Objective  Mountainous soil carbon (C) pool is an important component of terrestrial C pool, which plays a key role in adjusting C cycle of ecosystem and mitigating global climate change. Altitude gradient is a governing factor in influencing the content and property of mountainous soil C. Exploring the distribution characteristics of organic C and fractions in mountainous soil of different altitude gradients is of great significance for reasonably estimating the soil organic C pool and predicting its sensitivity to climate change. This study aims to investigate the variation characteristics of organic C and fractions in soils along the altitude gradient of subtropical mountainous and the related influencing factors.

  Method  Gully evergreen broad-leaved forest (< 800 m), montane evergreen broad-leaved forest (800-1200 m), theropencedrymion (1200-1500 m), alpine meadow and montane elfin (> 1500 m) at different altitude gradients and depths (0-10, 10-20, 20-40, 40-60, and 60-100 cm) in Nanling national natural reserve were selected as the study objects, the distribution and density characteristics of soil organic C and various fractions (Water-dissolved soil organic C, WSOC; readily oxidizable carbon, ROC; particle organic carbon, POC, recalcitrant carbon, RC) were examined, as well as soil physicochemical properties including pH, mechanical composition, soil moisture content and nutrient indicators (total nitrogen, total phosphorus and total potassium).

  Result  The results showed that: (1) the soils in Nanling mountains were extremely acidic or strongly acidic, and the pH values were relatively higher in the soil of montane evergreen broad-leaved forest. Sand was the predominant fraction of the mechanism composition and the proportion of clay particles decreased gradually with the increase of altitude. The contents of total nitrogen and total phosphorus in alpine meadow and montane elfin soils were significantly higher than those in the soils at other altitudes, while the content of total potassium increased first and then decreased with the increasing of altitude; (2) the content of SOC, ROC, POC and RC was relatively higher at high altitudes and the maximum values appeared in the soil of gully evergreen broad-leaved forest with low altitude, while the maximum values of WSOC appeared in the soil of gully evergreen broad-leaved forest with relatively low altitude. The content of soil organic C and fractions decreased obviously with the increase of depth. Among the various soil C fractions, WSOC varied the most with altitude, while POC varied the most with depth. The variation regularity of the proportions of different organic C fractions in total organic C were also different to some extent; (3) the soil organic C density of Nanling mountains varied in the range of 8.81-26.59 kg m⁻², which was slightly higher than that of the nearby mountains overall. The variation tendency of different organic C fractions with altitude and depth was similar to that of total organic C; (4) pH, clay content, total nitrogen showed good relevance with soil organic C and fractions. The RDA results showed that TN, TP and soil moisture content had high proportions in explaining the variation of oil organic C and fractions.

  Conclusion  There existed obvious difference in the organic C and fraction content of soils at different altitudes in Nanling mountains. Soil property is an important aspect influencing the distribution characteristics of organic C, while other environmental factors should also be considered in the future study.