Al In Acid Soils Research Articles

The Central Role of Soil Organic Matter in Soil Fertility and Carbon Storage

The aim of the paper is to give an overview on the chemistry of soil organic carbon (SOC) affecting nutrient availability, the emission of greenhouse gases and detoxifying harmful substances in soil. Humic substances represent the stable part of SOC, accounting for between 50 and more than 80% of organically bound carbon in soil. Humic substances strongly affect the soil solution concentration of several plant nutrients and may increase P-, Fe-, and Cu- solubility, thereby increasing their plant availability. Soil organic carbon, mainly humic substances, can detoxify monomeric Al in acid soils, can strongly bind toxic heavy metals, making them unavailable to the plant roots, and may strongly bind a vast variety of harmful organic pollutants. Increasing SOC is an important goal in agriculture. The inclusion of mixtures of semi-perennial plant species and cultivars may strongly increase SOC and humic substance content in soils. To increase SOC, farmyard manure and its rotted or composted forms are superior compared to the separate application of straw and slurry to soil. The storage of carbon, mainly in organic form, in soils is very important in the context of the emission of greenhouse gases. Worldwide, soils release about 10 times more greenhouse gases compared to fossil fuel combustion. Small increments in SOC worldwide will strongly affect the concentration of atmospheric CO2. The public discussion on soil fertility and greenhouse gas emissionshas been politically controlled in a way that leaves the important and positive contribution of soil organic carbon and mainly humic substances partly misinterpreted and partly underestimated.

Structural basis of ALMT1-mediated aluminum resistance in Arabidopsis.

The plant aluminum (Al)-activated malate transporter ALMT1 mediates the efflux of malate to chelate the Al in acidic soils and underlies the plant Al resistance. Here we present cryo-electron microscopy (cryo-EM) structures of Arabidopsis thaliana ALMT1 (AtALMT1) in the apo, malate-bound, and Al-bound states at neutral and/or acidic pH at up to 3.0 Å resolution. The AtALMT1 dimer assembles an anion channel and each subunit contains six transmembrane helices (TMs) and six cytosolic α-helices. Two pairs of Arg residues are located in the center of the channel pore and contribute to malate recognition. Al binds at the extracellular side of AtALMT1 and induces conformational changes of the TM1-2 loop and the TM5-6 loop, resulting in the opening of the extracellular gate. These structures, along with electrophysiological measurements, molecular dynamic simulations, and mutagenesis study in Arabidopsis, elucidate the structural basis for Al-activated malate transport by ALMT1.

Ecological Significance of Residues Retention for Sustainability of Agriculture in the Semi-arid Tropics

In South Asia, land degradation is primarily a monsoon mediated phenomenon restricted to 2-3 rainy months. The overall strategy for land degradation neutrality should (i) favour actions that keep soils covered with residues and (ii) plant kharif (rainy season) crop before the onset of monsoons to provide soil cover. Retention of anchored residues provides surface cover, increases microbial activity, carbon sequestration, and availability of nutrients. Surface retained residues reduce root zone salinization, detoxify phytotoxic monomeric Al in acidic soils and enhance the potential for use of brackish ground water in crop production. Residues covers save irrigation water and overcome the ill effects of poor agronomic and water management practices. Early direct dry seeding in surface retained residues has the potential of making kharif season planting independent of the onset of monsoon rains in South Asia and helps reduce acreages of Kharif and Rabi fallow lands. For improving carbon content in Indian soils, perhaps the most important priority is to devise tillage and crop residue management approaches that promote in situ rain water storage and its use for growing more crops. The paper summarises how crop residues fuel and drive soil functions and related ecosystem services and plant growth.

Application of measuring electrochemical characteristics on plant root surfaces in screening Al-tolerant wheat

To explore the relationship between Al phytotoxicity and the electrochemical characteristics of wheat root surfaces, a new chemical mechanism for tolerance of wheat to Al toxicity was initially proposed by conducting acute root elongation experiment, adsorption/desorption experiment, streaming potential determination, and infrared spectroscopy (ATR-FTIR) analysis respectively to classify the grade of Al tolerance of 92 wheat cultivars and quantitatively characterize the electrochemical properties of their root surfaces. Then a pot experiment was conducted with the screened wheat cultivars with different Al resistance grown on acid soils to verify their tolerance to Al toxicity. Results show that zeta potentials of the roots of 67 wheat cultivars at pH4.46 were significantly negatively correlated with Al(Ⅲ) adsorbed on the roots and their relative root elongation (P < 0.05), indicating that wheat roots with less negative charges is more tolerant to Al toxicity. Based on the mechanism, 14 Al-tolerant, 23 medium Al-tolerant and 30 Al-sensitive wheat cultivars were classified. The pot experiment reveals that the relative dry weight of Al-tolerant wheat cultivars was generally greater than that of medium Al-tolerant and Al-sensitive wheat cultivars and Al-tolerant wheat cultivars accumulate less Al in their shoots, which further verifies the relationship among charge characteristics, tolerance of wheat to Al toxicity, and Al uptake by wheat. The negative charges derived from organic functional groups on root surfaces could influence the exchangeable and complexed Al(Ⅲ) adsorbed on wheat roots and thereby affect Al tolerance of wheat cultivars. This finding not only provides a new perspective to screen Al-tolerant wheat cultivars and explain the mechanism of tolerance of wheat to Al toxicity, but is also useful for the prediction of differences in the uptake of Al in the shoots between Al-tolerant and Al-sensitive wheat cultivars, and finally contributes to the prevention of food security risk caused by Al in acid soils.

Phosphorus × Other Plant Nutrient Interactions, Reaction Products, Anion Exchange and Phosphate Fixation in Soil and Strategies to Increase Availability of the Native and Applied P to Crop Plants-A Mini Review and Critique

Phosphorus is a major plant nutrient obtained from non-renewable phosphate rock, which is not much available in India. Over and above in this its recovery efficiency in crops hardly exceeds 15-20 per cent. Utmost care is therefore required in its use. Phosphorus applied to soil gets fixed by the formation of insoluble reaction products by reacting with Fe and Al in acid soils and with calcium in saline and alkaline soils. Two techniques used for identification of reaction products are X-ray diffraction and solubility product principle. In addition, phosphate ions are also held on Fe and Al hydroxides by anion exchange. The reaction products identified are variscite and strengite minerals in acid soils and dicalcium phosphate and hydroxy apatite in calcareous soils. Methods to increase phosphorus recovery by crops include: 1) addition of organic matter to soils; 2) addition of sulphur to compost or to ground rock phosphate or directly to soil and use of phosphorus solubilising organisms including VAM/AM along with ground rock phosphate.

Al(III)-binding properties at the molecular level of soil DOM subjected to long-term manuring

Advanced knowledge of the binding of dissolved organic matter (DOM) with aluminum (Al) ions is essential for understanding the toxicity, mobilization, and fate of Al in acidic soils. In this study, parallel factor analysis (PARAFAC) and two-dimensional correlation spectroscopy (2D-COS) were combined to characterize the Al(III)-binding properties of DOM extracted from farmland topsoil (0–20 cm) subjected to long-term application of chemical fertilizers (CF) or manure (M). The total Al concentration of solid soil in the M treatment (7.57%) was lower than that in the CF treatment (8.56%). In contrast, the organic-complexed Al concentration of the aqueous DOM extract in the M treatment (79 μmol/L) was dramatically higher than that in the CF treatment (0.9 μmol/L). Compared with CF treatment, PARAFAC-derived components in soil DOM from the M treatment had stronger Al(III)-binding affinities (log KM value), suggesting that soil DOM from long-term manuring has stronger potential to mobilize Al in acidic soils than that from chemical fertilizer applications. Fourier transform infrared (FTIR) spectroscopy coupled with 2D-COS revealed that soil DOM from the M treatment exhibited more Al(III)-binding sites than that from the CF treatment. In addition, aliphatic –OH groups showed the fastest response to Al(III) binding. These results provide a more detailed theoretical basis, at the molecular level, of the interaction mechanism of Al(III) with soil DOM, which is helpful for a deeper understanding of DOM-influenced biogeochemical cycling in farmland subjected to long-term manuring.

Analysis of aluminum toxicity in Hordeum vulgare roots with an emphasis on DNA integrity and cell cycle.

Barley is one of the cereals that are most sensitive to aluminum (Al). Al in acid soils limits barley growth and development and, as a result, its productivity. The inhibition of root growth is a widely accepted indicator of Al stress. Al toxicity is affected by many factors including the culture medium, pH, Al concentration and the duration of the treatment. However, Al can act differently in different species and still Al toxicity in barley deserves study. Since the mechanism of Al toxicity is discussed we cytogenetically describe the effects of different doses of bioavailable Al on the barley nuclear genome—mitotic activity, cell cycle profile and DNA integrity. At the same time, we tested an established deep-water culture (DWC) hydroponics system and analyzed the effects of Al on the root system parameters using WinRHIZO software. We demonstrated the cytotoxic and genotoxic effect of Al in barley root cells. We showed that Al treatment significantly reduced the mitotic activity of the root tip cells and it also induced micronuclei and damaged nuclei. The DNA-damaging effect of Al was observed using the TUNEL test. We define the inhibitory influence of Al on DNA replication in barley. Analysis with the labelling and detection of 5-ethynyl-2‘-deoxyuridin (EdU) showed that the treatment with Al significantly decreased the frequency of S phase cells. We also demonstrated that Al exposure led to changes in the cell cycle profile of barley root tips. The delay of cell divisions observed as increased frequency of cells in G2/M phase after Al treatment was reported using flow cytometry.

Lucerne dry matter and N-fixation, when sown with or without lime and inoculant

This paper quantifies the effect of inoculation and lime on lucerne growth and nitrogen fixation over the establishment and following season. The field experiment at Ashley Dene, in Canterbury, had an initial soil pH of 5.2 and moderate exchangeable aluminium content (4.2 mg/kg). The 15N natural abundance (δ15N) method was used to quantify nitrogen fixation. The pH of the top-soil was increased to ca. 5.5 and the Al content was decreased to 2.0 and 1.3 mg/kg soil by the application of 1 and 2 t lime/ha, respectively. In Year 0 the dry matter yield increased from 3 to 4 t DM/ha with the addition of 2 t lime/ha. The nitrogen content of lucerne plants was 2.2% (w/w), and was unaffected by inoculant or lime in Year 0. Similarly, the δ15N was 0.81‰ and unaffected by inoculant or lime. In Year 1, the N percentage of inoculated lucerne plants was 4.2% (w/w) compared with 3.6% in uninoculated plants. The δ15N value was 0.81‰ in uninoculated plants compared with -0.23‰ in inoculated lucerne plants. The inoculated lucerne yielded 7.8 t DM/ha compared with 2.4 t DM/ha when uninoculated. The calculated proportion of legume N derived from atmospheric N2 (%Ndfa) estimated that 70% of the nitrogen content in the inoculated lucerne shoots was derived from nitrogen-fixation in Year 1. There was no effect of lime on lucerne DM, or δ15N values in Year 1. This suggests lucerne rhizobia tolerated moderate levels of Al in acidic soils. The application of lime and inoculant are therefore recommended for lucerne, particularly in areas where there is no history of lucerne. The available soil N was sufficient to meet crop demand in the establishing year. Lucerne was then reliant on biological nitrogen fixation for yield in Year 1 which suggests lucerne preferentially used soil available N in Year 0, before commencing N fixation. Keywords: Aluminium, lucerne, lime, Medicago sativa L., nitrogen fixation.

Coadaptation of Plants to Multiple Stresses in Acidic Soils

Soil acidification is an increasing problem in agricultural production and ecological stability worldwide. An understanding of the mechanisms by which plants adapt to acidic soils would help address this problem. Poor plant growth in acidic soils results from a variety of factors, including proton toxicity; deficiencies in nitrogen, phosphorus, calcium, magnesium; and toxicities of various metals (aluminum (Al), manganese, iron). Aluminum toxicity is generally the primary limitation for plant growth in acidic soils. Although much research has focused on Al toxicity in recent decades, it is rarely the only limiting factor in acidic soils. The occurrence of Al toxicity depends on many other factors that coexist with Al in acidic soils. The interactions between Al and such factors are important determinants of Al toxicity to plants under field conditions. In this review, therefore, instead of focusing only on mechanisms of Al tolerance, we examine the effects of the interactions between Al and other limiting factors on plant adaptation to acidic soils. We summarize research on the interactions between Al and nitrogen, phosphorus, pH, and iron. Accordingly, three strategies are proposed to improve plant growth in acidic soils: a combined application of lime and nutrients, depending on the types of soils and plants; development of plant varieties tolerant to multiple stresses; and inoculation of plants with certain beneficial microbes. This review emphasizes the ability of plants to coadapt to multiple stresses in acidic soils.

Aspalathus linearis(Rooibos tea) as potential phytoremediation agent: a review on tolerance mechanisms for aluminum uptake

Aspalathus linearis (Burm. F.) R. Dahlg., commonly referred to as Rooibos tea, grows naturally in nutrient-poor, sandy, acidic soils (pH 3–5.3) with high aluminum concentration ranging from 110 to 275 μg Al g−1in the Cederberg’s mountainous areas in South Africa. Earlier studies found significant differences in Al concentration in organs of A. linearis, with roots having higher amounts (1262–4078 μg Al g−1), suggesting that the plant is capable of accumulating excess Al in acidic soils. Identification of the mineralogical constituents of organs of A. linearis using X-ray diffraction (XRD) analysis revealed the presence of an Al–Si complex (aluminosilicate or hydroxyaluminosilicate (HAS) species) in the shoot and root, possibly to internally ameliorate Al toxicity. In addition, A. linearis has specialized cluster roots that exude Al-chelating organic acid ligands such as citric, malic, and malonic acids. Organic acids can bind strongly to Al in the plant and rhizosphere to reverse its phytotoxic effects to the plants. Field and glasshouse studies revealed significant differences in pH between rhizosphere and nonrhizosphere soils of A. linearis and also showed that roots of the plant release OH−and HCO3−anions to raise rhizosphere pH possibly to immobilize Al through complexation. Furthermore, A. linearis is easily infected by arbuscular mycorrhizae (AM) fungi, but mycorrhizal associations are known to inhibit transport of metallic cations into plant roots. These features of A. linearis are perceived as good indicators for bioremediation; and the plant could, therefore, be a suitable candidate for phytoremediation technologies such as phytoaccumulation, phytostabilization, and phytodegradation. The environmental and economic implications of the potential of A. linearis to bioremediate Al-contaminated soils are briefly discussed. Furthermore, this review briefly highlights future studies investigating the utilization of the shoot of A. linearis as adsorbent for the removal of trace and (or) heavy metal from aqueous solutions.

Pengaruh Kombinasi Senyawa Humik dan CaCO3 terhadap Alumunium dan Fosfat Typic Paleudult Kentrong Banten

Decomposition of organic matter releases humic compounds, that can chelate metals include Al in acidic soils. This chelation of Al is important fordecreasing of Al activity and P fixation in acidic soils. This study was aimed to test the effect of humic compounds extracted from rice-straw compost and CaCO3 combination on aluminium activity and phosphate (available P) on an Ultisol (Typic Paleudult) collected from Kentrong Banten. A laboratory study was conducted by series consentration of CaCO3 (0, 500, 1.000, 2.000, dan 3.000 ppm) in 8 grams of acidic soil. The acidic soil had previously been subjected to continuously addition of humic compounds and none. Then, these mix of soil and CaCO3 were added 40 ml contained 5.000 ppm humic compounds to obtain 1:5 soil:suspension ratio and control was made by using aquadest. They were shaked for 2 hours every day and pH was measured. At 11st days incubation Alexch (N KCl) and soluble of P (PB and PC) were measured. The results showed that liming or addition of CaCO3 to the acidic soils decreased Alexch (precipitated to be Al(OH)3) linearly with formula y = -0.778x + 6.108; y = Alexch (Cmol kg-1), x =CaCO3 (M); R² = 0,916. Combination addition of humic compounds and CaCO3 was able to increase pH and Alexch up to not detected. The increased of P-soluble or desorption P untill 384% took place at addition of 0.0016 M CaCO3 and 5000 ppm humic compounds. More desorption of P was observed if the acidic soils has previously been subjected to continuously addition of humic compounds, such us the increased of soluble P up to 739% at addition of CaCO3 0.0008 M.

Tolerance to ion toxicities enhances wheat (Triticum aestivum L.) grain yield in waterlogged acidic soils

The high concentrations of Mn, Fe and Al in acid soils during waterlogging impair root and shoot growth more severely in intolerant than tolerant wheat genotypes. This study aims to establish whether this difference in vegetative growth and survival during waterlogging (1) is verifiable across a range of tolerant/intolerant genotypes and acid soils, and (2) results in improved recovery after cessation of waterlogging and enhanced grain yield. Wheat genotypes contrasting in their tolerance to ion toxicities were grown in four acid soils until 63DAS and maturity, with a 42-day waterlogging treatment imposed at 21 DAS. The shoot Al, Mn and Fe concentrations increased by up to 5-, 3- and 9-fold respectively due to waterlogging in various soils. Compared to the intolerant lines, Al-, Mn- and Fe-tolerant genotypes maintained a relatively lower increase in shoot concentrations of Al (79 vs. 117%), Mn (90 vs. 101%) and Fe (171 vs. 252%) and demonstrated better waterlogging tolerance at the vegetative stage expressed in relative root (38% vs. 25%) and shoot (62% vs. 52%) growth. After cessation of waterlogging and the continued growth to maturity, tolerant genotypes maintained a relatively lower plant concentration of Al, Mn and Fe, but produced a higher above-ground biomass (74% vs. 56%) and most importantly demonstrated improved waterlogging tolerance (a relative grain yield of 78% vs. 54%) compared to intolerant genotypes. Maturity following waterlogging stress was delayed less in tolerant than intolerant genotypes (114 vs. 124%, respectively), which would reduce the potential yield loss where post-anthesis coincides with drought. The results confirm the validity of a novel approach of enhancing waterlogging tolerance of wheat genotypes grown in acid soil via increased tolerance to ion toxicities.

Sugar-Beet-Processing Lime as an Amendment for Low pH Soils

In the Red River Valley, about 385,000 Mg of lime is used to help refine about 39,000 Mg of sugar beets. In a greenhouse study, sugar-beet-processing lime (spent lime; SL), commercial ag lime (AL), and reagent-grade lime (RL) were added to an acidic soil at rates of 0, 1.96, 3.93, and 7.86 Mg ECC ha−1. Wheat was planted, and aboveground biomass and root masses were determined after 44 d of growth. Soil pH increased the greatest with the highest lime application rates. Incorporation of lime uniformly increased soil pH in the pots, whereas with surface application, soil pH decreased with increasing depth. The SL yielded more aboveground soil biomass and root mass than the AL and RL. Use of SL for increasing soil pH and reducing exchangeable Al in acid soils would be a viable use for this industrial by-product.

Effects of buckwheat growth on variation of aluminum and major metals in root‐zone soil solutions

AbstractThe variation in and relationships between aluminum and other major metals including Ca, Mg, Mn, and K in root‐zone soil solutions were studied by growing buckwheat (Fagopyrum esculentum Moench. cv. Jiangxi), an Al accumulator, in pots filled with an acid soil amended with CaCO3 and unamended controls with a preinstalled nondestructive soil‐solution sampler. Soil solutions were collected every 7 d with a syringe connected to the sampler. The results showed that pH of the soil solution was not the only factor controlling the concentration of Al. Significant positive linear correlations were found between Al and Ca as well as between Al and Mg in soil solutions from the controls. The ratio of base cations (BC, Ca+Mg) to Al might partly explain the high Al resistance of buckwheat. Oxalate secretion from roots significantly activated Al in acid soils, which may be important for changes of Al in soil solutions, but it also caused decreased concentrations of Ca, Mg, and Mn. It is concluded that the variation of metal concentrations including Al, Ca, Mg, and Mn is a holistic effect involving competition for exchange sites among Ca, Mg, H, and Al, the chelation by oxalate secreted from roots, and metal uptake by buckwheat.

Use of agricultural by‐products to study the pH effects in an acid tea garden soil

AbstractThe amelioration of an acid Alfisol from a tea garden was studied by incorporating various plant materials: canola straw, wheat straw, rice straw, corn straw, soybean straw, peanut straw, faba bean straw, Chinese milk vetch shoot and pea straw prior to incubation for a maximum of 65 days. Soil pH increased after incubation with all the incorporated materials with the legumes causing the largest increases. The final soil pH was correlated with ash alkalinity (r2 = 0.73), base cations (r2 = 0.74) and N content (r2 = 0.93) of the applied materials. It was assumed that the incubation released the base cations in plant materials as they decomposed which ultimately increased the base cation saturation of the soil. Similarly, soil exchangeable Al was also decreased with the incorporation of the legume plant materials and corn straw and rice straw. Our investigation demonstrated that legumes are the preferred choice for controlling the soil acidity and also for reducing the toxicity of Al in acid soils.

Biogeochemical indicators to evaluate pollutant removal efficiency in constructed wetlands

Wetlands support several aerobic and anaerobic biogeochemical processes that regulate removal/retention of pollutants, which has encouraged the intentional use of wetlands for pollutant abatement. The purpose of this paper is to present a brief review of key processes regulating pollutant removal and identify potential indicators that can be measured to evaluate treatment efficiency. Carbon and toxic organic compound removal efficiency can be determined by measuring soil or water oxygen demand, microbial biomass, soil Eh and pH. Similarly, nitrate removal can be predicted by dissolved organic C and microbial biomass. Phosphorus retention can be described by the availability of reactive Fe and Al in acid soils and Ca and Mg in alkaline soils. Relationships between soil processes and indicators are useful tools to transfer mechanistic information between diverse types of wetland treatment systems.

Biogeochemical indicators to evaluate pollutant removal efficiency in constructed wetlands

Wetlands support several aerobic and anaerobic biogeochemical processes that regulate removal/retention of pollutants, which has encouraged the intentional use of wetlands for pollutant abatement. The purpose of this paper is to present a brief review of key processes regulating pollutant removal and identify potential indicators that can be measured to evaluate treatment efficiency. Carbon and toxic organic compound removal efficiency can be determined by measuring soil or water oxygen demand, microbial biomass, soil Eh and pH. Similarly, nitrate removal can be predicted by dissolved organic C and microbial biomass. Phosphorus retention can be described by the availability of reactive Fe and Al in acid soils and Ca and Mg in alkaline soils. Relationships between soil processes and indicators are useful tools to transfer mechanistic information between diverse types of wetland treatment systems.

Fractionation of extractable aluminum in acid soils: A review and a proposed procedure

Abstract Different forms of soil aluminum (Al) are involved in the retention of anions and cations, phytotoxicity of Al in acid soils, CEC reduction and soil physical properties such as aggregate stability and water infiltration. Therefore it is desirable to quantify the different forms of Al in soil especially acidic soils. A rationale was developed from a literature survey to identify the following fractions of Al: (a) exchangeable quantified by 1M KC1 extraction; (b) organic bound quantified by 0.1M CuCl2 + 0.5M KCl extraction; (c) sorhed Al extractable with 1M NE4OAc at pH 4.0; (d) amorphous Al oxide and hydroxide and amorphous aluminosilicates (if present) extractable with 0.2M ammonium oxalate at pH 3.0; and (e) interlayered Al extractable with 0. 33M sodium citrate at pH 7.3. Pools (a), (b), and (c) are extracted sequentially. Amorphous Al oxide and hydroxide (pool d) is calculated from ammonium oxalate extractable Al minus (a + b + c). Interlayered Al is calculated from sodium citrate extractable Al minus ammonium oxalate extractable Al. The latter two extractions are done on separate subsamples of soils. From preliminary studies and data for 13 soil samples it is suggested that this fractionation of soil Al is more meaningful than that obtained by the KCl ‐> K4P2O7 ‐> ammonium oxalate > citrate‐bicarbonate‐dithionite extraction sequence.

Effects of Va mycorrhizae on growth and mineral uptake of sorghum grown at varied levels of soil acidity

Abstract The effects of the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus deserticola (Trappe, Bloss, and Menge) on growth and mineral element uptake of sorghum [Sorghum bicolor (L.) Moench] were investigated in greenhouse experiments with acid Cecil sandy clay loam (clayey, kaolinitic, thermic, Typic Hapludults) at five pH values (4.1 to 5.8). Control experiments consisted of sterile VAM inocula. Dry matter yields and mineral element uptake were markedly depressed at pH 4.1, were relatively high at pH 4.5, and remained fairly constant from pH 4.5 to 5.8 in mycorrhizal plants. Dry matter yields, root lengths, and mineral element contents of nonmycorrhizal plants increased as soil pH increased, and these characteristics were similar to mycorrhizal plants only after the soil was above pH 5.0 to 5.5. Mycorrhizal plants had higher total and efficiency uptake of mineral elements (shoot element content/root dry matter ratio) than nonmycorrhizal plants. Available Al in acid soils may have been detrimental...

The retention of organic matter in soils

The turnover of C in soils is controlled mainly by water regimes and temperature, but is modified by factors such as size and physicochemical properties of C additions in litter or root systems, distribution of C throughout the soil as root systems, or addition as litter, distribution of C within the soil matrix and its interaction with clay surfaces. Soil factors which retard mineralization of C in soils are identified from correlations of C contents of soils with other properties such as clay content and base status. The rate and extent of C mineralization depends on the chemistry of the added organic matter and interaction with clays of the microbial biomass and metabolites. The organomineral interactions are shown to depend on cation bridges involving mainly Ca in neutral to alkaline soils, Al in acid soils and adsorption of organic materials on iron oxide surfaces. The various organomineral interactions lead to aggregations of clay particles and organic materials, which stabilizes both soil structure and the carbon compounds within the aggregates.