2.3. System of Cultivated Land PA, SA, and HA

The study takes a sample of 48,693 cultivated land parcels from Jiangyou City in 2019 as the evaluation unit. First, the projection transformation and vectorization of each factor were carried out using ArcGIS software. Second, the scores of each parcel were calculated based on the results of the first step, as shown in Table 2. Factors such as land surface slope, available soil depth, etc., are graded according to national arable land standards or related research [25,41]. Soil heavy metal is classified according to national standards [42]. Other factors such as farmland road density, fractional vegetation cover, etc., are graded according to the principle of equal spacing. Third, the scores of each cultivated land unite in the PA, HA, and SA systems were normalized [43]. Fourth, the four types of coupling coordination index values of each parcel were calculated [44,45]. The scores of each evaluation factor are as follows:

Comprehensive evaluation indicator system and quantification standards of cultivated land.

In recent years, the results of updating and improving the agricultural land quality classification have provided a rich and detailed theoretical and practical foundation for the establishment of the cultivated land PA system. For this study, the PA system of cultivated land was constructed according to the evaluation system of agricultural land classification (Table 2) [46]. Then, land surface slope, available soil depth, soil texture, soil organic matter, profile pattern, and soil pH were selected for the PA system.

The available soil depth is closely related to the soil quality of the cultivated land. Within a certain range, the thicker the soil layer, the more fertile the soil. This is because soil texture has a significant effect on soil water and heat conditions, fertility conditions, and root development. Notably, the quality of soil texture can be reflected in the soil’s clay content [28]. The degree of acidity and alkalinity is also an important factor in soil quality, as the normal growth of crops can be restricted by excessive acid or alkali content in the soil. This is expressed as a pH value, among which neutral soil is the most fertile. Meanwhile, the content of soil organic matter is positively correlated with soil fertility, soil structure, and soil buffer capacity. The index of each parcel unit is calculated by weighted superposition, after which the final score of the PA is expressed using the normalization method.

In this paper, the cultivated land patch is taken as the evaluation unit, from which the cultivated land productivity index map is created using the weighted superposition of the above eight factors. The calculation formula is shown in Formula (1):

where: Fi is the score of the ith parcel;fij is the score of the jth factor of the ith unit; ωij is the weight of the jth factor of the ith parcel, and n is the number of factors.

Cultivated land site conditions indicate a location’s potential for the stable and sustainable use of its cultivated land resources [47]. The strength of a site’s elements indicates the development potential of cultivated land resources and can be measured using he SA subsystem. This is evaluated using three indicators: location factors, farmland capital construction, and social and economic factors. The distance of a plot of land from the city center corresponds to the willingness of farmers to develop their cultivated land. The closer the land is to the city, the higher the farmers’ desire to develop. At the same time, a plot of land’s distance from the farmers’ market indicates how convenient it is for farmers to buy seeds, fertilizers, and tools. The patch shape index indicates the intensity with which cultivated land can be developed [48]. The more regular the cultivated land, the higher the degree to which it can be developed. The water network density indicates the ease with which the farmland can be irrigated. The higher the index, the easier it is to irrigate. The density index of the farmland road network indicates the convenience of reaching the cultivated land and if it is possible to sow and harvest the cultivated land mechanically. The per capita cultivated land represents the pressure of the population on the cultivated land. In the SA subsystem, each factor is first calculated individually, then the cultivated land SA index of the entire study area is calculated and analyzed using the weighted superposition method.

The degree of urban influence and degree of the agricultural market influence are calculated by a point-like influence formula. The point-like influence mode is expressed by concentric circle diffusion and is calculated using the linear attenuation method to express the score of point factors [49], as shown in Formula (2):

where: fi is the score of point factors on the evaluation unit at a certain relative distance; Mi is the scale index; di is the absolute distance from the evaluation unit to the factor; d is the influence radius of the factors; ri is the relative distance of the factor.

The patch shape index is comprehensively expressed by patch area and perimeter. The calculation formula is written as follows [50]:

where:fi is the i th patch shape index; Si is the patch area (m²); and Pi is the perimeter of the patch (m). The closer the fi value is to 1, the closer the cultivated land patch shape is to a banded quadrilateral, which is easy to cultivate. At the same time, the more irregular the patch shape, the more difficult it is to use.

The index of the water network density is the ratio of the total area of rivers and lakes to the total area of the assessed area. This represents the abundance of water resources. The calculation formula is shown in Formula (4) below:

where: fi is the ith water network density in village; Ariv is the normalized index of river length, Alak is the normalized index of lake area, sriv is the area of the river, slak is the area of the lake, S is the area of arable land in village.

The index of farmland road density is calculated using the ratio of the length of the farmland road network to the assessed area. This represents the abundance of roads in the region. The greater the index value, the more convenient it is to reach cultivated land. The calculation formula is shown in Formula (5):

where: fi is the index of cultivated land road density in village; Si is the area of arable land in village; Li is the total length of the farmland road in village.

Healthy cultivated soil can guarantee healthy agricultural production and maintain the multiple functions of the soil ecosystem. In this paper, the external characterization factor of cultivated land soil(FVC) and the internal factor (heavy metal pollution) were selected to evaluate the health of the cultivated land’s soil [51]. The index of the FVC indicates the growth of crops on cultivated land to a certain level. Beyond this, however, certain harmful human activities or other natural factors may lead to the enrichment of heavy metals in the soil, which ultimately threatens the soil quality, the safety of agricultural products, and human health. In this paper, five specific indicators were selected to evaluate the pollution degree of soil heavy metals: cadmium, mercury, arsenic, lead, and chromium.

The HA index is calculated by weighting the sum of Internal HA and External HA. The calculation formula is shown in Formula (6):

where: HCI represents the index of cultivated land health, P is the index of soil heavy metal pollution, F is the fractional vegetation coverage, Aint and Aext are the weights of P and F. The weight is calculated by entropy weight method (EWM) [52].

In this study, the index of soil heavy metal pollution was evaluated using the Nemerow pollution index. First, based on the spatial characteristics of the study area, we selected the five aforementioned heavy metal indicators of cadmium, mercury, arsenic, lead, and chromium for testing, with a total of 325 sampling points. Second, through the interpolation method, the heavy metal pollution index of the whole area was generated and the internal health index of cultivated land was calculated according to the Nemerow comprehensive pollution index method [53]. The Nemerow pollution index P was calculated as shown in Formulas (7) and (8).

where: P is the Nemero pollution index, Pimax is the maximum value of pollution index of heavy metal ith factor. Pi is the ith single factor pollution index of heavy metals in soil, Dsi the measured value of heavy metals in soil, and Dni is the standard value of heavy metals in soil.

For external health status, FVC is used to represent the external health characteristics of cultivated land [54]. The calculation of this index is shown in Formula (9):

where: F refers to the index of FVC; NDVImin is the value when there is bare soil or no vegetation cover, and NDVImax is the value of high vegetation coverage; NDVI is the normalized vegetation index.