Temporal variation in the concentration and nitrogen isotopic ratios of inorganic nitrogen from soils under Cormorant and Heron colonies

Abstract

Two soils (Motomiya, Fukushima Prefecture and Kurume, Fukuoka Prefecture) representing currently active colony of Great Cormorant Pharacrocorax carbo, Black-crowned Night Heron Nycticorax nycticorax and Grey Heron Ardea cinerea were taken and analyzed for temporal variations in the inorganic nitrogen content together with stable nitrogen isotopic signature. Unusually high concentrations of inorganic nitrogen (sum total of ammonium and nitrate-nitrogen) in surface soils (up to 8 g/kg dry soil)increased towards fledgling and tended to decrease quickly after breeding activity. Anobserved tendency showing the higher rate of nitrification in soils from the warmer soiltemperature region (Kurume), relative to those from the slightly cooler region (Motomiya) was interpreted as an indication of the limited nitrification under prevailing lower soil temperature regime. Pronounced denitrification, coupled with nitrification, was indicated by the unique nitrogen isotopic signatures of residual soil nitrate during the later stages ofbreeding activity. Continued use of forest sites for breeding and subsequent roosting byGreat Cormorants, facilitates mineralization of fecal nitrogen, relative to those under temporary heron colony.