Temporal variations in the concentration and isotopic signature of ammonium- and nitrate–nitrogen in soils under a breeding colony of Black-tailed Gulls (Larus crassirostris) on Kabushima Island, northeastern Japan

Abstract

Abstract Temporal variations in the concentration and N isotopic ratios of inorganic N (NH4– and NO3–N) as affected by the soil temperature regime together with the input of bird excreta were analyzed in a sedentary soil under a dense colony (1.6 nests/m2) of breeding Black-tailed Gulls (Larus crassirostris: a ground-nesting seabird). Surface soil samples were taken monthly from mid-March to late July 2005 from Kabushima Island, Hachinohe, northeastern Japan. The spatial concentration of inorganic N in the soils varied considerably on all sampling dates. There may be a statistically significant trend, showing increased NH4–N content from settlement up to early June when the input of fecal N attains its maximum, and then decreases towards the end of breeding activity (early August). Abundant NO3–N was observed in all soils, particularly in the later stage of breeding (up to 3800 mg-N/kg dry soil), refuting earlier claims that nitrification is unimportant in the soils. δ15N values of NH4 in the soils showed unusually high values up to +51‰, reflecting N isotope fractionation due to volatilization of NH3 during the mineralization. Mean δ15N values of the monthly collected totals of NH4 and NO3 were not significantly different at the 5% level based on ANOVA and significant differences were observed only among the three means of NO3–N collected in mid-March (settlement of colony: δ15N = −0.2 ± 3.5‰) and late July (later stages of breeding: δ15N = +22.1 ± 7.0‰, +23.3 ± 7.8‰) at the 1% and 5% levels by t-test, respectively. Such an observation of significantly increased δ15N values for NO3–N in soils from the fledgling stage indicates the integration of denitrification coupled with nitrification under a limited supply of fecal N.