Effects of nitrogen and phosphorus addition on the net ecosystem CO₂ exchange of meadow steppe in Hulunbuir, Inner Mongolia

Increasing atmospheric nitrogen and phosphorus depositions regulate net ecosystem CO₂ exchange (NEE) and carbon source/sink function by affecting the balance between photosynthetic and respiratory processes of plant ecosystems, and thus feeding back to global climate change. In this study, we conducted a manipulative nitrogen and phosphorus addition experiment to investigate their potential effects on carbon cycling processes and the carbon source/sink function of meadow steppe in Hulunbuir, Inner Mongolia. We found that nitrogen addition led to higher ecosystem respiration (ER) by the stimulation of autotrophic respiration (AR) and greater ecosystem CO₂ emission due to its stronger positive effects on ER than on gross ecosystem photosynthesis (GEP). In addition, adding phosphorus alone showed non-significant effects on GEP, ER, and NEE, but adding it together with nitrogen lowered CO₂ emission compared with adding nitrogen alone. Furthermore, all treatments appeared as net CO₂ sources potentially caused by seasonal droughts. Our results suggest that precipitation and nitrogen deposition interactively determine NEE of meadow steppe in Hulunbuir, Inner Mongolia and that the drying climate and increasing nitrogen deposition can turn this region into a net CO₂ source.