Winter climate affects long-term trends in stream water nitrate in acid-sensitive catchments in southern Norway

Abstract

Abstract. Controls of stream water NO 3 in mountainous and forested catchments are not thoroughly understood. Longterm trends in stream water NO 3 are positive, neutral and negative, often apparently independent of trends in N deposition. Here, time series of NO 3 in four small acid-sensitive catchments in southern Norway were analysed in order to identify likely drivers of long-term changes in NO 3 . In two sites, stream water NO 3 export declined ca 50% over a period of 25 years while in the other sites NO 3 export increased with roughly 20%. Discharge and N deposition alone were poor predictors of these trends. The most distinct trends in NO 3 were found in winter and spring. Empirical models explained between 45% and 61% of the variation in weekly concentrations of NO 3 , and described both upward and downward seasonal trends tolerably well. Key explaining variables were snow depth, discharge, temperature and N deposition. All catchments showed reductions in snow depth and increases in winter discharge. In two inland catchments, located in moderate N deposition areas, these climatic changes appeared to drive the distinct decreases in winter and spring concentrations and fluxes of NO 3 . In a coast-near mountainous catchment in a low N deposition area, these climatic changes appeared to have the opposite effect, i.e. lead to increases in especially winter NO 3 . This suggests that the effect of a reduced snow pack may result in both decreased and increased catchment N leaching depending on interactions with N deposition, soil temperature regime and winter discharge.

Land‐use dominates climate controls on nitrogen and phosphorus export from managed and natural Nordic headwater catchments

“…The seven natural catchments in Norway are part of the national program for monitoring effects of long-range transported air pollution (Garmo & Skancke, 2018). Monitoring started during the early 1970s in four stations (de Wit, Hindar, & Hole, 2008), and around 1990 in three other stations. The catchments are all located on acid-sensitive bedrock and land cover varies from forest, forest interspersed with peatland, and predominantly tundra or bare rock.…”

“…Declines in NO 3 concentrations in natural catchments dominated over change in totN. In natural catchments, most totN consists of organic N and the dynamics of organic N are closely linked to those of DOM, which is usually elevated during the summer and autumn (Lepisto, Kortelainen, & Mattsson, 2008) whereas NO 3 is highest during the dormant season (de Wit et al, 2008) and likely to be more strongly linked to atmospheric deposition (Vuorenmaa et al, 2018).…”

“…Given the profound contrasts in N concentrations between land use categories, and the lack of correlations between trends in N species and trends in climatic and hydrological variables, it is likely that the decline in N species is the concerted effect of various interplaying factors, including environmental policy. Other studies document longterm declines in reactive nitrogen from unmanaged and managed Nordic landscapes (Garmo et al, 2014;Rekolainen, Mitikka, Vuorenmaa, & Johansson, 2005) which are likely caused by changes in N deposition and climate-mediated changes in hydrology (de Wit et al, 2008;Kaste et al, 2020;Vuorenmaa et al, 2018). The decline in N deposition itself is related to reduced emissions to the atmosphere (Grennfelt et al, 2020).…”

“…Monitoring started during the early 1970s in four stations(de Wit, Hindar, & Hole, 2008), and around 1990 in three other stations. The catchments are all located on acid-sensitive bedrock and land cover varies from forest, forest interspersed with peatland, and predominantly tundra or bare rock.…”

Atmospheric deposition as a source of carbon and nutrients to an alpine catchment of the Colorado Rocky Mountains

“…is a trend of increasing nitrate export in surface waters that has been observed in many alpine and high-elevation catchments, such as those in the Colorado Rocky Mountains, USA , the Alps of northern Italy (Rogora, 2007), and southern Norway (de Wit et al, 2008), for which the process-level controls are just beginning to be explored. suggested that observed increases in nitrate export from the high-elevation Loch Vale catchment in the Colorado Front Range were the result of warmer temperatures that are melting ice in glaciers and rock glaciers.…”