The study looked at how different amounts of nitrogen fertilizer affect crop yield and soil nitrate levels in North China. They found that using 309 kg/ha of nitrogen gave the highest wheat yield, while 213 kg/ha was best for maize. Soil nitrate levels increased with higher nitrogen application rates. To balance high yields and prevent nitrogen buildup in the soil, it's recommended to use 250-300 kg/ha for wheat and 150-200 kg/ha for maize in this region.