Environmental aspects of acid sulphate soils - ROOT of content

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differ from Fe-rich acid sulphate soils in their behaviour after flooding and reduction,as discussed below.Reduction processesReduction of redox elements that are common in soils, such as Fe, Mn, S, and N,consumes protons and is responsible for the increase in pH commonly observed inacid soils after waterlogging. Increased pH upon flooding is the main reason whywetland rice cultivation is succesful on acid sulphate soils. While the increase in pHand associated decrease in dissolved Al3+ are favourable for the crop, other changesfollowing reduction are not. These include the formation of other potentially toxicsubstances, such as Fe2+, organic acids and HzS, in the course of soil reduction. Moreover,soil reduction or pH increase following soil reduction are often limited in acidsulphate soils.Reduction of Fe111 and its chemical consequencesIn most moderately acid soils (pH 4-6), waterlogging causes an increase in pH to avalue between 6 and 7 after several weeks of flooding. Usually, reduction of Fe(ll1)to Fe(I1) is quantitatively the most important process involved in this rise in pH (Ponnamperuma1972, Patrick and Reddy 1978).Equation 5 illustrates that protons are consumed in the reduction of Fe(I1I) oxides,with organic matter (‘CH,O’) as the electron donorFe20~+ 1/2CH,O(,,) + 4H+(,,, + 2Fe2+Cdq) + 1/2COz,g,,,) + 5/2Hz0,,) (5)Hypothetical changes in pH and the concentration of Fe2+ upon reduction of soilswith various proton donors are indicated in Figure I. In acid sulphate soils, theseprotons can be derived from free sulphuric acid and desorption of adsorbed sulphate,producing soluble ferrous sulphate, according to pathway d. In acid soils with littlefree acidity and sulphate, hydroxylation of exchangeable Al3+ during reduction ofFeIII produces Al hydroxide plus exchangeable FeZ+ (pathway a). In slightly acidto near neutral conditions, CO, may serve as a proton donor, producing HCOq plussoluble Fe2+ (pathway c), part of which may be exchanged for other cations (pathwayb). The increase in pH and in the concentration of dissolved Fe2+ stops when saturationwith ‘Fe(OH),’ has been reached. By that time, acid sulphate soils have a somewhatlower pH and much higher levels of dissolved Fe2+ (pathway d-c) than acid ‘nonsulphate’soils (pathway a-c), and than slightly acid soils with appreciable amountsof exchangeable bases (pathway b). Sulphate reduction, which may follow reductionof FeIII, produces HzS and HCO). This causes a further increase in pH but tendsto decrease dissolved Fe2+ by precipitation of FeS so that, ultimately, pH and dissolvedFe in flooded acid sulphate soils may reach the same values as in ‘normal’ floodedsoils.In most moderately acid wetland rice soils, the pH rises to equilibrium values between6.5 and 7 within a few weeks of flooding. The same is true for many very youngacid sulphate soils, where an increase in pH from 3-3.5 to 5.5-6 is associated with‘Fe(OH)2’ is a proxy for the unknown Fell-containing hydroxide precipitates which form the bulk ofsolid Fe11 in most seasonally-reduced soils, with an apparent solubility product of about (van Breemenand Moormann 1978)396
1 o3[Fe2’]mg.1-‘1 o210’I I I4 5 61 iPH..Figure 1 Hypothetical changes in the concentration ofdissolved Fe2+ and pH in the course of soil reduction(van Breemen.1988). Dotted lines refer to H2S concentrations (mg 1-’) in equilibrium with Fe2+.Maximum concentrations Fe2+ are determined by the solubility of ‘Fe(OH)2’, as indicated bythe bold line. .Different arrows refer to different proton donors (exchangeable Al, CO2 andadsorbed SO4), with and without formation of exchangeable Fe2+, as follows:a FeOOH + 1/4CH@ + 2/3 Al3+(,,,,) + 1/4H2O +Fe2+(exctq + 2/3AI(OHh(s) + 1/4co2b FeOOH + 1/4CH20 + 7/4C02 + M2+(,,,h) + 1/4HzO +Fe2+(,xdlj + M2+(,,) + 2HC03-(,,)c FeOOH + 1/4CH20.+ 7/4co2 + 1/4H20 +Fe2+(,,) + 2HC0-3(,,)d FeOOH + 1/4CH20 S042-c,d,j + 1/4H20 +Fe2+(,,) + + 1/4co2 + 20H-(,dS)a steep rise in dissolved Fe2+. In most older acid sulphate soils, however, the pHincreases very slowly after waterlogging and, sometimes, does not reach the valuesbetween 5.5 and 6.5 indicated in Figure I (Ponnamperuma 1972, van Breemen 1976).The reasons for this behaviour have not been studied explicitly. A slow rise in pHcan be attributed to (i) slow reduction or (ii) appreciable reduction (fermentation)in the absence of inorganic reducable substances, e.g Fe111 oxides (Ponnamperuma1972, van Breemen 1976, Munch and Ottow 1982). In case (i), neither Eh nor pHwill change much after flooding. In case (ii), the Eh will drop without concomittantincrease in pH. Slow reduction has often been observed, and has been attributed toa low content of metabolizable organic matter (Kawaguchi and Kyuma 1968, Ponnamperuma1972, Van Breemen 1976), and/or to adverse effects of low pH, high dissolvedAI, and poor nutrient status on the activity of anaerobes (Kawaguchi andKyuma 1968, van Breemen 1976). The strong reduction, and associated increase inpH upon flooding of very young acid sulphate soils, suggests that the presence ofrelatively undecomposed organic matter from the original mangrove vegetation and397
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