The article examines the synergistic interactions between nitrogen (N) and phosphorus (P) in regulating primary producer communities across freshwater, marine, and terrestrial systems. Using a meta-analysis of 641 studies, the authors found that over half of the studies exhibited some form of synergistic response to N and P addition. Strict definitions of co-limitation, where community biomass responded only to combined N and P addition or to both nutrients when added separately, were observed in 28% of the studies. The results highlight the importance of N and P interactions in controlling primary producer community biomass and suggest the need for further research to understand the mechanisms leading to different types of co-limitation. The study also discusses the implications of these findings for nutrient pollution and eutrophication in various ecosystems.The article examines the synergistic interactions between nitrogen (N) and phosphorus (P) in regulating primary producer communities across freshwater, marine, and terrestrial systems. Using a meta-analysis of 641 studies, the authors found that over half of the studies exhibited some form of synergistic response to N and P addition. Strict definitions of co-limitation, where community biomass responded only to combined N and P addition or to both nutrients when added separately, were observed in 28% of the studies. The results highlight the importance of N and P interactions in controlling primary producer community biomass and suggest the need for further research to understand the mechanisms leading to different types of co-limitation. The study also discusses the implications of these findings for nutrient pollution and eutrophication in various ecosystems.