Nanohumic Acid Enhances Maize Yield by ModulatingSoil Bacterial Communities and Grain Tryptophan Metabolism: A NovelPlant Biostimulant

Maize (Zea mays L.) is a major staple crop worldwide and plays a critical role in food security. Although humic acid (HA) is widely used to improve crop productivity, its effectiveness is often limited by poor dispersibility and bioavailability. Herein, nanohumic acid (NHA) with improved dispersibility was synthesized, and its effects on maize yield and underlying mechanisms were evaluated. The results showed that NHA significantly promoted maize growth and increased yield, with the highest yield under NHA15 (15 mg/kg; 31.8 g/plant), exceeding HA15 (12.8 g/plant) and CK (6.8 g/plant). Maize yield responded to NHA were concentration-dependent, showing stimulation at low doses but inhibition at high doses. NHA also enriched bacterial taxa associated with plant growth and nutrient availability and upregulated grain tryptophan metabolism, increasing IAA contents by 35.1% and 108.3% relative to CK and HA, respectively. These findings support the application of NHA for maize yield improvement.