1. INTRODUCTION
Land use and land cover change (LUCC) plays a crucial role in global carbon (C) and nitrogen (N) cycles by modifying soil C and N turnover, storages, and soil erosion (Post and Kwon, 2000; Watson et al., 2000; Lal, 2004; Ding et al., 2013; Han et al., 2019). After cultivating the native forests, soil organic carbon (SOC) may be rapidly decreased owing to degradation and erosion through human-disturbed activities (Harris et al., 2012; Mukhopadhyay et al., 2016; Wiesmeier et al., 2016). On the contrary, shifting from cropland to perennial vegetation was reported to increase SOC accumulation by transforming more atmospheric C into the soil and simultaneously decrease C loss from decomposition and erosion (Post and Kwon, 2000; Guo and Gifford, 2002; Berthrong et al., 2009; Laganiere et al., 2010; Duan et al., 2018; Nave et al., 2018). As another key factor in maintaining soil quality, total nitrogen (TN) and SOC generally exhibit similar responses to LUCCs by changing the microbial conditions and litter inputs in ecosystems (Xie et al., 2004; Luo et al., 2009; Xu et al., 2019), hydrothermal conditions at the land surface (Luo et al., 2009; Jerome et al., 2010; Fei et al., 2015) and plant species (Nie et al., 2014; Pan et al., 2015), which, if considering the impact of human activities (Xiao et al., 2009; Deng and Shangguan, 2017), jointly determined the changes in TN storage. Thus, appropriate land-use management practices (e.g., reforestation and land restoration) have been considered as one of the major strategies for mitigating climate change (Richard et al., 2006; Song et al., 2014; Hu et al., 2018).
The SOC and TN are important factors affecting ecosystem productivity and various physiological processes (Reich et al., 1997; Vitousek, 2005). Since the beginning of the 21st century, much attention has been paid to the effects of LUCC on SOC and TN storage (Post and Kwon, 2000; Guo and Gifford, 2002; Don et al., 2011; Poeplau et al., 2011; Van et al., 2015; Nave et al., 2018), specifically, leading to contradictive discussions. Previous studies have found that deforestation (DF) could cause SOC and TN losses, which depends on land use patterns (Guo and Gifford, 2002; Lal, 2005; Shirvani et al., 2010; Gamboa and Galicia, 2011; Van et al., 2015; Nyawira et al., 2016). In contrast, land restoration (LR) could increase SOC and TN storage (Guo and Gifford, 2002; Song et al., 2014; Deng et al., 2016; Hu et al., 2018; Nave et al., 2018). Overall, the obvious SOC and TN loss/gain were found to be sensitive to LUCC.
The fixed-depth method (FD) is widely used to calculate SOC and TN storages, which is simply multiplied by soil bulk density (BD), depth and SOC and TN concentrations (Lee et al., 2009). Nevertheless, the calculated SOC and TN storages under different LUCC patterns using the FD may induce errors (Lee et al., 2009; VandenBygaart et al., 2010; Palm et al., 2014; Du et al., 2017; Hu et al., 2018). Instead, the equivalent soil mass method (ESM) could more accurately quantify SOC and TN storages by calibrating soil mass per unit area among different treatments (Ellert and Bettany, 1995; Gifford and Roderick, 2003; Du et al., 2017). Since both the LUCC and soil depth may lead to soil BD changes, more precise methods (such as the ESM) need to be utilized in practice.
Karst landform is known to be one of the most ecologically fragile regions in China (together with the Loess Plateau) and the total area in southwest China is about 4.3 × 104km2. In particular, this fragile ecosystem is extremely sensitive to LUCC, which has become major ecological and environmental problems in the Karst areas. Since the late 1990s, China had launched the Returning Farmland to Forest Program in the southwest Karst region. The subsequent land use and cover changes (i.e., deforestation and land restoration) have potentially interfered the local ecosystems and induced severe environmental stress (Wang et al., 2010; Hu et al., 2018). Therefore, investigating the responses of SOC and TN storage to LUCC may provide some insights into the interactions between local ecosystems and human activities, and eventually provide scientific evidences to support policy making for stakeholders.
In the recent decades, the meta-analysis approach has been considerably utilized to evaluate the changes in SOC and TN storages responsive to LUCC (Virto et al., 2012; Powlson et al., 2016; Shi et al., 2016). Despite variations in sampling methods and experiment conditions, the meta-analysis could provide a comprehensive way by obtaining a large amount of data, which is helpful to explain the overall trend of SOC and TN storage in responses to LUCC. To our best knowledge, such systematic analyses have not been carried out to analyze the impact of LUCC on SOC and TN storage in southwest China Karst region. This study aimed to (1) assess the uncertainties of calculation methods (the FD vs. ESM) on SOC and TN storages, (2) quantitatively analyze the effects of specific land use and cover and soil layers on SOC and TN storages the in Karst areas, and (3) reveal the controlling factors that affect SOC storage.