FIGURE S1. Interannual distribution of monthly precipitation (mm; black bars), estimated monthly water applied by irrigations (mm; grey bars), mean monthly temperature (°C; solid line) during the experimental period (year 1 and year 2); and mean monthly temperature for the last 30 years (°C; dotted line).

FIGURE S2. Accumulated dry matter (DM) of the mixed swards in (a) year 1, and (b) year 2, for each fertilization treatment (0 N, 60 U, 120 U, 240 U, 60 M, 120 M and 240 M). Productivity is indicated for each component, legume (dark grey bar) and grass (light grey bar). Error bars indicates ±SE of the mean of the total forage harvested (n = 4). Different letters indicate significant differences (Tukey, 0.05).

FIGURE S3. Relationship between the average P and N concentration in the dry matter (DM) of the harvested grass (solid line) and legume (dashed line) forage components of the mixed swards for each fertilization treatment (0 N, 60 U, 120 U, 240 U, 60 M, 120 M and 240 M). Linear regression is given for each grass and legume component. Solid coarse line indicates the critical P curve (Pc = 0.065 Nc + 1.5; Duru & Ducrocq, 1997). Statistical significance (p-value, .05) is indicated for each variable analysed (g P kg⁻¹ DM grass and legume).

FIGURE S4. Soil microbial abundance/activity parameters of the mixed swards during spring and summer of Year 1: October 2018–May 2019 and Year 2: October 2019–May 2020, in response to the fertilization treatments: 0 N, 60 U, 120 U, 240 U, 60 M, 120 M and 240 M. In (a) total soil microbial activity (Fluorescein [F] diacetate hydrolysis in dry soil [DS]); (b) abundance of AMF spores; colonization of roots of the mixed sward by AMF measured as (c) total mycorrhizal frequency (AMC%); and (d) arbuscule frequency (A%). Error bar indicates ±SE of the mean (n = 4). Different letters indicate significant differences; n/s indicates not significant differences (Tukey, .05).