Above-ground litter, root exudates, MBC, and soil organic
matter decomposition were considered to be the sources of
DOC (Kalbitz et al. 2000). DOC is easily leached along with
soil water (Kalbitz et al. 2000). Our results found DOC in
downhill was significantly higher than that in uphill (Table 2),
which was probably due to DOC from uphill soil to downhill
soil. This also could explain why DOC concentration in
0–20 cm was lower than that in 20–40 cm in uphill slope.
Although DOC in different soil layers and topographies
was different, plantation had no significant effect on DOC.
Plantation significantly increased total soil C in whatever soil
layers or topographies (Table 2, Fig. 2-M and N). The effect
of soil layer on total soil C was also significant: total soil C in
0–20 cm was higher than that of 20–40 cm (Table 2, Fig. 2-M
and N). The vertical distributions of soil C was closely related
with above-ground litter C leaching and vertical root
distributions (Jobbágy and Jackson 2000).
Our results suggested soil NH4
+ and NO3
–
concentrations
in 0–20 cm were lower than that in 20–40 cm, which
was probably due to greater root biomass and nitrogen
uptake in 0–20 cm soil than in 20–40 cm soil. Microbes
are generally C-limited and labile C inputs could increase
microbial activities (Fisk and Fahey 2001). However, when
there are enough C for microbes, microbes also need to
uptake enough N for stoichimetry balance (Cleveland and
Liptzin 2007). Under this condition, microbes are N-limited.
Although the main effects of plantation treatment on
NH4
+ and NO3
–
concentrations were not significant, the
correlations between microbial respiration and NH4
+ and
NO3
– concentrations in CK and GBH were significant,
which suggested microbial respiration is N limited under
CK and GBH