Alterations of pathways in fertilizer N conservation and supply in soils treated with dicyandiamide, hydroquinone and glucose

- Inhibitors enhanced the effects of fixed NH4+ on applied-N conservation and supply.
- Glucose addition increased the corresponding effects of organic N pool.
- Glucose decreased N availability, which was enhanced by DCD, but mitigated by HQ.
- Immobilization was the main drive for fixed NH4+ release with glucose or DCD input.
- Effect of nitrification on fixed NH4+ release increased in soils amended with HQ.

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.