China's urbanization process is accelerating, and the agricultural labor force will inevitably transfer to the urban and non-agricultural sectors. During the 13th Five-Year Plan period of China, urban and rural areas will continue to have an active floating population. By 2020, the floating population of China will have gradually increased to 291 million, of which the total agricultural transfer population will account for 220 million, and family migration will continue to increase (China's migrant population development report, 2016). The high level of agricultural labor transfer has greatly promoted the rapid growth of non-agricultural sectors of the economy but has also deepened the aging of the agricultural labor force and increased the number of women involved, while the overall agricultural labor supply is decreasing. Agriculture not only has a strong dependence on the natural ecological environment but also impacts it. The input of agricultural production factors has produced environmental externalities, and the adverse impact of agricultural labor transfer on the environment has gradually emerged.

Farmers' environmental behavior is a key factor that must be considered in the formulation of agricultural environmental policies because it is directly related to their implementation cost. Under the current rural system in China, effectively guiding farmers' good environmental behavior and motivating them to voluntarily participate in environmental governance is the key to solving agricultural environmental problems, and any environmental policies that lack the participation of farmers would lessen their motivation and willingness to protect the environment (Wang, 1999; Greiner and Gregg, 2011; Lu et al., 2020). Luo et al. (2020) used the DID model to investigate the mechanism by which environmental policies influenced chemical fertilizer pollution in major grain producing areas, and found that the effect of environmental policies on reducing chemical fertilizer pollution was mainly due to the expansion of farmers' planting scale. China's environmental constraints are tightening, and it is inevitable that agricultural modernization will improve the agricultural ecological environment by improving the allocation structure of agricultural factor inputs. In general, the externalities of agricultural production will inevitably appear if property rights are not clear (Coarse, 1959, 1960), and the multifunctional nature of agricultural systems means that agricultural production activities not only produce positive externalities, but also produce negative externalities which often have the characteristics of imperceptibility, time lag and uncertainty, such as ANSP (Pretty and Frank, 2000).

Agricultural labor transfer is a rational choice for farmers pursuing income maximization. Will such behavior necessarily lead to environmental degradation? The existing research has not reached consensus on this issue. According to the farmer behavior theory, as the economic entity that pursues profit and utility maximization, farmers' resource endowments have a strong influence on their behavioral decision-making; and when their behavior is motivated by the moral goals such as fairness and altruism, it can reduce negative externalities to the environment (Singh et al., 1986; Colman, 1994). Some scholars have pointed out that agricultural labor transfer can alleviate the predicaments of agricultural production and further improve the technical efficiency of the food production environment (Tian and Zhu, 2018). However, farmers' economic behavior, at the micro level, is the direct cause of agricultural environmental pollution (Hu, 1997), and this effect may vary among farmers with different farm scales (Yao et al., 2017). Luan et al. (2020) used the Spatial Durbin Model to analyze the impact of labor transfer on fertilizer pollution, and found that labor transfer has a positive direct effect and a spatial spillover effect on fertilizer pollution, and also that the indirect spillover effect is greater than the direct effect. Based on 2000-2016 provincial data in China, Shao and Li (2020) used a Spatial Panel Model and found that labor transfer had a significant impact on agricultural pollution by changing the proportions of agricultural livelihood capital and cultivated land, however, Luo et al. (2020) discovered that the promotion of labor transfer and the expansion of farm scale are conducive to achieving the reduction of ANSP of chemical fertilizers through environmental policies.

ANSP refers to the pollution of the water environment caused by nutrients such as nitrogen and phosphorus, pesticides and other organic or inorganic pollutants through surface runoff and farmland leakage during agricultural production activities, mainly including chemical fertilizer pollution, pesticide pollution, centralized-breeding factory pollution, domestic sewage and garbage pollution. ANSP is the most widely distributed non-point source pollution at present and has become the primary source of pollution for the deterioration of the water environment quality in China. Recent efforts for controlling the pollutants caused by livestock and poultry farming through the regulation and supervision of large-scale livestock and poultry aquaculture have achieved remarkable results.

The excessive application of chemical fertilizers is an important cause of ANSP in China (First National Pollution Source Survey Report, 2014). According to 2015 data from the Food and Agriculture Organization of the United Nations (FAO), the total nitrogen input in China's food production in 2015 was 228.48 kg ha^-1, which is 3.7 times the world average of 61.89 kg ha^-1 and 2.9 times the total nitrogen application amount of 77.46 kg ha^-1 in the United States. The application intensity of chemical fertilizer in China is 1.6 times higher than the world average. In 2015, the utilization rate of chemical fertilizer for major crops in China was 35.2%, and it is forecasted to reach 40% by 2020. The inefficient application of chemical fertilizer is a common problem, which also leads to its excessive application (Agricultural Modernization Plan (2016-2020)). Shi et al. (2016) also concluded that the amount of chemical fertilizer applied in China's grain production exceeds the optimal economic application rate. The degrees of excessive fertilization in corn, wheat and rice production are 50.74%, 27.26% and 24.67%, respectively, based on a compilation of the cost-benefit data for these agricultural products over ten years.

Land, capital and labor are fungible to some extent in agricultural production. According to the theory of induced change, the relative scarcity of an effective agricultural labor force will induce farmers to make adaptive adjustments, such as saving relatively scarce resources and pursuing the minimum cost and maximum profit. How does the factor substitution generated by agricultural labor transfer influence ANSP? Most studies have found that labor transfer reduces the labor supply for agricultural production. Farmers increasingly want to improve the marginal productivity of labor; thus they tend to reduce the number of fertilization applications while increasing the amount of fertilizer applied each time to avoid yield loss and reduce risk in the context of increasing labor costs (Calhoun, 1989; Horowitz and Lichtenberg, 1993; Abdoulaye and Sanders, 2005; Jepson, 2005; Gong et al., 2010; Hu and Yang, 2015). Driven by time constraints and economic interests, farmers no longer rely on intensive farming, as they did predominantly in the past. Instead, they rely excessively on the input of modern factors such as chemical fertilizers (Ebenstein et al., 2011; Willinmson, 2011; Chang and Mishra, 2012; Pan, 2014). Some scholars believe that with the continuous transfer of the agricultural labor force, an increasing amount of land will be transferred for the cultivation of cash crops that demand more fertilizer (Peng et al., 2008; Xin et al., 2012). The increase in non-agricultural income also reduces farmers' financial constraints, allowing them to purchase more agricultural production materials and leading to more intensive fertilization (He et al., 2006; Huang et al., 2008; Ebenstein, 2012). Some studies have pointed out that young and educated farmers are more likely to accept agricultural environmental protection policies, so the transfer of a large number of young and middle-aged laborers will lead to an overall decline of farmers' awareness of ecological and environmental protection, which will hinder the promotion of environment friendly technologies such as soil testing formulas (Zhang, 2008; Vanslembrouck et al., 2012). Wei et al. (2012) considered the defects and shortcomings of China's existing agricultural land property rights system to be an important institutional source of ANSP. Different property rights systems will lead to different resource utilization behaviors. However, while an effective land property rights system means rationalizing people's economic behaviors, Wei et al. (2012) did not quantitatively test these behaviors from the micro level of the farmers.

Environmental pollution is a serious problem in China, and efforts have been made to govern ANSP. Agricultural labor transfer brought about a change in farmers' agricultural production behaviors and ultimately produced ANSP as an externality. Yet, what is the underlying influencing mechanism? What are the results of micro quantitative research? The existing literature on this issue is still relatively lacking. Therefore, in order to systematically analyze the impact of agricultural labor transfer on ANSP, and based on externality theory and induced change theory, this study demonstrates the mechanism of labor transfer on ANSP in China from the perspective of the factor substitution effect and the economies of scale effect, and verifies that mechanism by using micro survey data. The conclusions of this study have extremely important reference value for the control of ANSP in China against the background of the continuous agricultural labor transfer. The structure of this paper is as follows. First, the mechanism through which agricultural labor force transfer impacts ANSP is systematically analyzed, and an econometric model is constructed. Second, data sources are presented, and the key descriptive statistics of the data are provided. Finally, the results and conclusions are presented and discussed.

Agricultural environmental problems occur when technological and institutional changes lag behind changes in resource endowments. The theory of induced change argues that a change in the relative scarcity of resources will lead to adaptive technology selection and encourage organizational systems to use relatively abundant resources. Land, labor and capital are all substitutable to some extent in agricultural production. Motivated by profit maximization, farmers will reconfigure agricultural factors according to the changes in their prices. In essence, agricultural labor transfer brings about changes in the structure of agricultural factors and ultimately causes ANSP as a negative externality.

(1) Mechanism 1: Substitution effect

First, agricultural labor transfer reduces the amount of labor that farmers can access for agricultural production. When machinery is difficult to use at a large scale (for example, in hilly and mountainous areas), farmers tend to reduce the number of fertilization applications and increase the amount of chemical fertilizer applied each time to compensate for the adverse effect of the labor shortage, leading to excessive or inefficient use of the fertilizers and increasing ANSP. Second, the increase in non-agricultural income due to agricultural labor transfer increases household income. On the one hand, this increased income reduces the capital constraints on purchasing agricultural fertilizer, and the amount of fertilizer applied may increase. On the other hand, the increase in household income will weaken the dependence of farmers on agricultural production which may reduce the application of fertilizer. Third, the increase in total household income will enhance farmers' ability to resist risks, and the planting structure may shift from single-growing food crops to cash crops with greater fertilizer demands, driving an increase in chemical fertilizer application. Finally, as a form of natural production and a part of social reproduction, agriculture has a higher risk and higher uncertainty in terms of yield, and farmers tend toward risk aversion. The additional input of chemical fertilizers has become an important way for farmers to avoid risks and ensure stable agricultural yields.

(2) Mechanism 2: Economies of scale effect

As an important part of deepening China's rural reform, the transfer of agricultural labor improves the ratio of rural labor to land and promotes the development of a market for the transfer of farmland management rights; therefore, agricultural production has shown a trend toward large-scale farmland management. First, the integrity and stability of agricultural land property rights have been continuously improved as China's rural reform has advanced. Agricultural land in China has gradually shifted from low-efficiency farmers to become more concentrated among high-efficiency farmers, and agricultural production factors have improved in the direction of Pareto optimality. Second, the transfer of farmland management rights has brought about the expansion of farm size and the birth of a series of modes for organizing modernized agricultural production. Scaling the operations of agricultural land will reduce the marginal cost of factor inputs, which may reduce ANSP. However, the transfer of farmland management rights objectively causes a difference in land property in terms of contracted land and transferred land. The negative externalities of agricultural production will inevitably occur when the land property rights are not clear and exclusive (Corse, 1959, 1960), and will eventually cause ANSP. The analysis framework is shown in Fig. 1:

Stata13.0 (StataCorp LLC, College Station, TX, USA) software was used for the empirical analysis. To control for heteroscedasticity, autocorrelation and the possible influence of outliers, robust estimation was used for the regression. The F-test indicates significance at the 1% level, and the fit result of the model is good and shows that the model has strong explanatory power. The mean VIF is 1.11, and the model can be considered to be free of multicollinearity problems. The R² value is 0.125, and the result also shows a certain explanatory power for the cross-sectional data. In general, a correlation analysis is required when analyzing the causal relationship between two variables. However, Spearman statistics can only reflect the correlation between the two variables, but they fail to prove the causal relationship between the two variables. On the contrary, if a causal relationship exists between the two variables, there must be a correlation between them. The specific regression results and Spearman statistics are shown in Table 2.

The environmental problems caused by agriculture have become increasingly serious in China, and efforts have been made to better manage ANSP. Effectively guiding farmers' environmental behavior and encouraging them to voluntarily participate in environmental governance is the key to solving agricultural environmental problems in China. Farmers are the first group to directly and profoundly experience the deterioration of the agricultural environment, and they are also the source of agricultural environmental protection. For many years, the government in China has focused only on the technical path and the effect of implementing controls on ANSP, but it has ignored the effect of farmers' behavior on governing agricultural environmental pollution. With the on-going nonagricultural transfer of agricultural laborers, the agricultural production mode and livelihood structure of farmers in China have undergone great changes, and farmers are now highly differentiated and heterogeneous. Scale farmers will become the main force in agricultural development in China and so they will be the main force driving the control of ANSP in the future. Therefore, the government should adopt relevant incentive measures for scale farmers and further strengthen education and training in environmentally friendly agricultural technology in order to reduce the ANSP caused by agricultural inputs at the source. In addition, the government should also actively guide scale farmers to participate in agricultural pollution control and strengthen their demonstration role in governing ANSP reduction.

Agricultural labor transfer will promote the development of agricultural socialized service organizations and provide farmers with agricultural services such as tillage, raising seedlings, fertilization, machine insertion, harvesting and transportation, thus alleviating the problems caused by the shortage in the agricultural labor supply. The government could provide subsidies or incentives for agricultural social service organizations to expand the coverage and frequency of their agricultural social services. The Chinese government is vigorously implementing a rural revitalization strategy which aims to increase farmers' enthusiasm for retaining land, especially under the continuous advancement of the “separation of three powers” over agricultural land and policy guarantees on the stability of land contract rights. Farmers' understanding of land property rights continues to improve, and their long-term expectations for the land and the income from land appreciation are increasing. Therefore, government departments can make full use of the rural revitalization strategy period and vigorously publicize the positive role of ANSP treatment. Farmers should be encouraged to participate in the treatment of ANSP, and the treatment effect could also be improved.

Farmers vary in their resource endowments, levels of environmental awareness and abilities to adapt to the changing market environment. The government should also pay attention to the differences between farmers, reduce governance costs and increase implementation efficiency. Future research can further group farmers, combine the characteristics of different types of farmers and the characteristics of agricultural production, determine the key factors in farmers' agricultural decisions, and provide theoretical support and a practical foundation for the control of ANSP. This study has some shortcomings. For example, the degree of nonagricultural employment could vary, or nonagricultural employment could occur in different locations, such as within townships, in towns outside the county, or elsewhere in the county or province, all of which would change the impact of agricultural labor transfer on ANSP. A detailed analysis can be conducted based on this perspective in order to provide more targeted suggestions for governing ANSP.

Based on the micro survey data of farmers and econometric methods, this study investigated the impact of agricultural labor transfer on ANSP considering the substitution effect of agricultural factors and the effect of agricultural economies of scale. The main conclusions are as follows.

Firstly, the increase of the agricultural labor force will not be conducive to improving the agricultural environment during the process of agricultural modernization in China. In contrast, the income increase brought by agricultural labor transfer will promote the upgrading of the input structure of agricultural production factors and improve the agricultural environment.

Secondly, expanding the coverage area or increasing the number of agricultural social service programs, and strengthening agricultural technology training for farmers are conducive to improving the efficiency of agricultural inputs and alleviating the extensive agricultural management caused by an insufficient supply of agricultural labor.

Thirdly, due to terrain restrictions, the transaction cost of farmland transfer in hilly and mountainous areas is high, the scale operation of agricultural land resulting from the transfer of agricultural labor is difficult, and the environmental effects of the agricultural land scale operation are not obvious.

Finally, at present, the grain-growing purpose of selling is not conducive to improving the agricultural environment (Lu et al., 2019). In the context that the market cannot achieve both good quality and good prices for agricultural products, the pursuit of yield will become a priority for agricultural production, and farmers will adopt an extensive mode of agricultural operation to maintain stable yields.