Mineral nitrogen in a rhizosphere soil and in standing water during rice (Oryza sativa L.) growth: effect of hydroquinone and dicyandiamide

Inhibitors are applied to improve the efficiency of N fertilizer use and to reduce N losses from the soil. However, little information is available on the effects of a urease inhibitor, hydroquinone (HQ), and a nitrification inhibitor, dicyandiamide (DCD), on urea-derived mineral N in the rhizosphere of rice plants and in the standing water. Furthermore, this is important to understand the contribution of rice plants to gaseous N losses in the presence of inhibitor(s). A rhizobox was used to simultaneously study urea-derived mineral N in the soil at increasing distances from rice roots, and the effect of growing plants on the behavior of fertilizer-N in soil. The combined application of urea (U) with DCD showed a significantly positive effect on rice growth. The redox potentials in the rhizosphere soil were higher in the presence than in the absence of DCD, especially when rice plants entered their vigorous growth stage. During the entire experimental period, NH4+-N concentrations in the rhizosphere of rice plants were smaller in the U + DCD and U + HQ + DCD treatments than in the U and U + HQ treatments. On days 20 and 60 after fertilization, the U + DCD and U + HQ + DCD treatments showed a smaller concentration of (NO3− + NO2−)-N in soil less than 0.5 cm from the rhizobox, compared to the U treatment. During the early period of rice growth, the NH4+-N concentration of the standing water was higher in the presence than in the absence of DCD. However, there were no significant differences in NH4+-N among all treatments after 20 days. In the absence of rice plants, also, the application of DCD and especially DCD and HQ, always increased the level of NH4+-N and pH in the standing water. Thus, rice plants might reduce potential gaseous N losses from a urea-fertilized, flooded soil, due to the stimulating effect on rice growth in the presence of DCD or DCD + HQ. The N2O emissions from the urea-fertilized, rice-planted soil significantly decreased when DCD or DCD + HQ was used. From the results obtained, we conclude that DCD and DCD + HQ together with U can be an effective tool to improving crop growth and reducing N loss as N2O in rice cultivation, and that the use of a rhizobox is useful to study the interaction between plant uptake and the behavior of fertilizer-N in soil.