The researchers studied how adding nitrogen and phosphorus fertilizers to alfalfa plants in the Loess Plateau affects the nutrients in the plants and soil. They found that adding phosphorus fertilizer increased the carbon concentration in the plants, while nitrogen fertilizer decreased the nitrogen concentration. Adding phosphorus also increased the phosphorus content in the plants and soil. Overall, increasing phosphorus levels had the biggest impact on changing the nutrient balance in the plants and soil, helping alfalfa grow better in the area.