The temporal stability of above-ground biomass (AGB) and below-ground biomass (BGB) in grasslands is crucial for maintaining a continuous supply of ecosystem functions and services, particularly in the context of global changes. Nitrogen (N) addition is well known to affect AGB stability, however, we still lack knowledge of how N addition affect BGB stability. Furthermore, a crucial knowledge gap remains regarding which underlying mechanisms drive AGB and BGB stability, which obstructs our comprehensive awareness of biomass stability from both above- and below-ground perspectives simultaneously. Through a five-year manipulative experiment with seven N addition levels, we tested how AGB and BGB stability responded to N addition and which factors modulated the effects of N addition on stability. Using a framework developed to quantify changes in biomass stability, we investigated the relative contributions of dominant species, taxonomic diversity, functional diversity, species asynchrony, species stability and soil properties in driving AGB and BGB stability. We found that N addition enhanced AGB stability directly and indirectly through the modulatory effects of dominant species. N addition increased dominance of fast species, causing high community-weighted mean (CWM) fast-slow and further high species asynchrony. Large increases in species asynchrony and weak decreases in dominant species stability, rather than decreases in species richness, were crucial factors driving AGB stability. Additionally, increased CWM fast-slow enhanced BGB stability, although this positive effect was partially offset by a slight decrease in soil water content (SWC). Our results broaden the insurance hypothesis and the mass ratio hypothesis, providing new insights into how N addition affects biomass stability and underlying driving mechanisms in a temperate desert steppe. These findings emphasize that dominant species and CWM fast-slow play crucial modulatory roles in driving biomass stability. Therefore, it is very necessary to pay attention to dominant species and functional diversity, in order to provide guidance for the sustainable functions and services in species-poor drylands.
Keywords: Biomass stability; CWM fast-slow; Dominant species stability; Nitrogen deposition; Soil properties; Species asynchrony.
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