Numerous field studies have measured mycorrhizal dynamics under additions of nitrogen, phosphorus, or atmospheric CO₂ to test the hypothesis that plants should invest in mycorrhizal fungi when soil nutrients are limiting. I used meta-analyses to integrate nutrient responses across independent field-based studies. Responses were compared between ecto- and arbuscular mycorrhizal fungi, and among fertilizer types, methods of measurement, biomes, and lead investigators. Relationships between degree of response and study length, fertilization rates, total amounts of nutrients applied, and numbers of replicates were also tested. Across studies, mycorrhizal abundance decreased 15% under nitrogen fertilization and 32% under phosphorus fertilization. Elevated CO₂ elicited a 47% increase. Nitrogen effects varied significantly among studies, and phosphorus effects varied significantly among lead investigators. Most other factors did not affect mycorrhizal responses. These results support the plant investment hypothesis, and suggest that global standing stocks of mycorrhizal fungi may increase substantially under elevated CO₂ but decline moderately under phosphorus additions. Effects of nitrogen deposition may be difficult to predict for individual ecosystems, with a slightly negative influence overall.