Abstract: We explored the changes of soil organic carbon (C) and total nitrogen (N) storage along an altitudinal gradient and its coupling relationship with environmental factors in Wenshan Nature Reserve, to provide basic data for estimating ecosystem C and N fixation potential. Three typical vegetation types along an altitude gradient were selected to examine the distribution of C and N storage along the altitude gradient and soil profile. The principal component reduction analysis was used to explore the impacts of environmental factors (microclimate, vegetation, and soil) on C and N storage along the altitude gradient. The results showed that soil carbon and nitrogen storage of different vegetation types showed a significant increase along the altitude (P<0.05). The storage of C and N peaked in middle-montane humid evergreen broad-leaved forest of high altitude, which increased by 82.81% and 80.98% compared with monsoon evergreen broad-leaved forest in the low altitude. Soil C and N storage decreased with the deepening of soil layers (P<0.05). Compared with 0-10 cm layer, C and N storage in 40-50 cm layer decreased by 54.82% and 55.38%, respectively. Litter  thickness, Simpson index, soil total porosity, capillary porosity, moisture content, and total potassium increased along the altitude, while total phosphorus decreased first and then increased. The values of the above-mentioned variables were 53.29%-666.67% higher in middle-montane humid evergreen broad-leaved forest of higher altitude than that in the monsoon evergreen broad-leaved forest of low altitude. The Margalef index, Shannon index, Pielou index, soil non-capillary porosity, bulk density, temperature, pH and mechanical composition decreased along the altitude, the values of which were 15.97%-59.71% lower in the middle-montane humid evergreen broad-leaved forest of high altitude than in the monsoon evergreen broad-leaved forest of low altitude. Changes of environmental factors along the altitude significantly affected C and N storage (P<0.05). The increases of litter amount, annual precipitation, soil total porosity, capillary porosity, moisture content and total potassium along altitudes had a significant positive effect on C and N storage, while the decreases of plant diversity along the altitude had an extremely significant positive effect on C and N storage. In conclusion, soil C and N storage increased along the altitude in Wenshan Nature Reserve. The vertical distributions of microclimate, soil physicochemical properties and litter  thickness along the altitude are the key factors controlling C and N storage.

Key words: vegetation, altitude, soil organic carbon storage, soil total nitrogen storage, environmental factor.