Soil erosion is a global ecological issue that threatens ecosystem health and sustainable development. It is an important cause of reservoir sedimentation and soil nutrient loss, thus it has a significant impact on the eutrophication of water bodies and freshwater pollution, and is also a major threat to soil quality and agricultural productivity [1
]. Soil erosion is also a common ecological problem in China; according to the Ministry of Water Resources of the Peoples’ Republic of China, there was a total soil erosion area of 2.71 million square kilometers in 2019, accounting for 28.34% of the total area. Although soil erosion in hilly areas is not the most serious issue in China, the harm of soil erosion needs to be considered, as more land in these areas is involved in agricultural production, and the lower reaches of rivers are often important industrial and agricultural production bases and economic centers. Therefore, soil erosion and its control in hilly areas deserve more attention and study.
The first issue raised in the study of soil erosion is how to quantitatively estimate the intensity of soil erosion and its spatial distribution. An accurately quantitative assessment of soil erosion dynamics is crucial to understand the erosion process and contributes to soil and water conservation [7
]. Among the existing models, the universal soil loss equation (USLE) [9
] and the revised universal soil loss equation (RUSLE) [10
] models are highly recognized and widely used [3
]. It has been demonstrated that the soil erosion in the Lower Yangtze Basin ranged from 120 to 260 t·ha−1
from 2001 to 2014 based on the RUSLE method [15
]. However, using the data from 2017, the soil erosion modulus ranged from 0 to 50 t·ha−1
in most of the areas in Jiangsu Province [16
]. The results differed in terms of temporal and spatial heterogeneity. Indeed, soil erosion in a specific region needs to be estimated to establish ecological restoration measures.
How to reduce soil erosion is another issue that needs to be studied. A number of engineering projects can effectively alleviate the soil erosion of agricultural land, among which LC is worthy of attention but has been less studied. Large-scale LC has been carried out since the mid-1990s and has been subject to a national plan since 2008 in China [17
]. LC is carried out to reduce the fragmentation of land, to increase the quantity and quality of cultivated land, and to improve infrastructures and farming conditions in fields, thus enhancing the utilization efficiency of land, water, labor, machinery, and other production factors [18
]. Land use structure, landscape patterns, vegetation coverage, soil properties, as well as ecological functions are inevitably changed during the process of LC [22
]. Subsequently, either positive or negative impacts are exerted on soil erosion. On the other hand, the connotation of LC has been constantly expanding [14
], and eco-environmental protection has gradually become one of the goals of LC [28
]. Since the concept of the life community of mountains, rivers, forests, fields, lakes, and grasses was first proposed, the Green Development Concept has gone deep into all aspects of social development. As an agricultural engineering measure, LC should pay more attention to ecological protection, and it is necessary to analyze the impact of LC on soil erosion.
The hilly areas in China are typical LC regions, and greater importance needs to be attached to both soil erosion and its control in these areas. The purposes of this study were to: (1) Quantitatively assess the soil erosion in the study area; (2) quantify the impact and heterogeneity of LC on soil erosion; (3) discuss the positive effects and potential risks brought by LC to soil erosion reduction. These results are valuable for understanding how to reduce soil erosion under LC and to provide strategies for LC in the future.
4.1. Positive Effects of LC on Soil Erosion Reduction
In this study, compared to plots without LC, plots with LC showed lower soil erosion (Figure 5
a). The positive effects of LC on soil erosion reduction could be explained from the perspectives of vegetation cover and engineering projects.
The construction of protection forests, farmland shelterbelts, or vegetation protection systems is one of the basic requirements for LC [17
], and the retention and increase of vegetative cover of soil is a vital factor for maintaining soil stability [48
] and for reducing the dynamics of runoff [49
], thus it is conducive to soil and water conservation. During the process of LC in Lishui District, vegetation cover, such as trees, shrubs, and grasses, are planted to protect the surface of slopes and to reduce soil erosion.
The construction of differentiated land engineering is one of the major types of LC that is widely carried out in China. First, projects of land leveling can reduce the altitude difference between plots and can make land flatter, thus decreasing the flow rate and weakening water erosion. Second, with LC in sloped areas, especially areas with high hypsography, measures of terraces or other ecological slope protection projects are taken to improve water conservation, avoid land collapse, and control soil erosion [47
]. Third, gully consolidation projects create more farmland in gullies and reduce land reclamation on slopes, which is helpful for soil conservation on slopes [51
], and filling gullies for farmland could indeed reduce the soil erosion at the bottom of such gullies [53
]. Fourth, soil reconstruction projects can improve soil particle composition and profile structure [54
], and a favorable soil structure contributes to water and nutrient retention as well as a decrease in erodibility [55
]. Last but not least, drainage engineering carried out in appropriate locations is conducive to alleviating problems with ponding caused by short-term intense rainfall under heavy rain events [27
]. During the process of LC in Lishui District, different kinds of farmland consolidation, land reclamation, and land development were carried out. Soil erosion in the plots with different types of LC (farmland consolidation, land reclamation, or land development) was lower than that in the plots without LC, except for farmland consolidation in Sampling 3 and 4 (Figure 5
4.2. Potential Risks Brought by LC to Soil Erosion Reduction
LC in China is used to reduce land fragmentation, increase cultivated land, improve production capacity, and strengthen intensive land use [21
]. Due to these goals, there may be several potential risks that hinder the soil erosion reduction during or after LC.
First, different tillage systems have different impacts on soil compaction and soil erosion, while runoff and erosion reduce under decreasing tillage intensity [58
]. Compared to conventional tillage systems, no-till and conservation tillage could decrease soil erosion on sloping agricultural land [60
]. As a reverse process of intensive land use, the marginalization and abandonment of land could increase vegetation cover and reduce soil erosion [48
]. The results of Han et al. (2020) also showed that cultivated land experienced stronger erosion than abandoned land or forest–grass land [49
]. However, what needs to be noted is that abandoned land increases erosion when the soil is left bare [48
]. Land development may become beneficial if the abandoned/unused land in one area is bare and with the prerequisite of guarding against desertification and soil erosion [65
]. It may be one of the possible reasons for farmland consolidation in some areas leading to more severe soil erosion, while land development leads to slighter soil erosion (Models 11–12; Figure 5
Second, LC may lead to increases in the use of fertilizers, pesticides, and plastics when newly increased cultivated land is of poor quality [66
]. On the one hand, the misuse of fertilizers, pesticides, and especially herbicides may induce environmental problems and soil erosion [48
]. The study by Keesstra et al. (2016) also showed that the highest runoff and soil erosion was both identified in the herbicide-treated plots (compared to tillage plots and covered plots) [64
]. On the other hand, the plastic film mulching used in agricultural land could intensify soil erosion and embankment collapse due to the rapidly formed concentrated flow under heavy rain, especially with inappropriate drainage systems [49
4.3. Strategies for LC in the Future
LC has turned out to be more important in developing countries, including China, which has the characteristics of soil degradation, and with the continuous and increasing attention to ecological environment issues, ecological environment protection has become an indispensable part of LC. Strategies for LC in the future should fully take the economy, ecology, and sustainability into consideration, and the goals of improving the quantity, quality, and ecological function of land should be clear and definite.
On the one hand, the proper use of cultivated land after LC should be guaranteed. The cultivated land is mainly used for grain production, and the aim of LC is to improve the grain production infrastructure and increase productivity. The phenomenon of using cultivated land to plant fruit trees, tea plants, or nursery stocks is contrary to the original intention of LC, which needs to be eliminated. Moreover, suitable tillage methods should be adopted in cultivated land after LC. Conservation tillage is recommended for sloping cultivated land to reduce soil erosion. Additionally, LC is not a temporary project, and it is necessary to achieve the continuous management and protection of land after LC.
On the other hand, the concept of “mountains, waters, forests, farmlands, lakes, and grasslands are part of a community of life” and the laws of “natural recovery and ecological priority” [67
] should be complied with during the process of LC. In the current practice of LC, more attention is still paid to eliminating or weakening restrictive factors in development (low efficiency of resource utilization, restoration of ecological environment damage, etc.). LC in the future should attach importance to the combination of comprehensive regulation of fields, water, roads, forests, and villages and the construction of soil and water conservation, wind, and sand fixation in important ecological functional areas.
Additionally, the impact of LC shows obvious regional differences and measure differences. Differentiated measures should be taken to improve the quality of cultivated land, rationally exploit abandoned land, rationally construct engineer land, and realize the remediation of land degradation and restoration of land ecology.
The hilly areas of China experience soil erosion, which harms production, livelihoods, and ecology. Taking Lishui District, Nanjing City, as the study area, this study quantitatively evaluated the soil losses in the study area. The soil erosion ranged from 0 to 385.77 t·ha−1·yr−1 with spatial heterogeneity due to the topography, land cover, and vegetation cover.
The hilly areas in China are also typical LC regions, and LC is one of the important human factors affecting soil erosion. This study further analyzed the impact and its heterogeneity of LC on soil erosion. LC has reduced soil erosion in the study area, where the amount of soil loss in plots with LC is lower than that in plots without LC. Moreover, farmland consolidation could lead to more serious soil erosion compared to land development. Overall, LC is conducive to reducing soil erosion due to the construction of protection forests or farmland shelterbelts, and differentiated land engineering, which could increase vegetative cover, decrease flow rate and alleviate ponding problems, maintain soil stability, and improve soil structure, or protect slopes and avoid collapse. However, there may be potential risks brought by LC to soil erosion reduction, such as harm vegetation from intensive land use and herbicide or plastic misuse, runoff, and erosion. Therefore, strategies for LC in the future should harmonize the promotion of LC and the protection of the ecological environment.