Nitrogen enrichment is a major global change driver, which directly leads to higher biomass production but also to a loss of species richness and shifts in functional composition from slow growing (low specific leaf area, SLA) toward faster growing (high SLA) plant communities. These effects are typically studied in isolation: for instance many experiments have investigated the impact of diversity loss but its importance relative to other drivers of function is not well known. These different direct and indirect effects of nitrogen were investigated in a grassland field experiment that factorially manipulates nitrogen addition, plant species richness and plant functional composition. The PaNDiv experiment, located near Bern in Switzerland, comprises 336 2x2m plots sown with different numbers and compositions of 20 common grassland herbs and grasses. We measured aboveground biomass and leaf traits on each plot. We also conducted a decomposition experiment with different litter bags to investigate the individual and combined impacts of changes in soil and litter quality, caused by our treatments, on decomposition rate.
Results/Conclusions
Nitrogen addition and species diversity both increased biomass production but diversity effects were larger than those of nitrogen. Nitrogen addition also tended to reduce the positive effect of diversity because, although it led to slightly more positive selection effects, it strongly decreased the degree of complementarity between species, probably because it reduced niche dimensions. Functional composition was also an important driver of biomass and high SLA communities produced more biomass, however, this effect was smaller than that of N or diversity. Decomposition on the other hand was mostly affected by nitrogen fertilisation and functional composition: decomposition was faster in plots with nitrogen added and in those dominated by fast growing species (high SLA). Species diversity did have some effects on soil communities and increased soil decomposition potential. Our results clearly show that biodiversity was a major driver particularly of biomass production, even compared to the other experimental treatments such as nitrogen addition. This suggests that maintaining diversity in grasslands is crucial to maintain high functioning.