3.4. Relationships among the SOC fractions, enzyme activities,
physical and chemical characteristics
A correlation analysis (Table 4) demonstrated that the MBC content
displayed an extremely significant positive correlation with the POC and
catalase and displayed an extremely
significant negative correlation with EOC (with a correlation
coefficient was 0.911). The POC was significantly correlated with the
SOC, urease, sucrase, total N and P, however, no significant
correlations were observed with amylase, catalase, total porosity, and
bulk density. The SOC was significantly correlated with urease, sucrase,
total N, total P, total porosity, and bulk density. Soil sucrase
activity displayed an extremely significant correlaion with amylase and
urease,with respective correlation coefficients of 0.597 and 0.848.
Physical and chemical characteristics of the soil (i.e., total N, total
P, total porosity, and bulk density) displayed strong positive
correlation with urease and surase.
4. Discussion
4.1. Soil carbon fraction of
different type vegetations
Vegetation is one of the most important components of an ecosystem, and
its community succession has a significant effect on the SOC content
(Deng et al., 2018; Solomon et al., 2007). This study demonstrated that
SOC content in a forest was significantly higher than in shrublands and
grasslands (Figure 2D). Root exudates and litter from forest vegetation
both strongly affected the organic carbon content in the soil and
promoted the effectiveness of forest nutrients(Qiao, Miao, Silva, &
Horwath, 2014). At the same time, forest vegetation can also alter the
forest environment, reducing solar radiation and temperature
differences, increasing soil moisture(Özkan & Gökbulak, 2017), and
creating a stable environment for litter decomposition. All of this
causes the soil organic carbon content of forest to be higher than that
of shrublands and grasslands. Moreover, due to the higher coverage of
herbaceous vegetation and abundant species density (Table 1), more
surface litter increases the sources of organic carbon (Zhang et al.,
2019), making the SOC content of desert grassland vegetation higher than
that of both HR and CK vegetation. Meanwhile, the SOC content of the
four vegetation types was not only due to organic C inputs, but was also
affected by physical and chemical characteristics and enzyme activities
of the soil. A correlation analysis between SOC contents and soil
physical and chemical characteristics and enzyme activities further
confirmed these results (Table 4).
The MBC content in the soil of HR was significantly higher than in the
soil of GL (Figure 2A). On the one hand, HR vegetation has a wide
horizontal root structure and can quickly grow new shoots(Letchamo et
al., 2018). These new shoots increase soil porosity (Table 2) and oxygen
content during the growth process, and increased soil aerobic microbial
activity. On the other hand, the root nodules of Hippophae
rhamnoides can fix atmospheric nitrogen and improve soil fertility
(their annual average nitrogen accumulation is 17,475
kg/hm2)(Ruan & Li, 2002). Studies have shown that
increasing soil nitrogen can promote microbial activity and increase the
decomposition rate of soil organic matter(Nottingham et al., 2012;
Sistla, Asao,
&
Schimel, 2012), thereby reducing soil organic carbon content. The
partial shading effect of XS vegetation reduces the soil temperature and
the activity of soil microorganisms (Jiménez, Tejedor, & Rodríguez,
2007). Therefore, the soil MBC content is highest in HR vegetation.
The changes in soil POC and SOC are consistent (Figure 2C) across
different types of vegetation, while the changes in EOC and SOC differ
(Figure 2B). Since the soil in this study was obtained from different
types of vegetation, different physical and chemical properties (Table
2) regulate the decomposition rates in the soil (Xu et al., 2016).
Various surface litter can significantly change the input of soil
organic matter(Thorburn, Meier, Collins, & Robertson, 2012), which
affects the EOC content in the surface soil (DuPont, Culman, Ferris,
Buckley, & Glover, 2010). At the same time, the higher soil temperature
and the lower soil water content, which may potentially create more
beneficial conditions to enhance labile C fractions(Chen et al., 2016).
However, the decomposition of plant litter is the most complex
ecological process in the biosphere(Méndez, Martinez, Araujo, & Austin,
2019). Therefore, the soil active carbon fractions may be affected by
the physical and chemical properties of the soil, various environmental
factors, and the activity of microorganisms.
The content of activated carbon in the soil under the four vegetation
types was greater in the upper layer than in the lower layer. This was
mainly because the soil active organic carbon largely depends on the
total organic carbon content of the soil. Total organic carbon decreased
as soil depth increased (Figure 2D), however, the litter on the upper
layer not only provides a significant amount of organic carbon for the
soil, but also provides the surface soil with a high concentration of
nutrients (Table 2), providing stable conditions for growing fine roots
in the topsoil layer. Litter and root exudates have become an important
source of soil active organic carbon after they are decomposed by
microorganisms(Weintraub, Scott-Denton, Schmidt, & Monson, 2007).