Dominant species drive biomass and diversity responses to nutrient inputs

Global change is enriching terrestrial ecosystems with multiple nutrients
and amplifying interannual variation in precipitation. Grassland
productivity may be co-limited by combinations of nitrogen (N), phosphorus
(P), and potassium (K). How these nutrients may interact with each other
or with varying precipitation to influence the contributions of dominant
species and functional groups to aboveground net primary productivity
(ANPP) and species diversity is rarely considered. We fertilized a mesic
grassland for five years with all combinations of N, P, and K +
micronutrients in the first year (Kµ) to test which nutrients 1) limited
ANPP and functional group biomass, 2) reordered dominant species and
impacted plant species diversity, and 3) interacted with annual
precipitation to influence these responses. Adding N and P together
disproportionately increased ANPP, but adding both N and P or N and Kµ
disproportionately increased forb biomass to account for nearly all (90%)
of ANPP. Grass biomass was correlated with light availability, not
nutrients, and legume biomass decreased with added N, with or without
other nutrients. Nutrient combinations (mainly NP and NPKµ) causing the
greatest increases in forb biomass and ANPP also resulted in replacement
of dominant species by an annual forb and decreased species diversity
(Shannon index), evenness, and species richness. Nutrient combinations (P,
Kµ, PKµ) not increasing biomass favored dominance by C4 grasses, and
increased species richness. N effects on ANPP, species diversity, and
richness were greater in years with higher annual precipitation. Annual
precipitation interacted with all three nutrients to exert sometimes
positive and sometimes negative feedback on the abundance of the most
dominant species. Dominant species drive nutrient effects on community
productivity and species diversity. An expanded definition of nutrient
limitation incorporating constituent responses will improve understanding
of anthropogenic nutrient inputs on ecosystem productivity and related
ecosystem services.