Effect of Humic Acids in Regulating Microbial-Mediated Selenium Bioavailability and Uptake in Winter Wheat (Triticum aestivum L.): A Review

Selenium (Se) biofortification of staple crops like winter wheat is essential for addressing human nutritional deficiencies but requires precise management to avoid toxicity. The bioavailability of Se in soil-plant systems is directed by multifarious interactions between soil organic matter, pH, microbial activity, and fertilizer management. This review delivers a qualitative integrative synthesis of biochemical, microbial, and agronomic mechanisms by which humic acid (HA) and nitrogen (N) fertilization interact to regulate Se biogeochemistry and biofortification efficacy in winter wheat (Triticum aestivum L.). We examine the role of HA in buffering soil pH, shaping microbial community structure and function (particularly in mediating Se redox transformations), and acting as a plant biostimulant to improve root growth and exudation. The study additionally defines how these HA-mediated progressions interrelate with N forms ( $${\text{NH}}_{4}^{ + }$$ vs. $${\text{NO}}_{3}^{ - }$$ ) to modulate the kinetics of Se uptake, translocation, and partitioning to grain. Significantly, we highlight the dual role of HA in optimizing Se bioavailability in deficient soils while mitigating oxidative stress and Se toxicity in seleniferous conditions. The synthesis shows that co-application of HA with ammonium-based fertilizers can enhance grain Se enrichment in alkaline soils by decreasing excessive Se mobility. This integrated framework develops the fundamental understanding of the soil-microbe-plant continuum and delivers actionable insights for developing context-specific, sustainable agronomic approaches to improve Se biofortification and global nutritional security.