# Sloping Farmlands Conversion to Mixed Forest Improves Soil Carbon Pool on the Loess Plateau
## 1 Introduction  
Soils play an important role in the global carbon cycle [1, 2, 3]. Soil organic carbon (SOC) is an essential physical-chemical soil property and the most important indicator of soil quality [4]. Vegetation restoration is considered to be a potentially useful strategy for controlling soil erosion and improving soil quality in arid and semiarid ecosystems [5, 6, 7]. The conversion of farmland to forests or grasslands has been shown to increase SOC by increasing C derived from new vegetation, thus simultaneously decreasing C loss from decomposition and erosion [8, 9]. Thus, afforestation and revegetation have been proposed as effective methods for reducing atmospheric CO\({}_{2}\) due to C sequestration in soils.  
Soil physical-chemical properties have been extensively used to evaluate SOC; however, these properties usually change slowly, and thoroughly reflecting soil changes through these properties is impossible; thus, the selection of indicators that appropriately reflect the overall change in SOC is important. Previous studies have mainly focused on SOC dynamics during vegetation restoration [5], the effects of land use change and SOC [10, 11], and C-N relations [9], as well as the effects of aspect-vegetation complexes on the decomposition of SOC [12]. Moreover, many studies have focused on the effects of soil microbes [13], soil enzyme activities [14, 15], soil nutrients [16], soil aggregates and SOC fractions [17, 18],and soil mechanical components, e.g., sand, silt, and clay [19] on SOC. However, most of those studies only reported the effects of soil factors on SOC from one or a few aspects. For example, one study reported soil organic carbon variation determined by biogeographic patterns of microbial carbon and nutrient limitations [20], which can be reflected by the soil extracellular enzyme activities [6]. Soil enzyme-mediated mineralization of soil organic matter is a vital biochemical process within the soil C cycles [6]. And litter decomposition following vegetation restoration was linked to soil nutrient dynamics [21]. So, there is little available information on the combination of soil physical, chemical and biological factors to examine the effect of soil factors on SOC as a whole.  
In 1999, the Chinese government implemented the \"Grain for Green\" Program (GGP) by restoring degraded farmland to forests, shrubs and grasslands [5]. Although the initial goal of the GGP program was aimed at controlling soil erosion and restoring ecosystems, it has been instrumental in increasing both the rate and overall quantity of C sequestered in the soil. At present, the \"Grain for Green\" program is the first and still the most ambitious, ecosystem services program in the world [22, 23]. The Loess Plateau is the key zone for implementing the GGP. The process of natural and artificial restoration of abandoned farmland is underway on the Loess Plateau [5]. Although the initial goal of the GGP was to control soil erosion, the program strongly affects soil C cycling. Consequently, many studies have focused on changes in soil C accumulation following farmland conversion on the Loess Plateau [5, 13, 24]. However, those studies only focus on one simple type of vegetation restoration. The GPP includes forests, shrubs, grasslands and mixed forests. To date, there is still controversy regarding the best choice of vegetation restoration type for the Loess Plateau.  
Therefore, we are in need of a comprehensive study of soil C variations that considers different types of vegetation restoration (i.e., forests, shrubs, grasslands and mixed forests). The objectives of the study were to (1) explore the difference in SOC under different vegetation restoration types and (2) identify the soil factors that have the greatest effect on SOC.