Divergent drivers of above‐and below‐ground productivity responses to nitrogen enrichment across a 3500‐km transect

Abstract Nitrogen (N) enrichment is a key driver of global environmental change and strongly affects net primary productivity (NPP). However, research to date has largely focused on above‐ground NPP (ANPP), with limited attention to below‐ground NPP (BNPP). There is also limited understanding of how both ANPP and BNPP respond to N inputs across large spatial scales and under varying environmental conditions. To address this gap, we conducted a regional‐scale field experiment along a 3500 km transect across temperate grasslands in northern China. This allowed us to examine how N addition affects ANPP and BNPP and to identify the environmental factors driving their responses. We found that under non‐water‐limited conditions, N addition stimulates productivity primarily by altering community composition rather than species diversity. This indicates productivity as a sensitive ecological indicator and highlights the key role of water availability. The responses of ANPP and BNPP to N differed: ANPP followed an inverted U‐shaped pattern regulated by water availability, while BNPP increased linearly with N availability. Synthesis. These contrasting responses reflect the combined influence of optimal allocation strategies and shifting resource limitations—from water to N and then to other factors. Our findings reveal distinct mechanisms regulating above‐ and below‐ground productivity at large scales, offering new insights into carbon cycling processes linked to primary productivity and informing sustainable grassland management under global change.