Background/Question/Methods N and P have the potential to limit plant productivity, but we currently have a poor understanding of how N and P availability interact at geographic scales to constrain plant growth. To better understand how N and P availability interact in limiting productivity, soils were acquired from 99 grasslands across the US, from Washington to Florida. In each soil, individuals of Schizachyrium scoparium, a wide-spread C4 grass, were grown from seed and inorganic N and P added in factorial. After 76 d after germination of the first seeds, aboveground biomass was harvested to the soil level and soil washed from roots.

Results/Conclusions Among the 99 unfertilized soils, biomass varied over 60-fold. On average, N fertilization increased biomass by 81% over unfertilized soils while P fertilization increased biomass by 22% over control soils. The response of plants to nutrient addition was dependent on inherent soil nutrient availability. For example, low soil P availability constrained the response to N fertilization. Yet, plants growing on low-P soils were not necessarily more likely to respond greater to P addition—plants on low-P soils were also strongly N-limited. Fertilization with N and P increased biomass by 131% relative to controls, which was 29% greater than N-fertilized soils and 90% greater than P- fertilized soils. Plants fertilized with N and P had the lowest variation in total biomass, yet still varied 6-fold—raising questions about what soil factors besides N and P limit grass production.