Slow vs. Fast Pyrolysis Biochars: Mitigating Chromium Stress in Rapeseed Roots

Chromium (Cr)-induced stress is an escalating concern for plants, attributed to industrial activities within agricultural soils, and poses a substantial threat to plant growth. Biochar, an organic material known for its heavy metal immobilization properties, holds promise for ameliorating plant growth in heavy metal-contaminated soil. This study explores the repercussions of Cr stress, manifesting in non-toxic, 100, and 200 mg of K2Cr2O7 per kg soil, on the growth and physiological performance of rapeseed roots. Additionally, we examine the application of biochar, specifically 30 g per kg soil, produced through both slow and fast pyrolysis methods. Chromium stress is found to impede the growth and development of rapeseed roots. Biochar application elevates soil pH, cation exchange capacity, and nutrient absorption by plants such as nitrogen, phosphorus, potassium, calcium, and magnesium. Moreover, it enhances the synthesis of osmotic regulators, most notably glycine betaine and soluble proteins, alongside indole acetic acid in the root systems, thereby enhancing root development and water absorption even in the presence of Cr-induced stress. Furthermore, biochar usage demonstrates a marked reduction in the availability and absorption of Cr by rapeseed roots, resulting in a diminished production of stress-resistance hormones such as abscisic acid, salicylic acid, and jasmonic acid. Notably, slow pyrolysis biochar outperforms fast pyrolysis biochar in reducing Cr availability and absorption. The findings emphasize the efficacy of slow pyrolysis biochar in diminishing Cr absorption by plant roots in Cr-contaminated soil, holding promise for the development of sustainable strategies to counteract the detrimental effects of heavy metal contamination.