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.