Leaf-root trait relationships in Fabaceae and Poaceae forage species under contrasting nitrogen addition forms and levels

Introduction Nitrogen enrichment can reshape above- and belowground traits, but it remains unclear whether N form and supply level alter leaf-root coordination and whether such coordination is related to biomass. We therefore tested how ammonium and nitrate addition affect leaf and root traits, whether leaf-root relationships persist after accounting for species differences, and whether individual traits are independently associated with biomass. Methods Five perennial forage species, including two Fabaceae and three Poaceae, were grown for one field season under a control and three levels of ammonium or nitrate (6, 24, and 72 g N m - ²). We measured specific leaf area (SLA), leaf N concentration (LN), specific root length (SRL), root N concentration (RN), and total biomass. Trait responses were evaluated using treatment-versus-control contrasts and factorial models, and leaf-root relationships using raw, within-species centered, and N-induced response data. Species-standardized biomass responses were used as a robustness check. Results LN and SRL generally increased with N level, whereas SLA, RN, and biomass showed stronger species-dependent responses. Significant treatment-control differences occurred mainly at intermediate and high N addition. SLA-SRL covariation occurred within Poaceae in raw and centered data, but functional-group slopes did not differ. Raw LN-RN slopes differed between functional groups, but this difference disappeared after species mean centering. N-induced SLA-SRL responses were not associated, whereas LN-RN responses were positively related within Poaceae without a significant difference in slopes between groups. Relative biomass responses increased with N level and differed between N forms, but not between functional groups. No individual trait showed a strong independent association with biomass after accounting for species, N regime, and trait covariation. Discussion N enrichment produced trait-specific and species-dependent responses, while apparent leaf-root coordination depended on the source of variation considered. These findings highlight the importance of separating cross-species covariation from within-species responses when interpreting plant responses to N enrichment.