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 of 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 correlation 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
ofdifferent
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 (Fig. 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 SOC 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 carbon inputs, but was also affected
by enzyme activities and soil physical-chemical characteristics. A
correlation analysis between SOC contents and soil physical-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 (Fig. 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 increase soil aerobic microbial
activity. On the other hand, the root nodules of HR 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 N can promote microbial activity
and increase the decomposition rate of soil organic matter (Nottingham
et al., 2012; Sistla, Asao,
&
Schimel, 2012), thereby reducing SOC 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 (Fig. 2C) across
different types of vegetation, while the changes in EOC and SOC differ
(Fig. 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 SOC 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, due to the differences in tree species
composition, litter quantity and quality, soil microbial group
composition, soil moisture, temperature, and nutrient input, different
vegetation types have significant differences in soil active organic
carbon components (Yang et al., 2018; Soucémarianadin et al., 2018).
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 SOC decreased as soil
depth increased (Fig. 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).