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

Abstract

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.

The complexity of ecological impacts induced by great cormorants

“…Over time cormorants, especially in dense colonies, increase N and P content in soil layers to extreme extents (Ligęza & Smal, 2003;Hobara et al, 2005;Kolb et al, 2012;Rush et al, 2013;Litaor et al, 2014;Klimaszyk et al, 2015b). However, even a small cormorant flock can induce a short-term rise in their concentrations, usually restricted to period the birds are present (Klimaszyk et al, 2015b).…”

“…In this form, N accumulates mainly in the surface soil layer (Ligęza & Smal, 2003) and may undergo volatilization (Mizutani & Wanda, 1988;Mulder & Keall, 2001;Ligęza & Smal, 2003;Hobara et al, 2005). The intensity of this process depends, however, on variables such as pH, soil moisture and temperature (Loder et al, 1996).…”

Roosting Colony of Cormorants (Phalacrocorax Carbo Sinensis L.) as a Source of Nutrients for the Lake

“…It is highly plausible that the large quantities of nutrients observed in soils beneath a developing roosting colony are brought about by the greater density of great cormorants in comparison to perennial nesting colonies. Compared to other colonies (Ligęza et al 2001;Hobara et al 2001Hobara et al , 2005, the deeper soil layers under the roosting colony at Lake Góreckie contain smaller quantities of N and P. The impact of the great cormorants on soil chemistry is only traceable to the depth of 20 cm.…”

“…Some literature data suggested that the transfer of N and P from soils under great cormorant colonies into groundwater is minimal. In forest pressured by cormorants, volatilization of ammonia into the atmosphere is predominant whereas P is effectively accumulated in soils (Mulder and Keall 2001;Hobara et al 2005;Osono et al 2006). We found that surface runoff promotes the elution of N, P and other chemicals from soils beneath the colony.…”

“…Loads of N and P found in the top soil layer beneath the great cormorant roost at Lake Góreckie are only slightly lower than those noted in soils under perennial colonies (Ligęza and Smal 2003;Hobara et al 2005). It is highly plausible that the large quantities of nutrients observed in soils beneath a developing roosting colony are brought about by the greater density of great cormorants in comparison to perennial nesting colonies.…”