Increasing nitrogen (N) deposition and changing precipitation patterns in Neotropical savannas could alter plant growth, reproduction, and nutrients by altering soil nutrient and water availability. We examined the potential for simulated N deposition and increased dry season precipitation to have interactive effects on reproduction and growth of two abundant native Cerrado (Brazilian savanna) grasses—Loudetiopsis chrysothrix and Tristachya leiostachya—via feedbacks with soil nutrient status. Plant growth and reproduction responses consistently varied by species. Water addition led to more consistent increases in both growth and reproduction than nitrogen addition and the two treatments did have significant interactive effects. We expected that both treatments would affect plant growth and reproduction via positive effects on soil and plant N. Instead, we found that plant responses were linked to species-specific treatment effects on soil and foliar phosphorus (P). Structural equation models (SEM) confirmed that changes in soil P—rather than changes in soil N or increasing soil acidity—explained plant response to treatments. Our results imply that N deposition and precipitation change could impact Cerrado plant growth and reproduction via subtle effects on plant and soil phosphorus.