The ecosystem provides humans with food and raw materials, and it is indispensable for sustainable development (Luo et al., 2019). According to the Comprehensive Report on Millennium Ecosystem Assessment issued by the United Nations in 2005, the intensive use of ecosystem services by humans in the past 50 years has increased human well-being(Millennium Ecosystem Assessment, 2005); however, it has also significantly destroyed the Earth’s ecosystem, resulting in ecosystem degradation, the reduction of ecological assets and the decline of biodiversity (Carpenter et al., 2006; Narducci et al., 2019). Therefore, understanding the optimal coverage of ecosystem services (Alejandre et al., 2019), accurately assessing the ecosystem service value and making a full assessment of the relationship between ecosystem services and human well-being are important for achieving sustainable development (Parkes, 2006).

Farmland is a semi-natural ecosystem in which human beings produce agricultural products using natural resources and artificial inputs (Xie and Xiao, 2013). The farmland ecosystem has both natural attributes and socio-economic attributes, with the characteristics of purpose, openness, efficiency, variability, vulnerability, external input dependence, and others (Yin et al., 2006). Farmland ecosystem services include provisioning services, regulating services, supporting services and cultural services (Simpson, 2019). In addition, the farmland ecosystem also functions in forming social security, culture and entertainment for improving human well-being and other cultural services (Bai et al., 2010; Zhang and Qiu, 2018).

Since the end of the 20th century, great attention has been paid to evaluating ecosystem services (Costanza et al., 1997). At present, the research on ecosystem service value mainly focuses on natural ecosystems of forest, grassland, wetland (Costanza et al., 2014), farmland (Marinidou et al., 2019), and others. Early research on farmland ecosystem service value mainly focused on the provisioning service, as well as the quality and yield of agricultural products, while it did not fully recognize the regulating service, supporting service and cultural service (Porter et al., 2009; van den Belt and Blake, 2014), or the impact of land use and farmland abandonment on the farmland ecosystem services value (Lin et al., 2018; Sil et al., 2019). Some practical techniques for the quantitative assessment of farmland ecosystem services have been developed to meet the requirements of different end users (Tzilivakis et al., 2019), and the loss of farmland ecosystem services value caused by the negative environmental impact of agricultural production activities needs to be strengthened (Lu et al., 2016). Because the farmland ecosystem is greatly affected by human activities, it is difficult to measure the value of farmland ecosystem services, especially its non-market value (Yang et al., 2005; Du et al., 2008).

As an international metropolis with a large population, the protection and wise use of the farmland ecosystem is particularly important for Qingdao City, especially at a time when its industrial economy is developing rapidly. On the basis of the ecosystem services value system of the Millennium Ecosystem Assessment, the values of farmland ecosystem services in Qingdao City in 1997, 2002, 2007, 2012 and 2017 were evaluated. Analyzing the value changes of farmland ecosystem services caused by agricultural production and land use changes can provide a scientific basis for agricultural production decision-making, rational allocation of agricultural resources, formulation of farmland utilization management policy and calculation of farmland ecological compensation standards in Qingdao City. In addition, this analysis can also help the public to fully understand the ecological utility of the farmland ecosystem, enhance the awareness of social public protection and the rational use of farmland, and promote the sustainable development of regional agriculture.

Qingdao City, Shandong Province (35°35°-37°09°N, 119°30°- 121°00°E) is located in the southwest of Shandong Peninsula, northern China. It is bordered by the Yellow Sea in the southeast, and by Yantai City, Weifang City and Rizhao City in the northeast and southwest, respectively. The terrain is high in the east and low in the west. The main types of landforms include plains, hills, mountains and coastal lowlands, accounting for 46.33%, 41.04%, 9.34% and 3.29% of the total land area, respectively. Among them, the plain is the main production area of grains, peanut, vegetables and cotton. Most of the hills have been reclaimed into farmland, with plantings of economic forests or wheat, corn, peanuts, sweet potato and so on; the mountainous areas are mostly used for forest, orchards and tourism; and the coastal lowlands are mostly used for maricultural ponds and salterns (Fig. 1). The soils are classified into brown soil, fluvo-aquic soil, cinnamon soil, lime concretion black soil, saline soil, paddy soil, and others. Among them, brown soil is the largest, most widely distributed type in the low mountains and hilly areas, accounting for about 58.5% of the available agricultural land, and most of it has been reclaimed into farmland. The temperate monsoon climate of the research area has the characteristics of four distinct seasons, with abundant rainfall in summer. The average annual temperature of Qingdao City is 12.0 ℃, the average annual precipitation is 687.3 mm, and the precipitation is mostly concentrated from June to August. The rivers of Qingdao City belong to the Dagu River system, the Beijiaolai River system and the system of coastal rivers.

Qingdao City has 7 districts and 3 county level cities: Shinan District, Shibei District, Licang District, Laoshan District, Huangdao District, Chengyang District, Jimo District, Jiaozhou City, Pingdu City and Laixi City. In 2018, there are a permanent resident population of 939.48×10⁴ people in the city. At the end of 2017, there was 51.86×10⁴ ha of cultivated land, mainly distributed in Laixi City, Pingdu City, Jiaozhou City, Jimo District and Huangdao District. The main crops are wheat, corn, peanut, potato, vegetables and fruits, and the total grain production of Qingdao City accounts for 6.29% of that of Shandong Province.

According to the Millennium Ecosystem Assessment issued by the United Nations in 2005 and the actual agriculture in Qingdao City, the farmland ecosystem services in Qingdao City are divided into two categories, direct ecosystem services and indirect ecosystem services. Direct ecosystem services mainly refer to provisioning services, that is, agricultural product production and provisioning, while indirect ecosystem services mainly include regulating services, supporting services and cultural services. According to the above classification of farmland ecosystem services, the evaluation index system for farmland ecosystem services in Qingdao City was constructed, including 7 indexes and 10 calculation methods (Table 1).

The data of population, types and yields of agricultural products, soil characteristics, water resources and their utilization, agricultural production and living standards, and others, were obtained from the Qingdao Statistical Yearbooks, Shandong Statistical Yearbooks and China Price Yearbooks for 1998, 2003, 2008, 2013 and 2018; the Statistical Bulletin of National Economic and Social Development in Qingdao City, the Statistical Bulletin of National Economic and Social Development of Shandong Province in 1997, 2002, 2007, 2012 and 2017; as well as the History and Annuals of Qingdao—Agriculture Annuals, the History and Annuals of Qingdao—Physical geography/Meteorology annuals and other miscellaneous documents.

3.2.1 Value of product provisioning service

Agricultural product production and provisioning are the most important farmland ecosystem services. The farmland in Qingdao City mainly produces grains, vegetables, melons and fruits and economic crops, among which wheat, corn, soybean, sweet potato and potato are the main food crops, while peanut, cotton and tobacco are the main economic crops. The value of the product provisioning service of the farmland ecosystem is calculated by the market value method, according to the current average market price of all the relevant types of agricultural products.

3.2.2 Values of carbon fixation service and oxygen release service

Atmospheric carbon dioxide is fixed in the form of organic carbon in agricultural products and soil organic carbon; meanwhile, oxygen is released through photosynthesis. According to the moisture contents and economic coefficients of the various crops, the net primary production, carbon fixation and oxygen release of the different crops can be calculated (Yan et al., 2007), and then the values of the carbon fixation service and the oxygen release service of the farmland ecosystem in Qingdao City can be calculated by several methods: carbon tax method, afforestation cost method, industrial oxygen production method, etc. In these calculations, only the carbon fixation and oxygen release services provided by the farmland ecosystem during the growing period of the different kinds of crops are taken into account, while the sink or source effects of carbon in the harvest and in the process of transformation into various ecosystems are excluded (Yang et al., 2005).

3.2.3 Value of water conservation service

The farmland ecosystem can prolong the formation of surface runoff and reduce the surface water loss through crop interception of precipitation, and it can also make use of the water maintained by soil capillary force to meet the needs of current plant growth and future water supply in order to achieve the water conservation service. The water conservation service value of the farmland ecosystem is composed of the value of the crop interception of precipitation service and the value of the farmland soil water retention service. Because the water conservation service of the farmland ecosystem has a similar function to that of a reservoir, its value is calculated by the alternative cost method, using the cost of constructing a reservoir with the same water conservation capacity.

3.2.4 Value of climate regulating service

Crop transpiration and farmland evaporation increase the humidity in the atmosphere of the surrounding farmland. Mesoscale convection induced by thermal and dynamic effects of the farmland ecosystem can promote the formation of precipitation and produce a rain enhancement effect (Zhang et al., 2009). Because crop transpiration and farmland evaporation are difficult to observe, the equivalent factor method is used to evaluate the climate regulating service value of the farmland ecosystem in Qingdao City. Based on a questionnaire survey of 200 Chinese ecologists, and according to the estimated results of the value of global ecosystem services (Costanza et al., 1997), a value equivalence scale of the ecological services per unit area of the terrestrial ecosystem in China was developed. The economic value of yearly grain production of farmland per hectare was calculated with the national average grain yield level defined as 1 value equivalent factor for the farmland ecosystem services (Xie et al., 2003; Xie et al., 2005). One value equivalent factor of farmland ecosystem services represents the proportional contribution of the specific service to the food production service of the farmland ecosystem. For example, the value equivalence ratio of the climate regulating service to the food production service is 0.89:1; that is, the value of the climate regulating service is 0.89 times of that of the food production service.

3.2.5 Value of soil conservation service

The brown soil is distributed over the largest area in Qingdao City. The management level of the farmland ecosystem in brown soil areas is high, and the crop coverage rate is also high. The soil attached to crop roots can reduce the soil loss caused by raindrop splashing, wind erosion, surface runoff erosion and other factors (Sun et al., 2013). Therefore, the soil conservation service of the farmland ecosystem can reduce the loss of soil nitrogen, phosphorus, potassium and other elements caused by erosion, maintain soil fertility, reduce the amount of chemical fertilizer applied in farmland, and reduce the loss of farmland economic benefits caused by soil degradation. The value of the soil conservation service of the farmland ecosystem includes the service values of maintaining soil nutrients and reducing land abandonment, which are estimated by the shadow price method and the opportunity cost method, respectively.

3.2.6 Value of social security service

The farmland ecosystem provides all kinds of agricultural products for human beings, and it is an important guarantee for the farmer’s livelihood. The scale of the rural surplus labor force is large, but it is hidden, so it does not directly bring pressure to society that is as great as the urban unemployed population. If the rural surplus labor force does not have enough farmland for basic livelihood security, then the local government needs to pay the minimum level of living security for them. Therefore, we can regard the rural surplus labor force as the population who need to receive the minimum level of living security in rural areas, and according to the annual agricultural employment population in Qingdao City, China’s agricultural hidden unemployment rate is 48.6% (Sun et al., 2007). Using the standard of the minimum living security for urban residents in Qingdao City, and the ratio of rural residents’ annual consumption expenditure per capita to urban residents’ annual consumption expenditure per capita, the value of the social security service of the farmland ecosystem in Qingdao City can be calculated by the substitute cost method. The Civil Affairs Bureau of Qingdao City proposed the average standard of minimum living security for urban residents of Qingdao City in 2017 as 630 Yuan per capita (http://www.qingdao.gov.cn/n172/n24624151/n24625835/n24625849/n24625863/170324140910626625.html).

3.2.7 Value of entertainment and cultural service

In the process of the development of agricultural civilization, farmland not only provides the important means of industries and livelihood for human beings, but also forms diverse agricultural cultures. It provides people with places for leisure, entertainment, tourism and for returning to nature, enjoying pastoral scenery and meeting people’s specific aesthetic needs (Ye et al., 2012). In recent years, with the continuous improvement of urban residents’ recognition of and participation in leisure agriculture, suburban leisure agriculture has developed rapidly, and the entertainment and cultural service value of the farmland ecosystem has continued to rise higher. At present, the agricultural tourism income of Qingdao City is not counted separately, so the equivalent factor method is used to estimate the entertainment and cultural service value of the farmland ecosystem. The equivalence ratio of the entertainment cultural service value to the provisioning service value of the farmland ecosystem is 0.01:1, so the entertainment cultural service value of the farmland ecosystem per unit area can be obtained by multiplying the equivalent product provisioning service per unit area by 0.01.

4.1.1 Provisioning service value and its changes

The provisioning service value of the farmland ecosystem in Qingdao City showed an upward trend from 1997 to 2017, and only a slight decrease in 2007. The food producing service values of wheat, corn, peanut, vegetables and fruits accounted for a large portion of the total provisioning service values in the 5 years (Table 2). The provisioning service value is greatly affected by human activities. Optimizing crop planting structure and improving agricultural technology and farmland management are helpful for making full use of farmland productivity and improving the efficiency of agricultural production and the farmland ecosystem services value.

4.1.2 Values of carbon fixation service and oxygen release service and their changes

Based on the moisture content of various crops, economic coefficients and yearly actual yields per hectare, the total net primary productivities of various crops in Qingdao City in 1997, 2002, 2007, 2012 and 2017 were calculated (Table 3). Using the carbon tax method and afforestation cost method, the carbon fixation service values of the farmland ecosystem of Qingdao City in the 5 years were estimated according to the absorption of 1.63g CO₂ and the release of 1.19 g O₂ when producing 1 gram of dry organic matter by photosynthesis, and the average carbon tax rate and afforestation cost were taken as the carbon fixation costs of the farmland ecosystem. In 1990, the carbon tax rate in Sweden was 150 US dollar t^-1 C (Ouyang et al., 2004), and the exchange rates of Chinese currency against the US dollar in the 5 years were calculated at 8.20, 8.20, 7.60, 6.30 and 6.80 Yuan to 1 US dollar, respectively (Zhang et al., 2008). The afforestation cost is 260.9 Yuan t^-1 C (Chen et al., 2012). Because the research period is long and price fluctuations exist, in order to ensure the comparability of prices in different years, the final results were converted to the relative prices of 2017 according to the purchase price index of industrial producers of 336.3, 307.0, 427.5, 546.5 and 527.6 in the 5 years, respectively.

The oxygen release service value of the farmland ecosystem of Qingdao City was evaluated by the industrial oxygen production method and afforestation cost method, with the industrial cost of producing the oxygen of 400 Yuan t^-1 and the average afforestation cost of 352.93 Yuan t^-1 released by forest photosynthesis used as oxygen producing costs, respectively. In order to ensure the comparability of prices in different years, with 1985 as the base period, the calculated results of farmland ecosystem service values in the 5 years were translated into relative prices for 2017 based on the producer price index in 1985 for 1997, 2002, 2007, 2012 and 2017, namely 315, 292.6, 353.8, 393.4 and 376.2 Yuan t^-1, respectively.

The values of the carbon fixation service and oxygen release service of the farmland ecosystem in Qingdao City increased gradually from 1997 to 2017 (Table 4). Due to the increase of net primary productivity, the value of carbon fixation service increased by 8.38×10⁸ Yuan, and the value of oxygen release service increased by 45.39×10⁸ Yuan in the period of 1997-2017. In the farmland ecosystem in Qingdao City, the net primary productivities of wheat, corn, peanut, vegetables and fruits are larger than those of other crops, and the values of carbon fixation service and oxygen release service of these crops also account for a large proportion of the total value of farmland ecosystem services.

4.1.3 Value of water conservation service and its changes

Based on the area weighted average of annual precipitation of Qingdao City in 1997, 2002, 2007, 2012 and 2017, the thickness of the farmland soil layer was 22.88 cm, the soil non-capillary porosity was 11.94% (History and Annuals Office of Qingdao, 1997), and the cost of reservoir construction was 0.67 Yuan m^-3 according to the constant price in 1990. The water conservation service value of the farmland ecosystem of Qingdao City was calculated by the substitute cost method (Table 5). In order to ensure that the costs of reservoir construction in different years were comparable, the fixed asset investment price index in the 5 years (210.3, 222.3, 271.8, 343.9 and 262.4) was used to convert the construction costs of reservoirs into the relative prices of 2017, namely, 1.41, 1.49, 1.82, 2.30 and 1.76 Yuan m^-3, respectively. Because the value of the water conservation service of the farmland ecosystem is greatly affected by precipitation, it is higher in years with abundant precipitation and lower in years with less precipitation. In general, the value of the water conservation service shows an increasing trend, and the water conservation service value in 2017 was 1.25 times that in 1997.

4.1.4 Value of climate regulating service and its changes

According to the yields of major crops such as wheat, corn, potato and soybean of Qingdao City in the five years and the grain prices in the current year, the value of the food production and provisioning service of the farmland ecosystem per hectare was calculated; that is, the equivalent product provisioning service value was calculated, and then the equivalent climate regulating service value was calculated. The climate regulating service value of the farmland ecosystem was calculated by using the equivalent climate regulating service value and farmland area (Table 6). The climate regulating service value of the farmland ecosystem in Qingdao City increased from 29.35×10⁸ to 46.28×10⁸ Yuan in the period of 1997-2017.

4.1.5 Value of soil conservation service and its changes

The potential erosion modulus of cultivated land is 7060.42 t km^-2 yr^-1, and the actual erosion modulus is 335.37 t km^-2 yr^-1 in Qingdao City (Zhao, 2017). The annual land income is expressed as the agricultural added value per hectare in Qingdao City, and the nutrient contents of N, P and K of soil are 0.054%, 0.020% and 1.045%, respectively. According to the evaluation specification of forest ecosystem services issued by the former State Forestry Administration of China in 2008 (LY/T 1721-2008), N and P are mainly obtained from the chemical fertilizer diammonium phosphate, and K is mainly obtained from the chemical fertilizer potassium chloride. The price of diammonium phosphate is 2400 Yuan t^-1, with N and P contents accounting for 14% and 15%, respectively, while the price of potassium chloride is 2200 Yuan t^-1, with K content accounting for 50%. Based on these contents of N and P in diammonium phosphate and K in potassium chloride, the chemical fertilizer conversion coefficients for N, P and K are 7.14, 6.67 and 5.46, respectively. To meet the requirement of a comparable price in different years, with 1997 as the base period, the prices of chemical fertilizer in the 5 years were converted to the relative price of 2017 by using the price index of agricultural means of production in the corresponding year, i.e., 100.00, 80.78, 108.09, 158.84 and 145.49. Then the soil conservation service value of the farmland ecosystem in Qingdao City was calculated (Table 7). The values of the reducing land abandonment service and the maintaining soil nutrients service of the farmland ecosystem in Qingdao City increased from 1997 to 2017, and the value of the maintaining soil nutrients service accounted for a large proportion of the value of the soil conservation service.

4.1.6 Value of social security service and its changes

According to the population in the rural area which needs to receive the minimum social security, the annual consumption expenditure of rural residents per capita, the annual consumption expenditure of urban residents per capita and other relevant factors, the value of the social security services of the farmland ecosystem in Qingdao City was calculated by the substitute cost method (Table 8). The relevant data were collected from Qingdao City Statistical Yearbooks of 1998, 2003, 2008, 2013 and 2018 and Statistical Bulletin of National Economic and Social Development of Qingdao City in 1997, 2002, 2007, 2012 and 2017, and other sources. The results show that the social security service value of the farmland ecosystem in Qingdao City has a fluctuating and declining trend in general, due to the continuously improving rural employment and the decrease of the population which needed to utilize social security.

4.1.7 Value of the entertainment and cultural service and its changes

The value of the entertainment and cultural service of the farmland ecosystem in Qingdao City was estimated by the equivalent factor method, using the value equivalence ratio of entertainment and cultural service to food production service (product provisioning service) of the farmland ecosystem as 0.01:1. The economic value of the food production service per unit area was multiplied by 0.01 to obtain the value equivalent of the entertainment and cultural service of the farmland ecosystem (Table 9) in Qingdao City, which was fluctuating and increasing from 1997 to 2017, with the value in 2017 being 1.58 times of that in 1997. With the income improvement and the change of living concept, people gradually realized the value of the entertainment and cultural service of farmland, and are more willing to experience the use of the farmland ecosystem to carry out leisure tourism, entertainment and cultural activities.

In the farmland ecosystem service value of Qingdao City, the value of the product provisioning service accounted for the largest proportion of the total, followed by the regulating service value, the cultural service value and the supporting service value. Generally, the proportions of the values of different services to the total were relatively stable during the period of 1997-2017; although the proportions of the values of different services to the total farmland service value were significantly different. In the 5 years which we analyzed, the combined proportions of the values of product provisioning, carbon fixation, oxygen release, and soil conservation services to the total farmland ecosystem service value was higher than 85%. The proportion of the product provisioning service value to the total was the largest and showed an increasing trend with fluctuation, indicating that the product provisioning of crops, especially those of grains, are the main farmland ecosystem service of Qingdao City. From largest to smallest, the values of farmland ecosystem services in Qingdao City are: the product provisioning service, the carbon fixation and oxygen release service, the soil conservation service, the climate regulating service, the social security service, the water conservation service, and the entertainment and cultural service (Table 10).

In the period of 1997-2017, the total value of farmland ecosystem services in Qingdao City increased from 499.74×10⁸ to 681.74×10⁸ Yuan, and basically showed a steady increasing trend. Meanwhile, the value of the ecosystem services per hectare also increased, from 6.57×10⁴ in 1997 to 9.73×10⁴ Yuan ha^-1 in 2017 (Fig. 2).

From 1997 to 2017, the farmland acreage decreased from 76.03×10⁴ ha to 70.04×10⁴ ha. Meanwhile, the value of direct services of the farmland ecosystem (i.e. the product provisioning service), as well as the value of indirect services (which include the regulating service, the supporting service and the cultural service) of the farmland ecosystem in Qingdao City, both showed upward trends, and the ratios of the values of indirect to direct services were 1.39, 0.68, 0.85, 0.91 and 0.79, respectively, showing an overall downward trend (Fig. 3). These changes were mainly caused by the rise of agricultural product prices and the optimizing of crop planting structure. The results show that with the continuous improvement of farming technology and field management technology, the agricultural production efficiency in Qingdao City continued to improve.

The proportions of provisioning service value, regulating service value, supporting service value and cultural service value to the total farmland ecosystem value in Qingdao City are in descending order, which is similar to those of Huailai County, Hebei Province in 2012 (Zhang et al., 2016) and Beidaihe District, Hebei Province in 2014 (Liu et al., 2017). However, the proportion of entertainment and cultural service value to the total farmland ecosystem service value in Qingdao City is lower than that of Huailai County (Zhang et al., 2016). The values of farmland ecosystem services per hectare in Qingdao City are higher than those of Wuma River watershed in Guizhou Province (Wu et al., 2016) and the average standard value for China in 2003 (Sun et al, 2007).

The importance of the indirect services value of the farmland ecosystem should be well recognized, so farmland should be reasonably utilized and effectively protected. Agricultural production should be developed reasonably based on the natural conditions of different regions, and agricultural production efficiency should be continuously improved by adopting scientific planting technology and efficient management to ensure the quality of agricultural products. We should promote supply-side structural reform in agriculture, make full use of the indirect service value of the farmland ecosystem (such as supporting service value and cultural service value), speed up the construction of modern agricultural parks, and develop ecological leisure agriculture and agricultural eco-tourism, thereby achieving the goal of providing maximum welfare for the human population through the farmland ecosystem.

In evaluating the values of the climate regulating service and the entertainment and cultural service of the farmland ecosystem in Qingdao City, only the positive service value of the farmland ecosystem was considered, and the loss of value caused by unreasonable agricultural activities, such as excessive fertilization, was ignored. Therefore, the actual value of the farmland ecosystem services should be slightly less than that reported in this paper, and that discrepancy should be further analyzed and discussed in the future research.

The farmland ecosystem provides the direct service of supplying agricultural products, indirect services of regulating service, supporting service and cultural service, and the service values of agricultural product provisioning, carbon fixation and oxygen release, water conservation, climate regulation, soil conservation, social security service, entertainment and cultural service, etc. Various methods (market value method, carbon tax method, afforestation cost method, industrial oxygen production method, substitute cost method, opportunity cost method and equivalent factor method) were used to evaluate the values of the farmland ecosystem services in Qingdao City in 1997, 2002, 2007, 2012 and 2017. The total value of the farmland ecosystem services in Qingdao City increased from 499.74×10⁸ to 681.74×10⁸ Yuan between 1997 and 2017, and the value of ecosystem services per hectare increased from 6.57×10⁴ to 9.73×10⁴ Yuan ha^-1. The service values of product provisioning, carbon fixation and oxygen release, and soil conservation of the farmland ecosystem in Qingdao City accounted for the largest portion of the total ecosystem services value, which showed an upward trend, and they were the main farmland ecosystem services in Qingdao City. The supporting service value and the cultural service value of the farmland ecosystem in Qingdao City were low, but they still have great utilization potential.