Nitrogen losses from agricultural soils represent a major environmental challenge, particularly under unstable moisture, where intensified nitrification and leaching reduce nitrogen use efficiency and increase the risk of environmental contamination. This study assessed the effects of nitrogen inhibitors and nitrogen-stabilizing agents on soil mineral nitrogen dynamics (NH₄⁺-N and NO₃⁻-N) and maize grain yield under rainfed conditions in the Left-Bank Forest-Steppe of Ukraine during 2023–2025. Field experiments included four maize hybrids and five treatments: a humic substances-based stabilizer (T1), two DMPP-based nitrification inhibitor formulations (T2–T3), an Azotobacter-based biological product (T4), and a nitrapyrin-based inhibitor (T5), compared with a control (T0). Application of the tested products increased the soil pool of mineral nitrogen, indicating improved mineral N retention in the root zone under unstable moisture. The strongest effect was observed for T1, which increased NO₃⁻-N by 7.48 mg kg⁻¹ (≈46% vs. T0) and NH₄⁺-N by 6.18 mg kg⁻¹ (≈38%). Overall, NO₃⁻-N increased by 14–46% and NH₄⁺-N by 12–38% relative to T0; the smallest increases were observed for T3. Improved soil N retention was associated with increased maize grain yield, with the highest average increase for T1 (1.18 t ha⁻¹; ≈13%), followed by T5 (≈10%). Treatment effectiveness depended on inhibitor type, maize hybrid, and interannual climatic variability, emphasizing the need for environmentally adapted nitrogen management strategies.