Associate Professor University of Minnesota St. Paul, Minnesota, United States
Abstract: Biodiversity often contributes to ecosystem functioning, even under perturbations such as nutrient eutrophication. Many grassland ecosystems are experiencing nutrient enrichment, which generally increases grassland biomass production but causes diversity decline. These impacts can partly result from the increase of nutrient availability shifting species interactions from facilitation to competition with respect to nutrient acquisition, which may be exaggerated as N inputs and effects accumulate over many decades. However, it remains unclear whether and to what extent the impacts of nutrient addition on plant diversity, productivity as well as their relationships are reversible after the cessation of nutrient addition, and how plant community composition may contribute to any recovery. In a 4-decade grassland experiment at Cedar Creek Ecosystem Science Reserve in Minnesota, USA, we monitored the dynamics of plant species diversity, composition, and aboveground biomass production, in plots that had either: (1) no nutrient addition (control plots), (2) 40 years of continuous N addition at 8 rates from 0 to 270 kg/ha/year, (3) 10 years of N addition followed by a 30-year opportunity for recovery after cessation of N addition or (4) 30 years of N addition followed by a 10-year opportunity for recovery after cessation of N addition.
Our experiment showed a hysteretic recovery in species richness 30 years after cessation of N addition. With low and medium levels of previous N addition, species richness showed over and full recovery compared to control plots, and with high levels of previous N addition species richness showed partial recovery. In contrast, the effect of N addition on biomass production vanished shortly after cessation and remain low for 30 years after cessation of nutrient addition. Most of the recovered species are native species and non-legume forb, leading to communities that are less productive. On the other hand, few C3 grasses species remains dominance after the cessation of high-levels N addition. These composition changes likely lead to irreversible diversity-productivity relationship after cessation. In addition, compared to those received 10 years of N addition, the plots that received 30 years of N addition before cessation showed larger effects of N addition on species richness and community biomass, which fluctuated less with duration after cessation, indicating longer time needed for recovery after higher cumulative N addition. These findings help inform agricultural management and conservation efforts of the potential for unamended recovery in plant diversity, which are helpful for preserving diversity and its effect on ecosystem functioning.