Trinity assessment method applied to heavy-metal contamination in peri-urban soil-crop systems: A case study in northeast China

Identification and assessment of heavy-metal (HM) contamination in soil-crop systems are prerequisites for treating contamination of cultivated land. Traditional assessment methods focus primarily on total soil HM concentrations, whereas the assessment of HMs that occur in available forms or are accumulated by crops is increasingly considered essential for evaluating land contamination. Until now, there has been a lack of methods to adequately quantify the contamination status of soil-crop agroecosystems. We propose a 'trinity' assessment method covering total, available, and accumulated HMs involving a catastrophe-progression method and fusion algorithm. A typical urban-rural transition zone in NE China was selected for demonstration of the method and to identify HM spatial patterns associated with the impact of urbanization using GIS-based bivariate Moran's I analysis. Results indicate a spatial mismatch between total, available, and accumulated HM concentrations. Traditional methods indicate that soil-crop systems generally have less contamination and lower risk levels, whereas contaminated areas defined more holistically by the trinity method have a decreasing urban-rural trend. Furthermore, the bivariate Moran's I analysis indicates that land-use conflicts cause spatial clustering of HM contamination in cultivated lands where urbanization and industrialization have led to HM accumulation. Cluster patterns are significantly associated with the transition from urban to rural areas. These findings suggest that the trinity soil-crop system provides a practical tool for the quantitative assessment of the HM contamination status of peri-urban agriculture in major grain-producing areas, aiding the development of managerial strategies to prevent the occurrence of ecological hazards.