Background/Question/Methods: Nutrient addition influences grassland ecosystem structure and composition, typically manifested by reduced species richness and increased productivity. Quantifying long-term impacts of nutrient addition on grasslands contributes to understanding and predicting impacts of eutrophication and nitrogen (N) deposition on grassland, particularly for grasslands adapted to low soil nutrient status. A nutrient addition experiment on a natural C
4 grassland was set up in 1950 on the Ukulinga research farm, South Africa (mean annual precipitation 838mm falling in summer; mean monthly maximum temperature 26.4˚C in February; winters mild with occasional frost). Continuously applied treatments on plots measuring 2.7 x 9 m include combinations of nitrogen at 0, 7, 14 and 21 g m
-2 per annum, phosphorus (P) at 0 and 2.8 g m
-2 per annum and lime (L) at 0 and 225 g m
-2 applied very 5 years. Nitrogen sources included Ammonium Sulphate (AS) (acidifying) and Limestone Ammonium Nitrate (LAN) (less acidifying). In 2018, soil samples taken at 0-200 and 200-400 mm depth were analyzed for all relevant soil parameters. In 2019 a full vegetation survey, including light measurements, was completed using a modified Daubenmire technique. We aimed to determine the long-term effect of nutrient addition on grassland productivity, species richness and soil chemistry.
Results/Conclusions: Control plots were less productive than enriched plots. Increasing levels of N increased productivity, except at the highest level of applied N where the marked reduction in vegetation cover, and consequent increase in bare ground area, counteracted the higher soil nutrient status. The impacts of N on productivity were enhanced in the presence of P addition. Soil nutrient status strongly influenced species richness and composition, with richness declining under higher nutrient status. Addition of P strongly influenced species composition, and in the presence of N was associated with tall, broad leafed grass species. Soil acidity increased with AS addition, and influenced species richness and composition, with acidity associated with lower richness, particularly forb richness. Intermediate vegetation and soil surveys throughout the 69 years of nutrient addition indicate rapid initial vegetation composition change, followed by continual reduction in richness coupled with invasion of species adapted to high levels of above-ground competition. Results facilitate predictive modelling of grassland change under conditions of increased soil nutrient status, with selected species traits (including specific leaf area, plant height, growth form) associated with varying levels of productivity and light availability.
