Sown species compositionThis file contains information on which species were sown into each of 27 experimental plots (2x2 m). The first column (‘plot.id’) gives the ID of the plot, linking the table to the other tables. All other columns contain presence/absence information of the plant species. 1, species sown; 0, species not sown.Sown.txtLevel of nutrient supply and biomass of experimental plotsContains information on manipulated level of nutrient supply (low, moderate or high) of the plots and their aboveground biomass (g/m2) in the third year (2015). The first column (‘plot.id’) gives the ID of the plot, linking the table to the other tables.NutBiom.txtObserved species composition in first growing season (2013)Presence/absence information of plant species in 675 0.1x0.1-m squares belonging to 27 permanent plots of 0.5x0.5 m, sampled in the first growing season (2013). Each permanent plot contained 25 squares (5 rows and 5 columns), which were numbered from left to right, beginning in the first row of the permanent plot. The first column (‘square.id’) is a running number containing the square ID. The second column (‘plot.id’) gives the ID of the plot, linking the table to the other tables. The third column (‘square.no’) gives the number of the square in the respective permanent plot. All other columns contain presence/absence information of the plant species. 1, species present; 0, species absent.permplot2013.txtObserved species composition in second growing season (2014)Presence/absence information of plant species in 675 0.1x0.1-m squares belonging to 27 permanent plots of 0.5x0.5 m, sampled in the second growing season (2014). Each permanent plot contained 25 squares (5 rows and 5 columns), which were numbered from left to right, beginning in the first row of the permanent plot. The first column (‘square.id’) is a running number containing the square ID. The second column (‘plot.id’) gives the ID of the plot, linking the table to the other tables. The third column (‘square.no’) gives the number of the square in the respective permanent plot. All other columns contain presence/absence information of the plant species. 1, species present; 0, species absent.permplot2014.txtObserved species composition in third growing season (2015)Presence/absence information of plant species in 675 0.1x0.1-m squares belonging to 27 permanent plots of 0.5x0.5 m, sampled in the third growing season (2015). Each permanent plot contained 25 squares (5 rows and 5 columns), which were numbered from left to right, beginning in the first row of the permanent plot. The first column (‘square.id’) is a running number containing the square ID. The second column (‘plot.id’) gives the ID of the plot, linking the table to the other tables. The third column (‘square.no’) gives the number of the square in the respective permanent plot. All other columns contain presence/absence information of the plant species. 1, species present; 0, species absent.permplot2015.txtAdditional species in 2013Additional species found in 2013 outside the 0.5x0.5-m permanent plots but within the 2x2-m experimental plots. The first column (‘plot.id’) gives the ID of the plot, linking the table to the other tables. All other columns contain presence/absence information of the plant species. 1, species present; 0, species absent.addplot2013.txtAdditional species in 2014Additional species found in 2014 outside the 0.5x0.5-m permanent plots but within the 2x2-m experimental plots. The first column (‘plot.id’) gives the ID of the plot, linking the table to the other tables. All other columns contain presence/absence information of the plant species. 1, species present; 0, species absent.addplot2014.txtAdditional species in 2015Additional species found in 2015 outside the 0.5x0.5-m permanent plots but within the 2x2-m experimental plots. The first column (‘plot.id’) gives the ID of the plot, linking the table to the other tables. All other columns contain presence/absence information of the plant species. 1, species present; 0, species absent.addplot2015.txt

Niche-based selection and stochastic processes can operate simultaneously to generate spatial and temporal variation in species composition. Yet, the conditions under which ecological dynamics are dominated by niche-based vs. stochastic processes are poorly understood. Using a field experiment in early-successional temperate grassland and null models of beta diversity, this study investigates the effects of soil nutrient supply on the relative importance of niche-based selection vs. stochastic dynamics for variation in species composition among sites. Nutrient availability was manipulated experimentally, individual seed mixtures with 25 species were sown in each experimental plot, and then stochastic and deterministic niche-based assembly processes were allowed to happen. We found that compositional variation among grassland plots with low nutrient supply was driven by stochastic immigration and extinctions. In contrast, nutrient enrichment reduced the importance of stochasticity and imposed a deterministic environmental filter that homogenized communities through the selection of few species with greater competitive ability for light. This demonstrates that soil nutrient availability is a critical environmental feature that dictates the degree to which terrestrial plant communities are controlled by niche-based selection vs. stochastic assembly processes. Our study shows further that alternative states of eutrophic grasslands emerge from initial stochastic variation in the composition of a particular functional group of species that can become dominant at high nutrient supply. We discuss potential mechanisms underlying the shift from stochastic to niche-driven dynamics along soil nutrient gradients.