4 | DISCUSSION
The highest TOC, SOM, TN, TC, NH4–N, and AP content
levels were identified in the soil samples at the 0–20 cm depth,
indicating that the chemical characteristics of the soil were in balance
(Wang et al., 2017). The low-yielding G. jasminoides + C.
oleifera forest had significantly increased TOC, HC, SOM, TN, TC
NH4–N, AN, and AP content levels, and the availability
of soil nutrients subsequently increased. This could be because
agroforestry trees provide nutrients that fulfill agricultural demands.
Another reason may be the increased abundance of related bacteria,
including rhizobia, phosphate-solubilizing bacteria, and
potassium-solubilizing bacteria, in soil nutrient cycling. However,G. jasminoides intercropping in a new afforestation area had
marginal effects on these indicators. This supports the hypothesis that
anthropogenic activities could considerably alter the original
distribution of nutrients in low-yield forests by changing the planting
patterns and altering the growth conditions to modify the soil nutrient
status. This investigation found that slope position had few effects on
the soil physicochemical properties.
Understanding the impacts of
forestry practices on the soil microbiota will aid in the development of
sustainable forestry practices. Our results support the hypothesis that
intercropping alters the below-ground microbial community and
composition. G. jasminoides supplementation in a new area ofC. oleifera afforestation increased the soil bacterial abundance
and improved the bacterial alpha diversity. The soil depth and the slope
position had no significant effect on the alpha diversity of C.
oleifera. These findings were in contrast with previous studies,
suggesting that bacterial diversity was significantly greater at the
lower slope position compared with the upper slope position (Sun et al.,
2018). Different bacterial taxa showed variable abundance with varying
soil depths (Eilers et al., 2012). This suggests that forest types had
more pronounced effects on the bacterial communities than the depth and
the slope position in the same plot.
Based on high-throughput sequencing, Proteobacteria, Chloroflexi, and
Acidobacteria were identified as the main bacterial phyla in the soil
communities, and this was consistent with the findings of previous
studies (Li et al., 2018, 2019). Proteobacteria and Acidobacteria are
the primary soil bacteria taxa associated with SOM decomposition.
Changes in the SOM content are highly correlated with the abundance of
Proteobacteria (Fierer et al., 2007; Banerjee et al., 2016). The
Proteobacteria abundance is highest in disturbed forest soils (Noble et
al., 2020), implying a prominent role in carbon turnover. Proteobacteria
were also found to be more abundant in the organic layer of deciduous
forests (Eilers et al., 2012;, Mundra et al., 2021). This could be
because higher levels of soil compaction in the deeper layers reduces
O2 availability, limiting the growth and activity of
many microorganisms (Hartmann et al., 2014). However, the reason behind
the increase in the abundance of Proteobacteria in the 40–60 cm soil
layer requires further investigation. Acidobacteria is the common
bacterial phyla in soil and can degrade the cellulose and lignin of
plant residues and reduce nitrate, nitrite, and potentially nitric
oxide, which play a major role in the carbon cycle and nitrogen circuits
(Ward et al., 2009; Kalam et al., 2020). Previous studies have
demonstrated that Acidobacteria diversity is inversely related to soil
depth. The abundance of Acidobacteria is highest in soils in unmanaged
forests (Kuske et al., 2002; Sheng et al., 2019), which was largely
confirmed in this study. This could be attributed to the Acidobacteria
oligotrophic nature or the ecological K-strategy (Ward et al., 2009;
Kielak et al., 2016. Chloroflexi have been identified in many
environments through marine and freshwater sediments, and they exhibit
biological activity in extreme
soil environments (Neilson et al., 2012). Chloroflexi efficiently
degrades chlorides (Zhang et al., 2020a) and can metabolize additional
complex carbon sources (McGonigle et al., 2020). It is likely that
Chloroflexi species play a role in the material circulation of the soil
bio-chemical layers.
In our research, the dominant soil microbes significantly differed in
their relative abundance among the different forest types. Our results
confirmed that the G. jasminoides plantation altered the relative
abundance of the dominant phyla and that there were no significant
differences in the bacterial communities in relation to the depth and
slope positions. These
observations suggested that different forest substructures affected the
soil ecological environment. A study on the effects of leguminous
supplementation on the resilience of soil bacterial communities and
nutrient content in Chinese fir plantations showed that functional plant
supplementation significantly increases the diversity and richness of
soil microbes, accelerates the transformation and absorption of soil
nutrients, and promotes the growth of Chinese fir (Zhang et al., 2020c).
Complex bacterial community structures increase soil resistance to
adverse environmental factors and are important for protection of the
soil ecosystem. As key components of the soil ecosystem, soil
physicochemical properties and microorganisms have an irreplaceable
role. These interspecific interactions can help maintain the stability
and ecological functions of microbial communities and re-construct
stable core microbial communities. Based on the LEfSe analysis, the
variable distribution of these biomarkers across different forest types
indicate that the influence of the soil bacterial community structure onC. oleifera may be highly important.
SEM analysis showed that the G. jasminoides plantation
significantly influenced soil bacterial communities by changing
NO3–N and secondarily affecting pH and EC. However,
these implications in the new C. oleifera afforestation were less
pronounced than in the low-yielding C. oleifera forest treatment
and, may not have had as strong a stimulatory effect on the bacterial
activity as expected. This indicates that the effects of the G.
jasminoides plantation in C. oleifera forest may be more
important than the depth and slope position when considering bacterial
function in the ecosystem.