Ontogenetic trait variability and nitrogen stoichiometric homeostasis explained high stability of Artemisia frigida-dominated grassland

The degraded grassland is characterized by changes in dominant species and decreased soil resource supply capacity, limiting community foraging by carbon capturing and/or resource absorption. In this study, we used a manipulative greenhouse experiment to explore the plant trait variations of three species representing three degradation succession stages in the Inner Mongolian grassland of China. We examined variability in key trait values during early, middle and late growing stages and assessed the nitrogen stoichiometric homeostasis (HN) of different plant organs growing on loamy (undegraded) and sandy (degraded) soils. The results showed that plant root length (RL) shifted significantly from the early to middle stage, where Artemisia frigida presented the longest RL, and Setaria viridis had the shortest RL. Leyums chinensis exhibited the largest root-shoot ratio (RSR) with a well-developed rhizome, while A. frigida had smaller but variable RSR which increased 1.57-2.15 times, changing from the resource acquisition to the resource conservation strategy from the early to late stage. The shoot nitrogen content (SNC) and root nitrogen content (RNC) decreased over the growing season. No obvious differences were observed between plant traits of L. chinensis or A. frigida when growing on loamy or sandy soils, despite the fact that the nutrient supply in loamy soil was better. Shoot HN showed higher values for plants growing on loamy soils than on sandy soils. The degradation indicator species A. frigida presented the highest shoot HN under sandy soil and held relatively stable root HN for both soils (6.71 vs 6.00), indicating a higher adaptability to sandy soils. Our results explained the occurrence of the stable degraded grassland dominated by A. frigida and could shed light on methods for its restoration.