Supplement 1. The complete data set.

File List fertilization_treatments.csv (MD5: 89bc45540bf3305cb2ef152cc68eaa0f) root_abundance_and_morphological_traits.csv (MD5: 31d7fb5a0a3858148df72683d227b914) root_nutrients.csv (MD5: 168940dbbfac94021d6be522d4978e9a) mycorrhizal_colonization.csv (MD5: f83d44888e6ffd45b60544ae605873f1) Description fertilization_treatments.csv is a comma-separated text file containing the fertilization treatments of the Gigante experiment. Column definitions are: treatment is the fertilization treatment, where N = nitrogen, P = phosphorus and K = potassium. plot is the plot number associated with each treatment. root_abundance_and_morphological_traits.csv is a comma-separated text file containing the root abundance and morphological trait dataset. Column definitions are: plot is treatment plot. core is core sample number. size_class is root diameter size class, where 0 = 0–1 mm, 1 = 1–2 mm and 2 = 0–2 mm. biomass is root biomass in g/m2 length is root length in m/m2 t_den is root tissue density in g/cm3 srl is specific root length in cm/g avg_diam is average root diameter in mm. root_nutrients.csv is a comma-separated text file containing the nutrient content of root tissue samples. Column definitions are: plot is treatment plot size_class is root diameter size class, where 0 = 0–1 mm, 1 = 1–2 mm and 2 = 0–2 mm. N is root tissue nitrogen concentration (%). C is root tissue carbon concentration (%). P is root tissue phosphorus concentration (ppm). K is root tissue potassium concentration (ppm). mycorrhizal_colonization.csv is a comma-separated text file containing mycorrhizal colonization dataset. Column definitions are: plot is treatment plot. arb_per is colonization of arbuscules expressed as a percent of root length ves_per is colonization of vesicles expressed as a percent of root length hyp_per is colonization of hyphae expressed as a percent of root length tot_per is total colonization expressed as a percent of root length arb_len is colonization of arbuscules expressed as root length (cm). ves_len is colonization of vesicles expressed as root length (cm). hyp_len is colonization of hyphae expressed as root length (cm). tot_len is total colonization expressed as root length (cm).