1. Introduction
Soil organic carbon (SOC) can reflect soil health, and plays an
extremely important role in increasing soil carbon storage, improving
soil fertility, and promoting plant growth (Sollins et al., 2007; Li et
al., 2018). However, the SOC group consists of sub-groups with variable
turnover rates, each with a different sensitivity to environmental
changes (Guo, Wang, Wang, Wu, & Cao, 2018). Soil active organic carbon
typically includes microbial biomass
carbon (MBC), easily oxidized carbon (EOC) and particulate organic
carbon (POC). While the proportion of soil active organic carbon to soil
total organic carbon is low, this ration can reflect the changes in soil
carbon groups due to soil management measures and environmental changes
(Jha et al., 2012; Sahoo, Singh, Gogoi, Kenye, & Sahoo, 2019). The
soil
active organic carbon is directly
involved in the biological and chemical conversion process of soil (Sun
et al., 2014), plays a vital role in the cycling of soil nutrient, and
stores soil nutrients (Simard, Fyles, D, & Nguyen, 2001). Furthermore,
soil active organic carbon is easily affected by plants and
microorganisms in a significant way (Chen, Zhou, & Xiao, 2010; Kimura,
Murase, & Lu, 2004). However, variation in soil active organic carbon
contents across different vegetation types is poorly understood.
Soil
enzyme activities are involved in the biochemical processes of the soil
system and are linked to “plant-soil enzymes-soil nutrients” (Araújo
et al., 2013; da Silva et al., 2012; Lino et al., 2015; Nannipieri et
al., 2012). In particularly, enzyme activities (i.e., amylase, catalase,
urease and sucrase) related to the soil carbon cycle and serve as
important indicators of soil fertility. Amylase and sucrase are involved
in the conversion of soil carbohydrates and can hydrolyze organic matter
into glucose and sucrose for plant growth and microbial activity (Xie et
al., 2017; Ge et al., 2011). Urease acts on carbon-nitrogen bonds in
organic matter, and produces carbon dioxide and water by hydrolyzing
ammonia or amino salts, while catalase is related to the redox ability
of the soil (Baddam et al., 2016; Nowak et al., 2004). These enzyme
activities have an important influence on the carbon cycles in soil
ecosystems (Bergstrom et al., 1999; Burns et al., 2013). Previous
studies have shown that plants can not only directly influence soil
enzyme activities by secreting exogenous enzymes, but also affect the
composition and diversity microbial species by releasing exudate and
oxygen into the rhizosphere, which indirectly affect enzyme activity
(Singh & Kumar, 2008). Moreover, plants also indirectly mediate enzyme
activities in the soil by controlling the volume of aboveground litter
(Caravaca, Alguacil, Torres, & Roldán, 2005). Therefore, these
enzyme activities were often chosen
to understand the variations in SOC and soil quality (Acosta-Martínez et
al., 2007; Chen et al., 2016).
The Loess Plateau is located in the north-central China and has one of
the highest concentration of loess on earth, with a total area of 64,000
square kilometers. It also has a high rate of soil erosion and is one of
the most ecologically fragile environments in the world, meaning
vegetation is important to enhance fertility levels and the soil’s
ability to hold water (García, Hernández, & Costa, 1994). Over the past
few decades, extensive efforts at restoring the environment have
improved the fragile natural ecosystems on the Loess Plateau
(Intergovernmental Panel on Climate Change, 2014).
Vegetation restoration not only
benefit for water preservation and reduction of soil erosion (Ran, Lu,
& Xu, 2013), but significantly improve the properties and quality of
soil (Zhang et al., 2019). Studies have shown that returning farmland to
forests not only improves the SOC reserves and quality, but also
improves the conversion trend of soil SOC-related fractions (Deng et
al., 2019; Xun et al., 2010; Liu et al., 2014). However, due to a
difference in vegetation types, environmental factors, and regional
variation, there is a lack of information on the relationship between
enzyme activities and soil carbon fractions across different vegetation
types. Therefore, we selected vegetation from forests (Xanthoceras
sorbifolia ), shrublands (Hippophae rhamnoides and Caragana
korshinskii ), and grasslands on the Loess Plateau to study the
distribution characteristics of soil active organic carbon components
and soil enzyme activities under different vegetation types. This
provided a reference value for the sustainable ecological restoration of
the Loess Plateau and subsequent improvement of the soil. We
hypothesized that both SOC components and enzyme activities in the
forest were higher than that of shrublands and grasslands, and that the
SOC components and enzyme activities in the surface layer of all
vegetation types were higher than in the lower layer. This study
provides additional insight into nutrient cycling processes in the
ecological restoration of vegetation, and improves the sustainability of
ecosystem restoration by identifying effective vegetation to plant.