University of Tehran Press Iranian Journal of Soil and Water Research 2008-479X 56 6 2025 08 23 Investigation of the effects of leonardite-derived humic substances on nickel desorption kinetics and chemical forms in a calcareous nickel-contaminated soil Investigation of the effects of leonardite-derived humic substances on nickel desorption kinetics and chemical forms in a calcareous nickel-contaminated soil 1555 1569 103838 10.22059/ijswr.2025.393048.669918 FA Iman Seifollahi Department of Horticultural Sciences, College of Agriculture and Natural Resources, Karaj Branch, Islamic Azad University, Karaj, Iran Ali Khanmirzaei Department of Soil Science, Karaj branch, Islamic Azad University, Karaj, Iran 0000-0003-1439-1375 Vahid Reza Saffari Research and Technology Institute of Plant Production, Shahid Bahonar University, Kerman, Iran 0000-0002-0540-0439 Peyman Foroozesh Department of Agronomy and Plant Breeding, College of Agriculture and Natural Resources, Karaj Branch, Islamic Azad University, Karaj, Iran 0009-0006-4746-3282 Mahboub Saffari Department of Environment, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran Journal Article 2025 04 07 The present study investigates the effects of different humic substances—including humic acid, fulvic acid, and their combination—on the behavior of nickel (Ni) in a calcareous soil. In this research, humic substances were extracted from leonardite obtained from Kerman Province and separately applied to soil contaminated with Ni (at a concentration of 120 mg/kg). After a three-month incubation period under moist conditions, Ni desorption kinetics were assessed using EDTA extraction, and the chemical forms of Ni were evaluated through a sequential extraction method. The results showed that the application of humic acid led to a 52% increase in Ni desorption. In comparison, the application of the humic acid–fulvic acid combination and fulvic acid alone resulted in 13% and 35% decreases in Ni desorption, respectively, compared to the control. The two first-order kinetic model fit the Ni desorption data well, indicating its capability to describe the desorption process in the treated soils. Furthermore, the impact of humic substances on the chemical forms of Ni revealed that humic acid and the humic acid–fulvic acid combination significantly (at the 1% level) increased the exchangeable and carbonate-bound fractions of Ni. In contrast, fulvic acid significantly reduced these mobile forms compared to the control. Overall, the findings demonstrate the high potential of humic substances in altering the bioavailability of Ni in calcareous soils and highlight the influence of humic substance type on the behavior of this heavy metal in the soil environment. These results suggest the applicability of humic substances in developing sustainable strategies for managing heavy metal pollution, such as chemical immobilization or phytoremediation processes.  The present study investigates the effects of different humic substances—including humic acid, fulvic acid, and their combination—on the behavior of nickel (Ni) in a calcareous soil. In this research, humic substances were extracted from leonardite obtained from Kerman Province and separately applied to soil contaminated with Ni (at a concentration of 120 mg/kg). After a three-month incubation period under moist conditions, Ni desorption kinetics were assessed using EDTA extraction, and the chemical forms of Ni were evaluated through a sequential extraction method. The results showed that the application of humic acid led to a 52% increase in Ni desorption. In comparison, the application of the humic acid–fulvic acid combination and fulvic acid alone resulted in 13% and 35% decreases in Ni desorption, respectively, compared to the control. The two first-order kinetic model fit the Ni desorption data well, indicating its capability to describe the desorption process in the treated soils. Furthermore, the impact of humic substances on the chemical forms of Ni revealed that humic acid and the humic acid–fulvic acid combination significantly (at the 1% level) increased the exchangeable and carbonate-bound fractions of Ni. In contrast, fulvic acid significantly reduced these mobile forms compared to the control. Overall, the findings demonstrate the high potential of humic substances in altering the bioavailability of Ni in calcareous soils and highlight the influence of humic substance type on the behavior of this heavy metal in the soil environment. These results suggest the applicability of humic substances in developing sustainable strategies for managing heavy metal pollution, such as chemical immobilization or phytoremediation processes.  https://ijswr.ut.ac.ir/article_103838_5d235ecc6e7a72af353990ac554239e8.pdf