Soil Properties and Aging Processes Regulate Cr(VI) Toxicity to Caenorhabditis elegans

Chromium (Cr) is a highly toxic heavy metal, yet its effects on soil invertebrates—particularly Caenorhabditis elegans (C. elegans)—remain insufficiently understood, especially regarding how soil properties and Cr speciation change regulate its bioavailability and toxicity. In this study, the toxicity of Cr(VI) to the growth, fertility, and reproduction of C. elegans was assessed in six representative agricultural soils following 7, 60, and 120 days of spiked soil aging, following ISO 10872 guidelines. Substantial differences in toxicity were observed among soils after 7 days of aging, with toxicity ranking from low to high as black soil < yellowish-red soil < red soil < yellow–brown soil < fluvo-aquic soil < purple soil. After 60 days of aging, Cr(VI) toxicity decreased markedly, with EC₅₀ values for growth, fertility, and reproduction increasing by 1.04–2.32, 1.04–2.34, and 1.40–2.20 times, respectively. Organic matter (OM) and amorphous aluminum oxides (Al_AO) were identified as the principal soil properties that were significantly correlated with Cr(VI) toxicity and were useful for explaining and estimating toxicity thresholds within the range of soils examined in this study. In addition, the magnitude of the aging effect showed significant positive correlations with both amorphous aluminum oxides (Al_AO) and total aluminum (Al_total), suggesting that Al-bearing minerals may contribute to the time-dependent immobilization of Cr(VI) under the experimental conditions of this study. These findings expand the ecotoxicological database for chromium, improve the prediction of toxicity thresholds under diverse soil conditions, and provide a scientific basis for refining soil environmental quality standards and developing targeted management strategies for Cr-contaminated agricultural soils.