A suitable organic fertilizer substitution ratio stabilizes rainfed maize yields and reduces gaseous nitrogen loss in the Loess Plateau, China

The application of organic fertilizers has become an increasingly popular practice in maize production to reduce the gaseous nitrogen (N) loss and soil degradation caused by inorganic fertilizers. Organic fertilizer plays a key role in improving soil quality and stabilizing maize yields, but few studies have compared different substitution rates. A field study was carried out in 2021 and 2022, based on a long-term trial initiated in 2016, which included five organic fertilizer N substitution rates with equal inputs of 200 kg N ha^–1: 0% organic fertilizer (T1, 100% inorganic fertilizer), 50.0% organic+50.0% inorganic fertilizer (T2), 37.5% organic+62.5% inorganic fertilizer (T3), 25.0% organic+75.0% inorganic fertilizer (T4), and 12.5% organic+87.5% inorganic fertilizer (T5), as well as a no fertilizer control (T6). The results of the two years showed that T3 and T1 had the highest grain yield and biomass, respectively, and there was no significant difference between T1 and T3. Compared with T1, the 12.5, 25.0, 37.5, and 50.0% substitution rates in T5, T4, T3, and T2 significantly reduced total nitrogen losses (NH₃, N₂O) by 8.3, 16.1, 18.7, and 27.0%, respectively. Nitrogen use efficiency (NUE) was higher in T5, T3, and T1, and there were no significant differences among them. Organic fertilizer substitution directly reduced NH₃ volatilization and N₂O emission from farmland by lowering the ammonium nitrogen and alkali-dissolved N contents and by increasing soil moisture. These substitution treatments reduced N₂O emissions indirectly by regulating the abundances of AOB and nirK-harboring genes by promoting soil moisture. Specifically, the 37.5% organic fertilizer substitution reduces NH₃ volatilization and N₂O emission from farmland by reducing the ammonium nitrogen and alkali-dissolved N contents and increasing moisture, which negatively regulate the abundance of AOB and nirK-harboring genes to reduce N₂O emissions indirectly in rainfed maize fields on the Loess Plateau of China.