This study examines the environmental impacts of excessive nitrogen (N) fertilization in two of China's most intensive double-cropping systems: waterlogged rice/upland wheat in the Taihu region and irrigated wheat/rainfed maize on the North China Plain. Current agricultural N practices, which apply 550-600 kg of N per hectare annually, do not significantly increase crop yields but lead to about twice the N losses to the environment compared to knowledge-based optimum N fertilization. The higher N loss rates and lower N retention rates indicate that residual N is not effectively utilized by the succeeding crop in rice/wheat systems, while wheat/maize systems show better N management. Periodic waterlogging in upland systems causes large N losses through denitrification in the Taihu region, while ammonia volatilization and nitrate leaching are the main N loss pathways in wheat/maize systems due to calcareous soils and concentrated summer rainfall. Over-apply N inputs have led to severe environmental degradation, including eutrophication, nitrate pollution, acid rain, and greenhouse gas emissions. The study suggests that adopting knowledge-based optimum N fertilization techniques, controlling primary N loss pathways, and improving agricultural Extension services can achieve a better N balance without sacrificing crop yields while significantly reducing environmental risks.This study examines the environmental impacts of excessive nitrogen (N) fertilization in two of China's most intensive double-cropping systems: waterlogged rice/upland wheat in the Taihu region and irrigated wheat/rainfed maize on the North China Plain. Current agricultural N practices, which apply 550-600 kg of N per hectare annually, do not significantly increase crop yields but lead to about twice the N losses to the environment compared to knowledge-based optimum N fertilization. The higher N loss rates and lower N retention rates indicate that residual N is not effectively utilized by the succeeding crop in rice/wheat systems, while wheat/maize systems show better N management. Periodic waterlogging in upland systems causes large N losses through denitrification in the Taihu region, while ammonia volatilization and nitrate leaching are the main N loss pathways in wheat/maize systems due to calcareous soils and concentrated summer rainfall. Over-apply N inputs have led to severe environmental degradation, including eutrophication, nitrate pollution, acid rain, and greenhouse gas emissions. The study suggests that adopting knowledge-based optimum N fertilization techniques, controlling primary N loss pathways, and improving agricultural Extension services can achieve a better N balance without sacrificing crop yields while significantly reducing environmental risks.