The challenge of soil loss control and vegetation restoration in the karst area of southwestern China

Journal Pre-proof The Challenge of Soil Loss Control and Vegetation Restoration in the Karst Area of Southwestern China Ligang Zhou, Xiangdong Wang, Zhaoyan Wang, Xiaoming Zhang, Cheng Chen, Huifang Liu PII:

S2095-6339(19)30198-4

DOI:

https://doi.org/10.1016/j.iswcr.2019.12.001

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ISWCR 193

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International Soil and Water Conservation Research

Received Date: 16 August 2019 Revised Date:

12 December 2019

Accepted Date: 13 December 2019

Please cite this article as: Zhou L., Wang X., Wang Z., Zhang X., Chen C. & Liu H., The Challenge of Soil Loss Control and Vegetation Restoration in the Karst Area of Southwestern China, International Soil and Water Conservation Research, https://doi.org/10.1016/j.iswcr.2019.12.001. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 International Research and Training Center on Erosion and Sedimentation and China Water and Power Press. Production and Hosting by Elsevier B.V.

The Challenge of Soil Loss Control and Vegetation Restoration in the Karst Area of Southwestern China Ligang Zhou 1,2, Xiangdong Wang1,2,*, Zhaoyan Wang1,2, Xiaoming Zhang1,2, Cheng Chen 1,2, Huifang Liu1,2 1

State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Research and Hydropower Research, Beijing 100048, China; 2 Research Center of Water and Soil Conservation Ecological Engineering and Technology, Beijing 100048. * Correspondence: [email protected]; Tel: +86-010-6878-6645

Abstract: Soil loss, both from surface soil loss and subsurface soil leakage, in the karst regions of southwestern China is a serious environmental problem that threatens sustainability in that region. The surface soil loss has been extensively studied, and many studies have been conducted to investigate the causes, impacts and mechanisms involved, but the study of subsurface soil leakage has received little attention due to the difficulties in studying the natural conditions. There is no consensus on the overall proportions between surface soil loss and subsurface soil leakage. To control soil loss, improve ecological restoration, and help locals out of poverty, the Chinese government carried out a series of ecological restoration projects in the karst regions of southwestern China starting in the 1980s. As a result, the intensity and areal extent of soil loss continues to decrease and the ecological situation is steadily improving. However, because of the fragile ecosystem in the karst regions, the soil loss control is a long-term task, and the soil loss in some karst regions continues to be a problem. Subsequently, we put forward some suggestions for the policy makers relative to conservation of soil loss and vegetation restoration. These suggestions include: (1) government, private organizations and individuals are encouraged to raise funds for soil loss control and vegetation restoration; (2) nature reserves should be established to increase biodiversity; (3) engineering projects such as small reservoirs, ponds, and flow diversion channels should be constructed in marginal karst regions. Keywords: karst; soil loss; vegetation restoration; southwestern China 1. Introduction Karst topography is defined by the geological action of water on soluble rocks, such as carbonate, gypsum, limestone and dolomite, which is dominated by chemical dissolution and supplemented by the mechanical actions of erosion or collapse due to water and is a general term for the phenomena arising from these actions (Wang et al., 2018a; Guo et al., 2007). The total area of global karst landforms is approximately 22 million km2, accounting for approximately 12% of the total area in the world, and these areas are inhabited by nearly 1 billion people (Jiang et al., 2014). The karst regions of southwestern China is the largest karst region in the world and experiences the strongest effects from the karst processes, it is also the most complex and fragile ecological environment among the global karst regions. With an area of approximately 0.53 million km2 that is largely in Guizhou Province (0.12 million km2, with 61.17% of the region’s total land area), Yunnan Province (0.11 million km2, 30.07%) and Guangxi Zhuang Autonomous Region (0.08 million km2, 34.73%) (Cao et al., 2011; Zeng et al., 2018) (Fig.1), the karst areas have a population of nearly 200 million, of which minority groups account for approximately 30 million. The population and land pressure have led the farmers to increasingly expand into areas with steeply sloping topography and as a result, large parts of the forests in the karst areas of southwestern China have been deforested, which has accelerated the soil loss. Consequently, a significant area of karst (~0.13 million km2) that was previously covered by shrubs and trees was gradually converted to bare soil (Brandt et al., 2018).

Fig.1. The major provinces in the karst regions of southwestern China

Soil loss is a serious environmental problem that threatens the sustainability and strong development of human society (Gao & Wang, 2019; Li, et al., 2017; Zhao & Hou, 2019). In the karst regions of southwestern China, which is a distinctive geological environment, the long-term effects of karst process make the carbonate rocks highly soluble, and causes air pockets to form under the rocks, which makes the soil loss unique, complex, and difficult to observe. The soil loss not only includes surface soil loss, but also subsurface soil leakage along underground conduits, such as pores, fissures, funnels, and even underground rivers. Due to worldwide concern about soil loss, soil loss models play an important role in estimating soil loss at large scale. The models not only provide both quantitative and qualitative estimation of the phenomenon under various conditions but also estimate the spatial and temporal patterns of soil loss (Li et al., 2015; Phinzi & Ngetar, 2019; Teng et al., 2019). Consequently, the soil loss models serve as guides for policy formulation and the implementation of effective strategies for the conservation of soil and water resource at large scale (Brinkmann & Parise, 2012; Peng & Wang, 2012; Phinzi & Ngetar, 2019). Several models have been developed in karst regions of southwestern China for assessment of soil loss, including empirical models, such as the universal soil loss equation (USLE), revised universal soil loss equation (RUSLE) and its enhancement (Chen et al.,

2017; Gao & Wang, 2019; Phinzi & Ngetar, 2019), and physical models, such as the water erosion prediction project (WEPP) (Flanagan et al., 2007; Laflen et al., 1991; Zhang et al., 2005), Limburg soil erosion model (LISEM)(Jia & Zheng, 2004) and European soil erosion model (EUROSEM) (Sogon et al., 1999). Empirical models have been widely used in low-slope regions at a relatively large spatial scale due to their simple structures and few required parameters, and the strong applicability of geographic information system (GIS) and remote sensing technologies (Gao & Wang, 2019). The RUSLE model has also been applied to the karst areas of southwestern China (Chen et al., 2017; Feng et al., 2016; Zeng et al., 2017; Zeng et al., 2018; Zeng et al., 2011), however, this application ignored karst fissures, including less erodible soil in areas with severe rocky desertification, and erosion-resistant bedrock outcrops, and thus may have overestimated karst soil loss(Gao & Wang, 2019). To control soil loss, improve ecological restoration, and help locals out of poverty, the Chinese government carried out a series of ecological restoration projects in the karst regions of southwestern China starting in the 1980s. However, in the case of thin soils, insufficient soil nutrients, and low land productivity, the primary consideration for ecological restoration is whether the soil nutrients in shallow soil can meet the nutrient requirements for vegetation growth. Vegetation restoration and ecological reconstruction in karst areas are systematic and large-scale projects; understanding how native plant communities overcome nutrient limitations in the characteristically thin, nutrient-poor soils of karst systems may provide solutions for the strategic restoration of the degraded karst regions of southwestern China (Sophie M. Green, 2019). The goal of this paper was to comprehensively analyze the characteristics of soil loss, the impact factors (natural and human activities), and the challenges of soil loss control and vegetation restoration in the karst regions of southwestern China. In addition, we put forward several policy suggestions for decision-makers for further control of soil loss and vegetation restoration. 2. Characteristics of soil loss in the karst areas of southwestern China 2.1 The imbalance of soil formation and soil loss rate The soil-forming material in the karst regions of southwestern China is derived from the residual material after the weathering and leaching of carbonate rocks. As the carbonate rocks are highly soluble and approximately 90% of the substances are dissolved by water and taken away, so the content of insoluble residual matters are generally less than 10%. According to an analysis of data 132 samples in Guizhou Province, southwestern China, the average content of residual substance was only 3.9%(Guangbin et al., 2013). Subsequently, only a few undissolved minerals (<10%) are available for soil formation. Many scholars have studied the rate of soil formation. For example, Wang et al. (2004) and Su (2012) reported that the formation of 1 cm soil from undissolved carbonate bedrock under the current climate conditions and without anthropologic interface would take approximately 2.5~8 thousand years, with an average of approximately 4 thousand years; that is, 0.0025 mm of soil is produced every year, and approximately 2.5 m3 km-2 yr-1 is of soil formed (Wang et al., 2004; Su, 2002). Li et al. (2008) also reported that the soil formation rates in the karst region of southwestern China were approximately 80 times slower than those in non-karst areas. However, human activities, such as the use of large quantities of acid fertilizers and cultivation practices; natural processes; and extreme climates, such as extreme drought or extreme rainfall events, play important roles in soil formation. A study by Sophie et al. (2019) showed that the growth activities of plant roots, the application of

inorganic fertilizers and farming activities on steep slopes all contributed to the physical and chemical weathering of the carbonate rocks, which exacerbated the soil formation(Sophie M. Green, 2019). However, all soil loss rates and the total soil losses are high and are far greater than the soil formation rate in the karst regions of southwestern China. Teng et al. (2019) used satellite images, field samples, and the RUSLE model to quantitatively estimate the soil loss in China, and their results showed that the average soil loss rate and the average total soil loss in the karst regions of southwestern China were 3.02 t ha-1 yr-1 and 78.52×106 t yr-1, respectively (Table 1). The rate of soil loss was far greater than the maximum soil loss tolerance (0.5 t ha-1 yr-1) in the karst regions. In general, the rates of soil formation and soil loss in the karst regions of southwestern China are extremely unbalanced. Table 1 Soil erosion rate and soil loss in the karst areas of southwestern China. Derived from Teng et al. (2019) Region

Province/Region

Southwestern China

Sichuan Chongqing Yunnan Guizhou Guangdong Guangxi Hubei Hunan

Total Average China

Soil loss rate (t ha-1 yr-1) 5.73 5.07 4.54 2.72 1.88 1.71 1.41 1.14 24.20 3.02 1.44

Total soil loss (×106 t yr-1) 261.03 39.96 168.32 47.07 28.37 38.15 22.76 22.49 628.15 78.52 1180.28

2.2 The coexistence of surface soil loss and subsurface soil leakage Surface soil loss and subsurface soil leakage are typical soil loss patterns in the karst regions of southwestern China (Fig.2). Severe surface soil loss and subsurface soil leakage not only lead to land degradation, reduction of soil depth and soil nutrients loss but also exacerbate the occurrence of drought, floods, landslides and other disasters (Zeng et al., 2017). In recent decades, many scholars have studied soil loss and determined its causes and spatial evolution (Bai et al., 2013; Borrelli et al., 2017; Brandt et al., 2018; Jiang et al., 2009; Li et al., 2015; Qin et al., 2016; Zhou et al., 2017). Loss force (Tang et al., 2019), loss processes(Dai et al., 2017a; Hu et al., 2015; Li et al., 2011; Luo et al., 2018), soil degradation (Breg Valjavec et al., 2018), soil leakage ratio (Chen & Lian 2016; Feng et al., 2016; Kereselidze et al., 2013; 1999; Wei et al., 2016; Yan et al., 2018; Wei et al., 2010), and soil loss mechanism (Zhou et al., 2012) have also been explored. However, studies on soil loss have been mainly concentrated in non-karst areas, and few studies have investigated the fragile ecological-geological environment in the karst regions of southwestern China. Only a few scholars have conducted preliminary studies on surface soil loss and subsurface soil leakage in the areas with karst landform. For example, Bai et al. (2011) using the 137Cs technique to assess the sediment and soil loss rates in a Chinese polygonal karst depression, which indicated that the sediment deposition rates ranged from 0.91 to 1.97 mm yr-1 from 1963 to 2007, respectively, and the average soil loss rate was 20 t km-2 yr-1, respectively. Chen et al. (2017) assessed soil loss using a strategy involving alternative submodels based on the RUSLE model in a karst basin of southwestern China and found that the RUSLE model estimated a mean annual soil loss rate of 30.24 Mg ha-1 yr-1 from the 1980s to 2000s. Feng et al. (2016) also found that the average soil loss rates on hillslopes simulated by the RUSLE model were 0.22 and 0.10 Mg ha-1 yr-1 from 2006 to 2011 in a partially cultivated peak-cluster

depression basin and an undisturbed peak-cluster depression basin of northwestern Guangxi, southwestern China, respectively. Although the soil loss, both from surface soil loss and subsurface soil leakage, in the karst regions plays an important role in economic and social sustainable development, the subsurface soil leakage has received little attention at present due to the great difficulty of making direct observations on them. Furthermore, because of the lack of theoretical research, effective technical methods or data support, researchers have ignored the contributions and impacts of subsurface soil leakage on vegetation growth and ecological construction (Yan et al., 2019). At present, the soil leakage phenomenon is not well understood in China, and the two main viewpoints about soil loss rates are that they are either high or low in the karst areas. Many scholars (Bai et al., 2013; Bai., 2011; Chen et al., 2017; Xiong et al., 2012) have suggested that the soil leakage ratios in the karst regions of southwestern China are high. A study of a karst gorge showed that the volumetric ratio of the surface soil loss (75%) was 3 times greater than that in the underground loss (Wei et al., 2016). However, Feng et al. (2016) and Joris et al. (2013) proposed that soil leakage ratios in the karst regions are very low at a large spatial scale, and only soil-filled cracks in the rock were noticed at a small spatial scale (de Vente et al., 2013; Feng et al., 2016). The debate about the soil leakage ratio ongoing.

Fig.2. Severe soil loss in the karst regions of southwestern China. Note: a indicates that severe soil loss caused surface topsoil to be almost lost, forming a karst rocky desertification landscape; b, c, and d indicate subsurface soil loss.

3. Factors affecting soil loss in karst areas The occurrence and development of soil loss in karst regions of southwestern China are mainly caused by geological and geomorphology, hydrology, vegetation cover and human activities, which can be attributed to two aspects: natural processes and human factors. 3.1 Natural processes 3.1.1 Geology and geomorphology Due to the Mesozoic Yanshan tectonic movement, the karst regions of southwestern China were superimposed with the generation of new movements from the Himalayan mountains, forming a steep and broken landscape pattern (Wang et al., 2004; Tang et al., 2010). The carbonate rocks are highly soluble and long-term leaching forms unique geomorphological features, such as surface fissures, pipelines, and funnels; thus, the water and soils on the slopes are easily lost along these conduits. Luo et al. (2008) reported that from the top of a karst peak to the bottom of the depression, the soil loss on the slopes is dominated by subsurface soil leakage, while soil loss on the depression is dominated by surface soil loss. The rate of soil formation in the karst regions of southwestern China is extremely low; 4 ~ 8.5 thousand years are needed to form 1 cm soil, which is 10 ~ 80 times slower than the non-karst regions (Jiang et al., 2014; Zhao & Hou, 2019; Su, 2002). The soil loss in some regions is exhausted and even bedrock is bare after several years of soil erosion, and it is extremely difficult to recover in the short term (Zhang et al., 2011). Generally, most soil profiles in the karst regions of southwestern China lack a C-horizon, which cause the soils to be in direct contact with the bedrock, and the soil loss easily occurs under light rainfall because the adhesion between the topsoil and bedrock is greatly reduced (Zeng et al., 2018). In addition, the thickness of the soil layer decreases, but the amount of soil loss increases as the slope increases. Some studies report that soil loss increases sharply with increasing the slope gradient. For example, when the slope gradient increases from 10° ~ 15° to 40°, the thickness of the soil decreases from 120 cm to less than 20 cm but the amount of soil loss increases from 286 t km-2·yr-1 to 11,700 t km-2·yr-1(Jiang et al., 2014; Zhang et al., 2016). However, Zhang et al. (2018) found that when the slope gradient increased, the soil loss on the slopes followed the pattern of increasing, decreasing, and increasing again. 3.1.2 Hydrological conditions Rainfall and temperature are drivers of soil loss and act as chemical and mechanical factors for soil loss in the karst regions of southwestern China (Dai et al., 2017b; Yang et al., 2010). A subtropical monsoon climate is dominant in this region, with a mean annual temperature of 15-20°C and a mean annual rainfall of 1200-2000 mm, which creates favorable hydrothermal conditions for soil loss due to the warm and humid climate and the concurrent rain and heat. Yuan et al. (2016) and Dai et al. (2017) through indoor simulation experiments, revealed that the runoff and sediment yield of the sloped cropland were dominated by underground runoff and sediment yield under light rainfall intensity (< 50 mm h-1) and moderate rainfall intensity (50 ~ 70 mm h-1), while the surface runoff and sediment yield were generally greater than the subsurface runoff and sediment yield. Li et al. (2011) studied in a karst area of Spain showed that the antecedent soil water content had a significant impact on the runoff and sediment generation; wet soil can generate surface runoff and sediment with only 12 mm of rainfall. A study by Lorena et al. (2010) also reported that the antecedent soil moisture had a significant impact on runoff and sediment production. Currently, the hydrological cycle and ecological environment are faced

with severe challenges due to global climate change. The frequency of extreme climate events, such as drought or extreme rainfall, have increased in the past 50 years in some of the karst regions of southwestern China. During the extreme drought that continued for three consecutive years in Qujing City, Yunnan Province, southwestern China, the area of karst rocky desertification increased by 6.8% annually from 2005 to 2011(SFA, 2012). These extreme changes in the climate and environment exacerbate soil loss in the karst regions of southwestern China. 3.1.3 Vegetation coverage Low vegetation coverage is another important factor affecting soil loss in karst regions of southwestern China. A study by Jiang et al. (2009) showed that rocky desertification caused by severely soil loss almost no longer occurs under the vegetation coverage exceeds 60%, while the soil loss is strong when the vegetation coverage is less than 20%. Li et al. (2011) also found that the soil loss is very weak even the rainfall is 55 mm h-1 when the vegetation coverage is higher than 50% in a typical karst region in Spain. In the karst regions of southwestern China, since the characteristics of thin soil layer, insufficient of soil nutrients such as organic matter and humus, few microbial species, high infiltration rate and difficult to effectively use groundwater resources, as a result, hinder the vegetation growth(Chen & Lian, 2016; Lv et al., 2007). 3.2 Human activities Problems in karst areas, karst management, and the sustainability of karst environments are receiving growing interest throughout the world due to the importance of natural resources and the need to protect and properly exploit these areas. (Brinkmann & Parise, 2012). However, unreasonable human activities such as cultivation on steep slopes (>25º), destruction of forests, overgrazing, and production, and construction activities can lead to land degradation and soil loss. In addition, excessive population pressure leads to serious soil loss. The depression of the economy in karst regions of southwestern China continues to worsen and the population continues to grow; the demand for food, energy and economic growth is constantly increasing. The population density in the karst regions of southwestern China was 207 people/km2 in 2017, which far exceeds the limit of a reasonable population capacity (~100 people/km2) at the current productivity level (SFA, 2018). To feed the population, local residents have no choice but to cut the short shrubs and trees to grow corn in steep areas, or to burn the bushes on the hillslopes to fertilize the hillslopes in the fall hillslope, resulting in vicious circle of the increasing the population leasing to excessive reclamation, which exacerbate soil loss and depresses the economically (Jiang et al., 2014). In particular, the Great Leap Forward resulted in great deforestation, as farmers cut large amounts of timber to aliment furnace to melt pig iron, and as forests were cut to increase the amount of farmland (Delang & Yuan, 2014). Deforestation also continued during the Cultural Revolution (Delang & Yuan, 2014). In addition, severe rural road erosion, housing excavation, and animal trampling can cause roads to collapse and, in extreme cases, cause landslides (Zhao & Hou, 2019). Tourism in the karst areas is an important mode of poverty alleviation and economic development. Tourism brings tourists and thousands of jobs that help to increase the income of the local residents’. However, as the numbers of tourists increases, the demand for water and other resources will increase. Although the karst regions of southwestern China have a subtropical monsoon climate with abundant rainfall, karst fissures develop, and precipitation is easily lost through these conduits (Zhao & Hou, 2019). Consequently, karst areas can experience severe water shortages, especially from September to

March. To maintain the quality of tourism year-round, engineering measures for water allocation, such as reservoirs, ponds, and water pipelines, are needed. Construction of these measures will inevitably change the local hydrological processes, thereby aggravating soil loss (Brinkmann & Garren, 2011; Zhao & Hou, 2019). 4. Vegetation restoration and challenges for soil loss control Soil loss is a common phenomenon throughout the world due to the impact of natural and human activities, especially in the karst areas of southwestern China. Teng et al. (2019) reported that the soil loss ratio in the karst regions of southwestern China (3.02 Mg ha-1 yr-1) was 2.1 times the national average soil loss ratio (1.44 Mg ha-1 yr-1). After experiencing the destruction of forests during the Great Leap Forward and the Cultural Revaluation, the Chinese government has attempted to become more influential in the economic and ecological fields and has become more interested in the soil loss and karst rocky desertification in the karst areas of southwestern China during the past few decades (Zhang et al., 2019; Zhao & Hou, 2019; Wang et al., 2018). Several major ecological conservation and construction projects, including the Green for Green Program, the Natural Forest Protection Program, The Fast-Growing and High-Yielding Timber Plantation Development Program in Key Regions and Planning Outline for Comprehensive Management of Rocky Desertification in Karst Areas (2006-2015), were implemented to reduce soil loss and to control karst rocky desertification starting in the 1980s. However, insufficient surface soil and soil nutrients are the main limiting factors for vegetation restoration, and it is unclear whether the fragile ecosystem in the karst areas of southwestern China (shallow soil, low soil fertility and high percentage of rock outcrops) can provide the water and nutrients needed for vegetation growth. It seems that the rock outcrop rate is negatively correlated with the level of soil nutrients, however, there were significant positive correlations between the percentage of rock outcrops and the majority of soil nutrients (Wang et al., 2018a). The main reason may be that rock outcrops can improve the cycling and accumulation of soil nutrients and redistribute the organic carbon and nutrients to nearby soil patches in the karst system. Wang et al. (2018a) also found that the N:P in the karst regions of southwestern China was 3.45, which is far lower than the global average (13.1) and the average for China (9.3), but significantly higher than those in some other fragile ecosystem in China, including degraded grasslands, deserts, and estuarine wetlands. Many studies have examined the effects of ecosystem restoration projects on individual soil parameters, including the soil water content (Yang et al., 2019) and soil organic carbon stock (Deng et al., 2014; Tong et al., 2018; Xiao et al., 2017), or only a few limited soil parameters, including the soil quality (Zhang et al., 2019) and soil nutrients (Tong et al., 2017; Vijith et al., 2017; Wang et al., 2018a; Wang et al., 2018b; Xiao et al., 2018; Zhu et al., 2017). Zhang et al. (2019) reported that vegetation restoration practices can greatly improve soil physical and chemical quality in karst landscapes of southwestern China (Zhang et al., 2019). Other scholars have also found that soil physical and chemical properties changed, and the nutrients increased significantly after vegetation restoration projects were implemented (Long et al., 2017; Pang et al., 2018; Peng et al., 2013). Tong et al. (2017, 2018) reported that tree-planting projects in the karst areas of southwestern China have increased the aboveground biomass carbon increased 9% (+0.05Pg C yr-1), which indicated that the positive effects of these ecosystem engineering projects may reduce the risks of soil loss and desertification by increasing the vegetation cover and reducing the sensitivity of the ecosystem to climate changes. Similarly, satellite images show going green and biomass increase in the karst regions of southwestern China (Brandt et al., 2018; Macias-Fauria, 2018). A few scholars, however, have reported that the ecological status did not show any sign of

improvement due to the lack of economic development and that the inappropriate human activities that have induced soil loss still exist in the karst regions of southwestern China (Bai, et al. 2013; Wang et al., 2004). At the same time, they also acknowledged that the soil loss and karst rocky desertification in the karst areas of southwestern China have not expanded, which implies that the soil loss and karst rocky desertification are essentially under control (Bai et al., 2013; Wang et al., 2004). Therefore, we can conclude that the soil nutrient conditions were sufficient for vegetation restoration and reconstruction in the fragile ecosystem. There have been positive outcomes from the implementation of these ecological restoration projects, including reduced pressure from the population and land requirement, and from the good hydrothermal conditions in the karst regions of southwestern China; the intensity and areal extent of soil loss continues to decrease and the ecological situation is steadily improving. The monitoring data showed that the average annual expansion rate of the karst rocky desertification area caused by severe soil loss was 1.86%, 1.67%, -1.27%, and -3.45% in 1999, 2002, 2011 and 2016, respectively (SFA, 2018). The artificial afforestation and protection of grassland vegetation played a leading role in the reversal of karst rocky desertification, with a contribution rate of 65.5%; the contribution rate of the natural vegetation restoration due to reduced population pressure and the adjustment of rural energy structure adjustment was 24.4%; the contribution rate of agricultural technology measures was 3.9%; and the contribution rate of other factors was 6.2%(SFA, 2018), which indicates that the overall trend of the expansion of soil loss has been effectively curbed, the areas of karst rocky desertification land in the karst regions of southwestern China has been continuously reduced, and the ecological situation has steadily improved. However, because of the fragile ecosystem in the karst regions, soil loss control is a long-term task, soil loss in some regions continues to. In summary, the challenges of soil loss control and vegetation restoration in karst areas mainly include the following: (a) The ecosystem is very fragile, which causes recovery to take a long time. The characteristics of the karst regions of southwestern China, such as the unique double-layer hydrological structure, high carbonate rock exposure, thin soil layer, poor water retention, and fertilizer retention capacity, make the fragile ecosystem vulnerable to disasters, easy to destroy and difficult to remediate. At present, the vegetation is dominated by shrubs (~ 56.5%) and the vegetation is still in the primary stage of succession. Zhou et al. (2018) report that it will take approximately 20 years for karst rocky desertification to recover from degraded herb communities to shrubs, approximately 47 years to arbor forests, and nearly 80 years to stable climax communities. (b) The areas require labor-intensive protection and is difficult to govern. At present, the area of shrubs and arbor forests that have been effectively protected in the karst regions of southwestern China is only 47% of the total area; 15 million hectares of shrubs and arbor forests need to be strengthened, and approximately 10 million hectares of karst rocky desertification land need to be treated (SFA, 2018). At the same time, with the advancement of these ecological projects, some karst rocky desertification lands with better site conditions have been gradually improved. The regions that need to be treated next will be more difficult, and the costs of governance are increasing. (c) The lack of economic development and the very high population and land pressure create difficulties. The karst regions of southwestern China, which are located in the middle and upper reaches of the Yangtze River and Pearl River, are an important ecological security barrier zone and typically have high levels of poverty. The per capita GDP is only approximately 71% of that of the country. Because farmers have fewer ways to increase their income, they are more dependent on agricultural land, and 211 counties continued to be improved in 2017 (http://www.stats.gov.cn/). The

population density of this region is 207 people/km2, which is 1.5 times the national average population density, and more than twice the theoretical maximum (100 people/km2) of the karst region (SFA, 2018). The most economically-deprived people in the region are the farmers, who forge a living by cultivating the characteristically thin soils that sit on top of the carbonate rocks in the sub-tropical karst ecosystem (Sophie M. Green, 2019). (d) Man-made damage and natural disasters still exist, and the partial deterioration of the land is difficult to eliminate. As the income from grain production and subsidies is much higher than the standard compensation for ecological benefits, deforestation and cultivation still occur on steep slopes. In remote villages, the use of fuelwood and overgrazing are also putting pressure on soil loss control. Coupled with the uncertainty of natural disaster and extreme climates, such as droughts and freezes, as well as landslide, forest fires, and forest pests, the consolidation and expansion of the soil loss control and vegetation restoration have always faced serious threats, and partial deterioration likely occurs without continuous monitoring. In the future, the most significant steps for controlling soil loss and vegetation restoration in the karst regions of southwestern China should be the following: (1) The government should increase its budget for studying the mechanisms and processes of soil loss so that it could construct an appropriate management system and provide scientific references for land management (Zhao & Hou, 2019). At the same time, we cannot solely rely on government investment; if the government no longer provides compensation for ecological improvements, farmers will become likely to reclaim land on the steep slopes (>25º); as a result, private organizations and individuals are also encouraged to raise funds for soil preservation and vegetation restoration. (2) In the karst regions of southwestern China, large amounts of water are stored due to the water resource are abundant, with an annual average precipitation of 1000-2000 mm. However, surface water supplies are often scarce due to the rapid percolation of the rainwater conduits within karst landscapes. Subsequently, the construction of small reservoirs and ponds, and flow diversion channels should be used to hold water resources. Dams are built to form reservoirs or lakes, which are used to introduce surface water and/or groundwater into low-lying areas for the irrigation of farmland and drinking water, or temporary pumping stations are built in cultivated areas such as depressions and valleys to meet the needs of spring irrigation and the water demands during drought (Zhou et al., 2009). A successful example of this type of practice is the construction of Hongfeng Lake in Guiyang City, Guizhou Province, southwestern China. Hongfeng Lake has an area of approximately 57.2 km2, a water depth of 10.52 m, a maximum depth of approximately 45 m, and a storage volume of 0.6 billion m3, and it provides drinking water for more than 300 million people (Longyang, 2019). Not only did it hinder the expansion of soil loss but it also provided sufficient soil water content for the restoration of the ecosystem in the karst regions. (3) Methods for increasing the incomes of poor farmers in the karst regions of southwestern China should be expanded, including increasing the output of rural surplus labor, developing ecological agriculture, continuously increasing compensation for returning farmland to forests or grasslands, growing economical plants, and developing ecotourism resources (Karst Geological Forest Park), to improve their economic conditions, thereby reducing the dependence of poor rural households on the agriculture land for income. (4) Nature reserves should be established in areas with severe soil loss and extensive karst rocky desertification to achieve the maximum protection from soil loss. In nature reserves, farming and other human activities that may cause soil loss without government permission should be forbidden (Zhao & Hou, 2019). A successful example of these practices is the construction of the Maolan Nature Reserver in Libo County (107°52-108°05 E, 25°09-25°20 N), Guizhou Province,

southwestern China. With an area of approximately 21,000 ha, the forest area is approximately 19,000 ha and the forest coverage is approximately 90%. There are 1,203 species of plants and 2,028 species of animals. The reserve not only has important scientific research value and great ecotourism value but also a very large biological gene pool (Liu et al., 2018; Zhengming et al., 2011). 5. Summary and conclusion Soil loss, both from surface soil loss and subsurface soil leakage, in the karst regions of southwestern China is a serious environmental problem that threatens sustainability. The main factors for soil loss are natural processes including geological, hydrological and vegetation cover, and unreasonable human activities including excessive population pressure, cultivation on steep slopes (>25°), production and construction activities. Surface soil loss has been extensively studied, but the study of subsurface soil leakage has received little attention at present due to the great limitations of natural conditions. In addition, there are two main viewpoints about soil loss rates that they are either high or low in the karst regions of southwestern China. Therefore, the soil loss phenomenon is not well understood in karst regions. As a result of the implementation of a series of ecological restoration projects and the good hydrothermal conditions in the karst regions of southwestern China, the intensity and areal extent of soil loss continues to decrease and the ecological situation is steadily improving. However, because of the fragile ecosystem in the karst regions, soil loss control is a long-term task, and the soil loss in some karst regions continues to. In the future, we must pay close attention to the following: first, the government should increase its budget and the private organization and individuals are also encouraged to raise funds for soil loss control and vegetation restoration; second, engineering measures such as small reservoirs, ponds, and flow diversion channels should be constructed in marginal karst regions; third, nature reserves should be established to increase biodiversity; and finally, methods for poor farmers to improve their economic conditions should be expanded, thereby reducing the soil loss caused by human activities. Reference

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