Study on ecological risk threshold of farmland soil in typical karst regions of Guangxi: A case study of Cadmium

Cadmium (Cd) is a non-essential element harmful to human health and is a primary pollutant in agricultural soils. It is important to assess the ecological risk threshold of Cd in soil–crop systems. Previous studies have demonstrated that soils in the karst regions of Southwest China are anomalously enriched in Cd due to geogenic processes. Therefore, it is imperative to propose an effective method for accurately evaluating ecological risk threshold of Cd in agricultural soils within karst regions. Rice is the most widely cultivated cereal crop in Daxin County and is also the crop with the highest Cd exposure in the human diet. In this study, 102 sets of rice crops and their rhizosphere soils were systematically collected from typical karst regions in Guangxi Province. With the use of the Bur Ⅲ distribution, the species sensitivity distribution (SSD) curves for Cd in agricultural soils of typical karst regions in Guangxi were fitted, and a soil Cd safety threshold was established to protect 95% of rice crops.

The results show that 90.2% of Cd in soil samples exceeded the screening value established by The National Environmental Quality Standard for Soil (GB 15618-2018). However, the Cd content in rice grains cultivated in those fields was quite low, and only 8.82% of the samples surpassing the Limit of Contaminants in Food (GB 2762-2022). This suggests a spatial mismatch in the Cd levels between the soil and the rice grains. Specifically, there are three primary scenarios: (i) the soil exceeds the Cd limit, but the rice remains safe; (ii) both the soil and the rice exceed the Cd limit; and (iii) the soil is within safe Cd levels, yet the rice exceeds the limit. In the study area, higher soil pH values correspond to reduced mobility of Cd, resulting in lower absorption and transfer of Cd by rice grains. Although the soil Cd form is mainly dominated by the content of bioavailable state, the soil pH is mainly weakly alkaline, which inhibits the absorption of soil Cd by plants. Under conditions where soil pH remains unchanged, changes in the soil will not pose a threat to food safety. This is one of the key reasons for the low Cd concentration in rice grown in high Cd soil. The existing standards of soil environmental quality are insufficient for the accurate evaluation of the level of Cd pollution in the soil of the study area. Given this, it is essential to establish the benchmark of soil environmental quality that can reflect the actual conditions. Therefore, using SSD, this study calculated the safety threshold for soil Cd to protect 95% of rice grains and evaluated the actual Cd pollution level in the study area. Based on SSD, the derived soil Cd safety thresholds for protecting 95% of rice grains under soil pH conditions of 5.5−6.5, 6.5−7.5 and 7.5−8.5 are 0.22 mg·kg⁻¹, 1.08 mg·kg⁻¹, and 6.4 mg·kg⁻¹, respectively. Compared with the limits of soil environmental quality stipulated by GB15618-2018, it is evident that the national standard is overly lenient for acidic soil (5.5 < pH ≤ 6.5) but overly stringent for neutral (6.5 < pH<7.5) and alkaline (pH > 7.5) soils. The derived value accuracy of evaluation results significantly increased from 23.5% to 92.1% with increasing pH values. In contrast, the screening value accuracy specified by national standards dropped from 47.1% to 6.3%. Therefore, the research results can be considered as more accurate evaluation thresholds, providing a scientific reference for safe production in Cd-contaminated rice paddies in typical karst regions of Guangxi.