ABSTRACT.- Background and aims:Since 1750, the species-rich grasslands native to the Río de la Plata basin in southern South America have sustained low-input livestock production. Typically, these grasslands have low herbage phosphorus (P) concentration, and P deficiency in livestock is not rare. Worldwide, P fertilization is used to increase the productivity and nutritive value of grasslands, but it is unknown whether the Río de la Plata grasslands are limited by P availability. -- Methods:We quantified the response of aboveground net primary productivity (ANPP) to P supply in a meta-analysis of 39 trials carried out between 1968 and 2016 comprising 285 independent comparisons (cases) between unfertilized controls and P-fertilized treatments. -- Results:The ANPP of unfertilized controls ranged from 400 to 17,500 kg dry matter (DM) ha-1 y-1. In 74% of cases, P fertilization did not increase ANPP. The average response was therefore low, at 399 kg DM ha-1 (relative effect + 15%), and largely unrelated to ANPP, vegetation, or soil characteristics. Conversely, in 59% of cases herbage P concentration did increase, particularly at high fertilization rates (average 0.43 mg P g DM-1; relative effect + 36%). -- Conclusions:Despite low herbage P concentration, the current ANPP of Río de la Plata grasslands seems little limited by P supply. Luxury consumption was common in response to P fertilization. Prevailing low herbage nitrogen-to-phosphorus ratios -below 11:1 in 65% of unfertilized controls- suggest nitrogen may be limiting ANPP. In this legume-scarce ecosystem, P fertilization is unlikely to boost productivity and may pose an environmental risk. © 2024 Springer Nature