Temporal trends in DOC concentration and flux were investigated at two geochemically distinct forested catchments in western Czech Republic. Mean discharge-weighted DOC concentrations averaged 18.8 ...mg L(-1) at the acidic Lysina catchment, and 20.2 mg L(-1) at base-rich and well-buffered Pluhuv Bor. Between 1993 and 2007 DOC in streamwater increased significantly in both catchments: the mean annual increase was 0.42 mg L(-1) yr(-1) (p < 0.001) at Lysina and 0.43 mg L(-1) yr(-1) (p < 0.001) at Pluhuv Bor, resulting in cumulative increases of 64 and 65%, respectively. These long-term increases in streamwater DOC were correlated with only modest increases in stream pH in both catchments, but large declines in ionic strength (IS), that resulted from declining atmospheric deposition. Neither catchment has undergone changes in soil-water pH, yet DOC concentrations tripled in the soil-water of both catchments. We conclude that changes in ionic strength of soil-water and streamwater, rather than acidity, are the primary drivers of changes in streamwater DOC in this region. Temperature, precipitation and discharge show no statistically significant trends during the study period, suggesting that climate change has played no role in the changes in DOC that we have observed.
The forests of central Europe have undergone remarkable transitions in the past 40 years as air quality has improved dramatically. Retrospective analysis of Norway spruce (Picea abies) tree rings in ...the Czech Republic shows that air pollution (e.g. SO2 concentrations, high acidic deposition to the forest canopy) plays a dominant role in driving forest health. Extensive soil acidification occurred in the highly polluted "Black Triangle" in Central Europe, and upper mineral soils are still acidified. In contrast, acidic atmospheric deposition declined by 80% and atmospheric SO2 concentration by 90% between the late 1980s and 2010s. In this study we oserved that annual tree ring width (TRW) declined in the 1970s and subsequently recovered in the 1990s, tracking SO2 concentrations closely. Furthermore, recovery of TRW was similar in unlimed and limed stands. Despite large increases in soil base saturation, as well as soil pH, as a result of repeated liming starting in 1981, TRW growth was similar in limed and unlimed plots. TRW recovery was interrupted in 1996 when highly acidic rime (originating from more pronounced decline of alkaline dust than SO2 from local power plants) injured the spruce canopy, but recovered soon to the pre-episode growth. Across the long-term site history, changes in soil chemistry (pH, base saturation, Bc/Al soil solution ratio) cannot explain observed changes in TRW at the two study sites where we tracked soil chemistry. Instead, statistically significant recovery in TRW is linked to the trajectory of annual SO2 concentrations or sulfur deposition at all three stands.
Temperate forest ecosystems store most of the organic carbon in soils (SOC), and changes in the soil carbon stock due to climate change or land management can potentially have a large influence on ...carbon balance. The most important factors controlling the SOC pool on a global scale are generally agreed upon; however, estimations of SOC pools differ significantly among studies at regional and local scales due to different sampling protocols and local scale variability. This study evaluates the SOC pool in the forest floor and mineral soil sampled down to a depth of 80 cm in 14 forested catchments with variable environmental conditions and soil acidification and eutrophication legacies, and determines the best explanatory variables of the SOC pool. The average SOC pool of 34 t ha−1 measured in the forest floor (O horizon) was best explained by measures of historical sulphur (S) deposition (i.e., soil acidification legacy) and forest type (conifer vs. broadleaf forest). An average total SOC pool of 132 t ha−1, combining both the carbon pool in the mineral soil down to 80 cm and the carbon pool in forest floor, was best explained solely by elevation, which reflects temperature and precipitation gradients. However, when considering the coupled SOC pool in the forest floor and upper half of the sampled mineral soil (down to 40 cm), natural environmental factors were outweighed by anthropogenic ones (soil acidification legacy and forest type). This has important implications for understanding potential SOC pool changes under ongoing global climate change, especially in regions currently or historically affected by soil acidification caused by acid deposition. The acidification effect on the SOC accumulation and subsequent soil recovery after acidification retreat might affect carbon balance.
Highlights
The SOC pool is dependent on soil acidification legacy, forest type and climatic gradient.
Anthropogenic factors outweigh the natural ones if shallow sampling is carried out.
Shallow sampling commonly carried out in forest soils underestimates the SOC pool.
Soil acidification caused SOC accumulation and subsequent soil recovery might lead to carbon loss.
Abstract
Surface water browning, the result of increasing concentrations of dissolved organic matter (DOM), has been widespread in northern ecosystems in recent decades. Here, we assess a database of ...426 undisturbed headwater lakes and streams in Europe and North America for evidence of trends in DOM between 1990 and 2016. We describe contrasting changes in DOM trends in Europe (decelerating) and North America (accelerating), which are consistent with organic matter solubility responses to declines in sulfate deposition. While earlier trends (1990–2004) were almost entirely related to changes in atmospheric chemistry, climatic and chemical drivers were equally important in explaining recent DOM trends (2002–2016). We estimate that riverine DOM export from northern ecosystems increased by 27% during the study period. Increased summer precipitation strengthened upward dissolved organic carbon trends while warming apparently damped browning. Our results suggest strong but changing influences of air quality and climate on the terrestrial carbon cycle, and on the magnitude of carbon export from land to water.
Acidification of lakes and rivers is still an environmental concern despite reduced emissions of acidifying compounds. We analysed trends in surface water chemistry of 173 acid-sensitive sites from ...12 regions in Europe and North America. In 11 of 12 regions, non-marine sulphate (SO₄*) declined significantly between 1990 and 2008 (−15 to −59 %). In contrast, regional and temporal trends in nitrate were smaller and less uniform. In 11 of 12 regions, chemical recovery was demonstrated in the form of positive trends in pH and/or alkalinity and/or acid neutralising capacity (ANC). The positive trends in these indicators of chemical recovery were regionally and temporally less distinct than the decline in SO₄* and tended to flatten after 1999. From an ecological perspective, the chemical quality of surface waters in acid-sensitive areas in these regions has clearly improved as a consequence of emission abatement strategies, paving the way for some biological recovery.
•Higher runoff of Mg, Ca and Sr than atmospheric input suggests geogenic origin.•Over 90% of geogenic Mg is supplied by biotite, most Ca and Sr comes from plagioclase.•δ26Mg, δ44Ca and 87Sr/86Sr ...ratios of these main source minerals dominate also runoff.•Whole rock is not a good proxy for Mg, Ca and Sr isotopes released to the ecosystem.•Analysis of 1–2 minerals suffices to represent bedrock in runoff origin studies.
Knowledge of the origin of magnesium (Mg) and calcium (Ca) in soil solutions and catchment runoff helps to predict forest ecosystems’ vulnerability to deficiencies in essential nutrients in an era of climate change, environmental pollution and bark-beetle calamities. Here we discuss isotope aspects of Mg, Ca and strontium (Sr) cycling in a spruce-forested headwater catchment in a relatively unpolluted part of Central Europe. We investigated to what extent Mg and Ca isotope signatures of runoff reflect the isotope compositions of specific Mg- and Ca-rich minerals that easily dissolve during the weathering of paragneiss, and compared the isotope variability of Mg and Ca in fresh bedrock minerals, soils and other ecosystem reservoirs. We also compared conclusions from Mg and Ca isotope systematics with inferences from catchment input–output mass budgets. Long-term input–output monitoring in the studied catchment situated near the Czech–German border (Central Europe) revealed 3.5–7 times higher outputs of Mg, Ca, and Sr via surface runoff relative to their present-day atmospheric inputs. It follows that hydrological exports of recent atmospheric Mg, Ca and Sr are minor. Release of geogenic base cations into the runoff results from the interplay between mineral abundances, concentrations of the studied elements in the minerals, and their dissolution rates. Chemical depletion fractions for the studied elements from bedrock to the soil were 50–70 %, and the losses of dominant soluble minerals in the soil were 30–80 %. Exports of residual Mg, Ca and Sr following partial incorporation of these elements into secondary phyllosilicates are probably low because newly-formed clay minerals are not abundant in the soil. Residual Ca following preferential incorporation of isotopically light Ca into growing tree biomass may contribute to the isotopically heavy runoff Ca. Isotope ratios of base cations were obtained for six minerals (plagioclase, orthoclase, biotite, muscovite, apatite, and ilmenite). Mineral fractions differ greatly in δ26Mg and δ44Ca values and 87Sr/86Sr ratios. 80–97 % of each of the three studied base cations are present in the bedrock in a single relatively easily dissolvable mineral: Mg in biotite, and Ca and Sr in plagioclase. The isotope composition of Mg in biotite was similar to the isotope composition of Mg in runoff. The isotope compositions of Ca and Sr in plagioclase were also similar to Ca and Sr isotope compositions in runoff. Thus, the dominant geogenic source of each of the studied elements (Mg, Ca and Sr) in the investigated paragneiss catchment can be represented by one relatively soluble mineral.
In 1994, a network of small catchments (GEOMON) was established in the Czech Republic to determine input–output element fluxes in semi‐natural forest ecosystems recovering from anthropogenic ...acidification. The network consists from 16 catchments and the primary observations of elements fluxes were complemented by monitoring of biomass stock, element pools in soil and vegetation, and the main water balance components. Over last three decades, reductions of SO2, NOx and NH3 emissions were followed by sulphur (S) and nitrogen (N) deposition reductions of 75% and 30%, respectively. Steeper declines of strong acid anion concentrations compared to cations (Ca, Mg, Na, K, NH4) in precipitation resulted in precipitation pH increase from 4.5 to 5.2 in bulk precipitation and from 4.0 to 5.1 in spruce throughfall. Stream chemistry responded to changes in deposition: S leaching declined. However at majority of catchments soils acted as a net source of S to runoff, delaying recovery. Stream pH increased at acidic streams (pH < 6) and aluminium concentration decreased. Stream nitrate (NO3) concentration declined by 60%, considerably more than N deposition. Stream NO3 concentration was tightly positively related to stream total dissolved nitrogen to total phosphorus (P) ratio, suggesting the role of P availability on N retention. Trends in dissolved organic carbon fluxes responded to both acidification recovery and to runoff temporal variation. An exceptional drought occurred between 2014 and 2019. Over this recent period, streamflow decreased by ≈ 40% on average compared to 1990s, due to the increases of soil evaporation and vegetation transpiration by ≈ 30% and declines in precipitation by ≈ 15% on average across the elevational gradient. Sharp decreases of stream runoff at catchments <650 m a.s.l. corresponded to areas of recent forest decline caused by bark beetle infestation on drought stressed spruce forests. Understanding of the interactions among legacies of acidification and eutrophication, drought effects on the water cycle and forest disturbance dynamics is requisite for effective management of forested ecosystems under anthropogenic influence.
Rapid recovery of precipitation chemistry was followed by steady recovery of acid‐base status at acidic streams (pH < 6) since 1994. Nitrate losses decreased proportionally more than measured declines in N deposition. Drought episodes altered solute fluxes and dissolved organic carbon export. Increased evapotranspiration losses caused dramatic declines in stream runoff at catchments under 650 m a.s.l. Based on documented drought and bark beetle induced forest declines, further alteration of forest biogeochemistry can be expected.
Norway spruce (Picea abies (L.) Karst.) stands in certain areas of Central Europe have experienced substantial dieback since the 1970s. Understanding the reasons for this decline and reexamining the ...response of forests to acid deposition reduction remains challenging because of a lack of long and well-replicated tree-ring width chronologies. Here, spruce from a subalpine area heavily affected by acid deposition (from both sulfur and nitrogen compounds) is evaluated. Tree-ring width measurements from 98 trees between 1000 and 1350m above sea level (a.s.l.) reflected significant May–July temperature signals. Since the 1970s, acid deposition has reduced the growth–climate relationship. Efficient pollution control together with a warmer but not drier climate most likely caused the increased growth of spruce stands in this region, the so-called “Black Triangle,” in the 1990s.
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•Long-term growth changes of Norway spruce are evaluated for the “Black Triangle.”•The ring width variations of Norway spruce reflect May–July temperatures.•Acid deposition reduced the growth–temperature relationships of Norway spruce.•This study suggests a complex interplay of multiple factors on forest decline.•Our results prove a recovery of forest growth in the 1990s.
Increased reactive nitrogen (N) loadings to terrestrial ecosystems are believed to have positive effects on ecosystem carbon (C) sequestration. Global “hot spots” of N deposition are often associated ...with currently or formerly high deposition of sulphur (S); C fluxes in these regions might therefore not be responding solely to N loading, and could be undergoing transient change as S inputs change. In a four-year, two-forest stand (mature Norway spruce and European beech) replicated field experiment involving acidity manipulation (sulphuric acid addition), N addition (NH4NO3) and combined treatments, we tested the extent to which altered soil solution acidity or/and soil N availability affected the concentration of soil dissolved organic carbon (DOC), soil respiration (Rs), microbial community characteristics (respiration, biomass, fungi and bacteria abundances) and enzyme activity. We demonstrated a large and consistent suppression of soil water DOC concentration driven by chemical changes associated with increased hydrogen ion concentrations under acid treatments, independent of forest type. Soil respiration was suppressed by sulphuric acid addition in the spruce forest, accompanied by reduced microbial biomass, increased fungal:bacterial ratios and increased C to N enzyme ratios. We did not observe equivalent effects of sulphuric acid treatments on Rs in the beech forest, where microbial activity appeared to be more tightly linked to N acquisition. The only changes in C cycling following N addition were increased C to N enzyme ratios, with no impact on C fluxes (either Rs or DOC). We conclude that C accumulation previously attributed solely to N deposition could be partly attributable to their simultaneous acidification.
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•Acidification alters soil carbon fluxes in spruce forest.•No effect of nitrogen on soil dissolved organic carbon concentration.•Acidification and nitrogen additions alter enzyme stoichiometry.•Acidification may stimulate soil carbon accumulation.
Acidification alters soil carbon fluxes in spruce and beech forests due to the altered soil pH. Limited effect of N addition on soil C cycling in both forest types.
Assessment of temporal trends and rates of change in hydrochemical parameters and forest cover has been conducted to elucidate key drivers of surface water acidification in glacial lakes in the Czech ...Republic. Since 1984, the key driver in acidification reversal was sulphate (SO₄) concentration (median decrease of −3.58 μeq L⁻¹ yr⁻¹) which fell in line with reductions in sulphur (S) deposition. Reduction of nitrogen (N) deposition was followed by proportional reduction in nitrate (NO₃) leaching although decline in NO₃ concentrations was more pronounced at two sites, the Čertovo Lake (CT) and Prášilské Lake (PR) until 2006; only Žďárské pond showed effective catchment N immobilization. Coherent decline of chloride concentration was detected across all sites. The decrease of strong mineral acids was partly compensated by decrease of inorganic aluminium (Alᵢₙ), especially at sites most acidified in the beginning of observations (ANC₁₉₈₄–₁₉₈₆ between −160 and −90 μeq L⁻¹ at CT, Černé Lake—CN and Plešné lake—PL) and by reductions of base cations and increases of pH. All lakes (CN, CT, PL, PR and LK) moved to the ANC range between −29 and 30 μeq L⁻¹ (2010–2012) where sensitivity of pH to further reductions in acid anions may be expected. Concurrently, charge of weak organic acids (OAs) increased and partly balanced the strong mineral acid decrease as a consequence of (i) significant DOC (dissolved organic carbon) increase (median change of 0.13 mgC L⁻¹ yr⁻¹ since 1993) and (ii) deprotonation of weak OAs caused by pH rise. Since 2000s, bark beetle induced forest decline accelerated NO₃ leaching at most of the catchments (by 200 % at LK, PL and PR). However, elevated N leaching was effectively neutralized by base cations (K, Mg, Ca) originating from decaying fresh litter, thus acidification recovery was not reversed, but slowed down. After cessation of NO₃ leaching we hypothesise that collapsed tree canopy across catchments (from 12 to 87 % compared to 1984) will cause lower total acid input in precipitation (S + N) and regrowth of vegetation may stimulate higher N immobilization (in biomass and soil); processes which could lead to further increase of ANC and pH, key indicators for biological recovery.
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