Nitrogen addition level is more influential to aboveground productivity than nitrogen addition frequency in an alpine grassland

Engineering notes

Community aboveground net primary productivity (ANPP) increased significantly with nitrogen addition level (F = 10.930, P< 0.001). Poaceae ANPP increased significantly with nitrogen addition level (p = 0.009), whereas Rosaceae and Fabaceae showed no significant response but exhibited progressive phosphorus limitation under high nitrogen levels.

Chinese explanation / 中文解读

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Original abstract

Aims The combined effects of climate change and human activities have greatly increased atmospheric nitrogen deposition globally, with profound impacts on plant growth and grassland productivity. This study investigates the mechanisms governing how plant aboveground productivity responds to nitrogen addition with varied addition levels and frequencies in Bayanbulak alpine grassland. Methods A field experiment was conducted with five N addition levels (0, 5, 10, 15, 20 g·m -2 ) and two frequencies (low and high). Soil properties, plant aboveground biomass and nutrient content were analyzed using ANOVA, Mantel tests, and structural equation modeling (SEM). Results Nitrogen addition level was the primary driver of changes in soil properties and plant productivity. Community aboveground net primary productivity (ANPP) increased significantly with nitrogen addition level (F = 10.930, P< 0.001). Poaceae ANPP increased significantly with nitrogen addition level (p = 0.009), whereas Rosaceae and Fabaceae showed no significant response but exhibited progressive phosphorus limitation under high nitrogen levels. Significant level-dependent increases were observed for soil nitrate nitrogen (soil NN) and available phosphorus (soil AP) (p< 0.05). Level × frequency interactions were significant for several soil variables, while addition frequency alone had limited main effects. SEM further revealed that the direct effect of nitrogen addition level on ANPP was significant for Poaceae under high-frequency addition, with additional indirect mediation via plant C:N. Nitrogen addition indirectly affected Rosaceae ANPP via plant TN, TP, C:P and N:P, with TP and C:P exerting negative effects; while for Fabaceae, no significant pathway detected, indicating its insensitivity to nitrogen enrichment. Plant nutrient content was a stronger predictor of ANPP than soil properties. Conclusions Nitrogen addition level exerts a stronger and more direct influence on aboveground productivity than frequency, indicating that total nitrogen load, rather than its temporal pattern, is the key driver of productivity changes. Functional groups show decoupled N–P dynamics: Poaceae benefit from alleviated N limitation, while Rosaceae and Fabaceae suffer progressive P limitation, and suppressed N fixation in Fabaceae. Predicting N deposition impacts thus requires considering these functional-group-specific responses.

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