I assessed colonisation using the McGonigle et al. (1990) intersect method and report it as the percentage of total root length colonised by fungal structures. Throughout the manuscript, I refer to the proportion of root length containingAMfungal structures (hyphae, arbuscules or vesicles) as ‘total colonisation’, and the proportion of root length colonised specifically by arbuscules as ‘arbuscular colonisation’. I measured root length colonised byAMfungi in four globally important crops (twoC3 and twoC4) and tested their relationships to plant biomass, phosphorus and phenolics. I hypothesised that: (1) the total colonisation and arbuscular colonisation would be positively associated with plant biomass, phosphorus and phenolic responses (responses being the change in each trait in plants with AM fungi relative to controls withoutAMfungi), particularly in theC4 crops; and (2) any relationships would be predominantly non-linear, reflecting threshold or saturation dynamics. While measures ofAMfungal colonisation remain a cornerstone metric in mycorrhizal research, their interpretation is often assumed rather than explored. This study offers empirical data across different traits and species to revisit what the root length colonised can, and cannot, tell us about symbiotic outcomes. I hope this serves as a timely prompt for deeper reflection on how we use colonisation metrics, and what we expect them to mean.