Nitrogen addition and ecosystem functioning: Both species abundances and traits alter community structure and function

Increased nutrient inputs can cause shifts in plant community composition and plant functional traits, both of which affect ecosystem function. We studied community‐ and species‐level leaf functional trait changes in a full factorial nitrogen (N), phosphorus (P), and potassium (K) fertilization experiment in a semi‐arid grassland. Nitrogen was the only nutrient addition to significantly affect leaf functional traits, and N addition increased community‐weighted specific leaf area (SLA) by 19%, leaf chlorophyll content by 34%, height by 26%, and leaf dry matter content (LDMC) decreased by 11% while leaf thickness and toughness did not change significantly. At the species level, most species contributed to the community‐weighted trait and increased inSLA, chlorophyll, height, andLDMCwith N addition. These intraspecific changes in functional traits account for 51–71% of the community‐level changes inSLA, chlorophyll, plant height, andLDMC. The remaining change is due to species abundance changes; the two most abundant species (Bouteloua gracilisandCarex filifolia) decreased in abundance with N addition while subdominant species increased in abundance. We also found annual variation inSLA, chlorophyll, plant height, andLDMCto be as important in influencing traits as N addition, likely due to differences in precipitation. Aboveground net primary productivity (ANPP) did not change significantly with N addition. However, N addition caused a 34% increase in leaf area index (LAI) and a 67% increase in canopy chlorophyll density. We demonstrate that nitrogen‐induced changes in both functional traits and species abundances magnifyANPPchanges inLAIand canopy chlorophyll density. Therefore,ANPPunderestimates N addition‐induced ecosystem‐level changes in the canopy vegetation.