Mowing does not redress the negative effect of nutrient addition on alpha and beta diversity in a temperate grassland

Abstract

Species loss due to an increasing the number of added nutrients has been explained by both light competition through biomass increase and by niche dimension reduction as a result of species-specific limiting soil resources trade-offs. Disturbances, by reducing community biomass, species dominance and increasing light availability, may counteract above-ground nutrient effects. However, it is unknown if diversity loss at local or spatial scales generated by increasing the number of added nutrients can be redressed with canopy disturbance. We evaluated if local (alpha) and spatial scale (beta) diversity loss generated by the number of added nutrients can be reverted by disturbances in Flooding Pampa grasslands, Argentina. In a 4-year replicated field experiment, we added soil resources combining nitrogen, phosphorus and potassium to obtain 0, 1, 2 or 3 nutrients and manipulated the regime of canopy disturbance by seasonal mowing and biomass removal. We found that the increasing the number of added nutrients strongly reduced local and spatial plant diversity, despite biomass and light changes generated by mowing. In mown plots, nutrient-driven local diversity loss intensified along time, thus increasing species dominance. While mowing did not affect dominant species loss, increasing the number of added nutrients promoted rare species loss and reduced spatial dissimilarity. Furthermore, mowing increased local and spatial diversity regardless of light or biomass effects, suggesting alternative pathway effects for disturbance. Synthesis. Our results demonstrate that even when disturbance generated a positive effect on local and spatial diversity, it did not completely counteract the negative effect of number of added nutrients. Thus, the relative importance of above- and below-ground resource competition may change when chronic disturbances alter community dominance. Under low light availability, above-ground competition may drive species richness loss but when disturbance reduces light limitation, the increasing the number of added nutrients may reduce niche dimensionality and thus species coexistence. In sum, faced with the need to manage eutrophized grasslands, our study showed that disturbance may not completely mitigate the negative effect of multiple nutrient inputs on local and spatial grassland diversity.