Scientists have long been concerned with soil carbon (C), because it is often the master variable determining soil fertility (Malhi et al. 1999; Johnson and Curtis 2001; Johnson et al. 2002). C enters the soil through both litterfall and rhizodeposition and leaves the soil mainly as CO₂ via root and microbial respiration (Sulzman et al. 2005; Cleveland et al. 2010; Díaz-Pinés et al. 2011; Sayer et al. 2011). However, as one of the most important green house gases, the tightly relationship between CO₂ and soil C is generally accepted in the context of global climate change. Thus, known soil C stock has become very important for assessing changes in atmospheric CO₂ concentrations and of global climate (Dixon et al. 1994; Schimel 1995; Søe et al. 2004). As the largest pool of terrestrial organic carbon in the biosphere, more C is stored in soil than is contained in plants and the atmosphere combined (Jobbágy and Jackson 2000). Global surveys of mineral soil organic carbon (SOC) indicate that the soil holds about 1500 Pg C in the upper meter of soil (Post et al. 1982; Eswaran et al. 1993; Jobbágy and Jackson 2000), and most of this SOC (roughly 70% of all SOC) is contained in forest soils (Dixon et al. 1994; Batjes 1996; Jandl et al. 2007).

The potential C sequestration of forest ecosystems is widely accepted (Batjes 1996; Jandl et al. 2007). In fact, by sequestering large amounts of atmospheric C, forest plays an essential role in the global C cycle and is thought to offer a mitigation strategy to reduce global warming (Dixon et al. 1994; Chiti et al. 2012). However, the extent to which the vegetation layer influences SOC stocks in natural mountain forest land of Three Gorges area is still poorly understood. Moreover, many articles about SOC have been focusing on its stock of a large area, for example, global scale, hemispheric scale or national scale (Eswaran et al. 1993; Dixon et al. 1994; Batjes 1996; Fang et al. 2001; Goodale et al. 2002; Li et al. 2004; Chiti et al. 2012). The SOC of smaller scale, such as forest communities and ecosystems, is not considered enough, especially in the aspect of relation between SOC and forest composition and diversity. Composition of tree species has a pivotal effect on soil processes, including the cycling and accumulation of C (García-Oliva et al. 2006; Díaz-Pinés et al. 2011). For example, trees drive litterfall inputs, rhizodeposition, animal manure and rainfall distribution, soil temperature, and consequently they shift soil microbial quantity and activity (Simón et al. 2013). Based on composition, the forest area can be classified into various types. Proportion of different species in the same plant community can be quantified through species composition investigation. Therefore, it is important to consider the influence of tree species composition on SOC stock at given sites, as it may provide a basis for quantifying C pool in forest, which plays a relevant role in the global C cycle (Mathers and Xu 2003; Chen et al. 2004). Our study will be added to the growing body of information on soil C storage in subtropical mountain forest of China. But beyond that, the relationship between plant diversity (i.e.: totality of genes, species, and ecosystems of a region) and biogeochemical process that regulates the ecosystem has been a central issue in both ecological and environmental sciences recently (Bunker et al. 2005; Chen 2006). Many studies have suggested that plant communities with high species diversity may promote more efficient use of resources compared with those of less species diversity and thus lead to greater net primary production, and consequently higher C sequestration (Saha 2008; Saha et al. 2009; Meier and Bowman 2010; Wang et al. 2011). However, Huston and Marland (2003) indicated that ecosystems with multiple species are not necessarily more productive than ecosystems with few species. Many natural ecosystems with low plant diversity, even near monocultures, are highly productive. Nevertheless, ecosystems with multiple species indeed provide some insurance that they may be steadier and continue to perform a particular function even if one of the species is lost. However, quantitative estimates of effects of tree species composition on SOC stocks under natural forest ecosystems remain scarce (Chapin III et al. 2000; Berger et al. 2002; Díaz-Pinés et al. 2011). Therefore, forest composition or biodiversity, which can be confirmed as the dominant effect on ecosystem C sequestration? The issue needs further researches.

In natural ecosystem, nitrogen (N) is a primary nutrient that limits vital activities of plant and microbe (Vitousek and Howarth 1991; Hu et al. 2001; LeBauer and Treseder 2008; Wei and Sun 2009). C cycling is consequently influenced by soil N and C:N ratio (Cleveland and Liptzin 2007; Cleveland et al. 2011), and both factors partly indicate activity of microbe and level of soil C decomposition by respirations of roots and microbes. Thus, both C stock and effects of C:N ratio have been hot spots of scientific interest in global change (Hungate et al. 2003; Chen 2006; Davidson and Janssens 2006).

Because of the alleviation effect on global warming, C sequestration ability of forest is expected for more and more focus (Wu et al. 2003; Lal 2004; Bonan 2008; Tarnocai et al. 2009). Especially in China, the large developing country all through the world, the conflict between environment and develop is becoming sharper and sharper. In order to reduce the green house gas, Chinese government has been struggling since a long time ago. Many measures have been conducted particularly in forestry. In last decades, although millions of hectares were planted (afforestation and reforestation) per year, making a huge C pool, the SOC stocks of forests in China have not restored from the continuously forestry C sequestration reducing since late 1940s (Fang et al. 2001). However, little attention was paid on the composition of tree species during silviculture and afforestation. It may cause inefficient C sequestration and cause unintended disastrous environmental consequences, especially in arid and semiarid regions (Gao et al. 2011). Nevertheless, C sequestration of natural forest should be studied in detail for “close-to-nature” afforestation and reforestation and finding the best forest management plan. Moreover, about 28 to 35% of forest C storage occurs in the southwestern region (including the provinces of Sichuan, Chongqing, Tibet, Yunnan, Guangxi, and Guizhou) which is the largest in China (Fang et al. 2001). Thus, forest C sequestration study in this region is important for afforestation and reforestation aiming at reducing green house gas in China. Since the end of 20^th century, the Natural Forest Protection Project has been conducted in Three Gorges of southwestern China (http://english.forestry.gov.cn/index.php/information-services). Vegetation coverage in this area was 35.62% by the end of 2007 and it was far greater than average of China (20.36%) (Zhao 2007). The natural forest ecosystems of the area are great potential for C stock. But study on this is still rarely showed. Therefore, the aims of this study are as follows: (1) Study the effects of forest composition on SOC concentration and stock. (2) Analyze the effects of tree species diversity on SOC concentration and stock. (3) Difference of SOC decomposition in different forests is showed by C:N ratio.

The main differences of SOC concentrations of F_m and F_b were presented in 0–15 cm and 15–30 cm soil with p value of 0.0101 and 0.0338 respectively. The reason may be that roots are mainly distributed in 0–50 cm soil horizon (Waisel et al. 1991; Upson and Burgess 2013). However, the difference of SOC concentrations was insignificant in 30–50 cm horizon (p=0.2068). The SOC stocks of F_m and F_b were only significantly different in 15–30 cm horizon (p=0.0294). Totally, SOC concentrations of 0–100 cm in the two forests (p=0.0016) were significantly and statistically different as well as SOC stocks (p=0.0052) (Fig. 1A, B and Table 2). Thus, tree species compositions of forest ecosystems could be considered as a reasonable factor for distinguishing SOC from each other especially in surface soil (0–30 cm). Currently, Chinese government has been carried out many protection programs of forest in order to build a healthy natural ecosystem, for example, Land Conversion from Farmland back to Forestland Project, Wildlife Protection and Nature Reserve Development Program and Natural Forest Protection Project, etc (http://english.forestry.gov.cn/index.php/information-services). Simultaneously, afforestation and silviculture are implemented all through the country to decelerate global warming. Ecological conditions of China have a continual improvement and C sink potential keeps increasing. However, problems are also existed: monoculture afforestation, lack of forest management, and contradiction between food shortage and returning crop land to forest. Forest quality is influenced by those problems. These issues should be properly solved. In Three Gorge area, because of our SOC stock estimates, mixed conifer-broadleaf forest (F_m) with the largest soil C pool (183.50 t/ha) may be the best choice for local afforestation and reforestation aimed at alleviating climate change.

Liu (2005) suggested that broadleaf forest was climax communities in succession process of Mt. Jinyun with mixed conifer-broadleaf forest being inferior community. Several works (Malhi et al. 1999; Marín‐Spiotta and Sharma 2013) suggested that both land use change and forest succession gradient were generally thought to have effect on SOC stocks, especially for surface soil. Nevertheless, the successional effects of forest communities in Three Gorges are still not understood well. Thus, the specific studies under local conditions are very necessary. Not only the 0–15 cm SOC concentrations were remarkably different (p=0.0101) between F_m and F_b, but also the 15–30 cm SOC concentrations were significantly different from each other (p=0.0338). Although insignificant, the difference of 0–15 cm SOC stocks between forests was indeed existent with p value of 0.0843. Nevertheless SOC stocks of 15–30 cm soil were remarkable different between F_m and F_b (p=0.0294). And the differences of SOC concentrations and stocks between F_m and F_b became weaker and weaker with increasing soil depth where roots of bottom soil were far less than surface soil. The above analysis showed that succession effects may be another reason which could control SOC stock of forest by influencing tree species composition.

Our total C stock estimates of 0-100 cm mineral soil under the two forest ecosystems (100.44–183.50 t/ha, Fig. 1B and Table 2) were beyond the range of values estimated for the mineral soil under forests of Mt. Dinghu with the similar climate (30.90-127.90 t/ha) (Fang et al. 2003), but included the estimate for the Ultisols soil (144.80 t/ha) in Chongqing city (Huang et al. 2005) (Table 4). In Mt. Dinghu (Fang et al. 2003), the SOC stocks in mixed conifer-broadleaf forests (30.90-107.10 t/ha) were less than those of evergreen broadleaf forests (95.00-127.90 t/ha). Chen (2007) suggested that mixed conifer-broadleaf forest (92.33-127.13 t/ha) sequestrated less C than evergreen broadleaf forest (151.63-290.82 t/ha) in Three Gorges region (Table 4). The results were contrary to our data. However, study in Spain showed that mixed conifer-broadleaf forest caught more SOC than evergreen broadleaf forest as well as in other regions of Mediterranean conditions which belongs to subtropics as the same as our research area (Díaz-Pinés et al. 2011) (Table 4). Li et al. (2004) also suggested that SOC stock of evergreen broadleaf forest (129.2 t/ha) in China was less than that of mixed conifer-broadleaf forest (225.70 t/ha) (Table 4). Ni (2001) estimated the SOC stocks of the two types of forests in China: 124.00-142.00 t/ha for evergreen broadleaf forest and 130.00-150.00 t/ha for mixed conifer-broadleaf forest (Table 4). Mixed forest caught more SOC than evergreen broadleaf forest. The differences of C stock among regions may be also due to climatic (Díaz-Pinés et al. 2011; Chiti et al. 2012) and geologic conditions etc (Schaefer et al. 2009). Besides, sampling time may also affect estimating value of C stock in forest soil. However, in southwestern China, the average forest biomass C stock was 60 t/ha which was the largest all through the country (Fang et al. 2001). And that the SOC is far more than biomass C is widely accepted. It indicated that natural forest in this region is a great container for C. Our results were greater than the average SOC stock of Ultisols soil on the Earth (Eswaran et al. 1993), which also showed the strong C sequestration of forests in the research area (Table 4). However, Woodwell (1984) indicated that Ultisols soil under virgin and secondary forests on Earth stored more SOC (180.00-240.00 t/ha) than the two forests in this study (Table 4).

Both the average soil C:N ratios of F_m and F_b were decreasing with increasing soil depth. And the average C:N ratios of F_b were less than those of F_m in each soil horizons. The C:N ratio provides some indication about the relative quality and biochemical stability of soil organic materials (Díaz-Pinés et al. 2011; Bui and Henderson 2013). The C:N ratio hinted a weak SOC decomposition in our studied forests. Therefore, SOC stock of study area was larger than the average value of Utisols soil on the earth (Eswaran et al. 1993) as well as forest soil in southwestern China (Fang et al. 2001). However, the C:N values of F_m and F_b were not statistically different (p=0.3879). On the other hand, SOC concentrations (p=0.0474) and stocks (p=0.0116) of F_m and F_b were remarkably different. The results showed that SOC differences between F_m and F_b were influenced by C:N ratio little, which indicated that SOC decompositions of F_m and F_b were similar. The composition of tree species may be a rational factor for distinguishing the differences between C sequestrations of forests in study area as above analysis.

The relationship between tree species diversity and SOC under studied forest ecosystems was not linear in our study. However, Chen (2006) suggested that SOC stocks were linearly increased with growing H indices of forests in Northeastern China. Nevertheless, in Sichuan Province of southwestern China (closely located in the west of our study area), the correlations between SOC and R and H of forests were as the same as our findings (Zhang et al. 2011). Kirby and Potvin (2007) did not find any linear relationship at soil profiles under forests in Eastern Panama either. The environmental factors of different region (Ewel et al. 1991; Berendse 1998; Forrester et al. 2006) and various forest productivities (Vandermeer 1989; Tilman et al. 1997) may cause the different relationships. And more studies are needed to explain the correlation between biodiversity and SOC in order to develop forest management and establish forest with great C sequestration. However, the statistical difference between biodiversity indices of F_m and F_b was not significant (Table 3). Thus, in contrast to plant species composition, biodiversity may not make difference in forest soil C sequestrations.

Tree species composition significantly and statistically influenced SOC concentrations and stocks of F_m and F_b. In first two soil horizons (0-15 cm and 15-30 cm), these differences were even more significant. However, SOC of F_m and F_b were not influenced by tree species diversity due to the very low linear coefficients. And the statistical difference between biodiversity indices of F_m and F_b was not significant. Thus, in contrast to plant species composition, biodiversity may not make difference in forest soil C sequestrations. The average C:N values of F_m in four different horizons were decreased with increasing soil depth as well as F_b. And the values were larger in F_m. But the difference between C:N ratios of F_m and F_b was not remarkable. C:N ratio contributed little to the difference between SOC of the two studied forests. Not only SOC concentrations of F_m and F_b were decreased with increasing soil depth but also SOC stocks reduced from surface soil to bottom. F_m showed a large capacity to store SOC rather than F_b in the area. Thus, mixed conifer-broadleaf forest may be the best choice for local afforestation and reforestation aimed at alleviating climate change in Three Gorges region. However, conflict issues can still be found in the relation between SOC and tree species diversity in studies all over the world. It needs more detail researches in different scale to explain.