Nitrogen saturation drives shifts in response patterns of non-structural carbohydrate pools in a meadow steppe after...

Non-structural carbohydrates (NSC) in grassland plants represent the outcome of carbon (C) allocation processes and serve as integrative indicators of plant C status and nutrient balance. However, their responses to different nitrogen (N) addition rates and to the cessation of long-term N addition remain unclear. In a temperate meadow steppe, we conducted a field experiment with six levels of N addition. We compared changes in the NSC pools and its components (soluble sugars, SS; starch, ST) at the community and functional-group levels. Community NSC and ST pools increased with increasing N addition rates, peaking at 20 g N m^-² yr^-¹, and subsequently declined after six years of continuous N addition. Three years after ceasing N addition, this single-peak response pattern changed, with no decline observed above the peak rate of 20 g N m^-² yr^-¹. NSC reserves were constrained by a saturation threshold of community N concentrations under N enrichment and N cessation scenarios. Before reaching this threshold, N addition increased community N concentrations and promoted the accumulation of the ST-dominated NSC pools. Above the threshold, the C-N coupling relationship weakened, and excessive N addition had negative effects on NSC and ST pools. From N enrichment to N cessation, the saturation threshold of community N concentrations shifted from 20 to 50 g N m^-2 yr^-¹, thus mitigating the adverse effects of high N addition on NSC pools. This study reveals the regulatory mechanisms of grassland C dynamics under the contrasting scenarios of N addition and N cessation and clarifies the differential responses of NSC reserves under N saturation constraints.