Calculating critical S and N loads and current exceedances for upland forests in southern Ontario, Canada

1996 ◽  
Vol 26 (4) ◽  
pp. 696-709 ◽  
Author(s):  
Paul A. Arp ◽  
Tõnu Oja ◽  
Marius Marsh
Keyword(s):  
Critical Loads ◽  
Base Cations ◽  
Soil Nutrient ◽  
Base Cation ◽  
N Deposition ◽  
Balance Model ◽  
Southern Ontario ◽  
Soil Weathering ◽  
Forest Sites ◽  
Soil Solutions

A steady-state mass-balance model was used to calculate critical loads of S and N deposition for maintaining acceptable long-term acidity levels within upland forests in southern Ontario. Preliminary estimates about critical S and N loads were obtained using existing information about soils, vegetation and atmospheric ion deposition from 12 forest sites, all located within provincial parks or conservation areas. The following were considered: wet atmospheric deposition of all major cations and anions; availability for plant uptake of N, Ca, Mg, and K in the rooting space of each soil; nutrient uptake and storage in the growing woody biomass of the forest stands; estimates of soil weathering; and mean annual air temperature, precipitation, and evapotranspiration. From this, regional isopleth maps were generated to depict the following: (1) current deposition patterns; (2) critical acidification loads and their current exceedances (or nonexceedances) for two acidification effects criteria for soil solutions, namely (i) acceptable Al concentrations ([Al]leach,crit) and (ii) acceptable Al to base cation concentration ratios ([Al]/[BC]leach,crit); (3) critical N-eutrophication loads and their current exceedances for acceptable levels of NO3-N concentrations in soil solutions ([NO3-N]leach,crit). It was found that the northern part of the study area (part of the Canadian Shield) is currently subjected to atmospheric S and N deposition in excess of critical loads, with [Al]leach,crit set at 0.02 mequiv./L or [Al]/[BC]leach,crirt set at 0.15 equiv./equiv. This sensitivity to acid precipitation is, as calculated, primarily due to shallow and weathering-resistant soils and soil parent materials (mostly granitic). The middle portion of the study area is calculated to receive N slightly in excess of the N eutrophication limit, when [NO3-N]leach,crit is set at 0.1 mequiv./L. Considerable co-deposition of base cations (Ca, Mg, K) in the middle and southern part of the study area alleviates some of the atmospheric acidification stress. This stress is further neutralized by the soils and bedrock of this region (predominantly calcareous).

scholarly journals Current, steady-state and historical weathering rates of base cations at two forest sites in northern and southern Sweden: a comparison of three methods

2020 ◽  
Vol 17 (2) ◽  
pp. 281-304 ◽  
Author(s):  
Sophie Casetou-Gustafson ◽  
Harald Grip ◽  
Stephen Hillier ◽  
Sune Linder ◽  
Bengt A. Olsson ◽  
...  
Keyword(s):  
Steady State ◽  
Soil Depth ◽  
Base Cations ◽  
Base Cation ◽  
Mineral Weathering ◽  
Soil Horizon ◽  
Soil Horizons ◽  
Weathering Rate ◽  
Weathering Rates ◽  
Forest Sites

Abstract. Reliable and accurate methods for estimating soil mineral weathering rates are required tools in evaluating the sustainability of increased harvesting of forest biomass and assessments of critical loads of acidity. A variety of methods that differ in concept, temporal and spatial scale, and data requirements are available for measuring weathering rates. In this study, causes of discrepancies in weathering rates between methods were analysed and were classified as being either conceptual (inevitable) or random. The release rates of base cations (BCs; Ca, Mg, K, Na) by weathering were estimated in podzolised glacial tills at two experimental forest sites, Asa and Flakaliden, in southern and northern Sweden, respectively. Three different methods were used: (i) historical weathering since deglaciation estimated by the depletion method, using Zr as the assumed inert reference; (ii) steady-state weathering rate estimated with the PROFILE model, based on quantitative analysis of soil mineralogy; and (iii) BC budget at stand scale, using measured deposition, leaching and changes in base cation stocks in biomass and soil over a period of 12 years. In the 0–50 cm soil horizon historical weathering of BCs was 10.6 and 34.1 mmolc m−2 yr−1, at Asa and Flakaliden, respectively. Corresponding values of PROFILE weathering rates were 37.1 and 42.7 mmolc m−2 yr−1. The PROFILE results indicated that steady-state weathering rate increased with soil depth as a function of exposed mineral surface area, reaching a maximum rate at 80 cm (Asa) and 60 cm (Flakaliden). In contrast, the depletion method indicated that the largest postglacial losses were in upper soil horizons, particularly at Flakaliden. With the exception of Mg and Ca in shallow soil horizons, PROFILE produced higher weathering rates than the depletion method, particularly of K and Na in deeper soil horizons. The lower weathering rates of the depletion method were partly explained by natural and anthropogenic variability in Zr gradients. The base cation budget approach produced significantly higher weathering rates of BCs, 134.6 mmolc m−2 yr−1 at Asa and 73.2 mmolc m−2 yr−1 at Flakaliden, due to high rates estimated for the nutrient elements Ca, Mg and K, whereas weathering rates were lower and similar to those for the depletion method (6.6 and 2.2 mmolc m−2 yr−1 at Asa and Flakaliden). The large discrepancy in weathering rates for Ca, Mg and K between the base cation budget approach and the other methods suggests additional sources for tree uptake in the soil not captured by measurements.

Cycling of acid and base cations in deciduous stands of Huntington Forest, New York, and Turkey Lakes, Ontario

1992 ◽  
Vol 22 (2) ◽  
pp. 167-174 ◽  
Author(s):  
N.W. Foster ◽  
M.J. Mitchell ◽  
I.K. Morrison ◽  
J.P. Shepard
Keyword(s):  
New York ◽  
Soil Acidity ◽  
Mineral Soil ◽  
Base Cations ◽  
Base Cation ◽  
Native Soil ◽  
Soil Solutions ◽  
Acid And Base ◽  
Turkey Lakes Watershed ◽  
Dominant Cation

Annual nutrient fluxes within two forests exposed to acidic deposition were compared for a 1-year period. Calcium (Ca2+) was the dominant cation in throughfall and soil solutions from tolerant hardwood dominated Spodosols (Podzols) at both Huntington Forest (HF), New York, and the Turkey Lakes watershed (TLW), Ontario. There was a net annual export of Ca2+ and Mg2+ from the TLW soil, whereas base cation inputs in precipitation equaled outputs at HF. In 1986, leaching losses of base cations were five times greater at TLW than at HF. A higher percentage of the base cation reserves was leached from the soil at TLW (5%) than at HF (1%). Relative to throughfall, aluminum concentrations increased in forest-floor and mineral-soil solutions, especially at HF. The TLW soil appears more sensitive to soil acidification. Deposited atmospheric acidity, however, was small in comparison with native soil acidity (total and exchangeable) and the reserves of base cations in each soil. Soil acidity and base saturation, therefore, are likely only to change slowly.

Effects of grassland afforestation on exchangeable soil and soil solution aluminium

Soil Research ◽  
2001 ◽  
Vol 39 (5) ◽  
pp. 1003 ◽  
Author(s):  
M. L. Adams ◽  
M. R. Davis ◽  
K. J. Powell
Keyword(s):  
Forest Soil ◽  
Soil Solution ◽  
Base Cation ◽  
Pinus Nigra ◽  
Base Saturation ◽  
Conifer Forest ◽  
Forest Sites ◽  
Hill Slope ◽  
Soil Solutions ◽  
The Impact

The impact of land use change from grassland to conifer forest on the aluminium (Al) concentrations in soils and soil solutions was examined. Soils from grassland were compared with those from adjoining 15–19-year old forest stands at 3 contrasting pairs of sites in South Island, New Zealand. The site pairs were on a terrace [Pinus nigra/P. ponderosa, and grassland (CP)], and a hill slope [Pseudotsuga menziesii and grassland (CF)] in the Craigieburn range, Canterbury, and a hill slope in the Lammerlaw Range, Otago [P. radiata and grassland (LP)]. The sites had never been cultivated or fertilised, and for each pair the forest and grassland were similar in terms of soil and topography. The 1 M KCl exchangeable and 0.02 M CaCl 2 extractable Al levels at 0–10 cm were higher in forest than in grassland topsoil at CP and LP (P < 0.01). In soil solutions there was a trend for both ‘reactive Al’ and Al bound in labile organic complexes to be higher in forest soil at all sites, but site-pair differences were only significant at LP, and only for ‘reactive Al’. The increase in ‘reactive Al’ at this site was linked to the low pH and low base saturation. The ratios of exchangeable and soil solution Ca 2+ and Mg 2+ to ‘reactive Al’ were substantially lower in forest than grassland soils at all sites. Aluminium complexation capacity (Al-CC) values at all sites were higher in forest soil solutions than in grassland soil solutions. For the grassland and forest sites at LP, the Al-CC correlated strongly with the amount of soluble fulvic and humic matter present, as estimated from soil solution UV absorbance at 250 nm. In soils with the lowest percentage base saturation and buffering capacity (LP), afforestation of pastoral grassland with Pinus radiata significantly reduced soil pH and base cation levels, while increasing both soil and soil solution Al concentrations. Under such conditions (base saturation <20%), the increase in ‘reactive Al’ concentrations in soil solutions under fast growing conifer tree species may be sufficient to affect Mg uptake.

scholarly journals Aluminium and base cation chemistry in dynamic acidification models – need for a reappraisal?

SOIL ◽  
2018 ◽  
Vol 4 (4) ◽  
pp. 237-250 ◽  
Author(s):  
Jon Petter Gustafsson ◽  
Salim Belyazid ◽  
Eric McGivney ◽  
Stefan Löfgren
Keyword(s):  
Cation Exchange ◽  
Base Cations ◽  
Soil Nutrient ◽  
Base Cation ◽  
Cation Binding ◽  
Ph Buffering ◽  
Water Composition ◽  
Organic Complexation ◽  
The Difference ◽  
Ph Range

Abstract. Long-term simulations of the water composition in acid forest soils require that accurate descriptions of aluminium and base cation chemistry are used. Both weathering rates and soil nutrient availability depend on the concentrations of Al3+, of H+, and of base cations (Ca2+, Mg2+, Na+, and K+) . Assessments of the acidification status and base cation availability will depend on the model being used. Here we review in what ways different dynamic soil chemistry models describe the processes governing aluminium and base cation concentrations in the soil water. Furthermore, scenario simulations with the HD-MINTEQ model are used to illustrate the difference between model approaches. The results show that all investigated models provide the same type of response to changes in input water chemistry. Still, for base cations we show that the differences in the magnitude of the response may be considerable depending on whether a cation-exchange equation (Gaines–Thomas, Gapon) or an organic complexation model is used. The former approach, which is used in many currently used models (e.g. MAGIC, ForSAFE), causes stronger pH buffering over a relatively narrow pH range, as compared to state-of-the-art models relying on more advanced descriptions in which organic complexation is important (CHUM, HD-MINTEQ). As for aluminium, a “fixed” gibbsite constant, as used in MAGIC, SMART/VSD, and ForSAFE, leads to slightly more pH buffering than in the more advanced models that consider both organic complexation and Al(OH)3(s) precipitation, but in this case the effect is small. We conclude that the descriptions of acid–base chemistry and base cation binding in models such as MAGIC, SMART/VSD, and ForSAFE are only likely to work satisfactorily in a narrow pH range. If the pH varies greatly over time, the use of modern organic complexation models is preferred over cation-exchange equations.

scholarly journals Soil weathering rates in 21 catchments of the Canadian Shield

2012 ◽  
Vol 16 (3) ◽  
pp. 685-697 ◽  
Author(s):  
D. Houle ◽  
P. Lamoureux ◽  
N. Bélanger ◽  
M. Bouchard ◽  
C. Gagnon ◽  
...  
Keyword(s):  
Base Cations ◽  
Base Cation ◽  
High Reactivity ◽  
Mineral Weathering ◽  
Forest Growth ◽  
Canadian Shield ◽  
Geochemical Model ◽  
Soil Weathering ◽  
Weathering Rates ◽  
Profile Model

Abstract. Soil mineral weathering represents an essential source of nutrient base cation (Ca, Mg and K) for forest growth in addition to provide a buffering power against precipitation acidity for soils and surface waters. Weathering rates of base cations were obtained for 21 catchments located within the temperate and the boreal forest of the Canadian Shield with the geochemical model PROFILE. Weathering rates ranged from 0.58 to 4.46 kmolc ha−1 yr−1 and their spatial variation within the studied area was mostly in agreement with spatial variations in soil mineralogy. Weathering rates of Ca and Mg were significantly correlated (r = 0.80 and 0.64) with their respective lake concentrations. Weathering rates of K and Na did not correlate with lake concentrations of K and Na. The modeled weathering rates for each catchment were also compared with estimations of net catchment exportations. The result show that modeled weathering rates of Ca were not significantly different than the net catchment exportations while modeled weathering rates of Mg were higher by 51%. Larger differences were observed for K and Na weathering rates that were significantly different than net catchment exportations being 6.9 and 2.2 times higher than net exportations, respectively. The results for K were expected given its high reactivity with biotic compartments and suggest that most of the K produced by weathering reactions was retained within soil catchments and/or above ground biomass. This explanation does not apply to Na, however, which is a conservative element in forest ecosystems because of the insignificant needs of Na for soil microorganisms and above ground vegetations. It raises concern about the liability of the PROFILE model to provide reliable values of Na weathering rates. Overall, we concluded that the PROFILE model is powerful enough to reproduce spatial geographical gradients in weathering rates for relatively large areas as well as adequately predict absolute weathering rates values for the sum of base cations, Ca and Mg.

scholarly journals Recovery of acidified Finnish lakes: trends, patterns and dependence of catchment characteristics

2007 ◽  
Vol 4 (5) ◽  
pp. 3249-3283 ◽  
Author(s):  
J. Vuorenmaa ◽  
M. Forsius
Keyword(s):  
Organic Anion ◽  
Regional Scale ◽  
Base Cations ◽  
Base Cation ◽  
N Deposition ◽  
Humic Lakes ◽  
Acid Anion ◽  
Catchment Characteristics ◽  
South Central ◽  
Recovery From Acidification

Abstract. The regional-scale trends (south, central and north Finland) in key acidification parameters over the period 1990–2003 were studied in lakes used for monitoring of acidification (157 lakes), and the catchment characteristics that best discriminate between lakes showing recovery (significant increase in alkalinity) and those not showing recovery (no significant increase) were determined. A significant decline in sulphate concentrations occurred in 82–98% of the lakes, depending of the region. Base cation (BC) concentrations decreased for most lakes, but to a lesser extent than those of SO4. Consequently, a significant increase in Gran alkalinity occurred in 40–92% of the lakes. The recovery from acidification has been strongest in lakes in south Finland, where both levels and decrease of S (and N) deposition have been higher compared to other regions. A significant increase in pH was detected in about 50% of the lakes in the south. Here labile aluminium concentrations also decreased in the most acidic lakes. Recovery has occurred most strongly in lakes which have SO4 as a dominant acid anion, whereas recovery has been weaker in acidified humic lakes which have organic anion as a dominant acid anion. The non-recovering lakes in south Finland have higher proportion of exposed bedrock in the catchment, and higher TOC and lower BC concentrations. In central Finland the proportion of peatland and TOC concentrations were higher and the decrease of BC concentration was steeper in non-recovering lakes than in recovering lakes. In north Finland, catchment characteristics, trend slopes and concentrations did not separate the recovering and non-recovering lakes. The non-recovering lakes were also located in regions which are acid-sensitive based on bedrock type, soil properties, weathering rate and runoff. These factors have resulted in lower concentrations and steeper downward trends for base cations.

scholarly journals Soil weathering rates in 21 catchments of the Canadian Shield

2011 ◽  
Vol 8 (3) ◽  
pp. 5743-5768 ◽  
Author(s):  
D. Houle ◽  
P. Lamoureux ◽  
N. Bélanger ◽  
M. Bouchard ◽  
C. Gagnon ◽  
...  
Keyword(s):  
Base Cations ◽  
Base Cation ◽  
High Reactivity ◽  
Mineral Weathering ◽  
Forest Growth ◽  
Canadian Shield ◽  
Geochemical Model ◽  
Soil Weathering ◽  
Weathering Rates ◽  
Profile Model

Abstract. Soil mineral weathering represents an essential source of nutrient base cation (Ca, Mg and K) for forest growth in addition to provide a buffering power against precipitation acidity for soils and surface waters. Weathering rates of base cations were obtained for 21 catchments located within the temperate and the boreal forest of the Canadian Shield with the geochemical model PROFILE. Weathering rates ranged from 0.58 to 4.46 kmolc ha−1 yr−1 and their spatial variation within the studied area was mostly in agreement with spatial variations in soil mineralogy. Weathering rates of Ca and Mg were significantly correlated (r = 0.80 and 0.64) with their respective lake concentrations. Weathering rates of K and Na did not correlate with lake concentrations of K and Na. The modeled weathering rates for each catchment were also compared with estimations of net catchment exportations. The result show that modeled weathering rates of Ca were not significantly different than the net catchment exportations while modeled weathering rates of Mg were higher by 51 %. Larger differences were observed for K and Na weathering rates that were significantly different than net catchment exportations being 6.9 and 2.2 times higher than net exportations, respectively. The results for K were expected given its high reactivity with biotic compartments and suggest that most of the K produced by weathering reactions was retained within soil catchments and/or above ground biomass. This explanation does not apply to Na, however, which is a conservative element in forest ecosystems because of the insignificant needs of Na for soil microorganisms and above ground vegetations. It raises concern about the liability of the PROFILE model to provide reliable values of Na weathering rates. Overall, we concluded that the PROFILE model is powerful enough to reproduce spatial geographical gradients in weathering rates for relatively large areas as well as adequately predict absolute weathering rates values for the sum of base cations, Ca and Mg.

scholarly journals Recovery of acidified Finnish lakes: trends, patterns and dependence of catchment characteristics

2008 ◽  
Vol 12 (2) ◽  
pp. 465-478 ◽  
Author(s):  
J. Vuorenmaa ◽  
M. Forsius
Keyword(s):  
Organic Anion ◽  
Regional Scale ◽  
Base Cations ◽  
Base Cation ◽  
N Deposition ◽  
Humic Lakes ◽  
Acid Anion ◽  
Catchment Characteristics ◽  
South Central ◽  
Recovery From Acidification

Abstract. The regional-scale trends (south, central and north Finland) in key acidification parameters over the period 1990–2003 were studied in lakes used for monitoring of acidification (157 lakes), and the catchment characteristics that best discriminate between lakes showing recovery (significant increase in alkalinity) and those not showing recovery (no significant increase) were determined. A significant decline in sulphate concentrations occurred in 82–98% of the lakes, depending of the region. Base cation (BC) concentrations decreased for most lakes, but to a lesser extent than those of SO4. Consequently, a significant increase in Gran alkalinity occurred in 40–92% of the lakes. The recovery from acidification has been strongest in lakes in south Finland, where both levels and decrease of S (and N) deposition have been higher compared to other regions. A significant increase in pH was detected in about 50% of the lakes in the south. Here labile aluminium concentrations also decreased in the most acidic lakes. Recovery has occurred most strongly in lakes which have SO4 as a dominant acid anion, whereas recovery has been weaker in acidified humic lakes which have organic anion as a dominant acid anion. The non-recovering lakes in south Finland have higher proportion of exposed bedrock in the catchment, and higher TOC and lower BC concentrations. In central Finland the proportion of peatland and TOC concentrations were higher and the decrease of BC concentration was steeper in non-recovering lakes than in recovering lakes. In north Finland, catchment characteristics, trend slopes and concentrations did not separate the recovering and non-recovering lakes. The non-recovering lakes were also located in regions which are acid-sensitive based on bedrock type, soil properties, weathering rate and runoff. These factors have resulted in lower concentrations and steeper downward trends for base cations. Climate change may increase the mineralization of soil organic matter and change the frequency and magnitude of runoff and organic acid episodes. An increase of these types of confounding effects on pH and alkalinity recovery may therefore be anticipated in the future.

scholarly journals Current, steady-state and historical weathering rates of base cations at two forest sites in northern and southern Sweden: A comparison of three methods

2019 ◽  
Author(s):  
Sophie Casetou-Gustafson ◽  
Harald Grip ◽  
Stephen Hillier ◽  
Sune Linder ◽  
Bengt A. Olsson ◽  
...  
Keyword(s):  
Steady State ◽  
Mass Balance ◽  
Soil Depth ◽  
Base Cations ◽  
Base Cation ◽  
Mineral Weathering ◽  
Soil Layers ◽  
Weathering Rate ◽  
Weathering Rates ◽  
Forest Sites

Abstract. Reliable and accurate methods for estimating soil mineral weathering rate are required tools in evaluating the sustainability of increased harvesting of forest biomass. A variety of methods that differ in concept, temporal and spatial scale and data requirements are available for measuring weathering rate. In this study, release rates of base cations through weathering were estimated in podsolised glacial tills at two experimental forest sites, Asa and Flakaliden, in southern and northern Sweden, respectively. Three different methods were used: (i) historical weathering since deglaciation estimated with the depletion method, using Zr as assumed inert reference; (ii) steady-state weathering rate estimated with the PROFILE model, based on quantitative analysis of soil mineralogy; and (iii) base cation mass balance at stand scale, using measured deposition, leaching and changes in base cation stocks in biomass and soil over a period of 12 years. In the 0–50 cm soil layer at Asa, historical weathering of Ca, Mg, K and Na estimated by the depletion method was 4.7, 3.1, 0.8 and 2.0 mmolc m−2 yr−1, respectively. Corresponding values at Flakaliden were 7.3, 9.0, 1.7 and 4.4 mmolc m−2 yr−1, respectively. Steady state weathering rate for Ca, Mg, K and Na estimated with PROFILE was 8.9, 3.8, 5.9 and 18.5 mmolc m−2 yr−1, respectively, at Asa and 11.9, 6.7, 6.6 and 17.5 mmolc m−2 yr−1, respectively, at Flakaliden. Thus at both sites, the PROFILE results indicated that steady-state weathering rate increased with soil depth as a function of exposed mineral surface area, reaching a maximum rate at 80 cm (Asa) and 60 cm (Flakaliden). In contrast, the depletion method indicated that the largest postglacial losses were in upper soil layers, particularly at Flakaliden. With the exception of Mg and Ca in shallow soil layers, PROFILE appeared to produce consistently higher weathering rates, particularly of K and Na in deeper soil layers. In contrast, the depletion method appeared to to produce consistently lower rather than higher weathering rates, due to natural and anthropogenic variability in (reference) Zr gradients. The mass balance approach produced significantly higher weathering rates of Ca, Mg, and K (65, 23, 40 mmolc m−2 yr−1 at Asa and 35, 14 and 22 mmolc m−2 yr−1 at Flakaliden), but lower Na weathering rates similar to the depletion method (6.6 and 2.2 mmolc m−2 yr−1 at Asa and Flakaliden). The large discrepancy in weathering rates for Ca, Mg and K between mass balance and the other methods suggest that there were additional sources for tree uptake in the soil besides weathering and measured depletion in exchangeable base cations.

scholarly journals Aluminium and base cation chemistry in dynamic acidification models – need for a reappraisal?

2018 ◽  
Author(s):  
Jon Petter Gustafsson ◽  
Salim Belyazid ◽  
Eric McGivney ◽  
Stefan Löfgren
Keyword(s):  
Cation Exchange ◽  
Base Cations ◽  
Soil Nutrient ◽  
Base Cation ◽  
Cation Binding ◽  
Ph Buffering ◽  
Water Composition ◽  
Organic Complexation ◽  
The Difference ◽  
Ph Range

Abstract. Long-term simulations of the water composition in acid forest soils require that accurate descriptions of aluminium and base cation chemistry are used. Both weathering rates and soil nutrient availability depend on the concentrations of Al3+, of H+, and of base cations (Ca2+, Mg2+, Na+ and K+). Consequently, assessments of the acidification status and base cation availability will depend on the model being used. Here we review in what ways different dynamic soil chemistry models describe the processes governing aluminium and base cation concentrations in the soil water. Furthermore, scenario simulations with the HD-MINTEQ are used to illustrate the difference between model approaches. The results show that all investigated models provide the same type of response to changes in input water chemistry. Still, for base cations we show that the differences in the magnitude of the response may be considerable depending on whether a cation-exchange equation (Gaines-Thomas, Gapon) or an organic complexation model is used. The former approach, which is used in many currently used models (e.g. MAGIC, ForSAFE), causes stronger pH-buffering over a relatively narrow pH range, as compared to state-of-the-art models relying on more advanced descriptions in which organic complexation is important (CHUM, HD-MINTEQ). As for aluminium, a fixed gibbsite constant, as used in MAGIC and ForSAFE, leads to slightly more pH-buffering than in the more advanced models that consider both organic complexation and Al(OH)3(s) precipitation, but in this case the effect is small. We conclude that the descriptions of acid-base chemistry and base cation binding in models such as MAGIC and ForSAFE are only likely to work satisfactorily in a narrow pH range. If the pH varies greatly over time, the use of modern organic complexation models is preferred over cation exchange equations.

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