Nitrogen and phosphorus enrichment and balance in forests colonized by cormorants: Implications of the influence of soil adsorption

Although much concern has been directed at nitrogen (N) cycling in terrestrial ecosystems with bird colonies, little has been clarified on the processes of phosphorus (P) cycling itself, and few comparisons between P and N cycling in bird colonies have been made. On the Isaki Headland and Chikubu Island, which are located on or near the shore of Lake Biwa, Central Japan, a dramatic increase in the population of cormorants has occurred since the 1980s. There has been a concomitant increase in the transport of nutrients from the lake to the waterside ecosystems. We compared the pools and dynamics of N and P in the cormorant-colony forests in order to clarify the effects of differences in soil N and P dynamics on the N–P balance of these colony forests. The total N concentration in the forest floor at excrement-influenced sites was not significantly different from that at sites without such influence, in spite of the heavy load of cormorant-derived N. In contrast to N, forest floor P concentration at the sites with excrement influence was significantly higher compared to sites without such influence, resulting in the lower forest floor N/P ratio at the excrement-influenced sites even after colony abandonment. The site pattern of total N and P concentrations and N/P ratio for mineral soil was similar to that for the forest floor. It seems that the leaky character for N and the accumulative character for P are due to the high mobility of nitrate in soils and the tight absorption of inorganic P to clay minerals, respectively. The site pattern of N/P ratios observed for Chamaecyparis obtusa Sieb. et Zucc. leaves is consistent with that for the forest floor and/or mineral soil, suggesting that the soil geochemical property was reflected in the foliar N/P ratio. The chemistry of throughfall and soil solution was also changed due to deposition of cormorant excrement, and the changes continued for a few years after abandonment of the colony. The quantitative analyses for N and P suggested that the major part of N and P transported by cormorants was not retained in plant matter and the surface soil beneath the colony but instead leached into deeper soil layers. The influence of cormorant excrement on nutrient balance of the whole colony ecosystem is also discussed.