Abstract: We examined the role of photosynthesis in regulating soil CO₂ emission under nitrogen enrichment in Keerqin sandy grassland. Results showed that nitrogen (N) application could affect soil respiration rate by altering the allocation of photosynthetic products to the belowground. Gross ecosystem photosynthesis rate (GEP) was positively correlated with soil respiration rate (R_s). Nitrogen application reduced slope of the fitting function from 0.236 to 0.161, with the equation intercept difference (0.51 μmol·m^-2·s^-1) being similar to the nighttime soil respiration rate increment (0.52 μmol·m^-2·s^-1). From May to October, the difference of photosynthetic rate (differential ratio) caused by nitrogen application was significantly correlated with that of soil respiration (differential ratio). Results from partial correlation confirmed the essential role of photosynthetic rate difference (ΔGEP) in driving soil respiration rate difference (ΔR_s) caused by nitrogen application. In the nighttime, soil respiration rate was affected by the aboveground vegetation activities in daytime. The daily mean GEP was an important factor affecting the nighttime soil respiration rate difference (ΔR_s) (P<0.01). Photosynthesis, rather than soil temperature, was the main factor affecting soil respiration rate difference (ΔR_s) under nitrogen application. Thus, the role of photosynthetic assimilation-regulating may provide a novel supplement for elucidating the responses of soil respiration to nitrogen enrichment.