2.2.Experimental design
The experimental design consisted of a split plot experiment with two factors (monoculture and intercropping); the treatments included nitrogen application and no nitrogen application, with a total area of about 1,400 m2. Each intercropping treatment covered an area of 60 m2 (5 mⅹ12 m). The configured lines of intercropped mulberry and alfalfa were two and four, respectively. The planting distance of mulberry was 45 cm, with a row spacing of 60 cm (Figure 1). For alfalfa, row spacing was 30 cm. To avoid nitrogen leaching and seedling burn, the nitrogen was applied at the bud stage of alfalfa through an opening ditch. Phosphorus, potassium, and other essential elements were applied in mid-April. In particular, we used 23 kg·667 m-2 of superphosphate and 6.67 kg·667 m-2 of potassium chloride. To avoid interference by other elements, nitrogen was applied as urea (46% N). In both systems (monoculture and intercropping), the same amounts of nitrogen were applied. For alfalfa, we applied 5.86 kg·667 m-2 of urea, while for mulberry, 22 kg·667 m-2 were applied.