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.