Exceedances etc

Some recent papers

Atmospheric deposition and exceedances of critical loads from 1800 2025 for the conterminous United States
CHRISTOPHER M. CLARK et al., 2018. Ecological Applications, 28(4), 2018, pp. 978–1002 “Atmospheric deposition of nitrogen (N) and sulfur (S) has increased dramatically over pre-industrial levels, with many potential impacts on terrestrial and aquatic ecosystems. Quantitative thresholds, termed “critical loads” (CLs), have been developed to estimate the deposition rate above which damage is thought to occur. However, there remains no comprehensive comparison of when, where, and over what time periods individual CLs have been exceeded…In total, it is clear that many CLs have been exceeded for decades, and are likely to remain so in the short term under current policies. Additionally, we suggest many areas for improvement to enhance our under- standing of deposition and its effects to support informed decision making.”

The response of soil solution chemistry in European forests to decreasing acid deposition
Johnson et al., 2018 in Global Change Biology “Abstract Acid deposition arising from sulphur (S) and nitrogen (N) emissions from fossil fuel combustion and agriculture has contributed to the acidification of terrestrial ecosystems in many regions globally. However, in Europe and North America, S deposition has greatly decreased in recent decades due to emissions controls. In this study, we assessed the response of soil solution chemistry in mineral horizons of European forests to these changes. Trends in pH, acid neutralizing capacity (ANC), major ions, total aluminium (Altot) and dissolved organic carbon were determined for the period 1995­2012. Plots with at least 10 years of observations from the ICP Forests monitoring network were used. Trends were assessed for the upper mineral soil (10­20 cm, 104 plots) and subsoil (40­80 cm, 162 plots). There was a large decrease in the concentration of sulphate (urn:x-wiley:13541013:media:gcb14156:gcb14156-math-0001) in soil solution; over a 10-year period (2000­2010), urn:x-wiley:13541013:media:gcb14156:gcb14156-math-0002 decreased by 52% at 10­20 cm and 40% at 40­80 cm. Nitrate was unchanged at 10­20 cm but decreased at 40­80 cm. The decrease in acid anions was accompanied by a large and significant decrease in the concentration of the nutrient base cations: calcium, magnesium and potassium (Bc = Ca2+ + Mg2+ + K+) and Altot over the entire dataset. The response of soil solution acidity was nonuniform. At 10­20 cm, ANC increased in acid-sensitive soils (base saturation ¾10%) indicating a recovery, but ANC decreased in soils with base saturation >10%. At 40­80 cm, ANC remained unchanged in acid-sensitive soils (base saturation ¾20%, urn:x-wiley:13541013:media:gcb14156:gcb14156-math-0003 ¾ 4.5) and decreased in better-buffered soils (base saturation >20%, urn:x-wiley:13541013:media:gcb14156:gcb14156-math-0004 > 4.5). In addition, the molar ratio of Bc to Altot either did not change or decreased. The results suggest a long-time lag between emission abatement and changes in soil solution acidity and underline the importance of long-term monitoring in evaluating ecosystem response to decreases in deposition.”

Long-term decline of sugar maple following forest harvest, Hubbard Brook Experimental Forest, New Hampshire
Natalie L. Cleavit et al., 2018. Canadian Journal of Forest Research “Forest harvesting can impact site quality by removing essential nutrients, exacerbating the effects of historic base cation losses associated with acid deposition…The results support previous studies indicating that regeneration by sugar maple is severely compromised on base cation depleted soils. Lower survival of seedlings for sugar maple emphasized the importance of maintaining advance regeneration to favor desired species such as sugar maple. Foresters should consider that sites with low base saturation and exchangeable Ca are likely to exhibit regeneration failure for sugar maple in the long term, even those with initial dominance by this species.”

Identifying global trends and drivers of freshwater aluminium concentrations using GloFAD (Global Freshwater Acidification Database)
Rotteveel, Lobke ; Sterling, Shannon 2020 22nd EGU General Assembly, held online 4-8 May, 2020, id.10854
Aluminum is toxic to most aquatic and terrestrial organisms. Increased Al concentrations in soils and freshwaters are a direct result of human activity, via increases in acid deposition. Elevated Al concentrations pose a wide variety of threats to ecosystems and society, from causing human neurotoxicity, reducing carbon sequestration in forests, threatening biodiversity, and increasing the cost of water treatment. Freshwater aluminium concentrations increased across Europe and North America between the 1960s and 1990s, predominantly due to ecosystem acidification. Following acidic deposition reduction legislation enacted in the 1990s, the problems of acidification and increased freshwater aluminium concentrations were considered solved. However, recently and unexpectedly, Sterling et al. identified aluminum concentrations to be increasing across North America and Scandinavia. Sterling et al. proposed a conceptual model suggesting these widespread increases in freshwater aluminium concentrations resulted from a hysteresis of base cation and dissolved organic carbon (DOC) response to decreasing acidic deposition, where base cation increase is slow compared to that of DOC, resulting in elevated freshwater aluminium concentrations. This process can be exacerbated by further increases in DOC due to increasing global surface temperatures…The widespread decreasing base cation trends and strong correlation between decreasing base cation and increasing aluminium trends indicates that increasing aluminium concentrations may become more widespread, posing a threat to aquatic and terrestrial organisms, potentially including humans, reducing carbon sequestration in forests, threatening biodiversity, and increasing water treatment costs.

Widespread diminishing anthropogenic effects on calcium in freshwaters
Gesa A. Weyhenmeyer et al., 2019. Scientific Reports volume 9.” Our global analysis of Ca and carbonate alkalinity concentrations in lakes and running waters shows that Ca and carbonate alkalinity generally strongly co-vary with highest concentrations in waters with a pH between 8.0 and 9.0, and lowest concentrations in acidic water bodies. Under the influence of anthropogenic acid deposition, however, Ca concentrations became disproportionate and unnaturally high relative to alkalinity, often accompanied by higher sulfate concentrations. As acid deposition has been increasingly mitigated in North America and Europe and freshwaters recover from anthropogenic acidification, Ca concentrations rapidly decline towards a state where they again become balanced with carbonate alkalinity. Ca concentrations may even decline below pre-acidification concentrations due to a depletion of base cations stores in the catchment soils of acid-sensitive regions, or because of other stressors including timber harvesting. Since Ca concentrations are generally low in many freshwaters, in some regions critically low, further Ca concentration declines as freshwaters fully recover from acidification will likely have widespread consequences for biota and ecosystem processes.