Fungal genome size and composition reflect ecological strategies along soil fertility gradients

Genomic traits reflect the evolutionary processes that have led to ecological variation among extant organisms, including variation in how they acquire and use resources. Soil fungi have diverse nutritional strategies and exhibit extensive variation in fitness along resource gradients. We tested for trade‐offs in genomic traits with mycelial nutritional traits and hypothesize that such trade‐offs differ among fungal guilds as they reflect contrasting resource exploitation and habitat preferences. We found species with large genomes exhibited nutrient‐poor mycelium and low GC content. These patterns were observed across fungal guilds but with varying explanatory power. We then matched trait data to fungal species observed in 463 Australian grassland, woodland and forest soil samples. Fungi with large genomes and lower GC content dominated in nutrient‐poor soils, associated with shifts in guild composition and with species turnover within guilds. These findings highlight fundamental mechanisms that underpin successful ecological strategies for soil fungi. Fungi with large genomes exhibited nutrient‐poor biomass and more efficient nucleotide composition. Fungi with large genomes and lower GC content dominated in nutrient‐poor soils, associated with shifts in guild composition and with species turnover within guilds. Our results reveal fundamental mechanisms that underpin fungal genome size and nucleotide composition, which helps to understand fungal growth and mycelial construction along soil nutrient gradients.