Streamwater responses to reduced nitrogen deposition at four small upland catchments in Norway
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ENVIRONMENTAL EFFECTS OF A GREEN BIO-ECONOMY
Streamwater responses to reduced nitrogen deposition at four small upland catchments in Norway Øyvind Kaste
, Kari Austnes, Heleen A. de Wit
Received: 17 January 2020 / Revised: 21 April 2020 / Accepted: 8 May 2020
Abstract Reduced emissions of nitrogen (N) in Europe have resulted in decreasing atmospheric deposition since 1990. Long-term data (1988–2017) from four small Norwegian catchments located along gradients in N deposition, rainfall, and organic carbon (C) show different responses to 25–30% reductions in N deposition during the same period. At three sites the decreased N deposition caused reduced leaching of nitrate to surface water, whereas the westernmost site showed no decrease, probably due to thin soils with low C:N ratio, poor vegetation cover and high precipitation. The loss of total N to streamwater constituted 30–50% of the N deposition. Losses via denitrification are unknown but assumed to be low, as a major fraction of the catchments are well-drained. Hence, the study sites seem to continue to accumulate N, presumably mostly in soil organic matter. Although atmospheric N deposition has declined, ambient loads might still exceed long-term sustainable levels in these vulnerable ecosystems. Keywords Atmospheric deposition Catchments Long-term trends Nitrogen leaching Surface waters
INTRODUCTION Emissions of sulphur oxides (SOx), nitrogen oxides (NOx) and reduced nitrogen (NHx) in Europe have been reduced by approximately 90, 55 and 20%, respectively, since 1990 (EEA 2017). The reduction in sulphur (S) deposition has resulted in a clear improvement in the acidification status of acid-sensitive lakes and streams, with increasing acid neutralizing capacity (ANC) and improved conditions for fish and other freshwater organisms (Garmo et al. 2014). Reduced deposition of sulphate (SO4) and sea-salts has also
resulted in an increase in dissolved organic carbon (DOC) in surface waters (Monteith et al. 2007), a proxy for dissolved organic matter (DOM) which is a vector for transport of organic N. Nitrogen can contribute to acidification in the same way as sulphur if it leaks out of the soil as nitrate (NO3). During the early 1990s, before the atmospheric deposition of N started to decline, there was a concern that the excess N from atmospheric sources would lead to N saturation and massive leakage of NO3 and acidifying components from soil to water (Aber et al. 1989; Stoddard 1994). Many natural or semi-natural terrestrial ecosystems, especially in central Europe and eastern North America, showed signs of N saturation with increased leaching of NO3 (Wright et al. 2001; Kopacek et al. 2005). In some cases, drought incidents and insect attacks worsened the situation and led to forest decline and massive NO3 leaching (Oulehle et al. 2013; Seidl et al. 2017). In Fennoscandia there were reports of increasing N concentrations in upland surface waters ˚ gren 1983; Lepisto¨ 1995; during the 1980s and 1990s (A Kaste et al. 1997). Many of these ecosystems are p
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