Nutritional and physiological limitations shape the radiation-use efficiency response to legume proportion in C₄...

Legume introduction is effective for boosting primary productivity in C₄-grass-dominated subtropical and tropical grasslands by overcoming nitrogen (N) limitation and consequently improving radiation-use efficiency (RUE), a key metric underlying plant production. However, an excessive proportion of C₃ legumes may negatively affect RUE, especially in warm climates. We assessed the relationship between aboveground RUE and legume proportion of Paspalum notatum Flügge (C₄ grass) mixtures with a tropical legume (Arachis glabrata Benth.; 0%-80%) under different defoliation and N fertilizer treatments in two studies over 3 years in Florida, USA. Linking the field data to a conceptual model, RUE was optimized at 26%-30% legume proportion across studies and years. When pastures were N-fertilized, RUE plateaued at 26% legume (0.60 g MJ^-1) and linearly decreased with higher legume proportions. When pastures were unfertilized, RUE showed a quadratic relationship with legume proportion, being maximized at 30% legume (1.10 g MJ^-1), overyielding the RUE in only-grass and legume-dominated sites by 110% and 86%, respectively. These responses suggest that RUE is N-limited when legume is below 30% in unfertilized canopies and is physiologically limited when legume is above 30% due to replacement of the C₄ grass with a C₃ legume. These findings provide a robust rationale to target low-to-moderate legume proportions in tropical grasslands for optimizing production and other ecosystem services. We empirically demonstrated that optimum legume proportion is ∼30% in a C₄-grass-based tropical grassland compared with previous observations of ≥40% in C₃-grass-based temperate grasslands, relevant insights for the design and maintenance of grassland ecosystems.