Mechanistic insights into organic acid-mediated mitigation of nickel toxicity in maize (Zea mays L.) grown on mining-impacted soils.

BACKGROUND: The effects of oxalic acid, citric acid, and acetic acid (2.5 and 5 mmol) on nickel toxicity in maize (Zea mays L.) grown in nickel-contaminated soil from industrial and mining-affected sites were investigated in a pot experiment under controlled greenhouse conditions. While organic acids are commonly used as chelators to enhance metal uptake in phytoextraction strategies, this study demonstrates a contrasting mechanism: they reduced nickel bioavailability and uptake in maize, a non-hyperaccumulator crop, thereby promoting phytostabilization rather than phytoextraction. This distinction is important because it highlights the context-dependent role of organic acids and supports their safe application for crop production on moderately contaminated soils. RESULTS: High-dose oxalic acid (5 mmol) produced the strongest effects, significantly increasing growth parameters (shoot height + 43.7%, total biomass + 40%), with leaf area increasing 2.7-fold. Photosynthetic function was largely restored (photosynthetic rate + 78.8%, stomatal conductance + 79.6%), oxidative stress markers declined substantially (malondialdehyde - 56.5%, hydrogen peroxide - 51.4%), and antioxidant enzyme activities were enhanced (particularly ascorbate peroxidase + 82.8%). Shoot nickel accumulation decreased by 53.2% and the translocation factor by 30.2%, accompanied by reduced soil bioavailable nickel (- 27.9%) and pH, together with marked increases in available phosphorus (+ 99.6%) and microbial biomass carbon (+ 54.2%). Multivariate analyses (principal component analysis explaining 95.3% of total variation) confirmed strong positive associations among growth traits, antioxidant capacity, and improved soil quality under organic acid treatments. CONCLUSIONS: These findings indicate that oxalic acid is a sustainable and cost-effective amendment for mitigating nickel toxicity in mining-impacted agroecosystems through rhizosphere chelation, pH modulation, reduced metal bioavailability, and enhanced antioxidant defense, thereby supporting both crop productivity and long-term soil remediation.