Soybean root growth in relation to ionic composition in magnesium-amended acid subsoils: Implications on root citrate ameliorating aluminum constraints

Abstract

Hydroponic studies with soybean (Glycine max [L.] Merr.) have shown that µmol L^?1 additions of Mg²⁺ were as effective in ameliorating Al rhizotoxicity as additions of Ca²⁺in the mmol L^?1 concentration range. The objectives of this study were to assess the ameliorative effects of Mg on soybean root growth in acidic subsoils and to relate the soil solution ionic compositions to soybean root growth. Roots of soybean cultivar Plant Introduction 416937 extending from a limed surface soil compartment grew for 28 days into a subsurface compartment containing acid subsoils from the Cecil (oxidic and kaolinitic), Creedmoor (montmorillonitic) and Norfolk (kaolinitic) series. The three Mg treatments consisted of native equilibrium soil solution concentrations in each soil (50 or 100 µmol L^?1) and MgCl₂ additions to achieve 150 and 300 µmol L^?1 Mg (Mg150 and Mg300, respectively) in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO₃ treatment limed to achieve a soil pH value of 6. Subsoil root growth responses to the Mg treatments were less than for the lime treatments. Root length relative to the limed treatments for all subsoils (RRL) was poorly related to the activity of the soil solution Al species (Al³⁺ and Al-hydroxyl species) and Mg²⁺. However, the RRL values were more closely related to the parameters associated with soil solution Ca activity, including (Ca²⁺), (Al³⁺)/(Ca²⁺) and (Al³⁺)/([Ca²⁺] + [Mg²⁺]), suggesting that Ca could be a primary factor ameliorating Al and H⁺ rhizotoxicity in these subsoils. Increased tolerance to Al rhizotoxicity of soybean by micromolar Mg additions to hydroponic solutions, inducing citrate secretion from roots to externally complex toxic Al, may be less important in acid subsoils with low native Ca levels.     

Dynamics of aqueous aluminum species in a podzol affected by experimental MgSO4 and (NH4)2SO4 treatments

Chemistry of aqueous Al in a podzol on a Norway spruce (Picea abies [L.] Karst.) site in the Black Forest (SW Germany) and changes induced by experimental applications of MgSO₄ were studied. Soil solution taken from the O, E and BC horizons were analyzed for the fractions ‘labile monomeric Al’, ‘non-labile monomeric Al’, and ‘acid-reactive Al’. The activities of ‘inorganic monomeric Al’ species and the saturation indices (SI) of the soil solution with respect to Al-bearing minerals were calculated using the equilibrium speciation model WATEQF. On the untreated plot, soil leachates are characterized by Al_tot concentrations of 0.1 mg L^?1 (mineral soil). In the O horizon, the fractions ‘acidsoluble Al’ and ‘non-labile monomeric Al’ (mainly organically complexed Al) together comprise 80% of Al_tot. In the leachates from the mineral soil Al³⁺ prevails, being 50% of Al_tot. Al-F-complexes make up 5 to 10% in all horizons. MgSO₄ and (NH₄)₂SO₄ treatments resulted in an intense Al mobilization up to 50 mg L^?1. In this situation, 60% of Al_tot is covered by Al³⁺ and 40% by non-phytotoxic Al-SO₄-complexes. After rainfall events, mobilized Al is quickly translocated into the subsoil, with water flow through macropores then appearing to be an important mechanism. In both treatments, soil solution chemistry was favorable for the precipitation of the Al(OH)SO₄-type minerals alunite and jurbanite. However, a control of Al solubility by this process is not likely due to kinetic restraints. Application of MgSO₄ was followed by an increase of the Mg/Al molar ratio in the soil solution, whereas the Ca/Al ratio decreased. After treatment with (NH₄)₂SO₄ both the Ca/Al and the Mg/Al ratios deteriorated.     

Related Characteristics and Multivariate Analyses in the Evaluation of White Oat Aluminum Tolerance

Standard white oat genotypes were subjected to different methods and aluminum (Al) levels under hydroponic conditions to verify the relationship between plantlet characteristics and their Al tolerance using multivariate analyses. A completely randomized design with three replications was used, adopting three evaluation protocols: “complete nutrient solution” with 0, 8, 16, and 32 mg L^?1 of Al supplied as aluminum sulfate [Al₂(SO₄)₃·18H₂O]; “complete nutrient solution” with 0, 8, 16 and 32 mg L^?1 of Al supplied as aluminum chloride (Al₂Cl₃?6H₂O); and the “minimum nutrient solution” with 0, 1, 3, and 5 mg L^?1 of Al supplied as Al₂Cl₃?6H₂O. The performance of white oat plantlet genotypes subjected to excess Al in hydroponic conditions is greatly associated with root length, where the nutrient solution composition and the Al sources interfere in these associations. The study based on the joint analysis of characteristics at plantlet level does not allow an efficient discrimination of Al-tolerant and Al-sensitive white oat genotypes.     

ALUMINUM SPECIATION OF AMENDED ACID TROPICAL SOIL AND ITS EFFECTS ON PLANT ROOT GROWTH

Exchangeable and soluble soil aluminum (Al) is limiting plant growth in many soils worldwide. This study evaluated the effects of increasing rates of dolomite and magnesium carbonate (MgCO₃) on Al³⁺, pH, dissolved organic carbon, cations, anions, and Al speciation on oil palm Deli dura × AVROS pisifera root growth. Dolomite and MgCO₃ additions significantly raised linearly soil solution pH, magnesium (Mg²⁺), nitrate (NO₃ ^?) and chlorine (Cl^?) concentrations; exponentially decreased the activity of phytotoxic Al species [aluminum (Al³⁺), aluminum sulfate (Al₂SO₄), and aluminum fluoride (AlF₃)]; and reduced manganese (Mn) concentration and activity. High activity of those species exponentially reduced root dry weight. Optimum oil palm growth was achieved at: <50 μM monomeric Al, < 30 μM Mn, and <0.20 unit of the ratio Al+Mn to calcium (Ca)+Mg. High activity of Al species and Mn in acidic soil solution cause significant reduction of the root growth. Soil acidity alleviation either with dolomite or MgCO₃ mitigates the toxic effect of Al and Mn.     

Growth, Net Photosynthetic Rate, Nutrient Status and Secondary Xylem Anatomical Characteristics of Fagus Crenata Seedlings Grown in Brown Forest Soil Acidified with H2SO4 Solution

Dry matter production, net photosynthetic rate, leaf nutrient status and trunk anatomical characteristics of Fagus crenata seedlings grown in brown forest soil acidified by adding H₂SO₄ solution were investigated. The soil acidification leaded to decreased (Ca+Mg+K)/Al molar ratio in the soil solution. Dry mass per plant of the seedlings grown in the soil treated with H⁺ at 120 mg·L^?1 was significantly reduced compared with the control value at 0 mg·L^?1. When net photosynthetic rate was reduced in the seedlings grown in the soil treated with H⁺ at 120 mg·L^?1, the carboxylation efficiency and maximum net photosynthetic rate at saturated CO₂-concentration were lower than the control values. The addition of H⁺ to the soil at 120 mg·L^?1iinduced a reduction in the concentration of Ca in the leaf. By contrast, the concentration of Al in the leaf was increased with increasing the amount of H⁺ added to the soil. The annual ring formed in the seedlings grown in the soil treated with H⁺ at 120 mg·L^?1 was significantly narrower than that at 0 (control), 10, 30, 60 or 90 mg·L^?1. Based on the results obtained in the present study, we conclude that Fagus crenata is relatively sensitive to a reduction in the (Ca+Mg+K)/Al molar ratio of soil solution compared with Picea abies.     

Effects of acid irrigation and liming in a norway spruce stand (Picea abies [L.] KARST.)

Acid irrigation (pH 2.7 to 2.8; mean annual input 4.1 kmol H⁺ ha^?1 as H₂SO₄) has caused significant changes in the chemistry of the soil of a mature Norway spruce stand (Picea abies [L.] KARST.) after 4 years of treatment. In the surface humus layer around 20% of the exchangeable Ca, Mg, K and Mn ions were leached. This was connected with a decrease of pH and cation exchange capacity. In the mineral soil no changes of pH and cation adsorption were observed. However there was a significant increase of Al³⁺ ions in the soil solution, exceeding 20 mg L^?1, mainly caused by dissolution of Al-hydroxides and Al-hydroxosulphates. Also the concentrations of ionic Cu, Zn and Cd were nearly doubled. Manganese concentrations are fluctuating according to periods with and without acid irrigation, showing reduction and oxidation phases. In contrast to microorganisms, certain moss species and Oxalis acetosella, the mature spruce stand was not severely damaged up to now. It is hypothesized that Ca/Al and Mg/Al ratios of single horizons are insufficient for characterizing Al stress in the field. Liming (4 Mg ha^?1) led to a significant increase of dissolved organic C, which is associated with mobilization of metals such as Pb, Cu and Al in organic complexation. Also nitrification increased in the surface humus layer. As a consequence the nitrate concentrations in the seepage water exceeded 250 mg NO₃ L^?1.     

Chemical effects of pH 3 sulphuric acid on a soil profile

Cores of podzolic soil (monolith lysimeters) were treated for 4.8 yr with 1500 mm yr^?1 of either 0.5 mM H₂SO₄ at pH 3, equivalent to 24 g S m^?2 yr^?1 (acid treated) or distilled water (controls). The acid treatment was about 37 times greater than the average annual input of H₃O⁺ from rain at the site from which the monoliths were taken. Acid treatment acidified the litter (from pH(CaCl₂)3.4 to pH(CaCl₂)2.6) and the mineral soil to a depth of 80 cm (mean pH(CaCl₂) decrease of 0.2 unit). In the litter and upper A horizon, ion-exchange reactions provided the main neutralizing mechanism, resulting in a decrease in the reserves of extractable (in 2.5 % acetic acid) Ca, Mg, and Mn of about 70 to 80 %. Dissolution of solid phase Al from hydrous oxides provided most neutralization below this depth. Al3⁺ was the principal soluble Al species throughout the profile. In the litter and upper A horizon, some of the mobilized Al³⁺ was retained on cation exchange sites resulting in an increase in exchangeable Al. Deeper in the profile, where the exchange sites were effectively saturated with Al³⁺, no increase in exchangeable Al occurred, and Al³⁺ was, therefore, available for leaching. Some reversible adsorption of SO₄ ^2?, associated with hydrous Al oxides, occurred in the Bs and C horizons. The results are discussed in relation to possible effects of acid deposition over regions of Europe and N. America.     

Effects of reduced atmospheric deposition on soil solution chemistry and elemental contents of spruce needles in NE—Bavaria,Germany

The decrease in anthropogenic deposition, namely SO₄^2— and SO₂, in European forest ecosystems during the last 20 years has raised questions concerning the recovery of forest ecosystems. The aim of this study was to evaluate if the long term data of element concentrations at the Fichtelgebirge (NE‐Bavaria, Germany) monitoring site indicates a relationship between the nutrient content of needles and the state of soil solution acidity. The soil at the site is very acidic and has relatively small pools of exchangeable Ca and Mg. The trees show medium to severe nutrient deficiency symptoms such as needle loss and needle yellowing. The Ca and Mg concentrations in throughfall decreased significantly during the last 12 years parallel to the significant decline in the throughfall of H⁺ and SO₄^2— concentrations. Soil solution concentrations of SO₄^2—, Ca and Mg generally decreased while the pH value remained stable. Aluminum concentrations decreased slightly, but only at a depth of 90 cm. Simultaneously a decrease in the molar Ca/Al and Mg/Al ratios in the soil solution was observed. Ca and Mg contents in the spruce needles decreased, emphasizing the relevance of soil solution changes for tree nutrition. The reasons for the delay in ecosystem recovery are due to a combination of the following two factors: (1) the continued high concentrations of NO₃^— and SO₄^2— in the soil solution leading to high Al concentrations and low pH values and, (2) the decreased rates of Ca and Mg deposition cause a correlated decrease in the concentration of Ca and Mg in the soil solution, since little Ca and Mg is present in the soil's exchangeable cation pools. It is our conclusion that detrimental soil conditions with respect to Mg and Ca nutrition as well as to Al stress are not easily reversed by the decreasing deposition of H⁺ and SO₄^2—. Thus, forest management is still confronted with the necessity of frequent liming to counteract the nutrient depletion in soils and subsequent nutrient deficiencies in trees.     

Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions

Alleviation by calcium (Ca) of inhibition of soybean [Glycine max (L.) Merr. cv. ‘Ransom'] root elongation by hydrogen (H) and aluminum (Al) was evaluated in a vertical split‐root system. Roots extending from a limed and fertilized soil compartment grew for 12 days into a subsurface compartment containing nutrient solution with treatments consisting of factorial combinations of either pH (4.0, 4.6, and 5.5) and Ca (0.2, 2.0, 10, and 20 mM), Al (7.5, 15, and 30 μM) and Ca (2.0,10, and 20 mM) at pH 4.6, or Ca (2, 7, and 12 mM) levels and counter ions (SO₄ and Cl) at pH 4.6 and 15 μM Al. Length of tap roots and their laterals increased with solution Ca concentration and pH value, but decreased with increasing Al level. Length of both tap and lateral roots were greater when Ca was supplied as CaSO₄ than as CaCl₂, but increasing Ca concentration from 2 to 12 mM had a greater effect on alleviating Al toxicity than Ca source. In the absence of Al, relative root length (RRL) of tap and lateral roots among pH and Ca treatments was related to the Ca:H molar activity ratio of solutions (R²≥0.82). Tap and lateral RRL among solutions with variable concentrations of Al and Ca at pH 4.6 were related to both the sum of the predicted activities of monomeric Al (R²≥0.92) and a log‐transformed and valence‐weighted balance between activities of Ca and selected monomeric Al species (R²≥0.95). In solutions with 15 μM Al at pH 4.6, response of tap and lateral RRL to variable concentrations of CaSO₄ and CaCl₂ were related to predicted molar activity ratios of both Ca:Al³⁺ (R²≥0.89) and Ca:3 monomeric Al (R²≥0.90), provided that AISO₄ and AI(SO₄)₂ species were excluded from the latter index. In all experiments H and Al inhibited length of lateral roots more than tap roots, and a greater Ca:H or Ca:Al concentration ratio was required in solutions to achieve similar RRL values as tap roots.     

Effects of phosphogypsum or calcium sulfate on aluminon reactive aluminum in solutions at varying pH

Abstract

Growing evidence of positive crop responses to gypsum or phosphogypsum (PG) application in acid soils strongly support the use of these amendments as an ameliorant of subsoil acidity. Although gypsum improves Ca availability in subsoils, its role in alleviation of Al toxicity needs careful attention. In the current study, either PG, CaSO₄.2H₂O or CaCl₂.2H₂O was added (to supply 12 mM Ca) to solutions containing 40 μM Al at pH 4.1 + 0.1. Solution pH was gradually raised to 4.5, 4.8 and then to 5.3 at various time intervals during 25 d aging of the solutions at 25 + 1^OC.

Concentration of Al measured by aluminon method without preacidification and preheating, referred to as “reactive Al”; in this paper, was 16 μM in 2 g L^‐1PG solution without added Al. This accounted 38% of total soluble Al in PG solution. Addition of 2 g L^‐1PG to solution containing 40 μM Al, resulted in only 42% of total Al in solution present in forms reactive with aluminon. According to MINTEQ speciation model, Al in solution was present as an entirely complexed form with F^‐. An increase in solution pH up to 5.3 had no effect on measured concentration of reactive Al or predicted distribution of Al species.

Addition of CaSO₄.2H₂O to 40 μMAl solutions had no effect on the concentration of reactive Al within pH 4.1 ‐4.8, however, up to 62% of total Al was in a form complexed with SO₄ ^2‐, as predicted by MINTEQ model. The concentration of reactive Al decreased by 60% at pH 5.3. Addition of CaCl₂.2H₂O also had no effect on the concentration of reactive Al within pH 4.1 ‐ 4.8. Nearly 73 ‐ 94% of total Al was present in Al³⁺form. An increase in pH to 5.3, decreased the concentration of reactive Al by 27%. The results suggest that ion‐pairing of Al with F^‐would appear to be a possible mechanism for alleviation of Al toxicity by PG at pH range 4.1 ‐ 5.3. With regard to CaSO₄.2H₂O, at pH 4.1 ‐ 4.8 ion‐pairing with SO.₄ ^2‐appears to be possible mechanism for the alleviation of Al toxicity. In addition, at pH 5.3 a considerable decrease in reactive Al was evident which would further alleviate Al toxicity.     

Effects of aluminium on growth and nutrition in birch seedlings under magnesium- or calcium-limiting growth conditions

Growth and nutrition of birch seedlings (Betula pendula Roth) with and without 0.5 mM Al³⁺ were studied. The seedlings were cultivated at growth-limiting Mg- or Ca-conditions by adding one of these elements at relative addition rates (R_Mg, R_Ca) of 0.15 day^?1. The concentration of Ca²⁺ in the Mg-limited treatments was 0.01.0.1 or 1 mM and Mg²⁺ was given in equal molarities in the Ca-limited treatments. The relative growth rates, R_G, of the plants and plant parts attained values close to R_Mg or R_Ca and were not affected by the concentration of Ca²⁺, Mg²⁺ or Al³⁺ in the culture solution. Uptake of Ca, at Ca-limitation, increased significantly in the roots after addition of Al, and decreased in all plant parts when the Mg concentration of the culture solution was raised. Less clear effects were found on Mg uptake at Mg-limitation. The inhibitory effect of Al on Mg uptake decreased with time as a result of the growth technique. The root mass fraction was significantly larger in plants limited in Ca (24%) than in plants limited in Mg (20%). The usefulness in using the balance between Ca and Al or ratios between Ca, Mg, K and Al as diagnostic tools for assessing the nutritional status of trees is discussed.     

Effects of selected cations and anions on pK-0.5pCa values for a Mn oxide system

We studied the relative influences of Mn²⁺, Al³⁺ NO₃^? and SO₄^2? on the K-Ca relationship, as expressed by a pK-0.5pCa value, in a Mn oxide suspension system using a double indicator electrode technique. With an increase in the quantity of solution Mn²⁺ and Al³⁺ in the system, Mn²⁺ ions showed a stronger effect than Al³⁺ on increasing pK-0.5pCa values, though this was not the case when the amount of the cations added was low. The stronger effect of Mn²⁺ on the K-Ca relationship was attributed to stronger specific adsorption of the Mn²⁺ ion on the Mn oxide as compared with Al³⁺. When K⁺ and Ca²⁺ were added after the addition of Mn²⁺ and Al³⁺, activities of K and Ca in equilibrium solution were higher than those when K⁺ and Ca²⁺ were added first, indicating that the exchange reaction of the cations on the Mn oxide was not fully reversible. Compared to NO₃^? added systems, the pK-0.5pCa values for SO₄^2? added systems were lower when the initial K/Ca molar ratios were 0.5 and 2. This effect of SO₄^2? on the pK-0.5pCa values was also related to the quantity of Ca ions in the system since no significant difference in pK-0.5pCa values was observed between the SO₄^2? and NO₃^? added systems when the K/Ca molar ratio was 8. The present results revealed that both cationic and anionic environments had a profound influence on the adsorption of K and Ca and, thus, the pK-0.5pCa values in a Mn oxide system. The findings of this study help attain a better understanding of the chemistry of K in soils, particularly those with variable charge.     

Soil Solution Chemical Response to Two Decades of Experimental Acidification at the Bear Brook Watershed in Maine

We examined long-term changes in soil solution chemistry associated with experimental, whole watershed-acidification at the Bear Brook Watershed in Maine (BBWM). At BBWM, the West Bear (WB) watershed has been treated with bimonthly additions of ((NH₄)₂ SO₄) since 1989. The adjacent East Bear (EB) watershed serves as a biogeochemical reference. Soil solution chemistry in the EB watershed was relatively stable from 1989?C2007, with the exception of declining SO₄?CS concentrations associated with a progressive decline in SO₄?CS deposition during this period. Soil solution chemistry in WB reflected a progressive change in acid-neutralization mechanisms from base cation buffering to Al buffering associated with treatment during this period. Total dissolved Al concentrations progressively increased over time and were ~4× higher in 2007 than in 1989. Treatment of WB was also associated with long-term increases in soil solution H⁺, SO₄?CS, and NO₃?CN, whereas soil solution dissolved organic carbon (DOC) was unresponsive to treatment. For solutes such as Ca, H⁺, and SO₄?CS, changes in stream chemistry were generally parallel to changes in soil solution chemistry, indicating a close coupling of terrestrial and aquatic processes that regulate the chemistry of solutions in this first-order stream watershed. For other solutes such as Al and DOC, solute concentrations were higher in soil solutions compared with streams, suggesting that sorption and transformation processes along hydrologic flow-paths were important in regulating the chemistry of solutions and the transport of these solutes.     

Evalution of Acid Deposition in Korea

This study was carried out to evaluate acid depositions and to understand their effect. Wet precipitation has been collected at twenty-four sites in Korea for one year of 1999. The ion concentrations such as H⁺, Na⁺, K⁺, Mg²⁺, NH₄ ⁺, Ca²⁺, Cl^?, NO₃ ^? and SO₄ ^2? were chemically analyzed and determined. Precipitation had wide range of pH(3.5～8.5), and volume-weighted average was 5.2. The contribution amounts of Cl^?, SO₄ ^2? and NO₃ ^? in anion were shown to be 54%, 32%, and 14%, respectively and those of Na⁺ and NH₄ ⁺ in cation were 32% and 25%. The ratios of Cl^? and Mg²⁺ to Na⁺ in precipitation were similar to those of seawater, which imply that great amount of Cl^? and Mg²⁺ in precipitation could be originated from seawater. The concentration of H⁺ is little related with SO₄ ^2?, NO₃ ^? and Cl^? ions, whereas nss^?SO₄ ^2? and NO₃ ^? are highly correlated with NH₄ ⁺, which could suggest that great amount of SO₄ ^2? and NO₃ ^? exist in the form of ammonium associated salt. The annual wet deposition amounts (g m^?2year^?1) of SO₄ ^2?, NO₃ ^?, Cl^?, H⁺, NH₄ ⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ were estimated as 0.88～4.89, 0.49～4.37, 0.30～9.80, 0.001～0.031, 0.06～2.15, 0.27～4.27, 0.10～3.81, 0.23～1.59 and 0.03～0.63.     

Chemical Composition of Precipitation, Throughfall and Soil Solutions at Two Forested Sites in Guangzhou, South China

Rain water at two forested sites in Guangzhou (south China) show high concentrations of SO₄ ^2?, NO₃ ^? and Ca²⁺ and display a remarkable seasonal variation, with acid rain being more important during the spring and summer than during the autumn and winter. The amount of acid rain represents about 95% of total precipitation. The sources of pollutants from which acid rain developed includes both locally derived and long-middle distance transferred atmosphere pollutants. The seasonal variation in precipitation chemistry was largely related to the increasing neutralizing capacity of base cations in rainwater in winter. Soil acidification is highlighted by high H⁺ and Al³⁺ concentrations in soil solutions. The variation in elemental concentration in soil solution was related to nitrification (H⁺, NH₄ ⁺ and NO₃ ^?) and cation exchange reaction (H⁺, Al³⁺) in soil. The negative effect of soil acidification is partly dampened by substantial deposition of base cations (Ca²⁺, Mg²⁺ and K⁺) in this area.     

Aluminium and sulphate in acidic soils and groundwaters on the Swedish west coast

Acidic groundwaters and soils in Halland County (Hailands län), southwest Sweden, have been investigated with respect to conditions of soluble aluminium (Al) and sulphate (SO₄ ^2?. Basic Al-sulphate, Fe-oxide, Al-oxide, Al-hydroxide and clay minerals, are discussed and evaluated in their roles for governing Al and SO₄ ^2? in the groundwaters. Based on this investigation, it is suggested that Al³⁺ solubility is controlled by amorphous Al-hydroxide. The SO₄ ^2? in the groundwaters will depend primarily on the H₂SO₄ input. The H₂SO₄ load enhances soil mineral weathering which enhances the production of Fe-oxides, i.e. anion exchange surface sites, to which groundwater SO₄ ^2? attain adsorption equilibra. The factors that control solubility of Al and SO₄ ^2? are both influenced by the acidity in the soil catenas which in the area largely depend on the H₂SO₄ input. Clay minerals such as illite, smectites, halloysite, and variable composition Al-silicates do not exert strong control on Al in the groundwaters investigated.     

Aluminium Toxicity Risk for Pinus pinaster in Acid Soils (Galicia,NW Spain)

The influence of bedrock on aluminium toxicity and aluminium speciation in the soil solution was studied in four Pinus pinaster plots. Growth and biomass parameters in the acidic soils were also evaluated in relation to different Al toxicity indices. The plots were developed over slate, biotitic schist, mica schist and granite. Samples of rhizospheric and non‐rhizospheric soil, 1‐year‐old needles and roots were collected in each study plot. Total Al, reactive Al, acid‐soluble Al, non‐labile and labile Al and Al species (Al³⁺, Al‐OH, Al‐F and Al‐SO₄) were determined in soil solution. Reactive Al dominated over the acid‐soluble Al, and the non‐labile Al predominated over the labile Al in all soils, but particularly over mica schist. In the biotitic schist soil, the Al forms and total Al were lower, whereas concentrations were always higher over mica schist. The Al forms considered most toxic were Al³⁺ and Al‐OH, and Al concentrations were highest over slate and mica schist. Al toxicity indices in soil, needle and roots showed a risk of toxicity in mica schist, slate and granite. The stand site indices over slate and mica schist were lower, consistent with the high labile Al and Al³⁺ + Al‐OH in soil solution. Despite the high stand site index over granite, the growth efficiency was low, in accordance with very low ratios of Ca/Al in needles or fine roots. This confirmed the adaptation of maritime pine to granitic substrates. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatio-temporal variations in nutrient concentration in soil solution under greenhouse tomato

The aim of this trial was to study the spatio-temporal variability in solution nutrient concentration under intensive greenhouse tomato production, to determine the number of suction-cups needed to obtain a representative sample and the influence by the position in the greenhouses. Twenty sampling points were selected within the greenhouse with one suction-cup per sampling point. One soil solution were sampled per point at weekly intervals to analyze for pH, electrical conductivity, chloride, nitrate, phosphate, sulfate, sodium, potassium, calcium, and magnesium (EC, Cl^?, NO₃^?, H₂PO₄^?, SO₄^2—, Na⁺, K⁺, Ca²⁺, and Mg²⁺) concentrations. The pH, Cl^?, H₂PO₄^?, and SO₄^2? concentrations showed no spatio-temporal variation but EC, NO₃^?, and K⁺ showed temporal variation. The spatial variability in EC, K⁺_, Na⁺, Mg²⁺, and Ca²⁺ can be influenced by microclimate and topography. The numbers of suction cups required for a representative sample ranged from 1 to 10 depending on nutrient.     

Are Indicators for Critical Load Exceedance Related to Forest Condition?

The aim of this study was to evaluate the suitability of the (Ca?+?Mg?+?K)/Al and the Ca/Al ratios in soil solution as chemical criteria for forest condition in critical load calculations for forest ecosystems. The tree species Norway spruce, Sitka spruce and beech were studied in an area with high deposition of sea salt and nitrogen in the south-western part of Jutland, Denmark. Throughfall and soil water were collected monthly and analysed for pH, NO₃-N, NH₄-N, K, Ca, Mg, DOC and Al_tot. Organic Al was estimated using DOC concentrations. Increment and defoliation were determined annually, and foliar element concentrations were determined every other year. The throughfall deposition was highest in the Sitka spruce stand (maximum of 40 kg N ha^?1yr^?1) and lowest in the beech stand (maximum of 11 kg N ha^?1yr^?1). The Sitka spruce stand leached on average 12 kg N ha^?1yr^?1 during the period 1988–1997 and leaching increased throughout the period. Only small amounts of N were leached from the Norway spruce stand whereas almost no N was leached from the beech stand. For all tree species, both (Ca?+?Mg?+?K)/Al and Ca/Al ratios decreased in soil solution at 90 cm depth between 1989 and 1999, which was mainly caused by a decrease in concentrations of base cations. The toxic inorganic Al species were by far the most abundant Al species at 90 cm depth. At the end of the measurement period, the (Ca?+?Mg?+?K)/Al ratio was approximately 1 for all species while the Ca/Al ratio was approximately 0.2. The lack of a trend in the increment rates, a decrease in defoliation as well as sufficient levels of Mg and Ca in foliage suggested an unchanged or even slightly improved health condition, despite the decreasing and very low (Ca?+?Mg?+?K)/Al and Ca/Al ratios. The suitability of these soil solution element ratios is questioned as the chemical criteria for soil acidification under field conditions in areas with elevated deposition rates of sea salts, in particular Mg.     

Ionic equilibria,selectivity and diffusion of cations in soil as influenced by anion additions

Abstract

An experiment was carried under controlled conditions to investigate the influence of the anions, H₂PO₄ ^‐. and Cl on the ionic equilibria, selectivity and effective diffusion of Rb, K, Na, Ca, Mg in two Indiana soils.

Additon of anions to the soils increased the concentration of cations in soil solution. In both the soils receiving H₂PO₄ ^‐, lower cation concentrations were found in the soil solution than in those receiving Cl^‐ . Additon of H₂PO₄ ^‐and Cl^‐ reduced the ion selectivity coefficient, k, for various homovalent (Rb/K, Rb/Na, K/Na, Ca/Mg) and mono‐divalent ion pairs (Rb/Ca, Rb/Mg, K/Ca, K/Mg). In Zanesville soil treatments receiving H₂PO₄ ^‐ had lower k values for mono‐divalent cations than treatments receiving Cl^‐. However, no such conclusions could be drawn for Raub soil. Soils treated with H₂PO₄ ^‐ had higher k values for homovalent cations than Cl^‐ treated soils. The differences in the selectivity of adsorption in these two soils might be attributable to the differences in the type and nature of exchange materials and cation concentrations on the exchange phase.

Addition of H₂PO₄ ^‐ or Cl^‐ enhanced the magnitude of effective diffusion coefficient. (De) of all the cations under considerations. The magnitude of effective diffusion coefficient for cations was lower for H₂PO₄ ^‐ treated soils than Cl^‐ treated soils. Such a reduction in De is related to the reduction in cation concentration in soil solution thereby increasing the buffer capacity for the ions under consideration.