Long-term sulphate and inorganic nitrogen mass balance budgets in European ICP Integrated Monitoring catchments (1990–2012)

Highlights

• Trends in runoff fluxes of SO₄ have increasingly responded to the decrease in S emissions.

• Forest catchments shifted from retention towards a net release of SO₄ in the late 1990s.

• Trends in inorganic N output fluxes and retention rates are still highly variable.

• Inorganic N deposition is effectively retained in undisturbed forest catchments.

• Disturbances in forest ecosystems can effectively disrupt the nitrogen cycle.

Abstract

Empirical evidence based on integrated environmental monitoring including physical, chemical and biological variables is essential for evaluating the ecosystem benefits of costly emission reduction policies. The international multidisciplinary ICP IM (International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems) programme studies the integrated effects of air pollution and climate change on ecosystems in unmanaged and calibrated forested catchments. We calculated site-specific annual input-output budgets for sulphate (SO₄) and total inorganic nitrogen (TINNO₃-N + NH₄-N) for 17 European ICP IM sites in 1990–2012. Temporal trends for input (deposition) and output (runoff water) fluxes and the net retention/net release of SO₄ and TIN were also analysed. Large differences in the input and output fluxes of SO₄ and TIN reflect important gradients of air pollution effects in Europe, with the highest deposition and runoff water fluxes at IM sites located in southern Scandinavia and in parts of Central and Eastern Europe and the lowest fluxes at more remote sites in northern European regions. A significant decrease in the total (wet + dry) deposition of non-marine SO₄ and bulk deposition of TIN was found at 90% and 65% of the sites, respectively. Output fluxes of non-marine SO₄ in runoff decreased significantly at 65% of the sites, indicating positive effects of the international emission abatement actions in Europe during the last 20 years. Catchments retained SO₄ in the early and mid-1990s, but this shifted towards a net release in the late 1990s, which may be due to the mobilization of legacy S pools accumulated during times of high atmospheric SO₄ deposition. Despite decreased deposition, TIN output fluxes and retention rates showed a mixed response with both decreasing (9 sites) and increasing (8 sites) trend slopes, and trends were rarely significant. In general, TIN was strongly retained in the catchments not affected by natural disturbances. The long-term annual variation in net releases for SO₄ was explained by variations in runoff and SO₄ concentrations in deposition, while a variation in TIN concentrations in runoff was mostly associated with a variation of the TIN retention rate in catchments. The net release of SO₄ from forest soils may delay the recovery from acidification for surface waters and the continued enrichment of nitrogen in catchment soils poses a threat to terrestrial biodiversity and may ultimately lead to a higher TIN runoff through N-saturation. Continued monitoring and further evaluations of mass balance budgets are thus needed.

Introduction

Due to the extensive emissions of sulphur dioxide (SO₂) and nitrogen compounds (NO_x, NH_x), acidification of sensitive ecosystems due to deposited acidifying compounds has received considerable attention as an international environmental problem in Europe and North America (e.g. Leivestad and Muniz, 1976, Rodhe et al., 1995, Schindler, 1988, Ulrich et al., 1980, Wright et al., 2005). A sustained accumulation of deposited inorganic N also poses a threat to ecosystems through nutrient enrichment and nutrient imbalance (Bergström et al., 2005, Bergström and Jansson, 2006, Lepori and Keck, 2012, Stevens et al., 2011). It also poses a threat to biodiversity, as a consequence of the eutrophication of sensitive ecosystems, shown by the results of the international networks of forested sites from both ICP IM (International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems) and ICP Forests (International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests) sites under UNECE CLRTAP (Dirnböck et al., 2014) and other studies (Bleeker et al., 2011, Bobbink et al., 2010, MEA, 2005, Sala et al., 2000). Deposition-induced N enrichment in forest soil may also deteriorate tree mineral nutrition (Jonard et al., 2014). In addition to airborne deposition, there is growing evidence that climate change causes impacts on the chemistry and ecology of freshwater ecosystems (e.g. Adrian et al., 2009, Shimoda et al., 2011). Many of the retention and release processes for sulphate and inorganic nitrogen are sensitive to climatic variables, and would, therefore, be affected by climate change (e.g. Dirnböck et al., 2016, Mitchell et al., 2013, Moore et al., 2010, Templer et al., 2012, Wright and Jenkins, 2001).

Observed detrimental effects of transboundary air pollution led to international negotiations on emission reductions under the Convention on Long-Range Transboundary Air Pollution (CLRTAP), signed in 1979 under the United Nations Economic Commission for Europe (UNECE, 1996). Since the 1980s, environmental regulations have led to declining emissions of air pollutants in Europe and North America. In Europe, overall emissions of SO₂ declined by 71% between the years 1990 and 2011 and those of nitrogen compounds by 42% (NO_x) and 31% (NH₃), resulting in a declining deposition of air pollutants (Schulz et al., 2013). As indicated, the emission reduction measures have been less successful for nitrogen, compared to sulphur, and nitrogen emissions have stabilized (NO_x) or increased slightly (NH₃) during the 2000s (Amann et al., 2013), and the decrease in inorganic nitrogen deposition has not been observed as strongly as for SO₄ (e.g. Waldner et al., 2014).

Following the drastic decrease in SO₄ deposition and SO₄ concentrations in surface waters, widespread recovery from the acidification of sensitive freshwater ecosystems has taken place in Europe and North America (Forsius et al., 2001, Garmo et al., 2014, Helliwell et al., 2014, Skjelkvåle et al., 2005, Stoddard et al., 1999, Wright et al., 2005). Accompanying a decreasing SO₄ deposition, forested catchments in Europe and North America have also shown a net release (output > input) of SO₄ from internal soil S sources. This process may be considered as a time-lagged recovery of terrestrial systems fuelled by the mobilization of legacy S pools accumulated during times of a high atmospheric SO₄ deposition (Augustaitis et al., 2010b, De Vries et al., 2003, De Vries et al., 2001, Forsius et al., 2005, Löfgren et al., 2001, Mitchell et al., 2013, Mitchell et al., 2011, Prechtel et al., 2001, Watmough et al., 2005).

In the absence of significant N deposition, nitrogen is generally the growth-limiting nutrient in many forest ecosystems, so that the biological uptake of available nitrogen compounds through plants and soil microbes is efficient (Tamm, 1991). Undisturbed forest ecosystems receiving low to moderate inorganic N deposition retain most of the nitrogen in the catchments; typically < 10% is leached in runoff, mostly as NO₃ (e.g. Hedin et al., 1995, Wright et al., 2005). However, ecosystem nitrogen dynamics are inherently complex with biological processes, long-term response lags and high spatial and seasonal fluctuations (Aber et al., 2003, Augustaitis et al., 2010a, Bernal et al., 2012, Butterbach-Bahl et al., 2011, Lovett and Goodale, 2011). Sustained high inorganic N deposition may cause N-saturation of terrestrial ecosystems (N input and available inorganic N exceeds biotic demand) and excess NO₃ leaching to surface waters (Aber et al., 1989, Curtis et al., 2005, Dise and Wright, 1995, MacDonald et al., 2002, Oulehle et al., 2012) or increased losses of greenhouse gases (Kreutzer et al., 2009). However, excess NO₃ leaching through N-saturation may not be indicative at all sites (Lovett and Goodale, 2011). Although an elevated NO₃ loss from catchments can be associated with a high N deposition, e.g. as shown at IM sites by Holmberg et al. (2013), and has reached elevated levels in forested areas which are prone to chronic N deposition (Corre et al., 2007, Kiese et al., 2011, Thimonier et al., 2010), time trends in NO₃ concentrations in surface waters do not show a regionally consistent pattern (Garmo et al., 2014, Helliwell et al., 2014, Lovett and Goodale, 2011, Watmough et al., 2005, Wright et al., 2001). In addition to concerns related to eutrophication, the relative importance of oxidized and reduced N-compounds as acidifying agents is increasing, as nitrogen emissions have decreased much less than sulphur emissions (Helliwell et al., 2014).

In order to improve our ability to anticipate potential trends and support mass balance models (e.g. Hettelingh et al., 2007, Sverdrup and De Vries, 1994) for the future surface water quality and SO₄ and inorganic N status of catchments in response to emission reductions, insights from long-term input-output budgets are useful. These budgets provide integrated, quantified responses to changes in deposition and allow the testing of hypotheses e.g. for changes in budget discrepancies of S and N in catchments with regard to other drivers, in particular climate. Previous mass balance budgets for SO₄ and inorganic N in ICP IM catchments in Europe were calculated as the average of three years (mainly 1997–1999) (Forsius et al., 2005). Thus, an extensive assessment of long-term changes in input-output budgets for S and N in IM catchments is lacking and, to our knowledge, little information on temporal changes in S and N mass balance budgets during the past 20 years in other forested catchments in Europe is available. In this paper, we calculate site-specific annual mass balance (input-output) budgets for sulphate (SO₄) and total inorganic nitrogen (TINNO₃-N + NH₄-N) and quantify the annual net retention or net release rates of SO₄ and TIN in the catchment, using available long-term data (longest time series in 1990–2012) collected in the international ICP IM network of forested research catchments in Europe. The main aims of the present study are: (i) to evaluate if deposition (input) and runoff (output) fluxes, and the retention and release rates of SO₄ and TIN have changed in the course of successful emission reductions in different regions in Europe; (ii) to assess if rates of changes in retention and release can be related to deposition levels and/or hydrology and/or catchment disturbance; and (iii) to assess the changes in fluxes and retention/release in the context of emission and deposition reduction responses.