Soil Nickel and Chromium Risk Thresholds for Safe Potato Production in High Geochemical Background Areas of Lijiang

To investigate the safe production of potatoes in the high chromium (Cr) and nickel (Ni) geochemical background areas in Lijiang, farmland soil samples from two regions (Gucheng District and Yulong County) and the potato variety Lishu-7 were used as experimental materials. The chromium content in edible potato parts was evaluated against the maximum permissible limit for contaminants in foods (GB 2762-2022, Cr < 0.5 mg·kg^-1), while the nickel content was assessed according to the Ni limit for vegetables (0.3 mg·kg^-1) specified in the Standard Manual of Physicochemical Testing for Food Hygiene (1998). The eco-environment effect method and species sensitivity distribution (SSD) method were applied to derive soil Cr and Ni risk thresholds to ensure potato quality and safety. The results show that the total Cr and Ni concentrations in the soil samples collected from Lijiang exceeded the soil pollution risk screening values for agricultural land (GB 15618-2018). Total Cr concentrations ranged from 545 to 907 mg·kg^-1 in naturally contaminated soil (T4-T6), all exceeding the screening value, with T5 (907 mg·kg^-1) surpassing the risk control value by 1.30 times. In T6, Ni content in edible potato parts exceeded the food standard by 3.23 times. Ni content in edible parts showed a significant positive correlation (P<0.05) with total Ni in naturally contaminated pot soils and a significant negative correlation with yield. Cr content in edible parts showed no correlation with total soil Cr but was significantly negatively correlated with yield. In artificially spiked soils, Cr content in edible parts was significantly positively correlated with available Cr (P<0.05), and Ni content was significantly positively correlated with both total and available Ni (P<0.05). Yield showed no correlation with soil Ni content. Based on a 10% yield reduction and a linear regression model, the risk threshold for Ni in soil (pH>7.5) was determined to be 631 mg·kg^-1. Using data from pot experiments with artificially spiked soils, risk thresholds for total Cr in soil (pH<7.5) were derived as 1 809 mg·kg^-1 (linear regression) and 1 547 mg·kg^-1 (SSD method). For soils with pH between 6.5 and 7.5, total Ni thresholds were 231 mg·kg^-1 and 393 mg·kg^-1 respectively. Although total Cr and Ni concentrations in the sampled soils significantly exceeded standard limits, the exceedance rate and magnitude in the edible potato parts were relatively low. Based on both potato quality safety and ≤10 % yield reduction, the derived soil Cr threshold (1 547 mg·kg^-1) was 1.19 times the existing risk control value for agricultural soils. This study suggests that the existing national standards for Ni and Cr in agricultural soils may be overly stringent for the high Ni and Cr geological background area of Lijiang. The risk thresholds derived under high background conditions provide a scientific basis for safe potato production in this region.