Study on Safe Usage of Agricultural Land in Typical Karst Areas Based on Cd in Soil and Maize: A Case Study of Northwestern Guizhou, China

Abstract

Cadmium (Cd) is an unnecessary dietary toxin that is harmful to human health. The Cd translocation in soil-crops system varies greatly depending on different soil matrices; therefore, a valuable method that could accurately evaluate soil Cd thresholds needs to be proposed immediately. In the southwestern part of China, a typical karst mountainous area of east Asia, the results of our survey of 492 soil-maize samples in the region showed high Cd accumulation in the soil, with concentrations ranging from 0.07 to 31.95 mg kg−1. The Cd concentrations in maize kernels planted in those fields, however, were quite low, and only 4 samples exceeded the national standard. A comparative study with nonkarst areas revealed a low bioaccumulation factor for soil Cd. This may be interpreted as the weathering soil-forming process of mineral-bearing rock systems, leading to high accumulation as well as low bioavailability of Cd in karst soils. A total of 172 soil-maize samples were evaluated inaccurately by the national standard evaluation procedure, accounting for 34.96% of the total. Therefore, we proposed the species sensitivity distribution model to address this inaccurate assessment. The results show that the hazardous concentrations of 95% and 5% in maize fields were 2.2 and 85.1 mg kg−1 for soil pH ≤ 5.5, 2.5 and 108.5 mg kg−1 for 5.5 < pH ≤ 6.5, and 3.0 and 161.8 mg kg−1 for 6.5 < pH ≤ 7.5, respectively. The total number of unsuitable samples according to the evaluation results decreased from 172 to 2 after modification. Therefore, this result could be considered a more accurate assessment threshold.