1. Introduction
Soil organic carbon (SOC) is a sensitive indicator of climate change,
which can be used to indicate the response of soil to climate
change (Don, Schumacher, &
Freibauer, 2011), and also play important roles in the carbon cycle
(Eclesia, Jobbagy, Jackson, Rizzotto, & Piñeiro, 2016). However, the
SOC pool consists of sub-pools with different turnover rates, which have
different sensitivity to environmental changes (Guo, Wang, Wang, Wu, &
Cao, 2018). Therefore, in order to estimate the response of the SOC to
environmental change more accurately, we need to separate the SOC into
different fractions. Soil active organic carbon mainly includes
microbial biomass carbon (MBC),
easily oxidized carbon (EOC) and particulate organic carbon (POC).
Although the proportion of soil active organic carbon to soil total
organic carbon is low, it can reflect the changes of soil carbon pool
caused by soil management measures and environmental changes (Jha et
al., 2012; Sahoo, Singh, Gogoi, Kenye, & Sahoo, 2019). The
soil
active organic carbon can be directly
involved in the soil biological chemical conversion process (Sun et al.,
2014), plays an important role in the soil nutrient cycling, and is the
storage of soil nutrients (Simard, Fyles, D, & Nguyen, 2001).
Furthermore, soil active organic carbon is easily and strongly affected
by plants and microorganisms (Chen, Zhou, & Xiao, 2010; Kimura, Murase,
& Lu, 2004). However, knowledge concerning the variation of soil active
organic carbon content under different vegetation types is poor.
Soil
enzyme activities participate in the biochemical processes of the soil
system and are the key link to the “plant-soil enzymes-soil nutrients”
(Araújo et al., 2013; da Silva et al., 2012; Lino et al., 2015;
Nannipieri et al., 2012). In particularly, the activities of soil carbon
cycle-related enzymes (i.e., amylase, catalase, urease and sucrase), as
important indicators of soil fertility, play crucial roles in the soil
organic matter circulation and energy transformation of the soil
ecosystem (Bergstrom et al., 1999; Burns et al., 2013). The enzyme
activities rapidly change if the soil nutrient status is altered
(Ciarkowska, Solek-Podwika, & Wieczorek, 2014). Plants can influence
soil enzyme activities by excreting exogenous enzymes, and affect
microbial species composition and diversity by releasing exudates and
oxygen into the rhizosphere that in turn indirectly affect enzyme
activity (Singh & Kumar, 2008). Plants can also indirectly mediate
enzyme activities in soil through controlling aboveground litter
quantity (Caravaca, Alguacil, Torres, & Roldán, 2005). Therefore, these
enzyme activities were often chosen
to understand the variations in SOC and soil quality (Acosta-Martínez,
Cruz, Sotomayor-Ramírez, & Pérez-Alegría, 2007; Chen et al., 2016).
The Loess Plateau is located in the north-central part of China. It is
one of the most concentrated and largest loess areas on the earth, with
a total area of 64,000 square kilometers. At the same time, it is also
one of the areas with the most serious soil erosion and the most fragile
ecological environment in the world, thus vegetation in this area is so
important to enhances soil water holding capacity and fertility levels
(García, Hernández, & Costa, 1994 ). Over the past few decades,
extensive environmental restoration have been implemented to improve the
fragile natural ecosystems on the Loess Plateau (Intergovernmental Panel
on Climate Change, 2014). Vegetation
restoration can not only preserve soil and water and reduce soil erosion
(Ran, Lu, & Xu, 2013), but also improve soil properties and quality
significantly (Zhang et al., 2019). Future climate change may have a
complex impact on the soil enzyme system in terrestrial ecosystems, thus
affecting the relevant soil SOC, nutrient processes. Sucrase supply
energy for crop roots and microorganisms growth (Hu et al., 2014; Kang,
Liu, Wan, & Wang, 2011). However, there is a lack of information on the
relationship between soil carbon fractions and enzyme activities in
different vegetation types. The goal of our study was to evaluate the
effect of vegetation types on soil carbon fractions and enzyme
activities in 40-cm soil layers on the Loess Plateau. We hypothesized
that both carbon fraction contents and enzyme activities of woodland in
the same soil layer are higher than that of bushes and grassland, carbon
fraction contents and enzyme activities under all vegetation soils are
higher in the surface layer than in the underlying layer.