Soil erosion by water is considered as one of the major threats to ecological security. Soil conservation is an important regulating service of ecosystems. The spatial pattern of soil conservation service is important for critical area identification, and is the basis and premise of soil erosion control and ecological protection. In recent years, the rapid development of economy and society, along with intensified human activities, resulting in great reduction in ecological functions. Meanwhile, the awareness of ecological issues prompts some countries to implement a series of ecological protection and construction projects. Assessing the effectiveness of soil conservation services can provide information for scientific decision-making for better soil conservation.
In this study, a meta-analysis of worldwide soil erosion field experimental measurements was presented. Then we assessed the global soil erosion using remote sensing, parameter calibration, model simulation and correction. Finally, we assessed the ecosystem’s soil conservation service, and analyzed the spatial pattern, trends and the driving factors. The main findings are as follows:
(1) Bare lands, croplands and orchards had higher soil loss rates (SLs) than that of grasslands, shrublands and forestlands. The SLs of the same land use type varies greatly between different climate zones, for example, the average SLs of bare lands in tropical and temperate climate zones were greater than 100 t·ha-1·yr-1, while in cold and arid climate zones, the SLs of bare land were 54.1 and 10.6 t·ha-1·yr-1 respectively; With respect to croplands, the SLs were also different: tropical (41.8 t·ha-1·yr-1) and temperate (17.8 t·ha-1·yr-1), cold (21.8 t·ha-1·yr-1) and arid (3.9 t·ha-1·yr-1).
(2) A comprehensive review was conducted to compare the effects of soil conservation techniques on water erosion control, the results showed that the soil conservation techniques were generally effective at reducing annual soil loss (84%), the efficiency of water erosion control varied as the terrain slope increased, and finally we quantified the soil conservation techniques factors.
(3) Based on the Universal Soil Loss Equation, we assessed the global soil erosion. The results show that there was about 1742.47 × 104 km2 land suffering from water erosion (SL > 2 t·ha-1·yr-1), with average soil erosion rate of 5.78 t·ha-1·yr-1. Croplands are the main source of soil erosion. Spatially, severe soil erosion are mainly located in Asia, Africa and South America, contributing 89.39% of the total amount of global soil erosion. The global water erosion was generally enhanced, with an average rate of 4.26×10-2 t·ha-1·yr-2.
(4) From 1992 to 2015, a soil conservation capacity of 228.22 t·ha-1·yr-1 was retained by ecosystems, with a soil retention rate of 95.87%. All kinds of ecosystems play important roles in soil conservation, especially the forest ecosystems, the total amount of soil conservation accounts for 68.74% of the total global soil conservation. The global soil remains a extremely important area of 244.12×104 km2, mainly located in Asia and South America, accounting for 76.68% of the global extremely important area, which are basically high mountain areas with good vegetation coverage.
(5) The ecosystems’ soil conservation service was generally enhanced, with an average rate of 0.93 t·ha-1·yr-2. In which, the trend exhibited a significant increase (p < 0.05) in 13.89% of global lands, with an average rate of 8.88 t·ha-1·yr-2; however, the trend exhibited a significant decrease (p< 0.05) in 4.02% of global lands, with an average rate of -23.43 t·ha-1·yr-2; The soil conservation capacity in the southern of Asia and South America varied greatly, while the soil conservation capacity in the cold climate zone generally increased, however the enhancement was relatively quietly.
（6) From 1992 to 2015, there were forest lands (103.94×104 km2), shrublands (24.82×104 km2) andgrasslands (23.35×104 km2) converted into farmlands, which decreased its soil conservation capacity by 14.64%, 16.24% and 14.84%, respectively; There were 63.9×104 km2 of farmlands converted into forest lands, and 42.01×104 km2 of bare lands converted into grasslands, which decreased its soil conservation capacity by 14.78% and 71.69%, respectively. The results show that the ecosystems’ soil conservation service was definitely enhanced under ecological protection and restoration programs, however, reclamation, through damage to ecosystem structures, is important cause of degradation in soil conservation service.
In short, the world is suffering from serious water erosion, especially the farmland ecosystem, which is the main source of global water erosion. While the soil conservation service of the global ecosystem gradually increased from 1992 to 2015, the implementation of ecological protection project and urbanization significantly enhanced the soil conservation service, however, the soil conservation service was significantly weakened by reclamation and desertification.