Input of seabird-derived nitrogen into rice-paddy fields near a breeding/roosting colony of the Great Cormorant (Phalacrocorax carbo), and its effects on wild grass

Terrestrial ecosystems near breeding/roosting colonies of piscivorous seabirds can receive a large amount of marine-derived N in the form of bird feces. It has been well demonstrated that N input from seabirds strongly affects plant communities in forests or coastal grasslands. The effects of nutrient input on plant communities in agricultural ecosystems near seabird colonies, however, have rarely been evaluated. This relationship was examined in rice-paddy fields irrigated by a pond system located near a colony of the Great Cormorant Phalacrocorax carbo in Aichi, central Japan. In the present study, spatial variations in N content (N %) and N stable isotope composition (δ¹⁵N) of soils and wild grass species together with the growth height of plants in paddy fields in early spring (fallow period) were examined. Soils had a higher N % and δ¹⁵N values in fields associated with an irrigation pond that had N input from cormorants. The δ¹⁵N values tended to be higher around the inlet of irrigation waters, relative to the outlet. These results indicate that cormorant-derived N was input into the paddy fields via the irrigation systems. Plants growing in soil with higher δ¹⁵N had higher δ¹⁵N in the above-ground part of the plants and had luxurious growth. A positive correlation in plant height and δ¹⁵N of NO₃–N was observed in soil plough horizons.