Ammonium utilization in constrained soils: Mechanisms and multi-dimensional regulation pathways

Nitrogen (N) is essential for plant growth, and soil ammonium (NH 4 + ) is a major inorganic N source directly utilized by plants. However, its availability and retention are strongly constrained in acidic, saline–alkaline, and sandy soils, limiting nitrogen-use efficiency and increasing environmental N losses. This review summarizes the dynamics and transformation processes of NH 4 + in these three types of constrained soils. In acidic soils, low pH and exchangeable aluminum (Al 3+ ) inhibit nitrification, thereby favoring the retention of nitrogen as NH 4 + rather than nitrate (NO 3 - ). In saline–alkaline soils, elevated pH promotes ammonia (NH 3 ) volatilization, while exchangeable sodium (Na + ) competition reduces NH 4 + uptake. In sandy soils, low clay and organic matter contents result in poor NH 4 + adsorption and unstable nitrogen supply due to leaching. To improve ammonium-use efficiency, we propose integrated strategies, including soil amendment to improve physicochemical properties and nutrient-retention capacity, ionic regulation to mitigate the adverse effects of toxic ions, genetic improvements in crop plants to enhance the capacities for NH 4 + transport and assimilation, and soil-microbial regulation to optimize nitrogen transformation and biological stabilization. We pay special attention to the rhizosheath, which serves as a potential interface regulating NH 4 + acquisition. Integrated plant–microbe–soil management is essential for improving ammonium-use efficiency in crops and for achieving sustainable nitrogen management in constrained soils.