Appropriate targets for soil erosion control at the national scale determined in China

Soil erosion causes topsoil loss of more than 75 billion tons per year globally. It results in reduced crop yields and other economic losses totaling approximately US$400 billion. The ecological and environmental issues, such as land degradation and water pollution, are aggravated, posing a serious threat to ecological security, river and lake health, and sustainable human development (Food Agriculture Organization of United Nations, 2015; Montgomery, 2007; Quinton et al., 2000). Soil erosion in China is particularly stern and the country has tackled this challenge unremittingly for over 70 years. The outcome speaks for itself: the national soil erosion area (with soil erosion modulus exceeding soil loss tolerance of corresponding region, e.g., 1000 t/km·a in the Northwest Loess Plateau, 500 t/km·a in the Southern Red Soil Area, and 200 t/km·a in the Northeast Black Soil Area) (MWR, 2008) has dropped from 3,670,300 km in the mid‐to‐late 1980s to 2,949,100 km in 2011 and further to 2,674,200 km in 2021, registering an accumulative reduction of over 1/4; and the proportion of soil erosion area at “moderate and above” level (with soil erosion modulus exceeding 2500 t/km·a) has decreased from 49.32% to 35.58%, meaning both erosion area and the intensity have declined (MWR et al., 2010; MWR, 2013, 2021). The Chinese government has attached great importance to ecological improvement amid its rapid economic and social development throughout the years. Both the understanding of this issue and the investment for it are growing. In 2017, China called for “fundamentally improving the ecological environment and largely achieving the goal of a beautiful China by 2035; and comprehensively advancing ecological progress by 2050”; and the ecological civilization concept of “pursuing green development and promoting harmony between humanity and nature” was again highlighted in 2022. As a fundamental measure for the ecological protection and restoration of rivers and lakes systems, soil erosion and water loss control is taken as an important element of pursuing ecological progress and has become a fundamental state policy to be upheld in the long run. Therefore, to what extent is soil erosion control considered adequate to meet China's goal of achieving ecological progress and the vision of a beautiful China by 2035 and 2050 is a question that warrants speedy response. In the meantime, as global ecological protection and governance continue to progress, studies have been carried out on appropriate targets for regional vegetation restoration (Feng et al., 2016), trade‐ offs for ecosystem services (Ouyang et al., 2016), and evolution and spillover effects of the social‐ecological system (Wu et al., 2020). These studies have shown that single ecological elements, processes, and their effects at a certain spatial and temporal scale may have relatively stable or reasonable threshold intervals for the system as a

Soil erosion causes topsoil loss of more than 75 billion tons per year globally. It results in reduced crop yields and other economic losses totaling approximately US$400 billion. The ecological and environmental issues, such as land degradation and water pollution, are aggravated, posing a serious threat to ecological security, river and lake health, and sustainable human development (Food Agriculture Organization of United Nations, 2015; Montgomery, 2007;Quinton et al., 2000). Soil erosion in China is particularly stern and the country has tackled this challenge unremittingly for over 70 years. The outcome speaks for itself: the national soil erosion area (with soil erosion modulus exceeding soil loss tolerance of corresponding region, e.g., 1000 t/km 2 ·a in the Northwest Loess Plateau, 500 t/km 2 ·a in the Southern Red Soil Area, and 200 t/km 2 ·a in the Northeast Black Soil Area) (MWR, 2008) has dropped from 3,670,300 km 2 in the mid-to-late 1980s to 2,949,100 km 2 in 2011 and further to 2,674,200 km 2 in 2021, registering an accumulative reduction of over 1/4; and the proportion of soil erosion area at "moderate and above" level (with soil erosion modulus exceeding 2500 t/km 2 ·a) has decreased from 49.32% to 35.58%, meaning both erosion area and the intensity have declined (MWR et al., 2010;MWR, 2013MWR, , 2021. The Chinese government has attached great importance to ecological improvement amid its rapid economic and social development throughout the years. Both the understanding of this issue and the investment for it are growing. In 2017, China called for "fundamentally improving the ecological environment and largely achieving the goal of a beautiful China by 2035; and comprehensively advancing ecological progress by 2050"; and the ecological civilization concept of "pursuing green development and promoting harmony between humanity and nature" was again highlighted in 2022. As a fundamental measure for the ecological protection and restoration of rivers and lakes systems, soil erosion and water loss control is taken as an important element of pursuing ecological progress and has become a fundamental state policy to be upheld in the long run. Therefore, to what extent is soil erosion control considered adequate to meet China's goal of achieving ecological progress and the vision of a beautiful China by 2035 and 2050 is a question that warrants speedy response. In the meantime, as global ecological protection and governance continue to progress, studies have been carried out on appropriate targets for regional vegetation restoration (Feng et al., 2016), tradeoffs for ecosystem services (Ouyang et al., 2016), and evolution and spillover effects of the social-ecological system (Wu et al., 2020). These studies have shown that single ecological elements, processes, and their effects at a certain spatial and temporal scale may have relatively stable or reasonable threshold intervals for the system as a whole. Soil erosion is a surficial process that works under the action of natural forces such as water, wind, gravity, and freeze-thaw, as well as human activities. The erosion area and intensity are dynamically changing within a certain area and a period of time. It is neither a static stock that can only be reduced through control, nor is it possible or appropriate to eliminate it completely. There must be a "ceiling" for scientific control and an "appropriate degree" determined by comprehensive judgment.
Therefore, to scientifically define soil erosion control targets nationwide and by regions, in 2020, the MWR came up with its first-ever indicator of "soil and water conservation rate," which is used to proactively and comprehensively characterize the regional soil and water conservation status. The threshold is defined as the appropriate degree of soil erosion control determined in accordance with the laws of nature and deemed necessary to meet the requirements of economic and social development. The indicator of "soil and water conservation rate" has been listed as one of the 22 indicators for the assessment of building a Beautiful China (National Development and Reform Commission [NDRC], 2020). During the period 2020-2022, MWR commissioned 10 research institutions, including the China Institute of Water Resources and Hydropower Research, to conduct a study on soil and water conservation rate thresholds nationwide and by types of areas (hereinafter referred to as the "Special Study"). Based on the eight national primary subdivisions of soil and water conservation, namely the Northeast Black Soil Area, the Northern Sandy Area, the Northern Rocky Mountain Area, the Northwest Loess Plateau Area, the Southern Red Soil Area, the Southwest Purple Soil Area, the Southwest Karst Area, and the Qinghai-Tibet Plateau Area, a comprehensive analysis of the natural geographic conditions, economic and social development levels and trends of each area were conducted, and the principles of anticipating long-term soil erosion control situation were proposed (Cao et al., 2021). The 30-m resolution grid was F I G U R E 1 Technical process for determining soil and water conservation rate thresholds. used as the spatial unit. Based on the superposition analysis of geospatial data such as soil erosion classification and grading, land use, elevation, topography, and vegetation cover, existing soil erosion areas were analyzed piece by piece as to which of them do not need to be controlled while others should be, and which can be fully controlled (i.e., the postcontrol soil erosion intensity can be reduced to below "mild" level, making them no longer counted as soil erosion area) and which cannot be fully controlled (i.e., the postcontrol soil erosion intensity can be reduced but remains at "mild or above" level, making them still counted as soil erosion area). Finally, the long-term soil erosion area thresholds nationwide and by types of areas were determined (see Figure 1 for the general technical process).
The results of the Special Study show that of the existing soil erosion areas in China in 2021, about 1.23 million km 2 either do not require control or are not inappropriate to control due to a combination of natural and social factors. These are mainly wind erosion area in the desert, the Gobi and the moving sandy hinterland in the northwest, and hydraulic erosion in the alpine and high-altitude sparsely populated areas such as the Qinghai-Tibet Plateau and the Hengduan Mountains; the remaining 1.44 million km 2 requires siteappropriate comprehensive and targeted control. The comprehensive analysis and weighing of soil erosion laws, physical and geographical conditions, as well as technical and economic factors in the long term (after 2050) indicate that the soil erosion intensity of about 520,000 km 2 of postcontrol area can be reduced to below "mild" level, which can count as nonerosion area according to the soil erosion classification standards; that of the remaining 920,000 km 2 can be reduced to varying degrees but remains at "mild or above" level, making them counted as erosion area in the long term. These are mainly wind erosion in the fixed sandy areas and the transition zone between oasis and desert in northwest China, hydraulic erosion on sloping arable land, garden land and steep-slope forest and grassland in the hilly areas of north, southeast, and southwest China, and the rolling hilly region of Northeast China, mixed hydraulic and gravity erosion on the F I G U R E 2 Distribution of existing soil erosion areas and classification of long-term soil erosion control in China. gullies and steep slopes of the Loess Plateau, and phased anthropogenic soil erosion caused by necessary production and construction activities (see Figure 2 for the distribution of existing soil erosion areas and the classification of long-term soil erosion control). Based on these studies, it is calculated that the national soil erosion area should and can be reduced to 2.15 million km 2 in the long term, and the threshold of soil and water conservation rate (i.e., the proportion of the area under sound soil and water conservation conditions to the national land area) can reach 77.5%. For the first time, appropriate science-based targets for soil erosion control at national scale in the context of advancing ecological civilization and building a beautiful China are determined.
The Special Study took a further step to determine the respective soil and water conservation rate thresholds and erosion control targets for each province based on the provincial spatial distribution data and relevant statistics. To advance soil erosion control in a systemic manner and urge the local governments to act on the responsibilities for soil and water conservation, MWR adopted both topdown and bottom-up approaches: a simultaneous review of all provincial soil and water conservation rate thresholds was conducted, the soil and water conservation rate targets by 2025, 2030, and 2035 in each province were identified, and the national targets for soil erosion control in each stage for the next 15 years was determined. In December 2022, the General Office of the CPC Central Committee and the General Office of the State Council issued the Opinions on Strengthening Soil and Water Conservation in the New Era (hereinafter referred to as the "Opinions"). It is clearly put forward in the Opinions that by 2025, anthropogenic soil erosion shall be effectively managed, the erosion in key regions be effectively treated, the overall erosion situation be continuously eased, and the national soil and water conservation rate reach 73%; by 2035, anthropogenic soil erosion shall be fully managed, the erosion in key regions be brought under full control, the national soil and water conservation rate reach 75%, and the soil and water conservation functions of ecosystems be significantly enhanced (The General Office of the CPC Central Committee the General Office of the State Council, 2022). As a navigating document for strengthening soil and water conservation work in China both for the present and the long run, the Opinions defines the appropriate targets for soil erosion control at the national level and provides leading objectives and guidelines in a scientific and orderly way. It is conducive to promoting highquality development of soil and water conservation in the new era and offers a template of addressing the common challenge of soil erosion globally.

ACKNOWLEDGMENTS
Major scientific and technological issues in water conservancy of the Ministry of Water Resources: "Research on the objectives and countermeasures of soil and water conservation in the new era"; National Natural Science Foundation of China: "Change mechanisms and thresholds for soil and water conservation rates in the Yellow River Basin" (U2243212).

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.
ETHICS STATEMENT None declared.