Functional group dominance-mediated N:P effects on community productivity depend on soil nutrient levels

Plant functional groups can mediate impacts of soil nutrient imbalances (i.e., a condition where an essential nutrient element is relatively deficient or excessive) on community productivity via the complementary roles of individual species. However, it is still unclear whether the relative dominance of plant functional groups can alter the effects of available N:P ratios on community productivity and whether such an effect depends on the total amount of nutrients available in the soil. We created experimental plant communities with six grassland species of three functional groups (two species of grasses, legumes, and non-leguminous forbs) in four types (grass-dominant, forb-dominant, legume-dominant, and even community) under three available N:P ratios (1.7, 15, and 135, i.e., low, intermediate, and high) and two nutrient levels (the total amount of nutrients was low and high). In the even, the grass-dominant, and the forb-dominant community, community biomass was significantly higher at the low than at the high N:P ratio, but it did not differ significantly among the three N:P ratios at the low-nutrient level. By contrast, in the legume-dominant community, although community biomass was also significantly higher at the low than at the high N:P ratio, it was significantly lower at the low N:P ratio than at the high N:P ratio at the low-nutrient level. Biomass of functional groups (grasses, legumes, and forbs) showed the similar pattern as community biomass, except in the legume-dominant community at the high-nutrient level, which did not differ between the three N:P ratios. Our results suggest that the dominance of plant functional groups can mediate the impacts of N:P ratios on the productivity of grassland communities, but such an impact varied depending on the total amount of nutrients available in the soil.