Soil Mineral Nitrogen Dynamics and Yield Stability in Maize Under Variable Precipitation: Comparing Leguminous Green Manures with Mineral Nitrogen Fertilization in a Two-Year Pilot Study

Regenerative farming practices, including the use of leguminous green manures, are increasingly recognized for their potential to improve soil quality, enhance nutrient cycling, and reduce nitrogen losses. In this study, we investigated the effects of two leguminous green manure crops, lupine (Lupinus albus L.) and common vetch (Vicia sativa L.), on maize (Zea mays L.) yield and soil mineral nitrogen dynamics in two soil layers (0–25 cm and 25–50 cm). The treatments were compared with mineral nitrogen fertilization (80 kg ha−1 N) and an unfertilized control under field conditions on humic sandy soil in Hungary between 2021 and 2023. The results showed that under extreme drought conditions (2022), maize yield in mineral-fertilized plots (0.90 t ha−1) was significantly lower than in the unfertilized control (1.61 t ha−1), while green manure treatments maintained substantially higher yields (4.11–4.21 t ha−1). Soil nitrogen dynamics were strongly influenced by precipitation patterns. Correlation and regression analyses revealed that increasing precipitation reduced mineral nitrogen content in the topsoil, while promoting its accumulation in deeper soil layers. This effect was particularly pronounced in mineral-fertilized treatments, where a significant negative correlation was observed in the 0–25 cm layer and a positive correlation in the 25–50 cm layer, suggesting greater downward movement of mineral nitrogen within the soil profile. Principal component analysis further indicated that mineral fertilizer-derived nitrogen was more strongly associated with precipitation variables than nitrogen originating from green manure treatments. The results suggest that the application of leguminous green manures promotes a more stable soil nitrogen distribution and reduces sensitivity to precipitation-driven nitrogen redistribution compared with mineral fertilization.