Fig. 1.
Effects of N addition on productivity and plant diversity in natural grasslands. (A) Nutrient enrichment promoted productivity (F7,176 = 54.5, P < 2.2 × 10−16), but this effect substantially diminished over time (F1,176 = 36.9, P = 7.48 × 10−9), especially in the plots that received the most fertilizer (N × Year: F7,176 = 2.8, P = 0.0096). (B) Nutrient enrichment also decreased the number of species (F7,176 = 72.4, P < 2.2 × 10−16), and this effect became increasingly negative over time (F1,176 = 92.1, P < 2.2 × 10−16) at all rates of N addition (N × Year: F7,176 = 1.6, P = 0.13). (C and D) Declines in the number of species were positively associated with declines in productivity. That is, productivity decreased most in plots that lost the most species over time (F1,7 = 42.9, P = 0.00032, R2 = 0.86) (D). Differences in biomass (A) or number of species (B) were quantified between enriched and control plots, with positive biomass differences indicating that enriched plots were more productive, and with negative species differences indicating that enriched plots had fewer species, than control plots. Arrows in C point from values observed during 1982, the first year of N addition, toward values observed during 2008, the most recent year that all fields were sampled. Changes in D were quantified over the same time interval. In D, for comparison, we also show the relationship between planted species richness and productivity in a nearby biodiversity experiment during 2008 (gray triangles and dashed line). Both lines in D are linear fits of the response on the ln-transformed predictor. Treatments C (control) and 0 differ in that non-N nutrients were added to the latter but not the former.