Elsevier

Science of The Total Environment

Volume 532, 1 November 2015, Pages 595-604
Science of The Total Environment

Salt in the wound: The interfering effect of road salt on acidified forest catchments

https://doi.org/10.1016/j.scitotenv.2015.06.034Get rights and content

Highlights

  • Road salt significantly affects acidity regime of road adjacent forest catchments.

  • Peak water concentrations of Na and Cl occurred during snowmelt periods (April/May).

  • Water concentrations of Na and Cl remained constantly high throughout the whole year.

  • Road salt seems to increase the leaching of soil cation nutrients (K, Mg and Ca).

  • Effects were detectible up to a distance of 830 m from the road.

Abstract

Atmospheric acidic depositions have strongly altered the functioning and biodiversity of Central European forest ecosystems. Most impacts occurred until the end of the 20th century but the situation substantially improved thereafter caused by legal regulations in the late 1980's to reduce acidifying atmospheric pollution. Since then slow recovery from acidification has been observed in forested catchments and adjacent waters. However, trends of recovery are inconsistent and underlying mechanisms diminishing recovery are still poorly understood.

We propose that the input of road salt can significantly affect acidity regime and acidification recovery of forest ecosystems.

By comparing the discharge hydro-chemistry and plant community composition of springs fed by forested catchments with and without high levels of salt input over two decades we observed a significant suppression of recovery and elevated levels of nutrient leaching (K+, Ca2 + and Mg2 +) in highly salt contaminated catchments.

We show that the pollution of near-surface groundwater (interflow) by road salt application can have lasting effects on ecosystem processes over distances of several hundred metres apart from the salt emitting road.

Introduction

During the second half of the 20th century, the emission of sulphuric and nitrogenous gases which peaked in the late 1980's resulted in large-scale acidification of forest soil and groundwater. Even remote forests and aquatic ecosystems were harmed in large parts of Europe and North America (Almer et al., 1974, Wright, 1983, Reuss et al., 1987, Matzner and Murach, 1995). The large scale deposition of acidifying pollutants strongly changed biogeochemical processes and equilibria in terrestrial and aquatic ecosystems (Rice and Herman, 2012). Sulphur and nitrogen oxides (SO42  and NO3) originating from airborne pollution caused the displacement of acidifying and toxic elements (H+ and Al3 +) and also the leaching of important nutrients (K+, Ca2 + and Mg2 +) form soils with detrimental ecological effects on forest and the subsequent freshwater ecosystems. Since then atmospheric deposition of acidifying pollutants decreased significantly as a consequence of the implementation of technology that is more efficient and emits less pollutants (Nyiri et al., 2009, Pihl Karlsson et al., 2011). However, forest and adjacent freshwater ecosystems are slowly recovering from this long-lasting impact (Hruška et al., 2002, Holmberg et al., 2013). Furthermore, patterns of recovery differ remarkably between sites (Pihl Karlsson et al., 2011) probably related to specific environmental settings.

Recovery processes were shown to depend on various environmental factors like the distance to the former source of acidic emission (Pihl Karlsson et al., 2011), which is probably related to critical loads, as well as on climate, soil, bedrock composition, and other environmental factors unrelated to anthropogenic acidification (Wright and Jenkins, 2001, Skjelkvåle et al., 2003). However, the full extent of environmental drivers which enhance or delay recovery processes is not sufficiently understood and major pieces are missing to resolve the puzzle of ecosystem recovery after acidification. Thus, human induced acidification is still a serious footprint of environmental pollution (Larssen and Holme, 2006).

Besides other environmental factors which are able to interfere with recovery, numerous studies from Scandinavia hint at a significant influence of sea salt deposition causing short-term acidification peaks that hinder long-term recovery of forested ecosystems from anthropogenic acidification (Hindar et al., 1995, Wright and Jenkins, 2001, Larssen and Holme, 2006, Skjelkvåle et al., 2007, Laudon, 2008, Akselsson et al., 2013). This so called ‘sea salt effect’ (Wiklander, 1975) is caused by cation exchange processes induced by an excessive input of sodium (Na+). While sodium is retained in the soil, H+ ions are exchanged which inevitably supports acidification of soil- and surface-waters.

Since the 1960's, road salt is the main de-icing agent in Europe and North America during winter (Green et al., 2007). Usually the most cost-efficient and widely used de-icing agent is sodium chloride (NaCl, c.f. Bayerisches Landesamt für Wasserwirtschaft (BayLfW), 1999, Green and Cresser, 2008). This widespread form of road salt is similar to sea salt in its chemical structure and ecological behaviour. The negative environmental effects of road salt application were perceivable since the very beginning of its application (Judd, 1969). However, most studies focused on short-term effects on particular organisms or habitats directly adjacent to the salted roads. Although there are some studies which focused on long-term trends of road salt in streams and groundwater not directly adjacent but still affected by road salt application (e.g., Godwin et al., 2003, Findlay and Kelly, 2011), there is a current lack of knowledge about its effects on distant forest areas that come in contact with salty groundwater. However, the topic of large-scale ecological effects of road salt application is of growing importance as road salt application continuously increased during the last decades.

The high density of roads and increasing traffic has caused high levels of road salt application during the last decades. To keep traffic flowing, between 0.61 and 3.5 million tons of road salt (1.6 million tons on average) was applied annually during winter on Germany's highways between 1992 and 2005 depending on weather conditions (Statista, 2015). Astebol et al. (1996) report 75% to 90% of applied sodium chloride to enter the road-adjacent environment by deposition and/or melting of contaminated snow. Several studies report a significant input of road salt in distant areas especially during spring-time via surface and sub-surface run-off or via groundwater (Bayerisches Landesamt für Wasserwirtschaft (BayLfW), 1999, Blasius and Merritt, 2002 and references therein). However, little knowledge exists about the effects of road salt application on the recovery rates of acidified forest ecosystems, although the ecological effects caused by road salt (NaCl) have to be assumed to be similar to the effects reported for sea salt deposition.

The ecosystems investigated in this study were heavily impacted by the deposition of acidifying pollutants (SO42  and SO2) during the 1970s to early 1990s which resulted in strong acidification of the forest and subsequent springs and riverine ecosystems in this region (Matzner and Murach, 1995). Since then the ecological effects of former acidification are slowly decreasing but are still detectable in the chemical properties and plant community composition of the spring/forest ecosystems under study (Alewell et al., 2000, Schweiger and Beierkuhnlein, 2014). The investigated springs are mainly fed by surface-near groundwater originating from predominantly forested catchments. By percolating through the soil layers of the catchments the water takes up soluble compounds originating from the whole catchment. At the spring site the upwelling groundwater with its solutes then reflects the geochemical traits of the catchments. Plants which inhabit these springs strongly rely on the prevalent environmental conditions (temperature, hydrochemistry) which are known to be very stable under pristine conditions (Strohbach et al., 2009, Audorff et al., 2011, Schweiger and Beierkuhnlein, 2014). Such specialized and highly adapted species react very sensitively to hydro-chemical changes of the spring water and, thus, shifts in biogeochemical processes in their catchments. In consequence, monitoring of spring hydro-chemistry and plant community structure allows for a spatially as well as temporally integrative assessment of biogeochemical processes of the forested catchments. When repeated multiple times in small catchments, long-term biogeochemical processes can even be assessed at the landscape scale in the absence of other acidifying processes such as forest clearing.

In this study we used springs as a (bio-) monitoring tool to assess how road salt application contributes to the behaviour of acidified forested catchments. In addition to repeated measurements of abiotic conditions we used plant indicator values of Ellenberg et al. (2001) as a bio-monitoring tool to quantify environmental conditions indicated by the species' occurrence. Based a multitude of field studies for all Central European plant species (including ferns, horsetails, mosses and liverworts) indicator values for their ecological behaviour regarding major site conditions are defined. One category of indicator values concerns species response to acidity regimes (R-value), which are important for nutrient solubility and thus availability but also for the solubility of toxic compounds. The availability of such values for all plant species is an extraordinary advantage in European vegetation science. This explains why this method is actually widely applied in environmental impact studies (e.g., Koch and Jurasinski, 2015, Kuechler et al., 2015).

We monitored water chemistry and plant community composition of 52 springs in the lower mountain ranges of Central Germany over a period of 25 years (1989 to 2014). All investigated springs are characterised by a constantly water saturated, thus, swampy (seepage) area in which the investigated plant communities have established. Resulting from the spatially diffuse emergence of groundwater, the investigated spring sites have a spatial extent of a few to several hundred square metres (Schweiger and Beierkuhnlein, 2014). Whereas most of these springs are more than 1000 m apart from main roads or in higher elevation, four of these springs are located between 150 and 830 m downslope of a road where salt is frequently applied during winter-time. In consequence, the discharge of these springs exhibits very high concentrations of sodium chloride in comparison with the control group of springs (n = 48). This situation provided an opportunity to study the effects of road salt on forested catchments.

Similar to the historic acidification caused by airborne pollutants, the major compound of de-icing agents, sodium (Na+) is not only reported to cause the release of acidifying elements (H+) but also to increase the leaching of important nutrients (K+, Ca2 + and Mg2 +). Continuous road salt application might, thus, continue the biogeochemical and subsequent ecological effects of historic acidification and, therefore impair the long-term recovery of forested and subsequent freshwater ecosystems. Though the term ‘recovery’ is ambiguous and not very precise we are using it in this study as follows: recovery does not mean total or complete restoration of a former state. First, such states have also been under transition, but very likely in different time scales and speed. Defining a precise reference state is difficult because the chosen reference time is arbitrary. Second, today's conditions will certainly differ not only according to one target variable (e.g., acidity) but also in other respect (e.g., climate). And third, we are not interested in a certain status but in processes that are going on. This is why we understand ‘recovery’ as a term that indicates the direction of detected trends, away from unfavourable states such as high acidity, high loads of heavy metals, or imbalanced nutrient availability. Like this it is feasible to talk about ‘recovery’.

Depending on the distance to the sources of application (roads) as well as on the predominant vegetation and land use, we assume the catchments and their springs to be differently affected by road salt contamination and the concomitant biogeochemical processes. We hypothesise that (1) similar to the ‘sea salt effect’ (Wiklander, 1975) road salt application significantly affects the acidity regime of forested catchments and impairs long-term recovery from former anthropogenic acidification. We furthermore hypothesise that (2) the strength of this ‘road salt effect’ decreases with increasing distance to the road.

Section snippets

Materials and methods

The monitored spring sites are located in the lower mountain ranges of Central Germany (Frankenwald, 50° 11′ to 50° 31′ N, 11° 15′ to 11° 40′ E). The sites were selected by considering comparable pedo- and geogenic conditions, forest composition and morphological site characteristics (see Table 1).

The four salt impacted spring sites are located downslope of a main road which runs along a mountain ridge (see Fig. 1c). As the application of road salt as well as snow clearing and traffic loads

Effects of road salt on the water chemistry and plant community composition of forest springs

Except for the spring closest to the road (spring 1), a clear spatial pattern was observable for the hydro-chemistry of the investigated springs. Concentrations of elements related to road salt input (Na+ and Cl) were exceptionally high for road adjacent springs but decreased with increasing distance from the salt emitting road. Furthermore, concentrations of important nutrients (K+, Mg2 + and Ca2 +) in the discharge of the springs were significantly elevated in road adjacent springs and

Discussion

Various environmental factors are known to affect long-term recovery of anthropogenically acidified forest catchments including the ‘sea salt effect’ (Wiklander, 1975), with episodic sea salt input significantly hindering long-term recovery of coastal forest ecosystems (Larssen and Holme, 2006, Skjelkvåle et al., 2007, Laudon, 2008, Akselsson et al., 2013). By increasing the net charge in the soil solution, episodic input of sea salt is described by experimental as well as field studies to

Conclusions

By conducting seasonal and also long-term monitoring of water chemistry and plant community composition of springs over two decades, we show that high loads of road salt input significantly suppress the long-term recovery of forested catchments from previous acidification. As increasing evidence stresses the widespread and persistent adverse effects of current de-icing practice, the call for alternatives becomes ever louder. While most alternative strategies including novel, more eco-friendly

Acknowledgements

We want to thank the Analytical Chemistry of the Bayreuth Center of Ecology and Environmental Research (BayCEER) for the laboratory work. This project is co-financed by the European fund for regional development of the European Union and the Bavarian State Ministry of the Environment and Consumer Protection (TEU01EU-63000).

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