Nitrogen deposition has significantly reduced forest soil respiration, with effects observed in temperate forests. This study, based on a meta-analysis of nitrogen-addition experiments and comparisons of sites with elevated or background nitrogen deposition, shows that nitrogen deposition reduces heterotrophic respiration and soil carbon dioxide efflux (SCE) by up to 15% and 17%, respectively. These reductions are attributed to decreased microbial activity and lower decomposition rates of soil organic matter (SOM). The negative effects of nitrogen on soil respiration are more pronounced in highly productive forests where nitrogen is not limiting, and less so in less productive areas. The study also found that nitrogen deposition can stimulate soil carbon sequestration, but this effect is offset by the reduction in soil respiration. The mechanisms underlying these effects include reduced below-ground carbon allocation, shifts in microbial community structure, and changes in enzyme activity that affect SOM decomposition. The study highlights the importance of incorporating nitrogen cycling into terrestrial carbon-cycle models, as nitrogen deposition has significant impacts on carbon cycling and ecosystem productivity. The findings suggest that nitrogen deposition may have negative effects on tropical forests, where nitrogen is not the most limiting nutrient, and that further research is needed to understand the long-term effects of nitrogen deposition on carbon cycling in these regions. The study underscores the need for a better understanding of nitrogen deposition effects to improve predictions of carbon cycling and ecosystem productivity in the face of climate change.Nitrogen deposition has significantly reduced forest soil respiration, with effects observed in temperate forests. This study, based on a meta-analysis of nitrogen-addition experiments and comparisons of sites with elevated or background nitrogen deposition, shows that nitrogen deposition reduces heterotrophic respiration and soil carbon dioxide efflux (SCE) by up to 15% and 17%, respectively. These reductions are attributed to decreased microbial activity and lower decomposition rates of soil organic matter (SOM). The negative effects of nitrogen on soil respiration are more pronounced in highly productive forests where nitrogen is not limiting, and less so in less productive areas. The study also found that nitrogen deposition can stimulate soil carbon sequestration, but this effect is offset by the reduction in soil respiration. The mechanisms underlying these effects include reduced below-ground carbon allocation, shifts in microbial community structure, and changes in enzyme activity that affect SOM decomposition. The study highlights the importance of incorporating nitrogen cycling into terrestrial carbon-cycle models, as nitrogen deposition has significant impacts on carbon cycling and ecosystem productivity. The findings suggest that nitrogen deposition may have negative effects on tropical forests, where nitrogen is not the most limiting nutrient, and that further research is needed to understand the long-term effects of nitrogen deposition on carbon cycling in these regions. The study underscores the need for a better understanding of nitrogen deposition effects to improve predictions of carbon cycling and ecosystem productivity in the face of climate change.