Replacement of Chemical Fertilizers with Partial Organic Manure Reduced N₂O Emission and Improved Sorghum Nitrogen Productivity in Saline-Alkali Soil

Large amount of synthetic N input resulted in high N₂O emissions and low N fertilizer productivity in agricultural production. Replacing inorganic N fertilizer with organic manure is a promising practice to reduce N₂O emissions and promote N use efficiency in farmland. However, the effect of synthetic N and organic manure combined uses on N₂O flux, crop yield, and N fertilizer productivity remains unclear in saline-alkaline soils. This study aimed to determine the suitable ratio of chemical N and organic manure for soil N₂O reduction and crop productivity enhancement, and identify the regulation mechanism of N₂O emission in salt-affected sorghum farmland. Synthetic N fertilizer (180 kg N/hm²) was replaced by organic manure N (decomposed cow dung) at ratios of 0 (N₁), 40% (N_0.4), and 60% (N_0.6) in crop two growing seasons of 2021 and 2022 in northwest China. Soil water content (SWC), available N, N₂O emission, yield, and N use efficiency of sorghum crops and their correlations were evaluated. SWC value at 0 ~ 60 cm for N_0.6 was superior to that measured from other two fertilization treatments (N_0.4 and N₁). Soil NO₃⁻-N contents at top-40 cm for N_0.6 were 8.0 ~ 12.6% and 10.5 ~ 13.4% higher than that of N_0.4. Soil NH₄⁺-N content at 0–40 cm accumulated in topsoil layers followed an order from high to low was N₁ > N_0.4> N_0.6. The combined use of synthetic N and organic manure help reduce N₂O emission, and the significant reduction of accumulative N₂O emission was N_0.6. As compared with N₁, the N_0.4 also enhanced N use efficiency, N partial fertilizer productivity, and agronomic efficiency of N by 36.9%, 12.9%, and 93.9%. The maximum accumulation rate of above-ground weight during the fastest growing point, and crop grain yield were significantly increased for the combined treatments. The substantially increased NH₄⁺-N promoted N₂O emission for N_0.4, whereas the contribution of NO₃⁻-N to N₂O was dominant for N_0.6. The N_0.4 (40% organic substitution of 180 kg N/hm²) was an optimum fertilization for reducing N₂O emission by adjusting soil N transfer processes at mid-late crop growth period and promoting crop N uptake and N use efficiency of sorghum crops in saline alkali arid district.