Nitrogen isotope composition of inorganic soil nitrogen and associated vegetation under a sea bird colony on the Hatana islands, Rotuma Group, Fiji

Abstract

Concentration together with the stable isotope composition (δ15N) of both ammonium- and nitrate-nitrogen were conducted for sedentary soils under the piscivorous seabird colony developed on Quaternary basaltic ejecta from the Hatana islands, Fiji, south Pacific. Higher inorganic nitrogen content (up to 2,420 mg-N/kg dry soil) was observed for soils under dense forest vegetation, relative to a lower nitrogen content for soils under sparse grass vegetation (270 mg-N/kg dry soil), reflecting the difference in the mulching effects of vegetative cover for leaching of nitrogen by meteoric water. Active mineralization of uric acid in the bird excreta followed by partial loss due to volatilization of ammonia, nitrification and denitrification were indicated by unique δ15N values of the inorganic nitrogen as high as +50‰. This unique process was facilitated by high soil surface temperature (24 to 32°C) in combination with the humid tropical climate (annual rainfall: 3560 mm). Distinct isotopic composition of the foliar nitrogen (δ15N = +13.6 to +36.7‰) indicated acquisition of the inorganic nitrogen derived from bird excreta in soils.