Fate of urea-¹⁵N as influenced by different irrigation modes

Fertilizer nitrogen (N) is a main pollutant in the agricultural ecosystem, while the fate of fertilizer N influenced by different irrigation modes is not well comparatively investigated. In this study, the distribution of fertilizer N in soil layers and tomato organs as well as its loss under drip, spray and flood irrigation with different quotas of 140, 180 and 220 m³ ha^-1 were evaluated quantitatively by using nitrogen-15 (¹⁵N) labeled urea (abundance of 19.6%) as fertilizer source. The results showed that the plant ¹⁵N, soil ¹⁵N and ¹⁵N loss accounted for 27.9-47.8%, 38.8-54.0% and 10.3-21.9% of the total applied ¹⁵N, respectively. The amount of ¹⁵N absorbed by plants was significantly (p < 0.05) higher under drip and spray irrigation in comparison to flood irrigation with the same irrigation quota. The maximum ¹⁵N use efficiency and the minimum ¹⁵N residual were detected under drip irrigation with quota of 180 m³ ha^-1, indicating that the supply and demand of urea-¹⁵N was more synchronized under such an irrigation mode. The ¹⁵N loss increased obviously as irrigation quota increased. Moreover, the correlation analysis between ¹⁵N loss and the possible impact factors indicated that the soil mineral ¹⁵N content after irrigation was one important factor influencing the ¹⁵N loss. Among the three irrigation modes, spray irrigation caused the lowest ¹⁵N loss of 10.3-13.1% when using the same irrigation quota. It was concluded that the irrigation modes have profound impacts on the fate of urea-¹⁵N. Irrigation could be used as a regulation pathway of plant N absorption and agricultural N output.