Figure 20.9 Increases in net primary productivity when nutrients are added to terrestrial ecosystems. Based on 141 experiments conducted around the world, researchers examined how NPP responds to different nutrient additions. To quantify the change in NPP, they determined the ratio of NPP in treatments with added nutrients compared to NPP in the control, which had no added nutrients. The addition of nitrogen and phosphorus increased the NPP of grassland, forest, shrublands, and tundra ecosystems. When both nutrients were added, the response ratio of NPP was often larger than when either nutrient was added separately. Error bars are standard errors.

Data from J. J. Elser et al., Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine, and terrestrial ecosystems, Ecology Letters 10 (2007): 1135–1142.