4.2. Soil organic content and storage
Throughout the stand growth, we significant increases in soil depth, which, in part, increased soil carbon pool. Our results showed the progressive increase in soil organic carbon storage proceeding from, younger to mature stages. In our results, the soil carbon contents and storage increased as the stand and A. neplalensis basal area increased, which indicates that A. nepalensis has a positive impact on the soil carbon content and storage. With forest development along the chronosequence, soil carbon and storage increased, which might be due to the nitrogen-rich litter of A. nepalensis. The litter of A. neplalensis , which is the dominant tree species, is known to produce nitrogen litter promoting soil carbon contents (Resh et al., 2002; Binkley., 2003). Soil Organic carbon is primarily derived from root exudates, plant residues, and litter and is influenced by forest species composition, forest history, and management (De Graaff et al., 2010). Litter biomass carbon and fine root biomass carbon were the main sources of soil carbon. Our study showed that litter biomass increased with stand basal area. Therefore, an increased nitrogen-rich litter amount leads to more soil organic carbon storage. Many studies have examined the effect of nitrogen-fixing tree species on forest soil (Binkley et al., 1992; Myrold et al., 1994; Resh et al., 2002; Binkley., 2003). Nitrogen-fixing tree species facilitate succession by increasing soil fertility, improving harsh environmental conditions, and possibly increasing soil organic carbon (Gómez-Aparicio et al., 2004; Macedo et al., 2008; Gómez‐Aparicio 2009). Also, we found that soil organic contents and storage decreases along with soil depth. Our results revealed that soil carbon contents and storage were highest in the 0-30 cm soil depth and decreasing with soil depth and indicate the presence of organic matters on the upper soil layer. The soil organic carbon storage of 3.31-210.14 Mg ha-1 in the present study is within the average range of 121 to 344 Mg ha-1reported for other forest ecosystems (Lal, 2004). In the central Himalaya, the upper layer (0-30 cm) soil organic carbon storage is higher (Sheikh et al., 2009). Similarly, a previous study showed that amount of soil organic carbon content and storage was higher in the upper soil layer (0-30 cm) than the lower soil layer (FAO 2017). This indicates that soil organic carbon accumulation was mainly in the upper surface in the early stages of A. nepalensis forest development; and in late stages, soil carbon accumulated in the deeper soil layer.