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.