[Altitude Gradient Characteristics and Influencing Factors of Forest Soil Carbon Fractions in Tianmu Mountain,...

A soil carbon pool constitutes the most substantial carbon reservoir within forest ecosystems, playing a pivotal role in regulating the carbon cycle of forest ecosystems and mitigating global climate change. In this study, an investigation was conducted at the southeast slope of Tianmu Mountain across five distinct altitude gradients, viz. 400, 800, 1 000, 1 300, and 1 500 m to explore the variation characteristics of soil carbon fractions and carbon pool quality and their influencing factors at varying altitudes. The results showed that: ① Soil water content and total nitrogen content fluctuated and increased with altitude, while soil particle size composition was predominantly silt, followed by sand particles, with clay particles being the least abundant. Soil average pH ranged from 3.94-5.46 and experienced a steady decline with increasing altitude. Furthermore, soil ammonium nitrogen content, C∶N, C∶P, and N∶P initially increased and then decreased with increasing altitude, reaching their peak values at 800 and 1 000 m, respectively, while the content of nitrate nitrogen and total phosphorus content showed the opposite trend. ② Soil total organic carbon, slow release carbon, and passive carbon exhibited a fluctuating "rise-fall-rise" pattern with increasing altitude. Conversely, soil active carbon initially increased and then decreased with altitude, reaching its peak at 1 300 m. ③ Soil carbon pool activity, carbon pool activity index, and carbon pool management index all reached their highest values at 1 300 m, while the carbon pool index peaked at 1 500 m. ④ Soil total nitrogen was identified as the principal factor affecting the carbon fractions and carbon pool quality of forest soil in Tianmu Mountain, followed by soil water content, silt content, sand content, pH, and N∶P. In conclusion, the soil carbon fractions and carbon pool quality in Tianmu Mountain exhibited distinct altitudinal gradients and were significantly influenced by soil physicochemical properties.