Renkou Xu

Effect of low-molecular-weight organic anions on surface charge of variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes. In the present paper, the effect of the anions of four low-molecular-weight organic acids on the surface charge of three variable charge soils was investigated. The results showed that the presence of organic anions led to an increase in negative charge and a decrease in positive charge. Positive charges decreased to a larger extent than negative charges. The effect of different anions on surface charge followed the order citrate > malate > oxalate > acetate. For hyper-rhodic ferrasol and rhodic ferrasol, the change of positive charge decreased with the increase in pH, while that of negative charge increased with the increase in pH. Among different soils the extent of change in surface charge was related to their iron oxides content. When free iron oxides were removed from the soil, the effect of organic anions on surface charge decreased sharply. These findings may be of practical significance for variable charge soils low in nutrient-retaining capacity by increasing the retaining capacity for cations such as potassium and calcium considerably but decreasing that for anions such as nitrate remarkably.

Acidity regime of the Red Soils in a subtropical region of southern China under field conditions

Abstract The acidity regimes of Red Soils in Yingtan, Jiangxi Province were examined by determining pH and pCa of the soil paste as well as determining pH in-situ. The results show that for upland soil profiles, the pH decreases gradually from the upper surface layer to the lower layer at a depth of 20 cm by 0.3 units, then decreases slightly until it does not change. For soil profiles under tea trees, the pH decreases gradually from the upper surface layer to the lower layer at a certain depth, then increases slightly with the increase in depth until it reaches a constant value. The distribution pattern of pH of the soil profiles under natural vegetation is similar to that of the soil profiles under tea trees. For paddy soil profiles, the pH increases from the upper layer to the lower layers within the depth of 0–40 cm by 1.37 units, then decreases gradually with increasing depth. For soil profiles under upland crops, tea trees, and natural vegetation, the pCa increases gradually from the surface layer to lower layers. However, in the paddy soil profiles, the pCa decreases from the upper surface downwards to a depth of 40 cm, then increases gradually with increasing depth. The lime potential (pH–0.5pCa) shows a similar trend as the pH. For a given soil, the measured pH value of the soil paste is lower by about 0.5 units than the value determined by the conventional method with a water-to-soil ratio of 5:1 or 10:1. The pH determined in-situ is even lower. The soil acidity status is principally determined by the balance between the leaching loss of base ions, especially calcium ions, and enrichment of these cations from the litter and agricultural measures.

EFFECT OF LOW MOLECULAR WEIGHT ORGANIC ANIONS ON ADSORPTION OF ALUMINUM BY VARIABLE CHARGE SOILS

To examine the effect of organic anions on adsorption of Al by variable charge soils at different pH values, the adsorption by three soils in the presence of three low-molecular-weight aliphatic carboxylic acids was investigated. The results showed that the effect depended on pH, the type of organic anions and their concentration. The presence of citrate and oxalate led to an increase in the adsorption of Al at low pH and low concentration of organic anions, with citrate showing a stronger effect than oxalate. For example, the maximum increments of Al adsorption in the presence of citrate were 131.9, 104.8 and 32.9% in the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol and the Ferric Acrisol, respectively, whereas in the presence of oxalate it was 36.1% in the Rhodic Ferralsol. At high pH or high concentration of organic anions, they showed an inhibiting effect on the adsorption of Al. For example, citrate caused the increase in Al adsorption by 164.0, 131.0 and 61.0% at pH3.85 and the decrease in Al adsorption by 15.2, 19.5 and 45.6% at pH 4.8 for the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol and the Ferric Acrisol, respectively. In the citrate and oxalate systems, the adsorption of Al increased with the increase in the concentration of organic anions, reaching a maximum values at about 0.4 mmol L−1, and then decreased. When the concentration of organic anions was higher than about 1.0 mmol L−1, both citrate and oxalate inhibited the adsorption of Al. The ability of organic anions in increasing the adsorption at low pH and decreasing the adsorption at high pH followed the same order: citrate > oxalate > acetate. The increase of Al adsorption at low pH is caused by the increase in soil negative surface charge as a result of the adsorption of organic anions by variable charge soils, while the decrease of Al adsorption at high pH and high concentration of organic anions is related to the competition of organic ligands for aluminum ions with soil surface. After the removal of free iron oxides from the soil, Al adsorption decreased in the presence of citrate, the anion species most strongly adsorbed by variable charge soils and complexed with aluminum ions. For example, for the Rhodic Ferralsol and the Ferric Acrisol, the removal of free iron oxides caused a decrease in the adsorption of Al in the presence of citrate at pH4.4 by 26.2 and 21.9%, respectively.

Effects of phthalic and salicylic acids on Cu(II) adsorption by variable charge soils

In the present study, the effect of two substituted benzoic acids on Cu(II) adsorption onto two variable charge soils was investigated, with the emphasis on the adsorption and desorption equilibrium of Cu(II). Results showed that the presence of organic acids induced an increase in Cu(II) adsorption onto the two soils. The extent of the effect was related to the initial concentrations of Cu(II) and organic acid, the system pH, and the nature of the soils. The effect of organic acids was greater for Oxisol than for Ultisol. Phthalic acid affected Cu(II) adsorption to a greater extent than salicylic acid did. The effect of organic acids varied with pH. The adsorption of Cu(II) induced by organic acids increased with increasing pH and reached a maximum value at approximately pHxa04.5, and then decreased. It can be assumed that the main reason for the enhanced adsorption of Cu(II) is an increase in the negative surface charge caused by the specific adsorption of organic anions on soils because the desorption of Cu(II) adsorbed in organic acid systems was greater than that for the control. The desorption of Cu(II) absorbed in both control and organic acid systems also increased with increasing pH; it reached a maximum value at pHxa0∼5.25 for control and salicylic acid systems and at pHxa0∼5.1 for a phthalic acid system, then decreased. This interesting phenomenon was caused by the characteristics of the surface charge of variable charge soils.

Inhibiting Effect of Dicyandiamide on Soil Acidification Induced by Application of Urea or Ammonium Bicarbonate

Heavy applications of ammonium-based fertilizers greatly accelerated the acidification of agricultural soils in southern China. The effects of dicyandiamide on soil pH and contents of ammonium nitrogen (NH4+-N) and [nitrate (NO3−) + nitrite (NO2−)-N] were investigated with incubation experiments to evaluate the inhibiting effect of dicyandiamide on soil acidification induced by ammonium-based fertilizers. Results showed application of urea or ammonium bicarbonate increased the pH of an acidic soil at the beginning of incubation, whereas nitrification led to a decrease in soil pH in the later stages. Dicyandiamide elevated soil pH through inhibition of nitrification. At the end of an 85-day incubation, soil pH was 1.30 and 2.20 units greater than the unfertilized sample, and 0.66 and 1.56 units greater if dicyandiamide was added with either 14.29 or 28.57 mmol N kg−1 of urea. Similar results occurred for ammonium bicarbonate. Therefore, application of urea or ammonium bicarbonate with dicyandiamide reduced the soil acidification caused by ammonium-based fertilizers.

Ameliorating Effects of Fungus Chaff and Its Biochar on Soil Acidity

A biochar was generated from fungus chaff at 300 °C, and the ameliorating effects of fungus chaff and its biochar on an acidic Ultisol were compared using incubation experiments. Incorporation of fungus chaff and its biochar significantly increased soil pH and soil exchangeable base cations but decreased soil exchangeable acidity. The ameliorating effect was greater for the biochar than the fungus chaff, and thus the biochar was a better amendment for acidic soil than its feedstock of fungus chaff. The biochar ameliorated soil acidity mainly through the release of its contained alkaline substances, while fungus chaff increased pH of acidic soils through two mechanisms: release of alkaline substances and inhibition of soil nitrification. The incorporation of fungus chaff increased soil-available organic carbon and thus accelerated the microbial assimilation of inorganic nitrogen, while incorporation of fungus chaff biochar enhanced nitrification due to increased soil pH.