Responses of two contrasting saline-alkaline grassland communities to nitrogen addition during early secondary succession

Question: (1) How do secondary successional dynamics differ in high and low saline-alkaline grassland communities in semi-arid lands; and (2) does nitrogen (N) enrichment promote plant secondary succession towards a perennial community under saline-alkaline stress? Location: Saline-alkaline grassland in northeast China. Methods: We selected two early successional communities differing in soil saline-alkaline stress and floristic composition: one 'high-stress community' codominated by the annual grass Chloris virgata and the annual forbs Artemisia scoparìa and Kochia sieversiana, and one 'low-stress community' dominated by the annual forb K. sieversiana. A complete randomized design experiment with five N addition levels was conducted in triplicate for four consecutive years (2008-2011). The vegetation variables (above-ground biomass of individual species, functional group, community level, community species richness) and soil characteristics (pH, electrical conductivity as a proxy for salinity) were investigated every year. Detrended correspondence analysis (DCA) was used to evaluate the successional trajectories. Results: DCA revealed clear differences in species composition and successional trajectories between the two communities. There were few changes in the species composition of the high-stress community, with annual species dominating all the plots over the 4 yr. However, most of the low-stress communities developed from the annual-dominated stage to the perennial-dominated stage. N addition had no effect on the relative biomass of the perennial functional group in the two communities. Thus, N addition did not obviously promote plant succession towards a perennial community, even under low saline-alkaline stress. Species richness did not respond to N addition in the high-stress community, whereas species richness declined across the N addition gradient in the lowstress community. Conclusion: Our results provided direct empirical evidence that high saline-alkaline stress limits plant community succession, and low saline-alkaline stress communities exhibit a succession direction from the annual-dominated stage to the perennial-dominated stage. Interestingly, replicates within the same treatment under low-stress conditions did not develop towards a similar community composition, indicating the importance of environment heterogeneity at a small spatial scale. N addition did not promote the expected perennial plant succession; however, N enrichment redu