Large amounts of marine-derived nitrogen are carried ashore by seabirds in their feces (ornithogenic N). Various topological, meteorological, and environmental factors can affect microbial transformation of ornithogenic N altering the physical and biochemical conditions of the soils. Tsunami may be a factor affecting ornithgenic soil N dynamics leading to changes in soil conditions. We examined the effects of the tsunami triggered by the 11 March 2011 earthquake off the East Coast of Honshu, Japan, on ornithogenic soil N in the Black-tailed Gull Larus crassirostris breeding colony on Kabushima (Kabu Island) four months after the disaster. There were no differences in the total N content in soils between tsunami splashed (upper half of the island) and flooded areas (lower half), and the visual topography was comparable to that reported before the tsunami. It seems that the tsunami may not have extensively removed accumulated ornithogenic N in the soils, nor did it induce deposition of organic matter derived from marine sediments. However, δ¹⁵N values of soil N were lower in the area flooded by the tsunami than in the splashed area, suggesting the possibility that the tsunami caused leaching of ornithogenic N from the soil. Soils flooded by the tsunami contained higher levels of marine salts than those from the splashed area. N isotopic differences between NH₄- and NO₃-N were smaller in the flooded soils than in the splashed area. This indicates that nitrification of ornithogenic soil N is suppressed in the flooded area because of the remarkable increase in the salt concentration of the soil.