The intensification of agriculture has led to a significant rise in nitrogen (N) fertilizer use to meet global demands for food, bioenergy, and fiber. Nitrogen is essential for plant growth and is mainly absorbed as nitrate (NO3⁻), ammonium (NH4⁺), and nitrite (NO2⁻). However, conventional nitrogen application practices are often inefficient, resulting in nitrate leaching, greenhouse gas emissions, and long-term environmental degradation. Nitrate leaching can increase by up to 50% under poorly managed nitrogen regimes, particularly in winter wheat systems. This review critically synthesizes the agronomic role of nitrogen in plant nutrition, the biochemical pathways of nitrification and nitrate leaching, methods for tracking soil nitrogen transformations and microbial processes involved in gaseous nitrogen losses. Special attention is given to nitrous oxide (N2O), a greenhouse gas with a 100-year global warming potential 273 times that of carbon dioxide (CO2). Sustainable practices such as optimized nitrogen scheduling, precision agriculture, and diversified crop rotations can reduce losses without compromising yields, although reported effects vary with crop, soil, climate and management context. The main conclusion is that sustainable nitrogen management must evaluate nitrate leaching, ammonia volatilization and N2O emissions together rather than as isolated loss pathways. This review aims to provide a critical synthesis of nitrogen dynamics and to identify practical priorities that improve nitrogen use efficiency (NUE) while minimizing ecological harm.