Genetic improvement of legume roots for adaption to acid soils

Acid soils occupy approximately 50% of potentially arable lands.Improving crop productivity in acid soils,therefore,will be crucial for ensuring food security and agricultural sustainability.High soil acidity often coexists with phosphorus(P) deficiency and aluminum(Al) toxicity,a combination that severely impedes crop growth and yield across wide areas.As roots explore soil for the nutrients and water required for plant growth and development,they also sense and respond to below-ground stresses.Within the terrestrial context of widespread P deficiency and Al toxicity pressures,plants,particularly roots,have evolved a variety of mechanisms for adapting to these stresses.As legumes,soybean(Glycine max) plants may acquire nitrogen(N) through symbiotic nitrogen fixation(SNF),an adaptation that can be useful for mitigating excessive N fertilizer use,either directly as leguminous crop participants in rotation and intercropping systems,or secondarily as green manure cover crops.In this review,we investigate legumes,especially soybean,for recent advances in our understanding of root-based mechanisms linked with root architecture modification,exudation and symbiosis,together with associated genetic and molecular strategies in adaptation to individual and/or interacting P and Al conditions in acid soils.We propose that breeding legume cultivars with superior nutrient efficiency and/or Al tolerance traits through genetic selection might become a potentially powerful strategy for producing crop varieties capable of maintaining or improving yields in more stressful soil conditions subjected to increasingly challenging environmental conditions.

Comparison of Highly-Weathered Acid Soil CEC Determined by NH₄OAc (pH = 7.0) Exchange Method and BaCl₂-MgSO₄Forced-Exchange Method

Cation exchange capacity (CEC) is one of the most important properties of soils. The NH4OAc (pH = 7.0) exchange method is usually recommended to determine CEC (CEC1) of all soils with different pH values, particularly for studies on soil taxonomy. But comparatively the BaCl2-MgSO4 forced-exchange method is more authentic in determining CEC (CEC2) of tropical and subtropical highly-weathered acid soils. But so far little is known about the difference between CEC1 and CEC2. In this study, the physiochemical data of 114 acid B horizon soils from 112 soil series of tropical and subtropical China were used, CEC1 and CEC2 were determined and compared, the influencing factors were analyzed for the difference between CEC1 and CEC2, and then a regression model was established between CEC1 and CEC2. The results showed that CEC2 was significantly lower than CEC1 (p < 0.01), CEC2 was 14.76% - 63.31% with a mean of 36.32% of CEC1. In view of the contribution to CEC from other properties, CEC2 was mainly determined by pH (45.92%), followed by silt (21.05%), free Fe2O3 (17.35%) and clay contents (12.76%), CEC1 was mainly decided by free Fe2O3 content (40.38%), followed by pH (28.39%) and silt content (27.29%;and the difference between CEC1 and CEC2 was mainly affected by free Fe2O3 (50.92%), followed by silt content (26.46%) and pH (21.80%). The acceptable optimal regression model between CEC2 and CEC1 was established as CEC2 = 2.3114 × CEC11.1496 (R2 = 0.410, P < 0.001, RMSE = 0.15). For the studies on soil taxonomy, the BaCl2-MgSO4 forced-exchange method is recommended in determining CEC of the highly-weathered acid soils in the tropical and subtropical regions.

Chemical Species of Aluminum Ions in Acid Soils

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Alleviation of Soil Acidity and Aluminium Phytotoxicityin Acid Soils by Using Alkaline-Stabilised Biosolids

A pot experiment was catried out to study alleviation of soil acidity and Al toxicity by applying analkaline-stabilised sewage sludge product (biosolids) to an acid clay sandy loam (pH 5.7) and a strongly acidsandy loam (pH 4.5). Barley (Hondeum vulgare L. cv. Forrester) was used as a test crop and was grownin the sewage sludge-amended (33.5 t sludge DM ha-1) and unamended soils. The results showed that thealka1ine biosloids increased soil pH from 5.7 to 6.9 for the clay sandy loam and from 4.5 to 6.0 for the sandyloam. The sludge product decreased KCl-extractable Al from 0.1 to 0.0 cmol kg-1 for the former soil andfrom 4.0 to 0.1 cmol kg-1 for the latter soil. As a result, barley plants grew much better and grain yieldincreased greatly in the amended treatments compared with the unamended controls. These observationsindicate that alkaline-stabilised biosolids can be used as a liming material for remedying Al phytotoxicity instrongly acid soils by increasing soil pH and lowering

Decreased Soil Nitrification Rate with Addition of Biochar to Acid Soils

This study was conducted to investigate the effects of mixed biochar on the nitrification rate in acidic soils. A15 N tracer experiment with(15 NH4)2 SO4 was conducted to determine the nitrification rates of 4 acidic agricultural soils with p H 4.03-6.02 in Yunnan Province, Southern China. The accumulation of15 N-NO3-and nitrification rates decreased with the addition of biochar at the end of incubation, suggesting that biochar could be a nitrification inhibitor in acidic fertilized soil. Nitrification rates in soil with p H 4.03 were evidently lower than those in soil with p H 4.81-6.02 with or without biochar. Decreased nitrification rates were detected in the acidic soils with biochar. Soil p H controlled nitrification more than biochar in certain strongly acidic soils.

Genotypic Differences of Forage Crop Tolerance to Acid Soils

Twenty eight species of forage crops were planted on acid soils derived from Quaternary red clay (pH 4.16)and red sandstone (pH 4.55) to study genotypic differences of the forage crops in tolerance to acid soils as affected by liming, phosporus and potassium fertilizer application. Eight forage species, Lolium multiflorum L., Brachiaria decumbens, Digitaria sumtisii, Melinis minutiflora, Paspalum dilatatum, Paspalum wettsteinii,Sataria viridis Beanv and Shcep’s Festuca, were highly tolerant to acid soils, and grew relatively well in the tested soils without lime application, whereas most of the other 20 tested forage species such as Lolium perenne L., Meadow Festuca and Trifolium pratense L. were intolerant to acid soils, showing retarded growth when the soil pH was below 5.5 and significant increase in dry matter yields by phosphrus fertilizer application at soil pH 6.0. Results showed that large differences in tolerance to acid soils existed among the forage species,and tolerance of the forage species to acid soils might be closely associated with their tolerance to Al and P efficiency.

Molecular approaches unravel the mechanism of acid soil tolerance in plants

Acid soil is a worldwide problem to plant production. Acid toxicity is mainly caused by a lack of essential nutrients in the soil and excessive toxic metals in the plant root zone. Of the toxic metals, aluminum(Al) is the most prevalent and most toxic. Plant species have evolved to variable levels of tolerance to aluminum enabling breeding of high Al-tolerant cultivars.Physiological and molecular approaches have revealed some mechanisms of Al toxicity in higher plants. Mechanisms of plant tolerance to Al stress include: 1) exclusion of Al from the root tips, and 2) absorbance, but tolerance of Al in root cells. Organic acid exudation to chelate Al is a feature shared by many higher plants. The future challenge for Al tolerance studies is the identification of novel tolerance mechanisms and the combination of different mechanisms to achieve higher tolerance. Molecular approaches have led to significant progress in explaining mechanisms and detection of genes responsible for Al tolerance.Gene-specific molecular markers offer better options for marker-assisted selection in breeding programs than linked marker strategies. This paper mainly focuses on recent progress in the use of molecular approaches in Al tolerance research.

Agronomic Potential of Partially Acidnlated Rock Phos－phates in Acid Soils of Subtropical China

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Effect of Electrolyte on the Dissolution of Aluminum from Acid Soils and the Distribution of Aluminum Forms inSoil Solution

KCl, CaCl2, NH4Cl, NaCl, K2SO4 and KF solutions were used for studying the effects of canons and anions on the dissolution of aluminum and the distribution of aluminum forms respectively. Power of exchanging and releasing aluminum of four kinds of canons was in the decreasing order Ca2+ >K+ >NH+4 >Na+.The dissolution of aluminum increased with the canon concentration. The adsorption affinity of various soils for aluminum was different. The aluminum in the soil with a stronger adsorption affinity was difficult to be exchanged and released by canons. The Al-F complexes were main species of inorganic aluminum at a low concentration of canons, while Al3+ became major species of inorganic aluminum at a high concentration of canons. The results on the effect of anions indicated that the concentrations of total aluminum, three kinds of inorganic aluminum (Al3+, Al-F and Al-OH complexes) and organic aluminum complexes (Al-OM)when SO2-4 was added into soil suspension were lower than those when Cl- was added. The dissolution of aluminum from soils and the distribution of aluminum forms in solution were edicted by the adsorption of Fon the soil. For soils with strong affinty for F- , the concentrations of the three inorganic aluminum species in soil solution after addition of F- were lower than those after addition of Cl-; but for soils with weak affinity for F- , the concentrations of Al3+ and Al-OM were lower and the concentrations of Al-F complexes and total inorganic aluminum after addition of F- were higher than those after addition of Cl-. The increase of F- concentration in soil solution accelerated the dissolution of aluminum from soils.

Relationship of Soil Qualities to Maize Growth Under Increasing Phosphorus Supply in Acid Soils of Southern Cameroon

A large array of soil properties influences plant growth response to phosphorus(P) fertilizer input in acid soils. We carried out a pot experiment using three contrasted acid soils from southern Cameroon with the following main objectives:i) to assess the main soil causal factors of different maize(Zea mays L.) growth response to applied P and ii) to statistically model soil quality variation across soil types as well as their relationships to dry matter production. The soils used are classified as Typic Kandiudox(TKO) ,Rhodic Kandiudult(RKU) ,and Typic Kandiudult(TKU) . Analysis of variance,regression,and principal component analyses were used for data analysis and interpretation. Shoot dry matter yield(DMY) was significantly affected by soil type and P rate with no significant interaction. Predicted maximum attainable DMY was lowest in the TKO(26.2 g pot-1) as compared to 35.6 and 36.7 g pot-1 for the RKU and TKU,respectively. Properties that positively influenced DMY were the levels of inorganic NaHCO3-extractable P,individual basic cations(Ca,Mg,and K) ,and pH. Their effects contrasted with those of exchangeable Al and C/N ratio,which significantly depressed DMY. Principal component analysis yielded similar results,identifying 4 orthogonal components,which accounted for 84.7% of the total system variance(TSV) . Principal component 1 was identified as soil nutrient deficiency explaining 35.9% of TSV. This soil quality varied significantly among the studied soils,emerging as the only soil quality which significantly(P < 0.05) correlated with maize growth. The 2nd,3rd,and 4th components were identified as soil organic matter contents,texture,and HCl-extractable P,respectively.

A Brief Study on Using pH_H₂Oto Predict pH_KClfor Acid Soils

pHKCl and pHH2O are two basic necessary indexes to reflect the acidity of asoil. Predicting pHKCl?directly from pHH2O?could save the cost of laboratory work. In this study, the values of pHKCl and of 442 and 310 horizon samples from 126 and 98 soil profiles (0 - 120 cm in depth) surveyed from 2014 to 2015 in Guangxi and Yunnan were used to establish the optimal correlation model between pHKCl and pHH2O. The results showed that: 1) pHKCl is lower than pHH2O, pHKCl?was 0.07 - 1.99 units with a mean of 0.99 units lower than for Guangxi, while 0.03 - 1.90 units with a mean of 0.89 lower than pHH2O?for Yunan. 2) There is significant positive correlation between pHKCl?and pHH2O, the optimal correlation models between pHKCl?(y) and pHH2O?(x) for Guangxi and Yunnan are y = 0.1963x2 &minus;1.0512x + 4.338, R2 = 0.836, p 0.1859x, R2 = 0.769, p pHKCl?with exchangeable H+ and Al3+ (R2 = 0.487, 0.716, p pHKCl?is dominated by exchangeable Al3+, followed by exchangeable H+, and their contribution to pHKCl?were 71.1% and 28.7%, respectively.

Effect of Saline Water on Soil Acidity, Alkalinity and Nutrients Leaching in Sandy Loamy Soil in Rwamagana Bella Flower Farm, Rwanda

The necessity to saline and sodic waters is sometimes used for irrigating agricultural activities under certain circumstances, but it is important to note that the use of these waters comes with specific considerations and limitations. One way to decrease undesirable effects of sodic waters on the physical and chemical properties of soils is to apply organic and chemical amendments within the soil. This study aimed to assess the effectiveness of saline water on soil acidity, alkalinity and nutrients leaching in sandy loamy soil at Bella flower farm, in Rwamagana District, Rwanda. The water used was from the Muhazi Lake which is classified as Class I (Saline water quality). Column leaching experiments using treated soils were then conducted under saturated conditions. The soil under experimental was first analyzed for its textural classification, soil properties and is classified as sandy loamy soil. The t-test was taken at 1%, 5% and 10% levels of statistical significance compared to control soil. The results indicated that the application of saline water to soils caused an increase in some soil nutrients like increase of Phosphorus (P), Potassium (K+), Magnesium (Mg2+), Sulphur (S), CN ratio and Sodium (Na+) and decreased soil texture, physical and chemical properties and remained soil nutrients. Consequently, the intensive addition of saline water leachates to soil in PVC pipes led to decreased of soil EC through leaching and a raiser Soluble Sodium Percentage (SSP). The rate of saline water application affected the increase accumulation of SAR and Na% in the top soil layers. The study indicated that saline water is an inefficient amendment for sandy soil with saline water irrigation. The study recommends further studies with similar topic with saline water irrigation, as it accentuated the alkalinity levels.

Differential Expression of Iron Deficiency Responsive Rice Genes under Low Phosphorus and Iron Toxicity Conditions and Association of OsIRO3 with Yield in Acidic Soils

With the hypothesis that iron(Fe) deficiency responsive genes may play a role in Fe toxicity conditions,we studied five such genes OsNAS1,OsNAS3,OsIRO2,OsIRO3 and OsYSL16 across six contrasting rice genotypes for expression under high Fe and low phosphorus(P) conditions,and sequence polymorphism.Genotypes Sahbhagi Dhan,Chakhao Poirieton and Shasharang were high yielders with no bronzing symptom visible under Fe toxic field conditions,and BAM350 and BAM811 were low yielders but did not show bronzing symptoms.Hydroponic screening revealed that the number of crown roots and root length can be traits for consideration for identifying Fe toxicity tolerance in rice genotypes.Fe contents in rice roots and shoots of a high-yielding genotype KMR3 showing leaf bronzing were significantly high.In response to 24 h high Fe stress,the expression levels of OsNAS3 were up-regulated in all genotypes except KMR3.In response to 48 h high Fe stress,the expression levels of OsNAS1 were3-fold higher in tolerant Shasharang,whereas in KMR3,it was significantly down-regulated.Even in response to 7 d excess Fe stress,the transcript abundances of OsIRO2 and OsNAS3 were contrasting in genotypes Shasharang and KMR3.This suggested that the reported Fe deficiency genes had a role in Fe toxicity and that in genotype KMR3 under excess Fe stress,there was disruption of metal homeostasis.Under the 48 h low P conditions,OsIRO2 and OsYSL16 were significantly up-regulated in Fe tolerant genotype Shasharang and in low P tolerant genotype Chakhao Poirieton,respectively.In silico sequence analysis across 3 024 rice genotypes revealed polymorphism for 4 genes.Sequencing across OsIRO3and OsNAS3 revealed nucleotide polymorphism between tolerant and susceptible genotypes for Fe toxicity.Non-synonymous single nucleotide polymorphisms and insertion/deletions(InDels) differing in tolerant and susceptible genotypes were identified.A marker targeting 25-bp InDel in OsIRO3,when run on a diverse panel of 43 rice genotypes and a biparental population,was associated with superior performance for yield under acidic lowland field conditions.This study highlights the potential of one of the vital genes involved in Fe homeostasis as a genic target for improving rice yield in acidic soils.

Effects of Microbial Fertilizers in Improving Acidic Tobacco-planting Soil and Tobacco Leaf Quality

[Objectives]This study was conducted to improve acidic soil and enhance the quality of tobacco leaves.[Methods]The effects of different microbial fertilizers on improving acidic tobacco-planting soil and tobacco leaf quality were investigated through plot experiments.[Results]The application of microbial fertilizers could improve the pH value of acidic soil,and composite microbial agent A showed the best application effect.The application of bio-organic fertilizer was beneficial to improving the contents of available phosphorus and available boron in acidic soil.The application of composite microbial agent A was beneficial to improving the contents of available phosphorus and available potassium in acidic soil,and could promote the growth of tobacco plants and improve the economic traits of flue-cured tobacco and the coordination of chemical components in tobacco leaves.The application of composite microbial agent B led to a downward trend in the content of available boron in acidic soil.The application of composite microbial agent B could promote the absorption of nutrients by tobacco plants,and improve their disease resistance and the quality of tobacco leaves.Due to the differences in functional microorganisms contained,the application effects of different microbial fertilizers in improving acidic tobacco-planting soil and improving tobacco leaf quality varied.Overall,the application of microbial fertilizers could increase soil pH,activate soil nutrients,promote tobacco growth,enhance disease resistance,increase tobacco output value,and improve tobacco quality.Microbial fertilizers have good application prospects in improving acidic soil and improving tobacco quality.[Conclusions]The application of microbial fertilizers to improve acidic tobacco-planting soil can ensure the normal growth and development of tobacco plants and the improvement of tobacco leaf quality,achieving high-quality and sustainable development of Zhaotong tobacco.

Calcium-Magnesium Ca/Mg Ratios and Their Agronomic Implications for the Optimization of Phosphate Fertilization in Rainfed Rice Farming on Acidic Ferralsol in the Forest Zone of Ivory Coast

This study is a contribution to improving rice productivity on acidic plateau soils of the tropical rainforest zone. It is based on taking into account the cationic balances of the soil in order to optimize the phosphorus (P) nutrition of rice on these acidic soils, where this nutrient constitutes a limiting factor for agricultural production. Three (3) pot trials were conducted in Adiopodoumé in the forested south of Côte d’Ivoire. The interactive effects of calcium carbonate (0, 25, 50 and 75 kg Ca ha−1) and magnesium sulfate (0, 25, 50 and 75 kg Mg ha−1) were evaluated on the response of NERICA 5 rice at doses 0, 25, 50 and 75 kg P ha−1 of natural phosphate from Togo, applied only once at the start of the experiment. Additional fertilizers of nitrogen (N) (100 kg N ha−1) and potassium (K) (50 kg KCl ha−1) were added to each of the tests in a split-plot device. The test results revealed a paddy production potential of approximately 3 to 5 t⋅ha−1 for NERICA 5 on an acidic soil, under the effect of the interaction of P, Ca and Mg. The quadratic response of rice yield to the doses of these fertilizers would be more dependent on their balance, itself influenced by Ca nutrition. For the sustainability and maintenance of rice production in agro-ecology studied, it was recommended doses of 38 kg Ca ha−1, 34 kg Mg ha−1 in a Ca/Mg ratio (1/1) with intakes of 41 kg P ha−1, overall in a ratio 1/1/1 (P/Ca/Mg) more favorable to the availability of free iron considered a guiding element of mineral nutrition. Thus, these promising results should be confirmed in a real environment for better management of the fertilization of rice cultivated on acidic plateau soils in Côte d’Ivoire.

The Preliminary Study on Screening and Application of Phthalic Acid-Degrading Bacteria

Phthalic acid is a main pollutant, which is also an important reason for the continuous cropping effect of tobacco. In order to degrade the phthalic acid accumulated in the environment and relieve the obstacle effect of tobacco continuous cropping caused by the accumulation of phthalic acid in the soil. In this study, phthalate degrading bacteria B3 is screened from continuous cropping tobacco soil. The results of biochemical identification and 16sDNA comparison show that the homology between degrading bacterium B3 and Enterobacter sp. is 99%. At the same time, the growth of Enterobacter hormaechei subsp. B3 and the degradation of phthalic acid under different environmental conditions are studied. The results show that the environment with a temperature of 30˚C, PH of 7, and inoculation amount of not less than 1.2%, which is the optimal growth conditions for Enterobacter sp. B3. In an environment with a concentration of phthalic acid not exceeding 500 mg/L, Enterobacter sp. B3 has a better effect on phthalic acid degradation, and the degradation rate can reach 77% in 7 d. The results of indoor potting experiments on tobacco show that the degradation rate of phthalic acid by Enterobacter B3 in the soil is about 45%, which can reduce the inhibitory effect of phthalic acid on the growth of tobacco seedlings. This study enriches the microbial resources for degrading phthalic acid and provides a theoretical basis for alleviating tobacco continuous cropping obstacles.

Growth and mineral nutrient analysis of teak (Tectona grandis) grown on acidic soils in south China

Teak （Tectona grandis L.f.） is widely planted in the world due to its high market demand, economic, ecological and social value. Its plantations have mostly been established and expanded into sites that are acidic to severely acidic in southern China. But, there are no available and specific evidence-based nutrient management techniques. To better recognize and understand the relationship between teak tree growth and nutrient content in the foliage and soil and establish nutrient norms are critical to optimally manage these young plantations. We studied the foliar nutrient and soil chemistry in 19 representative teak plantations aged 5-8 years. Regression analysis indicated that the mean annual increment of teak volume was linearly and positively correlated with foliar N, Ca, Fe and B concentrations, with soil base saturation percentage, available P and Zn concentrations, and negatively correlated with soil Al concentration. Only if the Ca and Mg contents in soil were enhanced, could the increase in soil base saturation percentage benefit teak growth. A revised classification of low-and high-yielding stands was established by using a sorting method of principal components over 6 foliar macro and 8 micro elements in a Diagnosis and Recommendation Integrated System （DRIS）. Specific DRIS norms for teak plantations in acid soils were derived. The nutrient balance of N, P, K Ca, Mg, Zn, B with Fe or A1, Ca with Mg, and Fe with AI provided a key to promote the growth of teak in acid soils. Meanwhile, soil Zn was also found as a primary trace element that affected teak growth in this study.

Tease out the future: How tea research might enable crop breeding for acid soil tolerance

Unlike most crops,in which soil acidity severely limits productivity,tea(Camellia sinensis)actually prefers acid soils(pH 4.0–5.5).Specifically,tea is very tolerant of acidity-promoted aluminum(Al)toxicity,a major factor that limits the yield of most other crops,and it even requires Al for optimum growth.Understanding tea Al tolerance and Al-stimulatory mechanisms could therefore be fundamental for the future development of crops adapted to acid soils.Here,we summarize the Al-tolerance mechanisms of tea plants,propose possible mechanistic explanations for the stimulation of tea growth by Al based on recent research,and put forward ideas for future crop breeding for acid soils.