The Effect of Simulated Acid Rain on the Decomposition Rate of Chinese Fir (Cunninghamia lanceolata) Litter Depends on Acid Rain Intensity and Litter Decomposition Stage

Acid rain is a severe global environmental issue, and clarifying its impacts on litter decomposition and underlying mechanisms is critical for accurately forecasting future climate change. Litter consists of components (e.g., non-structural carbohydrates, lignin, cellulose, and hemicellulose) with distinct decomposition resistance, but how acid rain affects these components to modulate overall litter decomposition across different decomposition stages remains unclear. Therefore, a microcosm experiment was conducted to determine decomposition rates of Chinese fir (Cunninghamia lanceolata) litter and its components based on litter mass loss under different simulated acid rain intensities (pH 4.5, moderate acid rain, MA; pH 3.0, severe acid rain, SA; and pH ≈ 7.0, tap water, CK) over two decomposition stages (0–5 months: initial decomposition stage; 6–16 months: late decomposition stage). Meanwhile, to analyze the factors influencing the litter decomposition rate, soil samples were collected at 5 and 16 months of decomposition for soil property analysis. Results showed that MA had no significant effect on litter decomposition in either stage. Conversely, SA led to a significant 43.7% increase in the litter decomposition rate in the initial decomposition stage, driven by its acid dissolution effect that accelerated the decomposition of cellulose and hemicellulose. However, SA significantly decreased the decomposition rate by 42.0% in the late decomposition stage by inhibiting the decomposition of lignin, cellulose, and hemicellulose, which was due to the reduced activities of soil peroxidase and xylosidase under soil acidification. Notably, neither MA nor SA significantly affected the litter decomposition rate over the entire decomposition period (0–16 months). This study indicates that acid rain’s effect on litter decomposition depends on its intensity and decomposition stage, emphasizing the necessity of distinguishing litter components and decomposition stages to explore its underlying mechanisms and precisely predict global climate change.