Implications of seasonal variation in nitrate export from forested ecosystems: a review from the hydrological perspective of ecosystem dynamics

Abstract

In recent decades, the seasonal variation in NO3− discharge from forest ecosystems has been used increasingly by ecologists and hydrologists as a diagnostic indicator of the nutrient status of ecosystems. Major factors underlying the seasonal patterns of stream NO3− concentration include seasonal variations in (1) ecosystem nutrient demand, (2) solute transport capability of the hydrological condition, and (3) in‐stream nutrient usage and supply. In this article I show how case studies have helped elucidate the dominant controlling factors by comparing data from Japanese catchments with previously compiled data from studies in North America and Europe. Moreover, I explain the different influences that hydrological and biogeochemical controls exert in regions with rainy (Japan) and dry (northeastern United States and Europe) summers. The seasonal variation in hydrological conditions is the predominant controlling factor in Japanese forests, whereas nutrient demand may predominate in the northeastern United States and Europe. I emphasize the importance of comparing seasonal patterns among regions with differing climates to obtain more universal explanations of seasonal variations in NO3−‐related biogeochemical and hydrological dynamics in a forest ecosystem. In addition, multi‐scale investigations will be needed to provide insight into the relative contributions of hillslope biogeochemical effects and in‐stream biological activities.

Drivers of atmospheric nitrate processing and export in forested catchments

“…Cirmo and McDonnell [] speculated that increased litter decomposition at the end of the growing season would create a significant dissolved inorganic N pool in soil waters, though such concentration increases were not observed during and after autumn leaf fall in a Vermont forest [ Sebestyen et al ., ]. Subsequent hydrologic flushing from that nitrification source during the dormant season may result in elevated stream nitrate concentrations [ Ohte , ] but not necessarily increased proportions of unprocessed atmospheric nitrate. However, Sebestyen et al .…”

Evidence for a Regime Shift in Nitrogen Export from a Forested Watershed

“…The change in both the source of N, from surface to deep soils, and the role of hydrologic connectivity between the soils and the stream is suggested by the altered relationship between export and discharge (Figure 3). Stoddard (1994) and Ohte (2012) suggested that seasonality of stream DIN concentration and export is an important diagnostic indicator of watershed nitrogen dynamics. Before clearcutting, WS 7 had a pattern of winter low and summer high DIN concentrations similar to reference watersheds at Coweeta, but we found very clear changes in DIN seasonality as a result of clearcutting WS 7.…”

“…For example, summer maximum nitrate concentrations have been observed in Japan (Mitchell and others 1997;Ohte and others 2001;Ohte 2012), Baltimore, MD (Tague and Band 2004), the Upper Susquehanna River basin in New York (Goodale and others 2009), eastern Tennessee (Lutz and others 2012), and western North Carolina at Coweeta Hydrologic Laboratory (Swank and Vose 1997;Worrall and others 2003). This summer peak has generally been attributed to high soil microbial nitrification under conditions of adequate soil water availability and warm soil temperatures during the growing season (Knoepp and Swank 1998;Goodale and others 2009;Brookshire and others 2011;Ohte 2012). Band and others (2001) suggested that this accumulated nitrate could then be flushed from riparian zones during summer, a time when there is also little overland flow to dilute streamwater concentrations.…”

Factors Impacting Water Quality of a Small Stream in a Mountain Forest

“…8 shows the change in concentration over distance for NO 3 -N. The concentration of NO 3 -N decreased over both intervals, but the decrease over the e-d interval on the upstream side was greater than the decrease over the b-a interval on the downstream side. Furthermore, the decrease in concentration was greater during the summer season when the air temperature is high [7,8]. Fig.…”