The Growth and Physiological Metabolic Responses of Cunninghamia lanceolata Saplings to Interactive Soil Acidification and Nitrogen Application and the Dominant Limiting Factor Determination

To reveal the relative effects and interaction mechanism of soil acidification and nitrogen regulation on the growth of Chinese fir (Cunninghamia lanceolata) and to provide a scientific basis for the prevention and control of Chinese fir plantation decline, we conducted an 18-month greenhouse pot experiment using 1-year-old Chinese fir saplings. The experiment combined three acid solution spraying gradients to simulate soil acidification (control S0, distilled water; low acidification LS, acid solution pH 4.5; high acidification HS, acid solution pH 3.5; the initial soil pH was 6.3; acidification was simulated by spraying a mixed acid solution of H2SO4 and HNO3 at a molar ratio of 5:1) with three nitrogen application levels (no nitrogen N0, 0; low nitrogen LN, 50 mg N·kg−1 dry soil; high nitrogen HN, 200 mg N·kg−1 dry soil), forming nine treatment groups in total. Correlation analysis and a structural equation model (SEM) were used to analyze the influencing pathways. Soil acidification significantly reduced soil pH and nitrate nitrogen content, inhibited sapling growth, impaired the function of the photosynthetic system, and excessively activated superoxide dismutase (SOD) activity, which showed an extremely significant negative correlation with growth rate (−0.83). The total inhibitory effect of soil acidification on sapling growth reached 0.792, operating through direct growth inhibition, impaired nutrient supply, and enhanced antioxidant defense responses. Nitrogen application significantly increased soil available nitrogen content and alleviated stress under low acidification conditions by improving chlorophyll synthesis and photosynthetic efficiency; however, it had no significant direct effect on growth rate (0.15) and slightly exacerbated soil acidification (−0.45). The alleviating effect of nitrogen was completely lost under high acidification treatment with acid solution pH 3.5. The structural equation model showed a good fit (p = 0.379, GFI = 0.940, RMSEA = 0.051), confirming that soil acidification was the core dominant factor restricting Chinese fir growth. These results provide theoretical support and technical reference for precise nutrient management and decline control of Chinese fir plantations in southern China.