Effect of Ionic Composition and Temperature on the Radiocaesium Fixation in Freshwater Sediments

The objective of this study was to verify in which way the ionic composition of the sediment and that of the overlying water column may have an effect on radiocaesium fixation, through possible structural modifications of the frayed edge sites (FES) pools of the sediments. Two experimental protocols have been considered: i) a condition in which sediments were homoionically saturated with either potassium, ammonium, calcium, magnesium or sodium ions. and ii) a mixed potassium-calcium scenario in the liquid phase. Nine freshwater sediments from four different locations were used in this study. For homoionic potassium and ammonium saturated sediments a nearly quantitative radiocaesium desorption (90–100%) was observed, whereas for calcium and magnesium the desorption yields scene about 20%. It appears that the action of strongly hydrated ions (Na⁺, Mg²⁺, Ca²⁺) leads to a pronounced enhancement of radiocaesium fixation in the solid phase, whereas poorly hydrated ions (K⁺, NH₄ ⁺) have the opposite effect and promote sorption reversibility. Another issue considered in this study concerns the effect of temperature and sediment drying on the radiocaesium fixation. Drying die sediments at 110°C leads to a significant increase in radiocaesium fixation levels, while drying the sediments at room temperature (25°C) has a very limited effect on radiocaesium fixation and appears to put a brake on the aging effects.     

Nitrogen fixation by free-living heterotrophic bacteria in an oak forest: The effect of liming

Nitrogen-fixing heterotrophic bacteria, mainly Clostridium butyricum and, less frequently, Enterobacter agglomerons and Klebsiella pneumoniae, are found throughout the aerial and soil layers of an oak forest. However they are only active in nitrogen fixation in the soil. Soil slurry experiments show that the main factors limiting N₂ fixation in the forest are low temperature, low pH and shortage of C sources. Raising the pH of the soil with lime to pH 6, which is the optimum for N₂ fixation by C. butyricum in soil slurries, more than doubles the rate of N₂ fixation (as measured by in situ¹⁵N₂ methods) from 7.84 to 16.1 kg N ha^?1yr^?1. The N fixed by C. butyricum can rapidly be taken up by oak seedlings and translocated to the actively-growing leaves.     

Alleviation of soil acidity improves the performance of oil palm progenies planted on an acid Ultisol

Abstract

Soil acidity is one of the main factors that limit profitable and sustained agricultural production. Oil palm (Elaeis guineensis Jacq.) is mainly planted in acidic soils. In the last years, there has been a stagnated yield and increases in disease incidence and severity worldwide that could be attributed in some extent to soil acidity. This study was conducted to determine the effects of soil acidity alleviation on oil palm seedlings. The effects of ground magnesium limestone or dolomite and magnesium carbonate (0, 1.1, 2.2, 3.3 and 4.4 t ha^?1) applied to an Ultisol dominated by kaolinite (pH in water 4.4) were evaluated on selected morphological, physiological and nutritional characteristics of hybrid (Deli dura × AVROS pisifera) and clonal (clone 366) oil palm progenies under nursery conditions for 8 months. Increasing rates of ground magnesium limestone and magnesium carbonate showed a significant effect on improving soil pH and lowering exchangeable aluminium. The hybrid oil palm showed significant either linear or quadratic trends for most of the parameters evaluated, indicating that the best responses for morphological and physiological traits were achieved from 2.5 to 4.23 t ha^?1 with ground magnesium limestone and 2.87 to 3.45 t ha^?1 with magnesium carbonate. Positive effects of increasing rates of ground magnesium limestone and magnesium carbonate were observed on nitrogen and magnesium uptake. Aluminium concentration in the third frond decreased significantly with increasing ground magnesium limestone rate. A significant reduction of manganese uptake was also observed with increasing rates of both ameliorants. The clonal oil palm progeny exhibited a better performance on un-amended treatment. This may be explained by the significant higher root growth of this progeny. Soil acidity alleviation improved the oil palm seedling growth. These results are important for the oil palm industry and could be applied in the nursery stage as well as extended to the immature stage.     

Soybean nodulation and nitrogen fixation on soil previously amended with sewage sludge

Summary A greenhouse study was conducted to examine the residual effects of sewage sludge on soybean Glycine max (L.) Merr., nodulation, and N fixation. Nodulating and nonnodulating isolines of Clark soybean were grown to the R2 stage in soils (Typic Paleudults) obtained from plots where heat-treated sludge had been applied in 1976 at rates equal to 0, 56,112, and 224 Mg ha^–1 high (7.0) and low (6.2) soil pH regimes were established by CaCO₃ additions. Sludge and soil pH treatments resulted in clearly defined differences in metal uptake by soybean shoots. Plant Zn, Cd, and Ni concentrations were greater on pH 6.2, sludge-amended soil than on the pH 7.0, amended soil. At low soil pH, soybean Zn and Cd concentrations, respectively, increased from 41 and 0.19 mg kg^–1 (control) to 120 and 0.58 mg kg^–1 at the 224 Mg hat sludge rate. At the high soil pH and 224 hg hat sludge rate, Zn and Cd concentrations were 45 and 0.15 mg kg^–1, respectively.Symbiotic N fixation provided 90% of the total N accumulation. Total N accumulation, shoot N concentration, dry matter, and N fixation by nodulating soybeans exhibited a significant linear increase with sludge rate. Total N accumulation, dry matter, and N fixation were significantly greater at high soil pH. For high and low soil pH, respectively, N fixation increased from 422 and 382 mg N per plant (control) to 614 and 518 mg N per plant at the 224 Mg ha^–1 sludge rate. While soybean nodulation also increased linearly on sludge-amended soil, a significant rate times pH interaction for nodule number indicated that nodulation was less strongly enhanced by sludge at low soil pH.     

Mapping Rain Composition in the East of Spain by Applying Kriging

This paper presents the results of applying kriging to the major inorganic ions from a bulk deposition network on the eastern coast of Spain that has been in operation for more than two years (October 1989 to December 1992). From the calculated variograms spatial correlation up to 70–130 km has been found, depending on the ion. The most suitable models of variograms have been found for sulphates and marine ions (chloride, sodium). Nitrates and pH correlates only at smaller distances. For calcium and pH additional to the correlation at smaller distances, a correlation at distances around 140 km was found. The nugget effect was high for most variograms, suggesting the importance of local influences in rain chemistry in the area. In general, most of the ions follow a spatial pattern that gives the highest values in the south and the lowest in the north inland part of the area of study. It is possible to discern different spatial patterns according to the ions and their main sources: A pattern following the coastline with decreasing values from the coast to inland was followed by chloride, sodium, magnesium and nitrate ions; a pattern divided into four areas, two with pH < 6.2 and less than 220 μeq L^-1 for calcium and another two zones that surpass these values. Finally total sulphate and non-marine sulphate follow a pattern half-way between the previous patterns.     

Nodulation response to molybdenum supplementation in alfalfa and its correlation with root and shoot growth in low pH soil

Molybdenum (Mo) is a critical micronutrient for nitrogen (N) fixation in legumes. Low pH limits the availability of Mo, thereby reducing nodulation and N fixation. This study investigates the effect of Mo supplementation on alfalfa nodulation and its correlation with root and shoot biomass in low-pH soil. Three experiments were conducted in the greenhouse, involving 14 genotypes of alfalfa subjected to four different treatments, unlimed low-pH soil (5.2) with Mo applied (lithium (Li)^?Mo⁺), low-pH soil (5.2) without Mo (Li^?Mo^?), limed soil (pH 7.3) with Mo applied (Li⁺Mo⁺), and limed soil (pH 7.3) without Mo (Li⁺Mo^?). Foliar application of Mo resulted in a significant increase in nodule counts in the 14 alfalfa cultivars grown in low-pH soil (5.2) even though to a lesser extent than in limed soil with neutral pH (7.3). The increase in number of nodules correlated positively with plant root weight and upper plant biomass.     

铜尾矿生物结皮的生物固氮及其影响因素研究

在铜尾矿生态系统自然恢复过程中,生物结皮广泛存在并成为尾矿生态系统演替早期的重要阶段。本文采用乙炔原位还原法对藻类结皮、藻藓混合结皮和藓类结皮的生物固氮特征进行了系统研究。结果表明:(1)生物结皮显著提高了铜尾矿总氮含量,同时降低了铜的含量。(2)不同类型生物结皮的固氮能力差别较大,其中藻藓混合结皮的生物固氮量最高,在N 4.36～30.39 kg hm2 a–1之间;藻类结皮和藓类结皮的固氮量分别为N 1.32～8.78、0～16.34 kg hm2 a–1。(3)生物固氮能力随季节变化明显,夏季的生物固氮量最高,春季次之,秋冬季节相对较低。(4)铜尾矿基质pH、NH4+-N和水溶性有机碳(WSOC)等与生物固氮量呈显著正相关(p<0.05),而土壤容重、NO3--N和总铜等与生物固氮量呈显著负相关(p<0.05)。     

Impacts of Acid Rain on Base Cations,Aluminum, and Acidity Development in Highly Weathered Soils of Thailand

The impacts of simulated acid rain on leachability of major plant nutrients, toxic element [aluminum (Al)], and acidity development in highly weathered tropical soils of Thailand were studied. Leaching experiments were conducted on soil columns with acidic solutions of pH 5.0, 4.0, 3.0, 2.0, and with water of pH 7.0 as a control treatment. Leaching losses of base cations from all soils increased with the decrease in pH associated with simulated acid rain (SAR) additions, and were found to be quite high under SAR with pH 2.0. The leaching removal of these cations was lesser at pH 3.0, 4.0, and 5.0 but greater than that in pH 7.0. The leaching of base cation from the soils depended not only on acid rain pH but also on soil properties, especially cation exchange capacity, soil texture, and initial base content. The significant losses of major plant nutrients [such as potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺)] from the plant root zone over extended periods could cause nutrient imbalance and lower soil productivity.     

Role of soil constituents in fixation of soluble Zn, Cu, Ni and Cd added to soils

Slow immobilization of trace metals in soil, termed ‘fixation’, affects their natural attenuation but it is still unclear which reactions occur. Twenty‐eight soils were selected to assess the role of Fe oxides and carbonates on fixation of Cu, Cd, Zn and Ni. Soils included samples from 2 toposequences (Vietnam, Spain) and 13 European topsoils with different soil characteristics (pH 3.4–7.7). Samples were amended with 250 mg Zn kg⁻¹, 100 mg Cu kg⁻¹, 80 mg Ni kg⁻¹ and 2.5 mg Cd kg⁻¹ as metal salts and incubated for 850 days. Fixation was measured as the increase of the fraction of added metals that were not isotopically exchangeable. Fixation increased with time and was, averaged over all the soils, 43% (Cu), 41% (Zn), 41% (Ni) and 28% (Cd) after 850 days. Metal fixation within samples from each toposequence was generally positively related to total Fe oxide concentration (Fe_d) for Zn, Ni and Cd. However, the fixation of Cd, Zn and Ni was mainly explained by pH and not by Fe_d when considering all soils. Fixation of Zn and Cd in soils with pH >7.0 increased with increasing concentrations of carbonates at initial ageing times. Fixed fractions of Zn, Ni and Cd were significantly released when experimentally removing 50% of carbonates by acidification. Fixation of Cu was most poorly related to soil properties. Our data suggest that fixation of Cd, Zn and Ni is related to a pH‐dependent diffusion into oxides and that of Cd and Zn also to diffusion and/or coprecipitation in carbonates. Fixation of Ni at neutral pH may also be related to stabilization of precipitates that form readily in soil.     

Measurement of asymbiotic N2 fixation in Australian agriculture

A wide range of bacteria capable of nitrogen fixation (free-living and associative) can be found in all agricultural soils across Australia, however measurement of their effectiveness in N₂ fixation has proved to be problematic because rates are low compared to symbiotic systems and quantitative methodologies barely adequate. It is generally believed that associative N₂ fixation rates may be greater than free-living N₂ fixation rates in ecosystems where grasses (including cereals) dominate, although this has not been unequivocally proven. Conditions promoting asymbiotic N₂ fixation are reduced availability of oxygen, high temperature and soil water, and large amounts of microbially available C in the soil. The most direct measure of N₂ fixation, incorporation of ¹⁵N₂, has rarely been used in undisturbed systems, and we can find no examples of its field application in Australia. Nitrogen balance calculations, based on long-term changes in total soil N of systems and crop N removal, have been used to infer asymbiotic N₂ fixation, but do not measure it directly. Such N balance studies can thus only give an indication of potential asymbiotic N₂ fixation over long periods of time, but cannot confirm it. There are no robust N balances published for Australian ecosystems. The acetylene reduction assay for nitrogenase activity has been used in Australia to study responses of both free-living and associative N₂ fixation systems to regulating factors. These studies have highlighted the importance of C supply, high soil water content and temperature in increasing asymbiotic N₂ fixation in soils. However significant methodological limitations do not allow field scale quantification using this assay. On balance we would concur with the authors of several earlier global reviews of this topic and conclude that (in Australia) contributions of nitrogen to crop growth from asymbiotic N₂ fixation are likely to be <10 kg N ha^?1 y^?1 and generally not of agronomic significance under low rainfall conditions. In tropical environments where higher rainfall and temperatures coincide, rates are likely to be greater if soil mineral N is low and carbon substrates are available for N₂ fixing microorganisms. If asymbiotic N₂ fixation is to be encouraged or profitably managed, there is a need for more reliable field measurement and a combination of methodologies including ¹⁵N might provide more definitive quantitative indications.     

Pinus radiata site quality study in the Basque Country (Spain) using nutritional and physiographical criteria

Nutrient status in Pinus radiata sites in the Basque Country (Spain) have been established. Current-year, 1-year-old needles, and soil samples were collected in 7 sites. Needle samples were analyzed for calcium (Ca), magnesium (Mg), potassium (K), phosphorus (P) and nitrogen (N). Soil samples were analyzed for pH (H₂O), N, P, Ca, Mg, K and aluminium (Al). The results indicate differences between sites and needles of different ages. In general, nutrient concentrations in needles are sufficient for the nutritive requirements, but some low concentrations of Mg and P found in 1-year-old needles, indicate that some trees may show signs of deficiency in the near future. In turn, N concentrations are, in many cases, higher than those usually found in this species and may originate growth disorders. These concentrations may be related to NO₃ ^? and NH₄ ⁺ deposition. Soil nutrients are sufficient for trees requirements, but the levels of K and Mg are low. Soil pH values are not very acid, and the value of the Ca/Al ratio is, in general, near 1. An arrangement of sites and physiographical and nutritional parameters based on correlation and principal component analysis is also presented.     

An evaluation of MES (2(N‐Morpholino)ethanesulfonic acid) and amberlite IRC‐50 as pH buffers for nutrient solution studies ∗

All buffering agents used to stabilize pH in hydroponic research have disadvantages. Inorganic buffers are absorbed and may become phytotoxic. Solid carbonate salts temporarily mitigate decreasing pH but provide almost no protection against increasing pH, and they alter nutrient absorption. Exchange resins are more effective, but we find that they remove magnesium and manganese from solution. We have tested 2(N‐Morpholino)ethanesulfonic acid (MES) as a buffering agent at concentrations of 1 and 10 mol m^‐3 (1 and 10 mM) with beans, corn, lettuce, tomatoes, and wheat. MES appears to be biologically inert and does not interact significantly with other solution ions. Relative growth rates among controls and MES treatments were nearly identical for each species during the trial period. The pH was stabilized by 1 mol m^‐3 MES. This buffer warrants further consideration in nutrient research.     

Effects of pH and aluminosilicate composition on KMg exchange selectivity of amorphous aluminosilicates

The KMg cation exchange selectivity of some allophanic soil clays, imogolite and a range of synthetic hydrous oxides and amorphous aluminosilicates was studied using solutions 0.05N with respect to KCl and MgCl₂. The preference of the aluminosilicates for Mg²⁺ increased with pH, which is in qualitative agreement with the predictions based on double-layer theory. Large quantitative discrepancies indicated, however, specific interaction of potassium with aluminosilicates of low and intermediate Al/(Al + Si) molar ratio. The potassium selectivity of these materials probably arises from swelling-pressure effects and steric restriction on the entry of the larger hydrated Mg²⁺ into channels in the aluminosilicate aggregates. Hydrous oxides of iron and aluminium were found to specifically adsorb magnesium at alkaline pH values and evidence indicates that specific adsorption by these materials also occurred in soil clays and in Fe(NO₃)₃-treated synthetic aluminosilicates.     

Kinetics of Competitive Fixation of Potassium and Ammonium Ions by Silt Component of Soils from Different Agro-Climatic Regions

Potassium (K⁺) and nitrogen [N, as the form of ammonium (NH₄⁺)] are major nutrients for plant growth. Although there have been a number of studies on the kinetic fixation of potassium and ammonium ions in soils and clays, however, investigations on the kinetics of competitive fixation of these ions have been few, if any, especially by taking into account silt component of the soils. In this study, the kinetics of potassium and ammonium fixation were examined in the silt components of several soil samples. The results revealed that considerable amounts of K⁺ and in lesser amounts, NH₄⁺ ions were fixed by silt components of the soils. Potassium fixation was strongly preferred over ammonium. To describe the fixation kinetics, seven mathematically models were evaluated. A comparison of the models showed that pseudo-second-order equation properly described the fixation of these ions by the silt components.     

THE MOBILIZATION AND FIXATION OF MOLYBDENUM, VANADIUM, AND URANIUM BY DECOMPOSING PLANT MATTER

Mo, V, and U are mobilized as anions by aerobically decomposing plant matter; the behaviour of the dissolved metals differs in several respects from what would be expected in inorganic systems. With respect to dialysis through cellophane, between pH I and 4 the mobilized Mo is fixed by colloidal organic decomposition products, with maximum retention at pH 1.5; V is retained between pH 1.5 and 7.0, with a maximum at pH 3. The specific fixation of Mo and V by soil organic matter was considerably less, and persisted over wider pH ranges–1.5-6.5 and 1.0-9.0 respectively. The fixation of U by both forms of humified organic matter increased sharply to a maximum around pH 4-5, and thereafter decreased slightly up to pH 8. The anionic forms of the three elements persisted when MOO:_2-⁴, VO;-³ and UO, were incubated with anaerobically decomposing plant matter; under these conditions V(V) was probably reduced to V(IV), and it seems that an anionic V(IV) complex was formed. Although the molecular size of the colloidal decomposition products of lucerne was somewhat less than that of organic matter extracted from Rothamsted top soil, acid hydrolysates of the two humic acids contained the same twenty-three amino acids, in much the same relative proportions.     

The effect of alkaline dust decline on the precipitation chemistry in northern Japan

The precipitation chemistry in northern Japan, especially Hokkaido, has been investigated since 1982. This area has often been found to have high concentrations of alkaline road dust (asphalt dust) in the air, caused by the use of studded tires during the winter. It is well known that the composition of precipitation in these areas is often dominated by asphalt dust including calcium bicarbonate. However, recently the concentration of asphalt dust in the air has decreased owing to a ban on the use of studded tires. Simultaneously, in precipitation, the lowering of pH values and the increase of hydrogen ion depositions have been occurring owing to the decrease of non-sea-salt calcium ions (nss-Ca²⁺) concentrations and depositions derived from asphalt dust. In addition, we found that a decrease of nss-Ca²⁺ firstly leads to a decrease of bicarbonate ions (HCO₃ ^?), the counter ion to nss-Ca²⁺ in asphalt dust. Therefore, the increase of H⁺ concentrations and depositions was great in comparison with the decrease of nss-Ca²⁺ concentrations and depositions in areas where the HCO₃ ^?concentrations, varied by pH, and depositions had been low. Furthermore, this variation was mainly observed in the ionic composition of snow cover and snowfall at sites along the Japan Sea in northern Japan during winter. In this area, the Acid Shock effect may become a serious problem from the decline of pH values in melting snow. Moreover, we found that ammonium ions and non-sea-salt sulfate ions depositions have also been decreasing in response to a decrease of nss-Ca²⁺ depositions, derived from asphalt dust. It seems that this phenomenon is caused by the decrease of asphalt dust concentrations in the air.     

Response of grass leys to applications of calcified seaweed

Abstract

Calcified seaweed has long been used as a soil conditioner in northern Europe, but supposed beneficial responses have not been experimentally substantiated. Field and glasshouse studies examined treatment responses on the characteristics of sandy silt loam Hapludalf soils and on the growth and elemental composition of Loliumperenne. Agricultural lime was a treatment in both experiments, being chemically similar to calcified seaweed. Calcified seaweed was applied at 2 t ha^‐1 and produced small increases in soil pH and extractable calcium (Ca). Significant increases in Lolium perenne growth were found in field studies after both calcified seaweed and lime applications. Smaller, but consistent, increases in growth were found in glasshouse pot studies. However, only one harvest showed a significant dry weight yield increase after calcified seaweed application compared with the untreated control. In pot studies, increases in soil extractable Ca were associated with increases in shoot elemental Ca. Decreases in shoot zinc (Zn) and manganese (Mn) concentrations were found after both calcified seaweed and lime applications. Total shoot element accumulation of Zn and Mn after calcified seaweed application were similar to those produced by the control, suggesting that decreases in shoot Zn and Mn concentrations resulted from dilution after increased shoot growth. However, total Zn and Mn accumulation decreased after lime application compared to the control and calcified seaweed treatments, probably resulting from fixation of available soil Zn and Mn through greater increases in soil pH.     

Landscape-scale estimates of dinitrogen fixation by Pisum sativum by nitrogen-15 natural abundance and enriched isotope dilution

Topography and slope position influence the soil and environmental factors that affect N₂ fixation by legumes. The present study was conducted to (1) estimate N₂ fixation by field peas in a gently rolling farm field using the natural ¹⁵N abundance and the ¹⁵N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N₂ fixation at the landscape scale. Whereas soil available water capacity, available NH _inf4 ^sup+ , total crop yield, and percent N derived from N₂ fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NO _inf3 ^sup- levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NH _inf4 ^sup+ but not with available soil water, pH, electrical conductivity, NO _inf3 ^sup- , or particle size. Estimates of the % Ndfa using enriched ¹⁵N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the ¹⁵N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched ¹⁵N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched ¹⁵N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N₂ fixation, large variations in N₂ fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N₂ fixation.Contribution No. R754 of the Saskatchewan Center of Soil Research     

Distribution and activity of hypolithic soil crusts in a hyperarid desert (Baja California, Mexico)

Widespread and ecologically important, biological soil crusts include those microbial communities living on the surface of the soil and those that live beneath semitranslucent rocks (a.k.a. hypolithic crusts). We examined the distribution, abundance, physiology, and potential soil N contributions of hypolithic, biological crusts in hyperarid ecosystems of the Baja California peninsula and islands in the midriff region of the Gulf of California, Mexico (Sonoran desert). Crusts were limited in distribution to areas with translucent quartz rocks less than 3 cm thick, were not found on areas of islands with seabird guano deposition, but covered as much as 1% (12,750 m²) of the surface area of one island. The percent of available rocks colonized by crusts was similar between the mainland (38%) and islands without seabird guano (26%). Carbon fixation rates in the field, which have not been previously reported, ranged between 0 and 1.23 μmol m⁻² s⁻¹, and in the lab ranged between 0.66 and 0.94 μmol m⁻² s⁻¹. Evidence of low rates of N fixation was inferred from δ ¹⁵N values of crust and soil. Hypolithic crusts were found to have minimal, if any, influence on soil salinity, pH, and , but may represent up to 14% of the biomass of primary producers on these islands and provide C and N to the belowground and possibly aboveground heterotrophic communities where crusts exist. The results of this study suggest a limited but potentially important contribution of hypolithic soil crusts to hyperarid ecosystems.     

Putting the Sodium Dominance Index to the Test as a Measure of Acid Sensitivity Across Diverse Geological Conditions and with Reference to the Influence of Plantation Forests

Traditionally, alkalinity has been widely used in order to determine the level of acid sensitivity, while pH values were often viewed as an indicator from sensitivity to impact. These methods of assigning acidity classifications are problematic, due to large temporal changes in flow conditions and various geological types, which influence the results. The Sodium Dominance Index (SDI) is essentially a ratio of cations, namely, the concentration of Na⁺, divided by the sum of the concentrations of Na⁺, Ca²⁺, and Mg²⁺ expressed as a percentage. Values of >60 SDI denoted sites ‘at high risk’ of acidification and were therefore classified acid sensitive. From a total of 248 sites (65 of which were forested) in Ireland, base flow sample results showed significant correlations between SDI, alkalinity and pH levels. Significant relationships for SDI to differing underlying geologies were also noted. The data suggested no effect of coniferous forest on the SDI results. It was found that SDI did not differ significantly between base and elevated flow conditions. It was concluded that a dilution effect of calcium (partially balanced by inputs of sodium and magnesium during storm events) along with a depletion of carbonate sources from soils resulted in small fluctuations in the Index between flow conditions.