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Unless properly managed, acid sulphate soils can exert a range of negative environmental impacts, including soil acidification and mobilization of metals and metalloids. Incorporation of organic matter in the form of plant mulches can substantially neutralize sulphuric soils and prevent the oxidation of sulphidic soils. These positive effects of dead plants are largely mediated by bacterial reduction of sulphates to sulphides, using the organic matter as a microbial nutrient source. However, very little is known about the effects of live plants on acid sulphate soils. In this study, we compared pH, Eh and sulphate content of sulphidic and sulphuric soils that were not planted (i.e. unplanted) with those soils planted with the following three common wetland plants: Phragmites, Melaleuca and Typha. Each of these plants is capable of growth in aerobic and flooded soils. In all our experiments, the presence of plants correlated with an increase in soil acidification rather than neutralizing soil acidity when compared to unplanted controls. The mechanism for this appears to be transport of oxygen down the soil profile by aerenchymatous tissue formed in these species, and the release of oxygen into the rhizosphere. [ABSTRACT FROM AUTHOR]

Abstract: In periodically flooded soils, interactions of Cu with biogenic sulfide formed during soil reduction lead to the precipitation of sparingly soluble Cu-sulfides. In contaminated soils, however, the amounts of Cu can exceed the amount of sulfate available for microbial reduction to sulfide. In laboratory batch experiments, we incubated a paddy soil spiked to ∼4.4mmolkg-1 (280mgkg−1) Cu(II) to monitor temporal changes in the concentrations of dissolved Cu and the speciation of solid-phase Cu during 40days of soil reduction and 28days of reoxidation as a function of initially available reducible sulfate (0.06, 2.09 or 5.92mmolkg−1). Using Cu K-edge EXAFS spectroscopy, we found that a large fraction of Cu(II) became rapidly reduced to Cu(I) (23–39%) and Cu(0) (7–17%) before the onset of sulfate reduction. Combination with results from sequential Cu extraction and chromium reducible sulfur (CRS) data suggested that complexation of Cu(I) by reduced organic S groups (Sorg) may be an important process during this early stage. In sulfate-depleted soil, Cu(0) and Cu(I)–Sorg remained the dominant species over the entire reduction period, whereas in soils with sufficient sulfate, initially formed Cu(0) and (remaining) Cu(II) became transformed into Cu-sulfide during continuing sulfate reduction. The formation of Cu(0), Cu(I)–Sorg, and Cu-sulfide led to an effective decrease in dissolved Cu concentrations. Differences in Cu speciation at the end of soil reduction however affected the dynamics of Cu during reoxidation. Whereas Cu(0) was rapidly oxidized to Cu(II), more than half of the S-coordinated Cu fraction persisted over 14days of aeration. Our results show that precipitation of Cu(0) and complexation of Cu(I) by reduced organic S groups are important processes in periodically flooded soils if sulfide formation is limited by the amount of available sulfate or the duration of soil flooding. The speciation changes of Cu described in this study may also affect the speciation and solubility of other chalcophile metals in redox-dynamic wetland soils. [Copyright &y& Elsevier]

Background: Oxidation-reduction and acid-base reactions are essential for the maintenance of all living organisms. However, redox potential (Eh) has received little attention in agronomy, unlike pH, which is regarded as a master variable. Agronomists are probably depriving themselves of a key factor in crop and soil science which could be a useful integrative tool. Scope: This paper reviews the existing literature on Eh in various disciplines connected to agronomy, whether associated or not with pH, and then integrates this knowledge within a composite framework. Conclusions: This transdisciplinary review offers evidence that Eh and pH are respectively and jointly major drivers of soil/plant/microorganism systems. Information on the roles of Eh and pH in plant and microorganism physiology and in soil genesis converges to form an operational framework for further studies of soil/plant/microorganism functioning. This framework is based on the hypothesis that plants physiologically function within a specific internal Eh-pH range and that, along with microorganisms, they alter Eh and pH in the rhizosphere to ensure homeostasis at the cell level. This new perspective could help in bridging several disciplines related to agronomy, and across micro and macro-scales. It should help to improve cropping systems design and management, in conventional, organic, and conservation agriculture. [ABSTRACT FROM AUTHOR]

Abstract: Soil science research has probably underestimated the significance that short-term, episodic cycles of reduction and oxidation has had on phosphorus (P) reactivity. Here, the effects of eleven pulsed reduction-oxidation (including wet-dry) cycles on soil P dynamics are compared for 12 soils having contrasting properties and all overfertilised with respect to P. The laboratory based incubation conditions attempted to simulate transient waterlogging of the soil profile and involved repeated sampling and analysis of both the solution and solid phase P forms. An initial increase in P concentration in solution that occurred up to and including the fourth full cycle was followed by a sharp decline in concentration for all but one soil. Accompanying changes in the main extractable forms of P, which appeared to be cumulative, could be summarised as a general decline in the organic P fraction and an overall increase in amorphous associated inorganic forms of P. The fact that up to 60% of the total soil P was demonstrated to change its sensitivity for a particular extractant suggests that these operationally defined P forms are susceptible to transformation as a consequence of changing environmental conditions. There was also a suggestion that certain of the changes in P forms were irreversible. While the laboratory conditions imposed do represent extreme conditions the soils only experienced cyclic changes in their moisture regime. If timing and frequency of intense precipitation events are likely to increase, as predicted in many climate change scenarios, then these results suggest that the effects of episodic redox pulses may have implications for P cycling in agricultural soils. [Copyright &y& Elsevier]

One possible explanation for unexplained disappearance of vinyl chloride (VC) from what are thought to be anaerobic subsurface environments is that the environments are, in fact, not anaerobic. Rather, they might be subject to low, steady influx of oxygen, and aerobic oxidation could be occurring at extremely low oxygen concentrations. Studies were conducted with VC-oxidizing transfer cultures derived from two chloroethene-contaminated sites, as well as with microcosms constructed from sediment and groundwater from one of these sites. Oxygen was steadily delivered to the experimental systems using permeation tubes to maintain low dissolved oxygen throughout the time-course of investigation. VC oxidation was sustained at dissolved oxygen concentrations below 0.02 mg/L in the two transfer cultures, and below 0.1 mg/L in the microcosms. This supports the possibility that—at least at some sites—apparent loss of VC from what are thought to be anaerobic zones might, in fact, be due to aerobic pathways occurring under conditions of low oxygen flux (e.g., via diffusion from surrounding aerobic regions and/or from recharge events). [ABSTRACT FROM AUTHOR]

Abstract: Reactive forms of nitrogen (Nr) are accumulating at local, regional and global levels largely due to human activities, particularly N-fertilizer production and use as well as fossil fuel combustion. This has resulted in a change in the nitrogen (N) cycle and excess Nr in the environment, which has negative environmental effects. Therefore, characterizing denitrification and the edaphic variables controlling denitrification and its products is the first step in predicting the long-term effects of Nr accumulation. In the present study, six forest soil types in different climatic zones were collected from East China and evaluated for denitrification products following a K15NO3 amendment and subsequent incubation. The results showed that denitrification, indicated by production of nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2), was higher in the studied temperate forest soils than in the studied subtropical and tropical forest soils and was negatively correlated with soil redox potential at the beginning of incubation (r = −0.94, P < 0.01), but not with soil pH. The ratios of NO/total N gas and N2O/total N gas produced during denitrification varied among the soils, and were generally higher in the subtropical and tropical soils. Spearman''s correlation analysis showed that the NO ratio was positively correlated with soil oxidation capacity (OXC) (r = 0.94, P < 0.01) and redox potential at the beginning of incubation (r = 0.86, P < 0.05), but negatively correlated with soil pH (r = −0.83, P < 0.05). The N2O ratio was not significantly correlated with these edaphic variables, but showed a significant correlation to NO ratio (r = 0.83, P < 0.05). These results suggested that the OXC value might be the key factor affecting denitrification rates in soils. One possible explanation for these effects is that large OXC values would result in a higher level soil redox potential, thus suppressing denitrification and enhancing NO and N2O ratios during denitrification. [Copyright &y& Elsevier]

Soil redox potentials (Eh) are commonly measured and operationally defined using platinum (Pt) electrodes, but there are variations in the methodology and instrumentation for measuring the associated voltages that may lead to imprecise and inaccurate estimates of oxidation-reduction intensity in the soil environment. Most field measurements of Eh are either made manually using a volt meter or automatically with a recording device such as a data logger. A mesocosm experiment was conducted to evaluate the impact of these two measurement approaches on resulting Eh values in three different wetland soils. Noticeable differences were observed between data collected using a multimeter (simple volt meter) and those collected with a data logger. Because the multimeter permitted the observed voltages to drift while the Pt and reference electrodes equilibrated following closure of the circuit, it was initially suspected that these were the more accurate data. By adding a 10-Mohm resistor (standard component of commercially available multimeters) to a closed circuit containing the Pt and reference electrodes, data loggers could be modified to collect data similar to those obtained using a multimerer. Further testing, however, demonstrated that the voltage drift observed when using a closed 10-Mohm circuit was an artifact of alterations in the electrochemical soil environment surrounding the Pt electrode. By using an instrument with very high (200 Gohm) input resistance, the artifactual drift could be essentially eliminated Our comparisons and experiments suggest that the most accurate measurements of soil Eh and soil redox status ate obtained using volt meters with high input resistance (e.g., 200 Gohm). Accurate Eh values also are obtained using data loggers (20 Gohm resistance) in the standard configuration that maintains an open circuit except during the actual instantaneous voltage measurement. [ABSTRACT FROM AUTHOR]

Describes the design of a polarographic assembly for multiple sampling of soil oxygen flux in the field. Outline of the sampling procedure used; Use of the equipment to monitor soil-oxidation-reduction potentials; Determination of the effective voltage of the equipment; Basis of the platinum-microelectrode technique on the reduction of oxygen at a platinum-wire cathode buried in the soil.

The article discusses the result of the study conducted regarding the oxidation-reduction reaction of soil particles. The spectroscopic study of soil particles have been sought to be an important factor in examining site contamination or potential toxic effects of the chemicals. However, the oxidation state of such chemicals has been found to influence its solubility, mobility as well as its toxicity. Furthermore, the quantitative investigation of the redox reactions in soil have been found to be disadvantageous for the lack of reliable analytical methods in studying reactions as well as the heterogeneity of the mineral components and redox systems in nature.