BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

SignificanceHow terrestrial plants use N and respond to soil N loading is central to evaluating and predicting changing ecosystem structure and function with climate warming and N pollution. Here, evidence from NO3− in plant tissues has uncovered the uptake and assimilation of soil NO3− by Arctic tundra plants, which has long been assumed negligible. Soil NO3− contributed about one-third of the bulk N used by tundra plants of northern Alaska. Accordingly, the importance of soil NO3− for tundra plants should be considered in future studies on N and C cycling in Arctic ecosystems where C sequestration is strongly determined by N availability.

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.

Abstract:Nitrogen dynamics were investigated in tandem with groundwater monitoring in a temperate forest catchment in Japan, to determine the effect of the partial dieback of dominant tree stands on nutrient cycling and streamwater chemistry, especially temporal and spatial changes in the NO 3 concentrations in soil and ground water. Decreased nitrogen uptake by roots and increased nitrogen supply from the litter fall that occurred with the 1992-94 pine dieback caused by pine-wilt disease brought on a threefold increase in nitrate concentrations in streamwater and subsurface groundwater. It was found that, from 1992 to 1996, seasonal peaks occurred in the stream NO 3 concentrations during the rainy season (July to August). This seasonal variation corresponded directly to groundwater levels in the riparian zone near the catchment outlet, suggesting that seasonal changes in groundwater levels were the dominant factor controlling temporal variation in stream NO 3 concentrations. The mechanism of the seasonal pattern found in this watershed emphasizes the importance of hydrological seasonality with the high precipitation, groundwater levels, and runoff rates that are seen in summer in Japan, whereas in forested watersheds in Europe and the eastern USA the smaller inorganic nitrogen pools and low transportation forces simultaneously affect NO 3 leaching during summer.