Facilitative N niche complementarity through grass-legume balance in the Anthropocene: A conceptual synthesis

Grasslands cover nearly 40% of the terrestrial surface and support essential ecosystem services, yet their biodiversity, productivity, and stability are increasingly threatened by climate extremes, land use change, and disruption of biogeochemical cycles in the Anthropocene. Nitrogen (N) limitation is a primary constraint on grassland functioning, and incorporation of N-fixing legumes can alleviate this limitation. However, system outcomes depend less on legume presence alone than on the balance achieved among grass and legume functional groups. Here, we present a conceptual synthesis integrating ecological theory with empirical evidence to examine how continuous variation in functional-group balance regulates N cycling, primary production, forage nutritive value, livestock performance and ecosystem resilience. Our synthesis advances four main points: (1) Ecosystem responses along species gradients are frequently nonlinear, with intermediate optima emerging from facilitative N niche complementarity interacting with physiological and competitive constraints. (2) These optima vary systematically with climatic context, species physiology and management intensity, with C₄ grass-based systems in warm climates typically reaching peak performance at lower legume proportions than cooler C₃ grass-dominated systems. (3) Climate change is likely to decouple vegetation dynamics from functional optima by disproportionately stimulating legume growth and altering water and nutrient efficiencies, increasing the risk of mismatches between botanical composition and ecosystem performance. (4) Integrating resource-ratio and state-and-transition frameworks, we discuss how grazing management, N inputs, and herbivore diet selectivity can shift systems toward target species proportions that enhance production efficiency while strengthening resistance and recovery capacity in mixed grassland. We conclude by outlining research and management agendas emphasizing gradient-based experimental and observational designs to better resolve continuous agroecological response functions, improve cross-system synthesis, and inform context-specific strategies for enhancing productivity, efficiency, and resilience of grasslands under increasing climatic variability.