Highly Dispersed Blast Furnace Sludge as a Source of Iron and Zinc for Sugar Beet: Effects on Oxidative Stress Markers and Micronutrient Bioaccumulation

Blast furnace sludge is a micro- and nano-dispersed metallurgical waste rich in iron and zinc, yet its accumulation poses a serious environmental challenge. Here we demonstrate its potential as a source of iron and zinc for sugar beet (Beta vulgaris L.), a crop with high micronutrient demand and economic importance. At application rates of 0.5–2 t ha−1 in alluvial-meadow soils with neutral pH, the sludge increased root yield by up to 1.5-fold and sugar content by up to 1.4-fold compared to untreated controls. The optimal dose (0.1 g kg−1 in greenhouse) significantly reduced the activity of oxidative stress markers—polyphenol oxidase (PPO) by 7.5-fold and peroxidase (POD) by 8-fold—indicating alleviation of cellular stress. The sludge also exhibited phytoprotective properties, reducing leaf necrosis under field conditions. A single application at these rates posed no food safety risks: lead and cadmium levels in beetroots and soil remained below international regulatory limits, and zinc accumulation in beetroots (≤10 mg kg−1) was an order of magnitude below the FAO/WHO guideline. However, repeated annual applications would gradually increase soil zinc; preliminary screening suggests that applying 2 t ha−1 annually could approach the soil MPC within 4–5 years under a linear accumulation scenario, necessitating long-term monitoring.