1 Introduction
To curb and restore the deteriorating ecological environment, Chinese government has introduced a series of ecological policies since the end of the 20th century (Yin et al., 2010). Grain for Green (GFG) is one of the most famous ecological policies. The GFG was to restore and improve the ecological environment quality at the expense of reducing the cultivated land that contracted by farmers. Since it was directly related to the interests of farmers, the evaluation of the implementation effect of GFG has been widely concerned (Bullock and King, 2011). Studies have shown that the implementation of GFG had promoted ecosystem services in relevant areas, such as the increase of vegetation coverage (Xiao, 2014), decrease of soil erosion (Lü et al., 2012), raised carbon sink (Wang et al., 2018), and enhanced per unit yield of cultivated land (Zhang et al., 2019). Due to the positive externalities brought by the improvement of ecosystem services, the surrounding regions can also benefit from the GFG (Wu et al., 2019). In addition, the GFG has also made positive contributions to the improvement of social and economic benefits such as poverty reduction and employment promotion (Peng et al., 2007; Bullock and King, 2011). On the contrary, some researchers also put forward negative views on the GFG, because large-scale afforestation was suspected of consuming too much soil moisture, which hurt soil conservation, soil quality, and biodiversity (Cao et al., 2009; Jiang et al., 2016; Jia et al., 2017). Despite different opinions on the evaluation of GFG, some researchers also pointed out the existing problems and the areas for improvement, the positive benefits, and the impacts generated by GFG were more generally recognized (Bryan et al., 2018; Chen et al., 2019).
In the Loess Plateau, the direct purpose of GFG was to control soil erosion, which was the most prominent ecological problem in the area (Wang et al., 2001; Qiu et al., 2002; Zhang et al., 2019). The causes of soil erosion included the evolution of regional natural characteristics and external interference factors such as human activities (Yang et al., 2003; Chen et al., 2019), among which the latter was more common in developing countries (Millward and Mersey, 1999; Onyando et al., 2005; Adimassu et al., 2017). From 1955 to 1989, about 56.6% of the Loess Plateau had an average annual soil loss of 2500 t·km-2or more, which was considered to be a moderate or above erosion level (Wang and Jiao, 2002). Serious soil erosion not only endangered the economic and social development and ecological environment security within the region but also caused a significant negative impact on the nearby areas (Chen et al., 2008). Since the implementation of GFG in the Loess Plateau, the soil conservation effects (SCE) has been significantly improved and soil erosion has been controlled to a certain extent through adjustment and optimization of land use structure, improvement of vegetation coverage and engineering measures (Deng et al., 2012; Wang et al., 2016).
In recent years, researchers have carried out detailed studies on the benefits of soil conservation brought by the GFG from different perspectives, mainly including empirical model and field observation. An empirical model was often used to analyze soil erosion at different stages and evaluate the SCE, such as USLE (Fu et al., 2011), SWAT (Yang et al., 2018), WATEM/SEDEM (Li et al., 2019), etc. On the basis, some researchers simulated soil erosion in different situations, thus expanding the application space of the empirical model (Hessel et al., 2003; Zhou et al., 2006; Han et al., 2016). Field observation was to use the data of river runoff, sediment content and sediment transport obtained from public release or experiments to analyze the impact of GFG on soil erosion in specific areas, especially at the river basin scale and made feasible suggestions (Gao et al., 2018; Yang et al., 2018; Deng et al., 2019). In general, the current evaluation on the impact of GFG on soil erosion focuses on the overall treatment effect, that is, the comparative analysis of the amount of soil erosion reduction before and after the implementation of GFG (Sun et al., 2014; Zhang et al., 2016). However, at the present stage of GFG, more attention should be paid to the coordination between the GFG investment and the SCE, so as to analyze whether the implementation of GFG is efficient or not. Theoretically, the GFG should be mainly invested in areas where there is a large gap between the current status of soil erosion and the theoretical minimum level of soil erosion, in order to obtain the optimal SCE. However, the existing research methods are unable to quantify this gap, so it is difficult to analyze the investment of GFG in different regions. This is not conducive to the effective evaluation of the GFG implementation in the past, but also hurts the formulation of more reasonable soil conservation plans in the future.
Based on the Revised Universal Soil Loss Equation (RUSLE), the soil conservation potential (SCP) model was established. The SCP model quantifies the difference between the current situations of soil erosion in different regions and at different times and the theoretical minimum level of soil erosion, calculates the range of improvement for soil erosion control, and provides a reference for the future investment planning of GFG. At the same time, the model can also evaluate the implementation effect of GFG in the past, and summarize experience and deficiencies. The significance of the SCP model lies in that it not only solves the theoretical defect of evaluating the GFG effect based on the amount of soil erosion reduction in previous studies, but also optimizes the input allocation of GFG, and strives to achieve the optimal implementation efficiency of GFG, so as to provide new ideas for researchers and decision-makers.
Yan’an, located in the northern part of Shaanxi province, China, was taken as study area in this study. According to the SCP model, the GFG investment and the SCE produced by the GFG in different regions were evaluated, the reasons for the mismatches in spatial locations of the two and the relationship with the SCP were analyzed, and the problems existing in the implementation of GFG at the present stage were discussed. Then the key implementation areas of GFG in the future were defined through the calculated SCP, and the dynamic input mechanism to achieve the optimal input efficiency was proposed. Finally, the optimization design of the future GFG in Yan’an was carried out.