Fig. 5

Seasonal variation of NH₄⁺ and NO₃^- content in soil layers 0-10, 10-20, and 20–40 cm under different levels of N addition. CK, 0 g/m²; N5, 5 g/m²; N10, 10 g/m²; N15, 15 g/m². EG early growth period, MG the peak growth period, LG the late growth period. Lowercase letters represent significant differences between treatments (P < 0.05).