دانلود رایگان مقاله حفاظت کود نیتروژن و عرضه در خاک تیمار شده با دی سیان دی آمید

Abstract

Nitrogen (N) immobilization by microorganisms and NH4 + fixation by soil minerals are common reactions responsible for fertilizer N retention in soils. However, the relationship between microbial immobilization and NH4 + fixation remains unclear to date, and the availability of immobilized or fixed fertilizer N has yet to be compared. Accordingly, we conducted a 96-day incubation experiment to study the effects of the nitrification inhibitor dicyandiamide (DCD), the urease inhibitor hydroquinone (HQ), and glucose adding on 15N-labeled urea-N partitioning in different N pools and on the subsequent remineralization of immobilized N and release of fixed NH4 + . Glucose significantly increased N retention in soil but decreased the availability of urea-derived N because a great proportion of urea-derived N was transformed into soil microbial necromass N (SMNN). In the non-glucose treatments, the effects of the fixed NH4 + pool on the conservation and supply of urea-derived N were 1.6-fold and 2.7-fold greater on average than those of the organic N pool (including soil microbial biomass N and SMNN), respectively, from the 12th day to the end of the incubation. In the glucose treatments, the corresponding effects of the organic N pool were 3.7-fold and 3.0-fold greater than those of the fixed NH4 + pool. Both inhibitors raised urea-derived fixed NH4 + but exhibited different influences on urea-derived organic N; in particular, DCD input increased urea-derived organic N, whereas HQ input decreased this parameter. The combination of glucose and DCD further decreased the availability of urea-derived N, but HQ alleviated the decline of urea-derived N availability induced by glucose input. Microbial immobilization and nitrification comparably contributed to the release of urea-derived fixed NH4 + in soil treated with urea alone. In the presence of glucose, microbial immobilization was the principal driving force of fixed NH4 + release, and this tendency was further enhanced by DCD but mitigated by HQ. The partitioning of released fixed NH4 + between the organic N pool (immobilization) and the mineral N pool (nitrification) can be clarified by comparing the path coefficients under different conditions. These results provide valuable information for combining the abiotic and biotic processes in N cycling after fertilizer N application and for quantifying N transformation in soils.

5. Conclusion

In general, the addition of inhibitors and glucose significantly altered the pathways in N conservation and supply in the test soil. Fixed NH4 + and SMBN pools exerted important reservoirs and suppliers for fertilizer N, and the former was more effective in the absence of glucose, but the latter was more efficient in the presence of glucose. This result indicates that fertilizer N has different ways of conservation and supply under different conditions. A major portion of immobilized fertilizer N was transformed into SMNN, which was responsible for the increased retention of fertilizer N in soil and the decreased availability of fertilizer N, especially in the glucose-treated soils. Both inhibitors increased urea-derived fixed NH4 + but exhibited different effects on the microbial immobilization of urea-derived N. In particular, DCD increased but HQ decreased the microbial immobilization of urea-derived N. Correspondingly, the combination of DCD and glucose further decreased fertilizer N availability, whereas HQ alleviated the decline of fertilizer N availability induced by glucose addition to some extent. Moreover, microbial effects on fixed NH4 + release were significantly influenced by the inhibitors and glucose additions. Glucose and DCD inputs enhanced the effect of microbial immobilization on fixed NH4 + release, whereas HQ input relatively increased the corresponding effect of nitrification. These microbial processes markedly affected the partitioning of recently released fixed NH4 + between the organic N pool and the mineral N pool. Further studies are also required to determine the rates and timing of organic substrate application in different soils, especially when external organic C is incorporated with different inhibitors. The results of these studies will be helpful in optimizing N management practices and synchronizing N supply with crop demand.