Alteration of soil pH induced by submerging/drainage and application of peanut straw biochar and its impact on Cd(II) availability in an acidic soil to indica-japonica rice varieties.

The effects of soil pH variations induced by submergence/drainage and biochar application on soil cadmium (Cd) availability to different rice (Oryza sativa L.) varieties are not well understood. This study aims to investigate the possible reasons for available Cd(II) reduction in paddy soil as influenced by biochar and to determine Cd(II) absorption and translocation rates in different parts of various rice varieties. A pot experiment in a greenhouse using four japonica and four indica rice varieties was conducted in Cd(II) contaminated paddy soil with peanut straw biochar. The results indicated that the submerging led to an increase in soil pH due to the consumption of protons (H+) by the reduction reactions of iron/manganese (Fe/Mn) oxides and sulfate (SO42-) and thus the decrease in soil available Cd(II) contents. However, the drainage decreased soil pH due to the release of protons during the oxidation of Fe2+, Mn2+, and S2- and thus the increase in soil available Cd(II) contents. Application of the biochar increased soil pH during soil submerging and inhibited the decline in soil pH during soil drainage, and thus decreased soil available Cd(II) contents under both submerging and drainage conditions. The indica rice varieties absorbed more Cd(II) in their roots and accumulated higher amounts of Cd(II) in their shoots and grains than the japonica rice varieties. The Cd(II) sensitive varieties exhibited a greater absorption and translocation rate of Cd(II) compared to the tolerant varieties of both indica and japonica rice. Biochar inhibited the absorption and accumulation of Cd(II) in the rice varieties, which ultimately lowered the Cd(II) contents in rice grains below the national food safety limit (0.2 mg kg-1). Overall, planting japonica rice varieties in Cd(II) polluted paddy soils combined with the use of biochar can effectively reduce Cd(II) content in rice grains which protects human health against Cd(II) toxicity.

Dissolved biochar fractions and solid biochar particles inhibit soil acidification induced by nitrification through different mechanisms.

Biochar can inhibit soil acidification by decreasing the H+ input from nitrification and improving soil pH buffering capacity (pHBC). However, biochar is a complex material and the roles of its different components in inhibiting soil acidification induced by nitrification remain unclear. To address this knowledge gap, dissolved biochar fractions (DBC) and solid biochar particles (SBC) were separated and mixed thoroughly with an amended Ultisol. Following a urea addition, the soils were subjected to an incubation study. The results showed that both the DBC and SBC inhibited soil acidification by nitrification. The DBC inhibited soil acidification by decreasing the H+ input from nitrification, while SBC enhanced the soil pHBC. The DBC from peanut straw biochar (PBC) and rice straw biochar (RBC) decreased the H+ release by 16 % and 18 % at the end of incubation. The decrease in H+ release was attributed to the inhibition of soil nitrification and net mineralization caused by the toxicity of the phenols in DBC to soil bacteria. The abundance of ammonia-oxidizing bacteria (AOB) and total bacteria decreased by >60 % in the treatments with DBC. The opposite effects were observed in the treatments with SBC. Soil pHBC increased by 7 % and 19 % after the application of solid RBC and PBC particles, respectively. The abundance of carboxyl on the surface of SBC was mainly responsible for the increase in soil pHBC. Generally, the mixed application of DBC and SBC was more effective at inhibiting soil acidification than their individual applications. The negative impacts of dissolved biochar components on soil microorganisms need to be closely monitored.

Application of chitosan- and alginate-modified biochars in promoting the resistance to paddy soil acidification and immobilization of soil cadmium.

To develop more green, practical and efficient biochar amendments for acidic soils, chitosan-modified biochar (CRB) and alginate-modified biochar (ARB) were prepared, and their effects on promoting soil pH buffering capacity (pHBC) and immobilizing cadmium (Cd) in the paddy soils were investigated through indoor incubation experiments. The results of Fourier transform infrared spectroscopy and Boehm titration indicated that the introduction of chitosan and sodium alginate effectively amplified the functional groups of the biochar, and improved acid buffering capacity of the biochar. Since there was a plateau region between pH 4.5 and 5.5 in acid-base titration curve of the CRB, adding this biochar to acidic paddy soils apparently improved the pHBC and enhanced the acidification resistance of the paddy soils. The addition of ARB enhanced the reduction reactions during submerging and weakened the oxidation reactions during draining, thus retarded the decline of paddy soil pH during drainage. Furthermore, the pH of the paddy soils with ARB addition was higher at the end of draining, which reduced the activity of soil Cd. Considering the environmental sustainability of chitosan and sodium alginate and convenience of preparation method, biochars modified with these two materials provided alternatives for acidic paddy soil amelioration and heavy metal immobilization. However, the additional experiments should be conducted under field conditions to confirm practical application effects in the future.

Critical values of soil solution Al3+ activity and pH for canola and maize cultivation in two acidic soils.

BACKGROUND: Aluminum (Al) toxicity caused by soil acidification is the main constraint for crop growth in tropical and subtropical areas of southern China. The critical values of soil solution Al3+ activity and pH for crops in acidic soils can provide a useful reference for soil acidity amelioration. RESULTS: A pot experiment in a greenhouse was conducted to investigate the critical values of soil solution Al3+ activity and pH for canola and maize in an Ultisol and an Alfisol. The critical values of soil solution Al3+ activity in Ultisol and Alfisol for canola were 1.5 and 10.0 µmol L-1 , and 13.9 and 30.4 µmol L-1 for maize, respectively. The Al tolerance varied with soil type for the same variety of crop. There was more biomass of roots and shoots and higher plant height under the same Al3+ activity, and thus greater critical values of soil solution Al3+ activity for both crops in Alfisol than those in Ultisol, owing to higher Ca2+ /Al3+ , Mg2+ /Al3+ and K+ /Al3+ ratios in soil solution caused by higher cation exchange capacity and exchangeable base cations in Alfisol, when compared with those in Ultisol. The critical values of soil solution pH for canola and maize in Ultisol were 5.09 and 4.72, respectively; while those in Alfisol were 4.87 and 4.54, respectively. CONCLUSION: The critical values of Al3+ activity were higher for maize than for canola and the critical values for both crops were higher in Alfisol than in Ultisol. The critical soil pH for both crops showed opposite trends to soil Al3+ activity. © 2022 Society of Chemical Industry.

Effects of pH variations caused by redox reactions and pH buffering capacity on Cd(II) speciation in paddy soils during submerging/draining alternation.

Incubation experiments were conducted to investigate the influencing factors of pH variation in different paddy soils during submerging/draining alternation and the relationship between pH buffering capacity (pHBC) and Cd speciation in ten paddy soils developed from different parent materials (including 8 acid paddy soils and 2 alkaline paddy soils). The soil pHBC and the changes in soil pH, Eh, Fe2+, Mn2+, SO42- and Cd speciation were determined. The results showed that there was a significant positive correlation between cation exchange capacity (CEC) and pHBC of these paddy soils, indicating that soil CEC is a key factor affecting the pHBC of paddy soils. The contribution of Fe(III) oxide reduction to H+ consumption is far greater than the reduction of Mn(IV)/Mn(III) oxides and SO42- during the submerging. For example, the contribution of the reduction of manganese oxides, SO42- and iron oxides to H+ consumption in the paddy soils from Anthrosol at 15 d submerging was 1.2%, 11.6% and 87.2%, respectively. This confirms that the reduction of Fe(III) oxides plays a leading role in increasing soil pH. Importantly, we noticed that during submerging, soil pH was increased and resulted in the content of available Cd in soils being reduced. This was due to the transformation of Cd to less active forms. Also, there was a significant positive correlation between the change rate of available Cd, the percentage of acid extractable Cd and pH variation. This suggests that the variation in soil pH was responsible for the transformation of Cd speciation. In addition, the change rate of available Cd and the percentage of acid extractable Cd concentration were significantly negatively correlated with soil pHBC. The soil with higher pHBC experienced less pH change, and thus the change rate of available Cd and the percentage of acid extractable Cd concentration were less for the soil. The results of this study can provide a basis for the remediation of Cd-contaminated acidic paddy soils.

The effects of H2O2- and HNO3/H2SO4-modified biochars on the resistance of acid paddy soil to acidification.

Biochar was prepared from rice straw and modified with 15% H2O2 and 1:1 HNO3/H2SO4, respectively. The unmodified biochars and HCl treated biochars for carbonate removal were used as control. The biochars were added to the acid paddy soil collected from Langxi, Anhui Province, China at the rate of 30 g/kg. The paddy soil was flooded and then air-dried, and soil pH and Eh were measured in situ with pH electrode and platinum electrode during wet-dry alternation. Soil pH buffering capacity (pHBC) was determined by acid-base titration after the wet-dry treatment. Then, the simulated acidification experiments were carried out to study the changing trends of soil pH, base cations and exchangeable acidity. The results showed that soil pHBC was effectively increased and the resistance of the paddy soil to acidification was apparently enhanced with the incorporation of H2O2- and HNO3/H2SO4-modified biochars. Surface functional groups on biochars were mainly responsible for enhanced soil resistance to acidification. During soil acidification, the protonation of organic anions generated by dissociation of these functional groups effectively retarded the decline of soil pH. The modification of HNO3/H2SO4 led to greater increase in carboxyl functional groups on the biochars than H2O2 modification and thus HNO3/H2SO4-modified biochars showed more enhancement in soil resistance to acidification than H2O2-modified biochars. After a wet-dry cycle, the pH of the paddy soil incorporated with HNO3/H2SO4-modified biochar increased apparently. Consequently, the addition of HNO3/H2SO4-modified biochar can be regarded as a new method to alleviate soil acidification. In short, the meaning of this paper is to provide a new method for the amelioration of acid paddy soils.

Effects of crop straw biochars on aluminum species in soil solution as related with the growth and yield of canola (Brassica napus L.) in an acidic Ultisol under field condition.

The toxicity of aluminum (Al) to plants in acidic soils depends on the Al species in soil solution. The effects of crop straw biochars on Al species in the soil solution, and canola growth and yield were investigated in this study. In a long-term field experiment, there were four treatments, which were a control, rice straw biochar (RSB), canola straw biochar (CSB), and peanut straw biochar (PSB). The soil solution was collected in situ, the Al species were identified, and the relationships between the concentration of phytotoxic Al and canola growth and yield were evaluated. The results showed that applying the three biochars resulted in significant decreases in the concentrations of total Al, monomeric Al, and monomeric inorganic Al (P < 0.05). The Al3+, Al-OH, and Al-SO4 proportions of the total Al also decreased. The abilities of the different biochars to reduce dissolved Al followed the order PSB > CSB > RSB, which was consistent with the alkalinity of these biochars. Application of the biochars significantly decreased the concentration of phytotoxic Al (Al3+ + Al-OH), which improved canola growth and increased the canola seed and straw yields. Plant height, leaf number per plant, area per leaf, chlorophyll content, and canola yield were negatively correlated with the Al3+ + Al-OH concentrations. Therefore, the results showed that crop straw biochars can be used to ameliorate soil acidity and alleviate Al toxicity in acidic soils, and that peanut straw biochar is the best amendment for acidic soils.

Biochar retards Al toxicity to maize (Zea mays L.) during soil acidification: The effects and mechanisms.

Biochar can effectively alleviate the Al phytotoxicity in acidic soils due to its alkaline nature. However, the longevity of this alleviation effect of biochar under re-acidification conditions is still unclear. In the present study, the maize root growth responding to the simulated re-acidification of two acidic soils amended by peanut straw biochar or Ca(OH)2 was investigated to evaluate the long-term effect of biochar on alleviating Al toxicity in acidic soils. Compared with Ca(OH)2 amendment, the application of biochar significantly retarded Al toxicity to plant during soil re-acidification. When 4.0 mM HNO3 was added, the maize seedling root elongation in an Oxisol with biochar was 99% higher than that in the Oxisol with Ca(OH)2. Also, the Evans blue uptake and Al content in the root tip in the biochar treatment were 60% and 51% lower than those in the Ca(OH)2 treatment. The retarding effect was mainly attributed to the slow decrease in soil pH during acidification and the release of dissolved organic carbon (DOC) in the soils amended by biochar. The slower decrease in soil pH resulting from the increased pH buffering capacity after biochar application inhibited the increase of soluble and exchangeable Al during re-acidification. The increased DOC after biochar application decreased the toxic soluble Al speciation at the same pH value and total Al concentration in soil solution. Therefore, given the re-acidification of soils, biochar presented a longer-term effect on alleviating Al toxicity of acidic soil than liming.

The mechanism for inhibiting acidification of variable charge soils by adhered Pseudomonas fluorescens.

Acidification in variable charge soils is on the rise due to increased acid deposition and use of nitrogenous fertilizers. The associated low pH and cation exchange capacity make the soils prone to depleted base cations and increased levels of Al3+. Consequently, Al toxicity to plants and soil infertility decrease crop yield. This study was designed to investigate the effect of Pseudomonas fluorescens on the acidification of two Ultisols. The simulated acidification experiment demonstrated that the pH of bacteria-treated soil was higher than that of control under similar conditions, suggesting that the adhered bacteria inhibited soil acidification. This observation was attributed to the association of organic anions (RCOO- or RO-) on bacteria with H+ to form neutral molecules (RCOOH or ROH) and reducing the activity of H+ in solution. The bacteria also inhibited the increase in soil soluble Al and exchangeable Al during soil acidification. The adhesion of bacteria on the soils increased soil effective cation exchange capacity (ECEC) and exchangeable base cations at each pH compared to control. The release of exchangeable base cations from bacteria-treated soil, and the decrease in soil ECEC and exchangeable base cations with decreasing pH confirmed that protonation of organic anions on adhered bacteria was mainly responsible for the inhibition of soil acidification. The change of zeta potential of the bacteria with pH and the ART-FTIR analysis at various pH provided more evidence for this mechanism. Therefore, the bacteria in variable charge soils played an important role in retarding soil acidification.

Beneficial dual role of biochars in inhibiting soil acidification resulting from nitrification.

The dual role of biochar for inhibiting soil acidification induced by nitrification was determined through two-step incubation experiments in this study. Ca(OH)2 or biochar was added respectively to adjust soil pH to the same values (pH 5.15 and 5.85), and then the amended soils were incubated in the presence of urea for 70 days. The results showed that compared with Ca(OH)2 treatment, both rice straw biochar and peanut straw biochar inhibited the decrease in soil pH and the increase in exchangeable acidity during the incubation. The application of biochars suppressed soil nitrification during the incubation, and thus reduced 7.5 mmol kg-1 and 1.4 mmol kg-1 protons released from nitrification compared to Ca(OH)2 treatments. Compared with Ca(OH)2 treatment, the ammonia-oxidizing bacteria population size was decreased by 8% and 12% in rice straw biochar and peanut straw biochar treatments respectively, which was the main responsibility for the inhibited nitrification after biochar application. In addition, the application of rice straw biochar and peanut straw biochar increased soil pH buffering capacity (pHBC) respectively by 22% and 32%. The increased pHBC played the main role (75%) in inhibiting the acidification of the soil amended with peanut straw biochar, while the rice straw biochar inhibited soil acidification mainly through suppressing nitrification during the incubation. Overall, compared with lime application, biochars can inhibit soil acidification caused by urea application through suppressing the nitrification process and improving the resistance of soils to acidification. The crop residue biochars presented a longer-lasting effect on ameliorating acidic soils than mineral lime.

Peanut straw biochar increases the resistance of two Ultisols derived from different parent materials to acidification: A mechanism study.

The mechanisms for increasing soil pH buffering capacity (pHBC) and soil resistance to acidification by peanut straw biochar were investigated by undertaking indoor incubation and simulated acidification experiments using two Ultisols derived from tertiary red sandstone and quaternary red earth. The biochar increased the pHBC and resistance of the two Ultisols to acidification. The addition of 3% biochar increased the pHBC of the two Ultisols by 76% and 25%, respectively. The increased resistance of the soils to acidification led to the inhibition to decrease in soil pH and the activation of soil Al during acidification. The protonation of carboxyl groups on the biochar surface was the main mechanism responsible for resisting acidification of the Ultisols when the pH was between 4.5 and 7.0. The higher soil pH (>6.0) after biochar application and the large number of carboxyl groups on the biochar surface were essential if biochar was to significantly increase the resistance of soils to acidification.

Incorporation of corn straw biochar inhibited the re-acidification of four acidic soils derived from different parent materials.

The effect of corn straw biochar on inhibiting the re-acidification of acid soils derived from different parent materials due to increased soil pH buffering capacity (pHBC) was investigated using indoor incubation and simulated acidification experiments. The incorporation of the biochar increased the pHBC of all four soils due to the increase in soil cation exchange capacity (CEC). When 5% biochar was incorporated, the pHBC was increased by 62, 27, 32, and 24% for the Ultisols derived from Tertiary red sandstone, Quaternary red earth, granite, and the Oxisol derived from basalt, respectively. Ca(OH)2 and the biochar were added to adjust the soil pH to the same values, and then HNO3 was added to acidify these amended soils. The results of this simulated acidification indicated that the decrease in soil pH induced by HNO3 was lower for the treatments with the biochar added than that of the treatments with Ca(OH)2 added. Consequently, the biochar could inhibit the re-acidification of the amended acid soils due to the increased resistance of the soils to acidification when the pH of amended soil was higher than 5.5. The inhibiting effectiveness of the biochar on soil re-acidification was greater in the Ultisol derived from Tertiary red sandstone due to its lower clay and organic matter contents and CEC than the other three soils. The incorporation of the biochar also decreased the potentially reactive Al, i.e., exchangeable Al, organically bound Al, and sorbed hydroxyl Al, compared with the treatments amended with Ca(OH)2. Therefore, the incorporation of corn straw biochar not only inhibited the re-acidification of amended acid soils through increasing their resistance to acidification but also decreased the potential of Al toxicity generated during re-acidification.

Higher cation exchange capacity determined lower critical soil pH and higher Al concentration for soybean.

Low soil pH and aluminum (Al) toxicity induced by soil acidification are the main obstacles in many regions of the world for crop production. The purpose of this study was to reveal the mechanisms on how the properties of the soils derived from different parent materials play role on the determination of critical soil pH and Al concentration for soybean crops. A set of soybean pot experiment was executed in greenhouse with a soil pH gradient as treatment for each of four soils to fulfill the objectives of this study. The results indicated that plant growth parameters were affected adversely due to Al toxicity at low soil pH level in all soils. The critical soil pH varied with soil type and parent materials. They were 4.38, 4.63, 4.74, and 4.95 in the Alfisol derived from loss deposit, and the Ultisols derived from Quaternary red earth, granite, and Tertiary red sandstone, respectively. The critical soil exchangeable Al was 2.42, 1.82, 1.55, and 1.44 cmolc/kg for the corresponding soils. At 90% yield level, the critical Al saturation was 6.94, 10.36, 17.79, and 22.75% for the corresponding soils. The lower critical soil pH and Al saturation, and higher soil exchangeable Al were mainly due to greater soil CEC and exchangeable base cations. Therefore, we recommended that critical soil pH, soil exchangeable Al, and Al saturation should be considered during judicious liming approach for soybean production.

Mechanisms for Increasing the pH Buffering Capacity of an Acidic Ultisol by Crop Residue-Derived Biochars.

The effects and underlying mechanisms of crop residue-derived biochars on the pH buffering capacity (pHbuff) of an acidic Ultisol, with low pHbuff, were investigated through indoor incubation and simulated acidification experiments. The incorporation of biochars significantly increased soil pHbuff with the magnitude of the increase dependent on acid buffering capacity of the biochar incorporated to the soil. Cation release, resulting from the protonation of carboxyl groups on biochar surfaces and the dissolution of carbonates, was the predominant mechanism responsible for the increase in soil pHbuff at pH 4.0-7.0 and accounted for >67% of the increased pHbuff. The reaction of protons with soluble silica (Si) in biochars derived from rice straw and corn stover also accounted for ∼20% of the pHbuff increase due to H3SiO4- precipitation. In conclusion, the incorporation of crop residue-derived biochars into acidic soils increased soil pHbuff with peanut stover biochar being the most effective biochar tested.

Amelioration of an acidic ultisol by straw-derived biochars combined with dicyandiamide under application of urea.

The rapid increase in agricultural pollution demands judicious use of inputs and outputs for sustainable crop production. Crop straws were pyrolyzed under oxygen-limited conditions at 400 °C for 2 h to prepare peanut straw biochar (PB), canola straw biochar (CB), and wheat straw biochar (WB). Then, 300-g soils were incubated each with urea nitrogen (UN) and UN + biochars with or without dicyandiamide (DCD) for 60 days. During the incubations, soil acidification induced by urea was somewhat inhibited by biochars, but nitrification of hydrolyzed NH4+ produced much more acidity than the neutralization potential of the biochars. In single UN (200 mg/kg) treatment, soil pH decreased drastically and the final pH after incubation was lower than the control. Antagonistic to UN, all three biochars neutralized the soil acidity, which was consistent to their inherent alkalinity. DCD inhibited nitrification which was obvious throughout the incubations, as 30 mg/kg DCD + 200 mg/kg UN combined with 1  % PB, CB, and WB retained 0.94, 0.79, and 1.19 units higher pH, respectively, and significantly reduced exchangeable acidity over the treatments without DCD (P < 0.05). The treatments of UN + biochars with and without DCD had highly significant effects on soil pH, exchangeable Al3+, NH4+-N, (NO3-+NO2-)-N, and available P (P < 0.05). Amplified NH4+-N retentions at higher rates of PB referred increased negatively charged sites for nutrient adsorptions. Applied UN transformations varied among different treatments, and the maximum amounts of total mineral N recovered were 218.3, 218.5, and 223.8 mg/kg in the presence of DCD by PB, CB, and WB, compared to 198.2, 201.6, and 205.2 mg/kg, respectively, in no DCD treatments. Urea induced severe soil acidification and even lowered the ameliorative effects of applied biochars. Thus, ammonium-based fertilizers must include nitrification inhibitor (DCD) and, if used in combination with biochars will offer a suitable choice to reduce the acidity, improve base saturation and fertility of soil for sustainable agriculture.