Nutrient enrichment reduces complementarity and increases priority effects in prairies managed for bioenergy

Prairies are a potential bioenergy feedstock that could benefit the environment while providing biomass for bioenergy. Increased diversity and nitrogen fertilization are two methods of increasing prairie productivity. Diversity may enhance complementarity of resource use within prairies, whereas the application of nitrogen fertilizer may cause priority effects whereby species with early phenologies or high responsiveness to fertilization are stimulated. We tested the effects of functional-group identity and nitrogen fertilization on resource capture and growth of prairie plants. To determine whether functionally-diverse mixtures of prairie plants exhibited complementarity and/or priority effects, we measured light interception, canopy duration, and aboveground biomass production of C3 grasses, C4 grasses, legumes, and multi-functional group mixtures grown without fertilization and with 150 kg ha−1 spring-applied nitrogen fertilizer. Under the conditions of our experiment, nitrogen consistently stimulated C3 grasses, whereas it stimulated C4 grasses only in the middle and late parts of the growing season and had no effect on legumes. In functionally-diverse mixtures, priority effects of C3 grasses in fertilized mixtures reduced overall resource capture and growth by suppressing growth of the more productive C4 grasses in the middle and late parts of the growing season. Complementarity occurred in unfertilized mixtures in which C3 and C4 grasses were dominant at different parts of the growing season. Prairies receiving more modest nutrient inputs than those used in this study or nutrient inputs later in the growing season may maximize the production of multiple ecosystem services by producing large amount of biomass while still capturing resources throughout the whole growing season.

► Prairies provide biomass for bioenergy and ecosystem services.
► Both diversity and nitrogen fertilization increase prairie productivity.
► Fertilization stimulated C3 grasses always and C4 grasses later in growing season.
► Unfertilized, diverse prairies had phenological complementarity of resource use.
► Fertilized, diverse prairies had priority effects due to C3 grass dominance.