Replacing chemical fertilizers with organic fertilizers at equivalent nitrogen levels enhances soil nitrogen transformation rates in Northwest China’s farmlands, consequently impacting crop yields

The effectiveness of substituting organic fertilizers for chemical fertilizers in enhancing crop yield and soil quality remains uncertain. This study conducted a two-year field experiment to investigate the effects of replacing chemical fertilizer nitrogen with organic fertilizer nitrogen at varying levels (0%, 30%, 60%, 100%) on soil nitrogen transformation and crop yield in potato and wheat cultivation. We measured soil nitrogen pools and key nitrogen-transforming enzyme activities to evaluate how various fertilization treatments affect the soil nitrogen cycling process. Results showed that replacing 60% of chemical fertilizer nitrogen with organic fertilizer (T2) significantly increased soil nitrate nitrogen content during the potato tuber expansion and wheat jointing stages. It also maintained high mineral nitrogen levels in the later growth period. Additionally, the T2 treatment significantly boosted soil urease (SU) activity by 14 - 38% and Soil alkaline protease (ALPT) activity by 9 - 22%, optimizing nitrogen transport to potato tubers and wheat grains. Compared to full chemical fertilizer treatment, T2 increased potato yield to 47.13 t·ha^-¹ and wheat yield to 6.06 t·ha^-¹, marking increases of 18.8% and 22.8%, respectively, and improved nitrogen use efficiency by 73.4 - 76.1%. This study demonstrates that substituting 60% of chemical fertilizer nitrogen with organic fertilizer effectively meets crop needs through a "quick-release and slow-release" coordinated nitrogen supply mechanism. It enhances nutrient release by boosting soil enzyme activities during critical fertilizer-demanding periods, offering a viable solution for reducing fertilizer use and increasing efficiency in arid and cool regions.