The researchers tested different types of rice plants to find ones that use nitrogen efficiently in South China's red soil. They grew 21 rice varieties with low and normal nitrogen levels to see which ones had high nitrogen efficiency. They found four rice types that produced over 6000 kg per hectare and had high nitrogen efficiency, and four types that didn't do as well. The key difference was the number of grains each plant produced. The high nitrogen efficient plants had more nitrogen in their leaves and photosynthesized better. They also had higher nitrate reductase activity, which helps them use nitrogen effectively.