Coupled responses of aboveground and belowground net primary productivity stability to nitrogen deposition in grasslands

The temporal stability of plant net primary productivity (NPP), defined as the ratio of temporal mean productivity to its standard deviation, is crucial for sustaining ecosystem services. Yet, little is known about whether the effects of nitrogen deposition on aboveground NPP (ANPP) stability are coupled with corresponding changes in belowground systems. Here we conducted a global meta-analysis based on 542 multi-year observations from 111 studies to assess the impacts of nitrogen deposition on the stability of ANPP and belowground NPP (BNPP). Nitrogen addition significantly decreased the stability of both ANPP and BNPP in a synchronized manner, reflecting a coupled response resulting from the allocation trade-off between mean ANPP and BNPP and their synchronized increase in variability. Further analysis showed that the declines in both ANPP and BNPP stability were associated with decreased plant species richness under nitrogen addition. Moreover, the negative responses intensified with longer treatment durations and higher nitrogen addition intensity. These findings highlight the importance of considering above- and below-ground interactions in future ecosystem management strategies under global change scenarios.