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It is commonly believed that nitrification is the dominant process for N2O production under aerobic conditions. However, this has been challenged by recent studies on acidic soils, where denitrification has been found to dominate N2O production. Analyzing the data collected from peer-reviewed literature, we found that pH was a critical factor regulating N2O production pathways under aerobic conditions. There is a pH threshold of approx. 4.4, below which denitrification dominated N2O production and vice versa. A decrease in soil pH can significantly increase the contribution of denitrification to N2O production. Overall, this mini-review increases our understanding of N2O sources in soils under aerobic conditions. [ABSTRACT FROM AUTHOR]

Abstract: On acid soils, no-tillage farmers are often advised to apply lime to the soil surface without incorporation by tillage. As such, it can take a number of years before the subsoil acidity is decreased. However, no-tillage seeders vary in the level of soil disturbance caused during seed placement. The consequence of such variations in soil disturbance for the effectiveness of lime in no-tillage cropping has not been explored. Our objectives were (i) to determine if the liming effect could be accelerated by increasing the rate of lime and level of soil disturbance during no-tillage seeding, and (ii) evaluate the effect of no-tillage seeding method, rate of lime and soil pH on yield of wheat, barley, canola and lupins. Three trials, each with a factorial design consisting of four seeding methods and four lime rates, were established in 1999 and continued until 2005. The soil had a texture contrast with about 15–20cm of sand over yellow sandy clay loam and the initial pH (CaCl2) at both 0–10 and 10–20cm was between 4.5 and 4.7. The four seeding treatments were: low disturbance zero-till disc openers (ZT), higher disturbance no-tillage tines with narrow knife-points (NT), higher disturbance full cut seeding with sweeps (FC) for the first three years followed by ZT from then on, and FC for the first year followed by NT thereafter. The four lime rates were 0, 1, 2 and 4tha−1. The liming effect was more rapid with the higher disturbance seeding of NT, FCZT and FCNT, compared with ZT; the seeding effect on soil pH diminished with time and was not detected in the subsoil after four years. Higher rates of lime increased this effect and there was no interaction between seeding method and lime rate. This indicated that soil pH under the different seeding methods responded in a similar manner to increased lime. Nonetheless, significant positive linear regressions were found between yield and soil pH for wheat and barley and a negative relationship for lupins. Canola showed no response to soil pH, possibly because establishment was affected by seeding method, and soil pH was not low enough to elicit a response. The ZT seeding method gave lower yields than the other methods for canola and lupins in some years, but had no effect on wheat and barley yields. Higher disturbance at the time of no-till seeding can, therefore, accelerate the liming effect, making earlier economic gains possible. [Copyright &y& Elsevier]

The persistence of fenazaquin (4-t-butylphenylethyl quinazolin-4-yl ether) was studied in three different soils, namely, Gangetic alluvial (pH 6.9), laterite (pH 5.3), and terai (pH 5.1) soil and in water at three different pH values (4.0, 7.0, and 9.2) under laboratory-simulated condition and samples were analyzed upto 60 days at regular intervals. Fenazaquin was applied at 5 and 10 µg g-1 for each soil and at 0.2 and 0.4 µg mL-1 to each water sample. Dissipation of fenazaquin in soil and water followed first-order kinetics irrespective of any treatments. The half-life of fenazaquin was found in the range of 47-64 days in soils and 3.49-37.6 days in water irrespective of dose. The persistence of fenazaquin in soil significantly increased in the order of Gangetic alluvial soil (pH 6.9) > laterite soil (pH 5.3) > terai soil (pH 5.1), whereas in water the trend of persistence was pH 9.2 ≥ pH 7.0 > pH 4.0. The dissipation of fenazaquin in soil and water was found to be dependent on pH irrespective of doses. [ABSTRACT FROM AUTHOR]

The effect of soil acidity on root and rhizosheath development in wheat and barley seedlings was investigated in an acid Ferrosol soil to which various amounts of lime (CaCO3) were applied to modify soil Al concentrations (pH (CaCl2): 4.22 to 5.35 and Al (CaCl2 extract): 17.7 to 0.4 mg kg−1 soil; respectively), and Ferrosol soil from an adjacent location at the same site which had a higher Al concentration (pH 4.19; 29.2 mg kg−1 Al). The cereal lines were selected on the basis of differences in their rate of root growth, Al-resistance and root hair morphology. Root morphology was assessed after 7 days of growth. The length of fine (mainly lateral) roots of Al-sensitive genotypes was more sensitive to soil Al concentrations than that of the coarse (mainly primary) roots. The experiments demonstrated that even where root growth was protected by expression of the TaALMT1 gene for Al-resistance, root-soil contact was diminished by soil acidity because root hair length (in many lines), and root hair density and rhizosheath formation (all lines) were adversely affected by soil acidity. In the case of Al-sensitive lines, fine root growth and rhizosheath mass were reduced over much the same range of soil Al concentrations (i.e. >3–6 mg kg−1 Al). Although Al-resistant lines could maintain fine root length under these conditions, they were similarly unable to maintain rhizosheath mass. This finding may help to explain why Al-resistant wheats which yield relatively well in deep acid soils, may also benefit from application of lime to the surface layers of the soil. [ABSTRACT FROM AUTHOR]

Grapes cultivated in warm zones have high sugar content and low acidity. The wines made with such grapes are more prone to fast evolution during maturation. Oxidation rate is pH-dependant. The purpose of this study was to compare oxidation risk in three different Malbec wines (pH levels 3.34; 4.14 y 4.33). Wines were made at the Experimental Winery, Enology I and II Chairs, of Agricultural Faculty, grapes were cultivated in the East (wine pH 4.14), North (wine pH 4.33) and South (wine pH 3.34) of Mendoza, Argentina. Assays were made in triplicate. The oxidation risk was determined by Singleton and Kramling test. Absorbance at 420 nm, absorbance at 520 nm and color density were measured. Statistical analysis was performed using ANOVA, Tukey test at the 95% confidence level. Higher pH wine registered a bigger color density increase than lower pH wines. High color density increase is related to high oxidation risk. Red wine oxidation risk can be evaluated by the modified Singleton and Kramling test. Absorbance at 520 nm must be registered and color density must be calculated. The test period might be shortened up to four days. [ABSTRACT FROM AUTHOR]

Abstract: The general objectives of this study were (i) to determine the kinetics and mechanisms of Mn(II) removal from solution by a representative plant-growth substrate; (ii) to assess the role of biotic components on Mn(II) cycling in this system; and (iii) to assess the roles of pH and temperature in determining Mn(II) partitioning between the solution and the solid phases. The Mn(II) solubility in typical growth medium (perlite) was predominantly controlled by pH-dependent reactions, i.e., adsorption and oxidation. The role of precipitation, including formation of new solid phases of Mn-P or Mn-carbonates, as possible contributing factors to Mn(II) removal from perlite suspensions was assessed and ruled out. Under acidic pH (below pH 6), Mn(II) was elevated above concentrations expected in an inert medium; it was attributed to releases from indigenous sources, on one hand, and both low adsorptive capacity and Mn-bacteria activity, on the other hand. The Mn(II) concentration declined sharply above pH 6, and at pH 7.7 dropped almost to zero, 48h after application. On a time scale of seconds to a few hours after application to perlite media the Mn(II) solubility was controlled by adsorption reactions, mainly onto the organic constituents accumulated from root excretions or decomposition and rhizosphere biota. With advancing time, biotic oxidation became important and it became the predominant mechanism of Mn(II) removal. [Copyright &y& Elsevier]

Abstract: A series of tests on the fermentation of vegetable kitchen wastes to produce hydrogen were conducted at pHs of 5.5, 6.0, 6.5, and 7.0 under a thermophilic condition. Microorganisms used in fermentation were enriched from kitchen-waste composts, and a high vegetable-content kitchen-waste was utilized. The initial substrate to microorganism ratio was kept at 10g COD/g VSS under a thermophilic (55°C) condition. A maximum specific hydrogen production rate of 0.48mmol H2/g VSS/h was achieved at pH=6.0, and the highest hydrogen yield of 0.57mmol H2/g COD occurred at pH=7.0. No hydrogen production occurred at pH=5.5. At pHs controlled at 6.0, 6.5, and 7.0, the major volatile acid produced was butyrate. Using a terminal restriction fragment length polymorphism (TRFLP) analysis for microbial identification, T-RFs of 227bps were found to be a dominant population when hydrogen was produced efficiently at pHs of 6.0, 6.5, and 7.0. [Copyright &y& Elsevier]

Abstract: Extracellular acidic pH-activated chloride channel I Cl, acid, has been characterized in HEK 293 cells and mammalian cardiac myocytes. This study was designed to characterize I Cl,acid in human umbilical vein endothelial cells(HUVECs). The activation and deactivation of the current rapidly and repeatedly follows the change of the extracellular solution at pH 4.3, with the threshold pH 5.3. In addition, at very positive potentials, the current displays a time-dependent facilitation. pH-response relationship for I Cl,acid revealed that EC50 is pH 4.764 with a threshold pH value of pH 5.3 and nH of 14.545. The current can be blocked by the Cl− channel inhibitor DIDS (100μM). In summary, for the first time we report the presence of proton-activated, outwardly rectifying chloride channel in HUVECs. Because an acidic environment can develop in local myocardium under pathological conditions such as myocardial ischemia, I Cl,acid would play a role in regulation of EC function under these pathological conditions. [Copyright &y& Elsevier]

Arsenic (As) concentrations in soils and waste material in two areas of former As mining and processing: Złoty Stok (Złote Mts.) and Żeleźniak (Kaczawskie Mts.), SW Poland, were measured to assess levels and environmental risk associated with possible increases in arsenic mobility under changing pH and redox conditions. Twenty six soil samples were collected from 12 sites, and represented a broad spectrum of soil properties and parent material origin, including natural soils, mine spoils, slags and tailings. Arsenic species were examined using a sequential extraction technique. The changes in As solubility were determined from extraction test solutions adjusted to solution pH values varying from 2 to 8. The effects of reducing conditions on As mobility were examined in incubation tests. Most of soils demonstrated extremely high concentrations of As, both of natural and anthropogenic origin. Total As concentrations in all samples were in the range 100–43,500 mg/kg. Sequential extraction techniques suggested that the main species of As in all soils were those bound to iron (Fe) oxides, in particular amorphous ones, whereas the contributions of mobile and specifically sorbed As forms were relatively low. In tailings and tailings-affected alluvial soils, As occurred mainly in residual forms, however these soils also had considerable amounts of mobile As. In all other soils, mobile As forms were very low. Arsenic mobilization could be possible at extremely low or high pH conditions (pH<2 or pH>8), and this may be of particular practical importance in the case of tailings, which are highly alkaline. High pH promotes solubilization of As forms in reducing conditions, whereas in neutral and acidic soils the risk of As release under reducing conditions is limited. [Copyright &y& Elsevier]

Abstract: pH is known to be a primary regulator of nutrient cycling in soil. Increasing soil acidity in agricultural systems has the potential to slow down N cycling and reduce N losses from leaching thereby enhancing sustainability and reducing pollution. We conducted a field experiment to investigate the impact of acidity on N leaching in arable and grassland agricultural systems. The results showed that nitrate (NO3 −) concentrations in soil water were greater under arable than under grassland. Soil acidification significantly lowered NO3 − concentrations in soil water over winter and spring under grassland, whilst in cereal plots a similar effect was only observed in spring. Our results suggest that soil acidification decreased nitrification causing an accumulation of NH4 + which was not subject to leaching. Dissolved organic nitrogen (DON) concentrations in soil water were significantly greater under arable than grassland. Soil acidification lowered concentrations of DON in soil water, usually to a greater extent in grassland than in arable plots. It was concluded that it may be possible to use careful soil pH management as a tool to control NO3 − leaching without compromising the quality of drainage water, and that this may be more effective on grassland than on arable crops. [Copyright &y& Elsevier]