## 2 Materials and Methods
### Research Framework  
Cultivated land is land developed by humans for crop production and represents a special type of land use [35]. Initially, the concept of CLQ primarily focused on basic soil fertility, but it has now expanded to include suitability, production potential, ecological safety, and sustainability [36; 37; 38]. The quality of black soil cultivated land depends on its ability to perform and maintain functions, as well as its resilience under external pressures. Therefore, CLQ evaluation should be primarily based on cultivated land functionality, while incorporating degradation risks that may affect these functions into the evaluation system. In this study, the research framework of \"Cultivated Land Quality-Function/Degradation Risk-Evaluation Indicators\" was established. The traditional evaluation of the quality of arable land in China has primarily focused on the quantity of resources and production functions, with less emphasis on multifunctionality and sustainable use. This study, therefore, emphasizes the productivity, stability, and sustainability of arable land, thereby addressing this shortcoming. This study employs an approach to indicator selection, encompassing both the functional aspects of cropland and the associated risk of degradation. The former evaluation index system is more comprehensive, theoretical, and systematic, focusing on the assessment of cultivated land function. In contrast, the latter addresses the actual problems of cultivated land protection in the black soil, highlighting the key challenges faced by regions in cropland utilization and construction. Cultivated land functions may be classified into three principal categories: production, ecology, and society. Of these, the production function is of the utmost importance. The northeast black soil benefits from a mild climate, four distinct seasons, simultaneous precipitation and heat, and sufficient light, which collectively provide favorable natural conditions for crop growth [39]. The higher effective cumulative temperature of \(\geq\)10 \({}^{\circ}\)C in the LHP, the high organic matter soil in the SJP and DXXXL, and the deep soil layer in the SNP create superior conditions for crop growth in terms of heat, nutrients, and root growth. Furthermore, the region's well-developed dry farming practices have reinforced the production function of its arable land. This function can be further categorized into three distinct categories: primary productivity (PP), soil nutrient and maintenance (SNM), and infrastructure regulation (IR) functions. The evaluation of primary productivity is based on terrain conditions that are not influenced by human activities. The maintenance and nutrient functions of the soil are crucial for the sustenance of crop growth and the attainment of sustainable production [40]. These functions are evaluated through the assessment of soil property indicators. Infrastructure regulation reflects human management and adaptation of cropland and is evaluated through indicators of human activities in cropland utilization. The sustainable utilization of cropland is contingent upon the maintenance of its ecological functions. Regarding soil attributes, soil microbial diversity is indicative of the complexity and stability of cropland ecosystems, thereby facilitating the maintenance of soil health and ecological functions [41]. In this study, the biodiversitysupply (BS) function was incorporated into the index system to reflect the capacity of arable land to sustain the ecosystem and biodiversity [42]. The social function reflects the social attributes of arable land. As a significant agricultural production region in China, the northeastern black soil area is directly associated with food security and agricultural output. The cultivation of high-yield arable land not only ensures the domestic food supply but also enhances the export potential of agricultural products, which has a positive impact on the national economy.