Acid rain, cation dissolution, and sulphate retention in three tropical soils

Surface and subsurface samples of three tropical soils were examined with respect to their interaction with dilute solutions of sulphuric acid of pH 3. In calcareous clayey samples with a large cation exchange capacity the H⁺ was replaced by an equivalent concentration of metal cations which remained in solution along with SO^2-₄ as counterion. In a coarse-textured neutral soil with small cation exchange capacity, there was less chemical interaction and a major proportion of the H₂SO₄ remained unchanged in the equilibrium solution. Another soil exhibited considerable ability to remove SO^2-₄ from solution and, therefore, the total ionic concentration was greatly reduced. Other samples showed behaviour which was intermediate to these three types.
The ability to adsorb SO^2-₄ is one of the most important factors which determines the nature of the interaction of soil with dilute sulphuric acid. This ability was shown to be affected by the content of hydrous sesquioxides and organic matter in these soils.     

Effect of liming and phosphate additions on sulphate leaching in soils

The effect of lime (CaCO₃) and phosphate additions on surface charge characteristics and their effect on the leaching of sulphate were examined for two soils (Patua loam and Tokomaru silt loam) which differed in their adsorption capacities for sulphate.
Incubation of soils with either CaCO₃ (0–600 mmol kg⁻¹) or phosphate (0-208 mmol kg⁻¹) resulted in a two- to five-fold increase in the net negative charge and a similar decrease in the adsorption of sulphate. The effect of either lime or phosphate addition on both the surface charge and sulphate adsorption was more pronounced for the allophanic Patua soil than for the Tokomaru soil containing mainly vermiculite.
In a column experiment, liming induced the leaching of sulphur either by the desorp-tion of adsorbed sulphate or by the mineralization of organic sulphur. During a miscible displacement study, addition of either CaCO₃ or phosphate resulted in an early breakthrough of sulphate in the leachate. In a pulse experiment, in which soils were incubated with sulphate (3.12 mmol kg⁻¹) for 1 week and subsequently leached with water, more added sulphate was lost in the leachate of the soils previously incubated with either CaCO₃ or phosphate.     

Acid inputs from the atmosphere in the United Kingdom

Abstract. Inputs of acidity to the ground arise through two distinct routes: wet deposition which includes all acidity deposited in rain and snow and dry deposition, the direct sorption of SO₂, NO₂ or HNO₃ gases by vegetation or soil surfaces. The acidity from dry deposition of SO₂ and NO₂ is created during the oxidation of deposited SO₂ and NO₂ to SO²₄ and NO₃⁻ respectively. The areas of Britain experiencing the largest wet deposition of acidity are the high rainfall areas of the west and north, in particular the west central highlands of Scotland, Galloway and Cumbria where inputs exceed 1 kp H⁺ ha⁻¹ annually. Wet deposited acidity in the east coast regions of Britain is in the range 0.3–0.6 kg H⁺ ha⁻¹ a⁻¹. Monitoring data for rainfall acidity at rural sites throughout northern Britain show a decline in deposited acidity of about 50% during the last six years. Dry deposition is largest in the industrial midlands and southeast England and in the central lowlands of Scotland, where concentrations of SO₂ are largest. In these regions the dry deposition of SO₂ following oxidation may lead to acid inputs approaching 3 kg H⁺ ha⁻¹ a⁻¹ and greatly exceeding wet deposition.     

Effect of rewetting air-dried soils on pH and accumulation of mineral nitrogen

The effect of rewetting a number of air-dried soils on pH and on accumulation of mineral-N was examined in a laboratory incubation study. When rewetted-soils were incubated at 25°C three patterns of change in soil _pH and in accumulation of mineral-N were observed. Ammonification and nitrification proceeded together in soils with pH values greater than 6.0; soil pH decreased whilst concentrations of nitrate rose and those of ammonium remained low. By contrast, in soils with pH values less than 5.0, although ammonification proceeded there was no appreciable nitrification; soil pH increased whilst concentrations of ammonium rose and those of nitrate remained very low. In a third group of soils with pH values between 5.0 and 5.5, there was a delay in nitrification, but ammonification was not retarded; soil _pH initially rose as concentrations of ammonium increased, but when nitrification subsequently commenced the _pH decreased, concentrations of nitrate rose and those of ammonium declined. When microbial activity in rewetted soils was inhibited by incubation at 3°C, or in a chloroform atmosphere at 25°C, there was little change in concentrations of ammonium and nitrate, and soil pH remained relatively constant.
Such changes in soil _pH, induced by ammonification and nitrification, are likely to have important consequences to soil chemical studies where _pH-dependent reactions are being studied using rewetted soils. Changes in pH can be minimized by using field moist rather than air-dried soils.     

Correlations between extractable Na, K, Mg, Ca, P & N from fresh and dried samples of two Aquults

Significant increases in extractable ions resulted from air-drying and grinding samples of two infertile Aquults. Effects of the sample preparation differed markedly between ions and between the two soils. Regression equations were calculated to predict extractable ions in dried, ground samples from extractable ions in fresh, unground samples and the relationships were compared between the two soil series. Regressions were significantly different between soils for extractable PO³₄, Mg⁺⁺, and K⁺, but not for Ca⁺⁺ and Na⁺. Extractable NH ⁺₄ and NO-₃ in fresh, unground samples were not correlated with those in air-dry, ground samples of either soil. Differences in response to preparation between soil types appeared to be related to the oxidative status of these soils in the field, wherein constituents of more poorly-drained soils may be less stable to the oxidizing conditions of air-drying and grinding. Such complexities suggest that effects of sample preparation should be considered when interpreting soil nutrient data for studies of forest nutrient cycling and forest soil fertility.     

Release of potassium from some benchmark soils of India

Release of potassium from 15 surface samples of benchmark Alluvial, Red and Black soils of India to 0.01 M solutions of BaCl₂, CaCl₂, NH₄Cl and NaCl was studied in soils either untreated or pretreated with 5 × 10⁻³ M KCl. In the untreated soils, the efficacy of the extractants declined in the sequence: BaCl₂ > NH₄Cl > CaCl₂ > NaCl. Cumulative K-release was greatest from Black soils, followed by Red and Alluvial soils. From soils pretreated with 5 * 10⁻³ M KCl, more K was released than retained, and more 'native' K was released than that from untreated soils. Increase in the release of 'native' K decreased in the sequence: Red > Alluvial > Black soils. The amounts of surface and internal K, desorption rate constants and parabolic diffusion constants were calculated from K release to the various electrolytes.     

Ion Transport and Permeability in an Allophanic Andisol at Low pH

Allophanic Andisols have a significant pH-dependent charge. The positive charge increases and the negative charge decreases as pH decreases; therefore, anion movement becomes slower and cation movement becomes faster as pH decreases in the soil. At low pH, soil dispersion occurs easily due to electric repulsive force. The permeability of the soil then decreases because of structural changes that occur when dilute HCl or HNO₃ is percolated in the soil. However, soil permeability does not decrease when dilute H₂SO₄ is percolated in the soil. This is because SO₄^2- strongly adsorbs on the soil surface at low pH and the soil remains flocculated.     

The soil solution chemistry of some Oxfordshire soils: temporal and spatial variability

The immiscible liquid displacement method was used to displace soil solutions from a variety of Oxfordshire soils. These included grassland, arable, and woodland soils. The results of detailed chemical analyses demonstrated that for most solutes there were significant seasonal differences as well as differences both within and between soil series. The variation over time intervals of a few days or weeks appeared to be relatively small in relation to sampling errors. Median concentrations (in mmol m⁻³) for soil solutions derived from 21 grassland sites and six arable sites sampled six or seven times throughout 1 year were: _pH 7.7, alkalinity 1810, Na 465, K 390, Mg 135, Ca 2120, Sr 1.9, Ba 0.16, Mn 0.52, Fe 3.4, Cu 0.25, TON 860, Cl 1590, S 327, P 64, Si 220, B 5.1, and DOC 4250. For many solutes, the individual soil solution concentrations spanned a range from approximately five-times less than these median values to approximately five-times more.     

Quasi-thermodynamic basis of competitive-adsorption coefficients for anionic mixtures in soils

Equilibrium competitive-adsorption coefficients derived from the application of the multi-component Freundlich-type Sheindorf-Rebhun-Sheintuch (SRS) equation appeared to have a soil-specific, quasi-thermodynamic basis. Using three soils, single-solute adsorption isotherms at 288, 298, and 308 K were constructed with arsenate, phosphate, and molybdate. A Clausius-Clapeyron-type equation was used to estimate apparent isosteric heats of adsorption values ( q _iso). The adsorption of the three anions was characterized by different q _iso values, and was generally in the order of H₂PO₄->H₂AsO₄->MoO₄^2-. The magnitudes of the SRS competitive coefficients derived from binary-solute mixtures were in agreement with the distributions of the heats of adsorption of the individual anion-soil systems. Higher competitive coefficients, indicating the preferential adsorption of solute i in a binary-solute mixture containing solutes i and j, tended to associate with higher q _iso values for solute i relative to solute j. However, the q _iso values for the adsorption of molybdate by one soil underestimated the extent of competitive interactions between arsenate and molybdate.     

The solution chemistry of podzolic soils from the eastern Canadian shield: a thermodynamic interpretation of the mineral phases controlling soluble Al3+andH4SiO4

Thirty-one soil solutions were extracted by immiscible displacement with CCl₄ under high speed centrifugation from sub-horizons of three podzolic soils from north-eastern Ontario, Canada. The solutions were analysed for major cations and anions and a speciation of dissolved Fe and Al was attempted to distinguish 'free', 'organically bound' and 'inorganically bound' species. Results indicated that the Ae (E) horizon solutions were of low pH and contained mainly organically bound Fe and Al. With depth, _pHs increased, ionic strengths decreased and the relative proportion of inorganically bound Fe and Al increased. Although application of phase diagrams permitted only a semi-quantitative interpretation of the data, all horizon solutions, with the exception of some Ae solutions, appeared supersaturated with respect to likely occurring crystalline and amorphous aluminosilicates [kaolinite, halloysite, allophane (Al:Si=l) and imogolite]. Of the phases considered, reactions involving imogolite-allophane, gibbsite-halloysite, gibbsite-allophane and gibbsite-imogolite all appeared reasonable in controlling the content of Al³⁺ and H₄SiO₄ in solution, although the presence of gibbsite and imogolite could not be definitely confirmed in these soils.     

Methane fluxes on agricultural and forested boreal organic soils

Abstract. Annual methane fluxes from an organic soil in eastern Finland, originally drained and planted with birch ( Betula pendula ) and then later cultivated, were studied for two years using a chamber technique. The agricultural soils growing grass or barley or without vegetation, generally acted as sinks for CH₄. Surprisingly, the agricultural soils emitted CH₄ during a warm dry summer. The CH₄ oxidation capacity and CH₄ uptake rate of the forested site was three times that of agricultural soils. Also, the forest soil better retained its capacity to take up CH₄ during a dry summer. Despite periods of CH₄ emission, the agricultural soils were annual sinks for CH₄, with uptake rate of CH₄-C varying from 0.1 to 3.7 kg ha⁻¹ yr⁻¹. The forested soil had a methane uptake rate of 3.9 kg CH₄-C ha⁻¹ yr⁻¹. All the soils acted as sinks for CH₄ during winter, which contributed up to half of the annual CH₄ uptake. The capacity of soils to transport gases did not explain the larger CH₄ uptake rate in the forest soil. At the same gas filled porosity, the forest soil had a much larger CH₄ uptake rate than the agricultural soil. Neither the soil acidity (pH 4.5 and 6.0) nor high ammonium content appeared to limit CH₄ uptake. The results suggest that CH₄ oxidation in agricultural organic soil is more sensitive to soil drying than CH₄ oxidation in forested organic soil.     

The importance of exchangeable cations and resin-sink characteristics in the release of soil phosphorus

The significance of exchangeable cations in the release of phosphorus by sequential extraction with water was evaluated in 11 acid (_pH 5.0–6.3) New Zealand soils contrasting in P status and P retention. The release of P from Na-saturated soil exceeded that from the original Ca-dominated soils by up to four-fold. Possible explanations for the larger P release in the Na system include: (i) desorption of P induced by increased surface negative potential associated with the exchange of Na for Ca/Mg, and/or (ii) accelerated dissolution of Ca phosphate compounds or complexes resulting from the creation of a sink for Ca.
The potential of a series of anion- and cation-exchange resin systems (AER and CER, respectively) as sinks for labile soil P was also examined. For all soils studied, P extracted by AER-HCO₃ 3 and that removed by sequential extraction with water. Also, the amounts of P extracted by AER-OH/CER-H and NaCl/ H₂O were closely correlated ( r = 0.95**), suggesting similar release mechanisms. The results obtained indicate that charge-balancing cations, particularly Ca which is the predominant exchangeable cation in the majority of soils, exerts a more significant control on soil P equilibria in acid soils than is commonly recognized.     

An approach for estimating when soils will reach maximum nitrogen storage

Abstract. Net accumulation of organic nitrogen in soil is constrained by the amount of organic matter and its minimum C:N ratio. Our objective was to estimate the potential for New Zealand soils to continue accumulating nitrogen within the soil organic pool. We calculated total carbon and nitrogen in the top metre of 138 representative soil profiles from the New Zealand National Soils Database. Carbon in these mainly pasture soils was assumed to be at steady state. The maximum nitrogen storage capacity was estimated by calculating the amount of nitrogen stored under assumed minimum soil C:N ratios of either 9, 10 or 11. The storage capacity remaining was determined as the difference between the amount of nitrogen currently stored and the maximum storage capacity. The length of time before a soil profile will reach the maximum capacity for nitrogen storage was calculated assuming net accumulation of 20, 50 and 100 kg N ha⁻¹ yr⁻¹. A C:N ratio of 9 (giving most storage capacity) and a conservative accumulation rate of 20 kg N ha⁻¹ yr⁻¹ showed that 12% of these soils would be at maximum storage within 40 years. A C:N ratio of 10 and a storage rate of 50 kg N ha⁻¹ yr⁻¹ would result in 54% of the soils reaching maximum storage within the next 40 years. As the capacity for nitrogen storage in soils declines, nitrate leaching is likely to increase with associated risk to the environment.     

Soil S availability in upland pastures of NE Scotland: relationship of extractable soil S and soil respiration to soil and site characteristics

Abstract. The mean extractable sulphur (S) concentration in 315 upland topsoil samples collected in 1988/89 from beneath pasture in NE Scotland was 13 μg S g⁻¹ (range 2–77 μg S g⁻¹). More than two thirds of the samples had S concentrations less than that acceptable for productive soils. Continued decreases in atmospheric S inputs may have increased this proportion subsequently. The analysis of herbage S also indicated that two-thirds of the samples were below 0.2% S. A 'respirometric index', namely CO₂ produced during cellulose decomposition without added S as a percentage of that produced with added S, was significantly less than 100% in a quarter of the soils. Results of three different extraction procedures suggested that sulphate in the soils was present mainly as free plus adsorbed rather than precipitated forms. Soil extraction identified a significant non-sulphate S fraction, presumably organic S. The variability in extractable S stemmed from a combination of geographical, depositional and local site and soil factors. Extractable S was significantly correlated with soil organic matter content and inversely with soil pH and together these factors explained 37% of the variability. While significant differences in mean concentrations between geographical area, soil association and drainage status were evident, no trends could be observed between the major soil subgroups or with altitude.     

Potassium balances for arable soils in southern England 1986–1999

Abstract. The behaviour of potassium (K) in a range of arable soils was examined by plotting the change in exchangeable K of the topsoil (Δ K_ex) at the end of a 3–5 year period against the K balance over the same period (fertilizer K applied minus offtake in crops, estimated from farmers' records of yield and straw removal). Based on the assumption that values for offtake per tonne of crop yield used for UK arable crops MAFF 2000) are valid averages, 10–50% of Δ K_ex was explained by the balance, relationships being stronger on shallow/stony soils. Excess fertilizer tended to increase K_ex and reduced fertilization decreased it, requiring between 1.2 and 5.4 kg K ha⁻¹ for each mg L⁻¹Δ K_ex. However, merely to prevent K_ex falling required an extra 20 kg K ha⁻¹ yr⁻¹ fertilizer on Chalk soils and soils formed in the overlying Tertiary and Quaternary deposits, despite clay contents >18%. Whereas, on older geological materials, medium soils needed no extra K and clays gained 17 kg K ha⁻¹ yr⁻¹. It is unlikely that the apparent losses on some soil types are anomalies due to greater crop K contents. Theory and the literature suggest leaching from the topsoil as a major factor; accumulation in the subsoil was not measured. Recommendations for K fertilization of UK soils might be improved by including loss or gain corrections for certain soil types.     

Effect of redox conditions on phosphate exchangeability and iron forms in a soil amended with ferrous iron

Application of iron (Fe) -rich amendments to soils has been proposed as a means of decreasing phosphorus (P) losses from soils. However, anoxic conditions following soil saturation are known to increase Fe and P solubility in soils, thus cancelling out the potential benefits. Our aim was to evaluate the effects of continuous oxic, continuous anoxic and alternating anoxic/oxic conditions on P exchangeability and Fe forms in soil amended with Ca(OH)₂ and FeSO₄. We incubated amended and unamended soils under these conditions for 8 weeks and measured Fe forms and P exchangeability. Under oxic conditions, addition of Ca(OH)₂ and FeSO₄ resulted in a strong decrease in P exchangeability and an increase in oxalate-extractable Fe. Mössbauer analyses suggested that an unidentified Fe oxide (D1oxide) with a strong sorbing capacity for P was precipitated. Under continuously anoxic conditions, P exchangeability and oxalate-extractable Fe increased with or without the amendments. Mössbauer analyses suggested that there was a partial dissolution of the D1oxide phase, precipitation of another unidentified Fe oxide (S3) and a reduction of structural Fe³⁺ in phyllosilicate, thereby increasing soil negative charge. These transformations resulted in a strong increase in rapidly exchangeable P. Alternating anoxic and oxic periods induced the dissolution and precipitation of iron oxides and the increase and decrease in P exchangeability. Implications of the results for limiting P losses from grassland soils are discussed.     

Factors affecting formation of methyl nitrite in soils

The formation of CH₃ONO in 11 soils treated with HNO₂ or NaNO₂ in a closed system, was studied by measuring the concentration in the gas space above the soil and by absorbing CH₃ONO in HI. The gaseous concentration of CH₃ONO increased and then decreased following additions of HNO₂ or NaNO₂, and the production of CH₃ONO increased with increasing concentrations of HNO₂ or NaNO₂ added to soils.
The amounts of CH₃ONO trapped in HI were 13.5 to 20.4 times higher than those determined by integrating under the net production curves. The evolved CH₃ONO amounted to 0.4 to 3.5% of added NO₂⁻, and 4.2 to 50% of the gaseous forms of N absorbed by acidic KMnO₄ solution. The CH₃ONO evolved from soils was positively correlated with the methoxy content of the soils, and inversely related to soil pH, with negligible amounts being evolved from alkaline soils. The results show that CH₃ONO is a product of NO₂⁻ decomposition in soils, and indicate that small concentrations of the gas may be produced in N–fertilized soils in which NO₂⁻ accumulates.     

Metal-humus complexes in A horizons of Thai and Korean red and yellow soils

Carbon, Al and Fe (C_pyr, Al_pyr and Fe_pyr) were extracted with 0.1 m Na₄P₂O₇ from 26 A horizon samples of tropical Thai and temperate Korean soils (Ultisols, Alfisols, Oxisols and Inceptisols). The soils, except for one Thai Inceptisol, had similar total C (0.35–3.29%) and C_pyr/total C ratios (0.20–0.41). There were approximately linear relationships between total C or C_pyr and clay content; two groups of soils gave different linear relationships. A curvilinear relationship between C_pyr and (Al + Fe)_pyr (milli-atom kg⁻¹) that can be approximated by an equation: C_pyr= 53 (Al_pyr+ Fe_pyr)^1/2– 24 was also found for most Thai and Korean soils. The above relationships indicated that total C and C_pyr would be close to zero at zero clay or zero (Al + Fe)_pyr. It was inferred that clay-humus interaction has a primary importance in the determination of humus content in red and yellow soils in tropical and temperate regions and that the main role of clay is to supply Al and Fe that complex and stabilize humus against microbial degradation.     

Effect of Low Molecular Weight Organic Anions on the Adsorption of NO3− by Variable Charge Soils

Low molecular weight organic acids exist widely in soils and have been implicated in many soil processes. The results in the present paper showed that the presence of organic anions led to a decrease in the adsorption of NO₃⁻. The effect of citrate was much larger than that of oxalate and malate. Among different soils, the effect for Hyper-Rhodic Ferralsol and Rhodic Ferralsol was larger than that for Haplic Acrisol, which was related to the content of iron oxides in these soils. The effect of organic anions decreased with the increases in pH value and the amount of organic anions added. The organic anions depressed the adsorption of NO₃⁻ through two mechanisms, the competition of the organic anions for adsorption sites with NO₃⁻ and the change of soil surface charge caused by the specific adsorption of organic anions.     

The effects of lime on adsorption and desorption of phosphate in five Colombian soils

The effects of lime (applied in the field) on the amounts of total and isotopically-exchange-able phosphate adsorbed from solutions were measured in five soils. The total amount of phosphate adsorbed without lime was in the range 200 to 1700 μg P per g of soil at 0.05 μg P cm⁻³ of solution. Lime diminished the amount of phosphate adsorbed at all concentrations of solution in an oxisol and a dystropept; in an ultisol and another dystropept, lime tended to increase sorption at small concentrations and diminish it at large concentrations; in a dystrandept that contained spheroidal allophane and a great deal of organic matter, lime increased adsorption at all concentrations up to 1 μg P cm⁻³. Lime increased the proportion of added phosphate that was isotopically exchangeable in the oxisol and one dystropept, had no effect in the other dystropept, and diminished the proportion in the ultisol and dystrandept.
Adsorbed phosphate was subsequently desorbed by suspending the soils in solutions without phosphate. After desorption the quantity of exchangeable phosphate in all soils was closely correlated with aluminium extracted by ammonium oxalate; buffer power was correlated in all except the dystrandept, in which it was larger per unit of aluminium than in the other soils; possibly the cause was aluminium associated with organic matter. In all soils lime diminished buffer power allowing a specific amount of exchangeable phosphate to maintain a larger concentration in solution. The beneficial effects of lime on exchangeable phosphate after desorption were consistent among soils, despite inconsistent results when the phosphate was adsorbed.