Effects of Organic Amendments with Different C/N Ratios and Application Rates on N Mineralization–Immobilization in Calcareous Cropland Soil

The application of organic amendments is increasing in intensive modern agriculture. However, the impacts of organic amendments with distinct quality and quantity on soil mineral nitrogen (N) dynamics remain unclear. In this study, we performed a laboratory incubation experiment for 90 days to investigate the effects of organic amendments with different carbon-to-nitrogen ratios (C/N) (7, 119, 506) and application rates (2, 4, 6 g C kg−1) on N mineralization–immobilization processes in calcareous cropland soil. The study showed that net N mineralization induced by low C/N amendment (rapeseed cake, 7) occurred in this soil. In contrast, net N immobilization was stimulated by glucose and high C/N organic amendments, including maize straw (C/N, 119) and cypress sawdust (C/N, 506). The immobilized N was in the order of glucose > maize straw > cypress sawdust regardless of application rates, which was ascribed to the discrepancy of their C availability. Both N mineralization and N immobilization increased with application rates during the incubation period. The net N mineralization from these organic amendments was significantly affected by C/N ratios (p < 0.001), application rates (p < 0.001), and their interaction (p < 0.001). The study indicated that organic amendments with a higher C/N ratio would promote soil N immobilization, which was further reinforced by higher application rates, thereby decreasing the risk of reactive N loss in calcareous soil.