Nutrient composition of Douglas-fir rhizosphere and bulk soil solutions

Rhizosphere soil solution is the direct source of nutrients for plant uptake. The nutrient composition of rhizosphere soil solution can be very different from that of bulk soil solution due to root exudation, nutrient uptake and rhizosphere microorganism activity. This study examined the nutrient composition of Douglas-fir rhizosphere soil solution in two soils belonging to the Nisqually and Pitcher soil series and compared rhizosphere solution with that of bulk soil solution. Fertilized and unfertilized Nisqually soils were also compared. Soil solutions were collected using centrifugation. Results indicated that nutrient concentrations in the rhizosphere solutions were typically higher than that of bulk soil solutions when no fertilizer was applied. Differences in the concentrations of nutrients between the rhizosphere and bulk soil solutions were masked by the addition of fertilizers. Rhizosphere solution pH also appeared to be affected by the concentration of NH₄ and NO₃ in the solution. With a higher concentration of NH₄ relative to NO₃ in the rhizosphere soil solution, the solution pH of the rhizosphere was lower than that of the bulk soil, but with a lower concentration of NH₄ relative to NO₃, the solution pH of the rhizosphere was higher than that of the bulk soil solution.     

Immobilization and mobilization of labelled sulphur in relation to soil arylsulphatase activity in rhizosphere soil of field-grown rape,barley and fallow

During plant growth, rhizosphere soils from fallow, barley (Hordeum vulgare L. cv Esterel) and rape (Brassica napus L. cv Capitole) grown in a calcareous soil were sampled 5 times (every fortnight) from May to July 2001 at plant maturity. In order to estimate the impact of C derived from photosynthesis, the aerial parts of rape and barley in an area of 1 m² were cut off about 2 cm from the soil surface, and left a fortnight before each sampling. Both soil arylsulphatase activity and a 1-week immobilization of S fertilizer in the sampled soils were then measured. The immobilization of S fertilizer was higher in fallow, followed by barley and rape rhizosphere soil. A strong positive linear correlation (r ²=0.71, P<0.001) was found between soil arylsulphatase activity and S fertilizer immobilized. Conversely, the mobilization of endogenous organic ³⁵S (obtained after leaching free and adsorbed SO₄ ^2–-³⁵S by 0.009 M Ca(H₂PO₄)₂) in the rhizosphere soil of each plant cover pooled at the end of the 5 samplings and materialized by ryegrass (Lolium perenne L. cv Massa) ³⁵S uptake, was about 3 and 2 times higher, respectively, in rape and barley than in fallow rhizosphere soil. Accordingly, strong inverse polynomial relationships were observed between soil arylsulphatase activity and ³⁵S uptake by the whole plant (r ²=0.904, P<0.02) and roots (r ²=0.970, P<0.01) of ryegrass. Plant cuttings affected both the immobilization and mobilization of S. It is concluded that the turnover of S freshly immobilized in rape rhizosphere soil was relatively high. Therefore, rape as a preceding crop in the rotations may have a beneficial effect by increasing S availability on the succeeding crop.     

Plant induced alteration in the rhizosphere and the utilisation of soil heterogeneity

Plant-soil interactions result in a special rhizosphere soil chemistry, differing from that of the bulk soil found only a few mm from the root. The aim of this study was to investigate adaptation mechanisms of herbs growing in acid soils through studying their rhizosphere chemistry in a greenhouse experiment and in a field study. Ten herbs were grown in acid soil (pH 4.2 in the soil solution) in the greenhouse. The concentrations of NO₃ ^-, SO₄ ^2-, phosphates, Ca²⁺, Mg²⁺, Mn²⁺, K⁺, Na⁺, NH₄ ⁺ and pH were analysed in soil solutions obtained by centrifugation. The general pattern found was a depletion of nutrients in the rhizosphere compared with their concentrations in the bulk soil. The pH increase (up to 0.7 units) in the rhizosphere soil appeared to be caused by plant uptake of NO₃ ^- (r²=0.88). The ion concentrations in the soil solution of the rhizosphere were dependent on plant species and biomass increase. Although species with a larger biomass and higher growth rates showed a higher degree of ion depletion (except for Na⁺, SO₄ ^2-) in the rhizosphere, there were also species specific responses. A field study of five herbs at five oak forest sites in Southern Sweden (Scania) was also carried out. In addition to the soil solution concentrations, the loss on ignition (LOI) and the concentrations of 0.1 M BaCl₂ extractable K⁺, Mg²⁺, Mn²⁺, Ca²⁺, and Al ions were measured. The amount of soil solution Al was determined as free ionic (quickly reacting) Al. For all species and sites, the LOI and the concentrations of exchangeable cations were higher in the rhizosphere than in the bulk soil, apparently due to the roots preferably growing at organic-rich microsites. The concentrations of the ions as measured in the centrifuged soil solution, were either higher in the rhizosphere than in the bulk soil or were the same in both, except for NO₃ ^- and quickly reacting Al. The lower concentrations of quickly reacting Al in the rhizosphere, compared with the bulk soil could indicate the uptake of Al by the plant or the exudation of complexing substances. The pH differences were only small and mostly non-significant. Plant-soil interactions and the ability of plants to utilise heterogeneity of the soil appear to be more important for plant growth in acid soils than recognised heretofore. Rhizosphere studies provide an important means of understanding plant strategies in acid soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

A technique for studying rhizosphere processes in tree crops: soil phosphorus depletion around camellia (Camellia japonica L.) roots

Rhizosphere studies on tree crops have been hampered by the lack of a satisfactory method of sampling soils at various distances in the rhizosphere. A modified root study container (RSC) technique developed for annual crops, grasses and legumes was used to study the mechanisms by which camellia plants (Camellia japonica L.) utilise soil P in the glasshouse and field. Plants belonging to the Camellia family (e.g. tea) have the ability to utilise P from relatively unavailable native P sources and for this reason camellia plants were selected for this study.In the glasshouse trial, the RSCs were filled with a Recent soil, treated with P fertilisers; North Carolina phosphate rock (NCPR), diammonium phosphate (DAP), mono calcium phosphate (MCP) and single superphosphate (SSP) at 200 g P g^-1 soil. A planar mat of roots was physically separated by a 24 m polyester mesh and the soil on the other side of this mesh was cut into thin slices parallel to the rhizoplane and analysed for pH, and different forms of P (organic, P_o and inorganic, P_i) to understand P depletion at different distances from camellia roots. In the field trial this technique was modified and used to study the rhizosphere processes in mature camellia trees fertilised with only SSP and NCPR.In both field and glasshouse trials, all P fertilisers increased all the bulk soil P fractions except NaOH-P_o over unfertilised soil with the greatest increases being in the H₂SO₄-P_i fraction in the NCPR treatment and NaOH-P_i in the SSP treatment. Resin-P, NaOH-P_i and H₂SO₄-P_i were significantly lower in the rhizosphere soil compared to the bulk soil whereas NaOH-P_o was higher in the rhizosphere soil than in the bulk soil. Plant and microbial P uptake were thought to be the major causes for the low resin-P rather than P fixation by Fe and Al because the NaOH-P_i fraction which is a measure of Fe-P and Al-P, also decreased in the rhizosphere soil. The rhizo-deposition of NaOH-P_o suggests that labile inorganic P was immobilized by rhizosphere microbes which were believed to have multiplied as a result of carbon exudates from the roots. A marked reduction in pH (about 0.2–0.4 in the glasshouse and 0.2 in the field trial) was observed near the rhizoplane compared to that in the bulk soil in all treatments. The pH near the rhizoplane as well as in the bulk soil was highest for NCPR treated soil. The increase in pH in the NCPR treatment over the control was consistent with the number of protons consumed during the dissolution of NCPR. In both trials, the dissolution of NCPR in the rhizosphere was higher than in the bulk soil due to lower pH and plant uptake of solution P in the rhizosphere. The RSC technique proved to be a viable aid to study the rhizosphere processes in tree crops.     

Studies on soil rhizosphere: speciation and availability of Cd

ABSTRACT

The rhizosphere soils of two durum wheat (Triticum turgidum var. durum L.) cultivars Kyle and Areola grown in two selected soils of southern Saskatchewan were collected both at 2-week and 7-week plant growth stages. The cadmium availability index (CAI), determined as M NH₄CI-extractable Cd, pH and the distribution of the particulate- bound Cd species of the soils were carried out and the data were discussed in comparison with those of the corresponding bulk soil. At the 2-week growth stage, the pH of the rhizosphere soil was less than that of the corresponding bulk soil and the CAI values were higher in the rhizosphere soil, indicating that more Cd was complexed with the low-molecular-weight organic acids (LMWOAs) at the soil-root interface and was extractable by M NH₄CI. Compared with the bulk soils, the CAI values were 2–9 times higher in the soil rhizosphere of the plots fertilized with Idaho monoammonium phosphate fertilizer at 2-week growth stage, which is attributed to the combined effects of the Cd introduced into the soil rhizosphere from the fertilizer (Cd content of the fertilizer was 144 mg kg^?1) and complexation reactions of phosphate and LMWOAs with soil Cd. At 7-week plant growth stage, such differences were not observed. The increased amounts of carbonate-bound and metal-organic complex-bound Cd species of the rhizosphere soils are due to the increased amounts of carbonate, a product of plant respiration, and the LMWOAs at the soil-root interface, respectively. Simple correlation analysis of the data showed that the CAI of the rhizosphere soils of the control plots correlated at least two orders of magnitude better with the metal-organic complex-bound Cd whereas the CAI of the rhizosphere soils treated with Idaho phosphate correlated better with carbonate-bound Cd species in comparison to other species.     

Phosphorus saturation and pH differentially regulate the efficiency of organic acid anion-mediated P solubilization mechanisms in soil

Exudation of organic acid anions by plants as well as root-induced changes in rhizosphere pH can potentially improve phosphate (P_i) availability in the rhizosphere and are frequently found to occur simultaneously. In non-calcareous soils, a major proportion of P_i is strongly sorbed to metal oxi(hydr)oxides of mainly iron (Fe) and aluminium (Al) and organic anions are known to compete with P_i for the same sorption sites (ligand exchange) or solubilize P_i via ligand-promoted mineral dissolution. Root-induced co-acidification may also further promote P_i release from soil. The relative efficiency of these different solubilization mechanisms, however, is poorly understood. The aims of this study were to gain a better mechanistic understanding of the solubilizing mechanisms of four carboxylates (citrate, malate, oxalate, malonate) in five soils with high and low P surface site saturation. Results indicate that at a lower P saturation of solid phase sorption sites, ligand-promoted mineral dissolution was the main P_i solubilization mechanism, while ligand exchange became more important at higher soil P concentrations. Co-acidification generally increased P_i solubility in the presence of carboxylates; however the relative solubilizing effect of carboxylates compared to the background electrolyte (KCl) control decreased by 20–50%. In soils with high amounts of exchangeable calcium (Ca), the proton-induced Ca solubilization reduced soluble P_i, presumably due to ionic-strength-driven changes in the electric surface potential favoring a higher P_i retention. Across a wider soil pH range (pH 3–8), P_i solubility increased with increasing alkalinity, as a result of both, more negatively charged sorption sites, as well as DOC-driven changes in Fe and Al solubility, which were further enhanced by the presence of citrate. Overall, the relative efficiency of carboxylates in solubilizing P_i was greatest in soils with medium to high amounts of anionic binding sites (mainly Fe- and Al-oxy(hydr)oxides) and a medium P sorption site coverage, with citrate being most effective in solubilizing P_i.     

Effects of rhizosphere remediation and bioaugmentation on carbofuran removal from soil

Rhizosphere soil contains important sources of nutrients for microorganisms resulting in high number of microorganisms capable of degrading various types of chemicals in the soil. Thus, this study investigated a carbofuran dissipation in rhizosphere soils of 6 weeds namely, umbrella sedge (Cyperus iria L.), fuzzy flatsedge (C. pilosus V.), small flower umbrella plant (C. difformis L.), tall-fringe-rush hoorah grass (Fimbristylis miliacea V.), cover fern (Marsilea crenata P.), and water primrose (Jussiaea linifolia V.). Rhizosphere soil of fuzzy flatsedge showed the shortest half-life (t_1/2) of carbofuran (15 days) among other soils. So, it was selected to be used in the bioaugmentation experiment using carbofuran degrader namely Burkholderia cepacia, PCL3, as inoculum in order to examine whether they would improve carbofuran degradation in soil. The results showed that the addition of PCL3 into rhizosphere soil did not improve carbofuran degradation suggesting that microorganisms in rhizosphere soil might be capable enough to remove carbofuran from soil. The number of carbofuran degraders in the rhizosphere soils was greater than in bulk soil 10–100 times which might be responsible to a rapid degradation of carbofuran in rhizosphere soils without the addition of PCL3. The ability of PCL3 to degrade carbofuran was evident in bulk soil (t_1/2 of 12 days) and autoclaved soils (t_1/2 13–14 days) when compared to soils without an inoculation (t_1/2 of 58 days) indicated that the addition of a degrader was useful in improving carbofuran degradation in soil.     

Role of root derived organic acids in the mobilization of nutrients from the rhizosphere

The role of organic acids in the mobilization of plant nutrients from the rhizosphere was assessed in seven contrasting soil types. The results indicated that malate was poor at mobilizing micronutrients from all the test soils, whilst citrate was capable of mobilizing significant quantities. Citrate was also capable of mobilizing P from one soil which possessed a large Ca-P fraction. This mobilization of P was due to both the complexing action of the citrate anion and due to the dissolution properties of the protons released from citric acid upon equilibrium with the soil solution. The reaction of citrate with cations was found to be near instantaneous with significant absorption to the solid phase in some soils at low concentrations. Soil decomposition studies indicated that citrate was rapidly broken down in organic soils but was more resistant to degradation in subsoil horizons. It was concluded that organic acids can be expected to be of little consequence in nutrient mobilization from high pH soils, whilst in acid soils they may be involved both in a more general mechanism for micronutrient uptake or as a potential Al detoxification mechanism.Abbreviations C₆H₈O₇ Citric acid and H-citrate indicates - C₆H₅O₇Na₃ whilst Na-citrate indicates     

氮沉降对杉木和枫香土壤氮磷转化及碳矿化的影响

氮沉降是全球变化的重大环境问题,根际是地下生态过程研究的前沿,但目前氮沉降对亚热带地区不同树种土壤氮、磷供应和碳矿化根际过程的影响及其机制尚不清楚。选取典型红壤区15a针叶树杉木(Cunninghamia lanceolata)和阔叶树枫香(Liquidamba formosana)为对象,野外原位开展10 g N m~(-2)a~(-1)氮沉降试验3a,于2014年8月收集杉木和枫香根际土壤和非根际土壤,测定其p H值、有效氮、速效磷、水溶性有机碳及其34 d有机碳矿化动态,并计算根际效应。结果表明:氮沉降显著降低两个树种土壤p H值和杉木根际土壤速效磷(P0.05);提高枫香非根际土壤NO~-_3-N和杉木非根际土壤水溶性有机碳含量。同时,氮沉降显著提高杉木土壤有机碳矿化速率,根际和非根际的增幅分别为71.2%和41.2%,降低枫香土壤有机碳矿化速率,根际和非根际的降幅分别为10.6%和44.1%。此外,氮沉降显著降低枫香土壤NO~-_3-N和有机碳前期矿化速率的根际效应,增强后期矿化速率的根际效应,而杉木对氮沉降响应不显著。可见,氮沉降可显著改变树木土壤养分供应和有机碳稳定性,且丘陵红壤区针叶树和阔叶树根际过程对氮沉降的响应模式有别。率先报道了亚热带不同树种根际碳、氮、磷耦合过程对氮沉降的响应格局,并较好地揭示了针叶树和阔叶树对氮沉降响应的分异机制。     

Influence of mature Douglas fir roots on the solid soil phase of the rhizosphere and its solution chemistry

Plants can induce significant changes in the rhizosphere through the uptake of water and ions, the exudation of organic compounds and the activities of micro-organisms. The aim of the present study was to assess the influence of tree roots on the chemistry (pH, exchangeable cations, total organic carbon) of both the solid phase of the soil and the soil solutions, extracted by centrifugation, under a mature Douglas fir stand over two distinct seasons (March and September, 1999). The chemical characteristics of either the solid soil phase or the soil solutions of the rhizosphere were found to be different from those of the bulk soil. The cation exchange capacity, base saturation and organic C were all greater in the rhizosphere than in the bulk soil, as a possible result of rhizodeposition, incorporation of decaying root material and micro-organism activity. The concentration of all elements increased in the rhizosphere solutions as compared to the bulk soil solutions, except for P. The pH was lower in the rhizosphere than in the bulk soil for both the solid soil phase and the soil solutions. Despite the greater overall Al concentration of the rhizosphere solutions, as compared to the bulk soil solutions, we suggest that in both, Al toxicity was efficiently restricted by both high Ca + Mg contents and Al complexation with various ligands.     

不同耐盐植物根际土壤盐分的动态变化

以甘肃秦王川引大灌区盐渍化土壤为研究背景,用盆栽根袋法对4种耐盐植物根际和非根际土壤pH和盐分离子的动态变化进行了分析比较。结果表明:4种待测植物随着培养时间的延长土壤pH和EC值呈降低趋势。新疆大叶(Medicago Sativa L.cv.Xinjiangdaye)、向日葵(Helianthus annuus)和霸王(Zygophyllum xanthoxylum)生长90 d后根际土壤pH明显低于非根际,而裸麦(Hordeum vulgare var. vulgare)根际较非根际pH差异不大。霸王和新疆大叶根际土壤EC值较非根际高,而裸麦和向日葵的根际与非根际差异不大。4种供试植物根际K⁺均出现亏缺,Ca²⁺、Na⁺、Mg²⁺、SO^2-₄和Cl^-在新疆大叶、霸王和向日葵3种植物根际均出现富集,对于裸麦:Ca²⁺、Mg²⁺和SO^2-₄ 3种离子在植物根际富集,而Cl^-和Na⁺在根际亏缺。随着待测植物培养时间的增加Na⁺/K⁺、Na⁺/Ca²⁺和Na⁺/Mg²⁺ 这3个比值呈降低趋势,说明Na⁺相对于K⁺、Ca²⁺和Mg²⁺的含量降低,生物措施对Na⁺的移除效果较显著。     

两种盐生植物根际土壤细菌多样性和群落结构

应用高通量测序技术对西北干旱区两种盐生植物黑果枸杞和里海盐爪爪根际土壤细菌的多样性和群落结构进行研究,旨在揭示两种耐盐植物根际土壤细菌之间以及根际与非根际细菌群落结构间的差异,为深入研究盐生植物根际土壤微生物与耐盐性之间的关系提供理论基础。结果表明:黑果枸杞、里海盐爪爪根际细菌多样性丰度高于非根际土,黑果枸杞根际土壤细菌多样性丰度高于里海盐爪爪。根际和非根际土壤细菌群落的组成和丰度存在差异,从黑果枸杞和里海盐爪爪根际土壤中分别检测出细菌21门289属和22门304属,而从非根际土壤中分别检测出28门285属和24门336属;在两种盐生植物根际土壤中,变形菌门和厚壁菌门均为优势门;拟杆菌门、放线菌门、蓝细菌门及浮霉菌门在根际土壤中的丰度显著高于非根际土壤,而厚壁菌门在根际土壤中的丰度低于非根际土壤。两种植物根际土壤中的细菌优势门和优势属的数量均高于非根际土壤,在黑果枸杞和里海盐爪爪的根际土壤中的细菌优势属分别有10个和9个,而二者非根际土壤中的细菌优势属各有4个,其中假单胞菌属是根际和非根际土壤中的共有优势属。黑果枸杞和里海盐爪爪根系细菌群落组成和丰度存在差异,只有假单胞菌属和盐单胞菌属是两种植物根际土壤中的共有优势属。Unifrac分析和聚类分析表明,两种盐生植物根际土壤细菌之间的相似性大于根际和非根际细菌群落间的相似性。细菌多样性与土壤有机碳、有机质、总氮正相关,与pH、电导率负相关,电导率和pH,有机碳和总氮分别是非根际土,根际土壤细菌群落物种组成的主要影响因素。     

Physiological and morphological adaptations of herbaceous perennial legumes allow differential access to sources of varyingly soluble phosphate

The aim of this study was to investigate the capacity of three perennial legume species to access sources of varyingly soluble phosphorus (P) and their associated morphological and physiological adaptations. Two Australian native legumes with pasture potential (Cullen australasicum and Kennedia prostrata) and Medicago sativa cv. SARDI 10 were grown in sand under two P levels (6 and 40 µg P g^?1) supplied as Ca(H₂PO₄)₂·H₂O (Ca‐P, highly soluble, used in many fertilizers) or as one of three sparingly soluble forms: Ca₁₀(OH)₂(PO₄)₆ (apatite‐P, found in relatively young soils; major constituent of rock phosphate), C₆H₆O₂₄P₆Na₁₂ (inositol‐P, the most common form of organic P in soil) and FePO₄ (Fe‐P, a poorly‐available inorganic source of P). All species grew well with soluble P. When 6 µg P g^?1 was supplied as sparingly soluble P, plant dry weight (DW) and P uptake were very low for C. australasicum and M. sativa (0.1–0.4 g DW) with the exception of M. sativa supplied with apatite‐P (1.5 g). In contrast, K. prostrata grew well with inositol‐P (1.0 g) and Fe‐P (0.7 g), and even better with apatite‐P (1.7 g), similar to that with Ca‐P (1.9 g). Phosphorus uptake at 6 µg P g^?1 was highly correlated with total root length, total rhizosphere carboxylate content and total rhizosphere acid phosphatase (EC 3.1.3.2) activity. These findings provide strong indications that there are opportunities to utilize local Australian legumes in low P pasture systems to access sparingly soluble soil P and increase perennial legume productivity, diversity and sustainability.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

Barley genotypes differ in activity of soluble extracellular phosphatase and depletion of organic phosphorus in the rhizosphere soil

In the present investigation we studied the extent of variation among barley genotypes (Hordeum vulgare L. cv. Alexis, Canut, Digger, Etna, Peel) in their ability: i) to induce activity of soluble extracellular phosphatase in rhizosphere soil. ii) to withdraw bicarbonate extractable organic phosphorus (NaHCO₃-P₀). All the genotypes induced 3–4 times higher phosphatase activities in rhizosphere soil as compared to bulk soil. Among the genotypes, there were significant (p>0.01) differences in soluble extracellular and non-soluble phosphatase activities and depletion of NaHCO₃-P₀ in soil near their root mats. Etna induced highest phosphatase activities and depleted most NaHCO₃-P₀ from the rhizosphere soil. A high correlation (r=0.79) was found between the activity of soluble extracellular phosphatase and the quantity of NaHCO₃-P₀ withdrawn from the rhizosphere soil by the barley genotypes.     

Organic acid extraction from rhizosphere soil: effect of field-moist, dried and frozen samples

This study investigates the effect of soil treatment and storage on organic acid extraction. For this study one clayey-loamy (Typic Udochrept) and one sandy-loamy (Aquic Ustifluvent) soil were selected and used to grow Lupinus albus L. plants in a climate chamber. After 4 weeks the rhizosphere soil was sampled and divided into five portions: (a) field moist, no storage; (b) air-dried; (c) oven-dried, (d) field-moist at +4°C for 8 weeks; (e) field-moist at ?20°C for 8 weeks. Organic acid extraction (1:4 w/v) was carried out for each soil portion both in water and in 10 mM NaH₂PO4. Organic acid concentration was subsequently determined by reversed-phase high performance liquid chromatography (HPLC). Oxalic, fumaric, malonic and α-ketoglutaric acid were identified in the rhizosphere of both soils but the extractable concentration was significantly higher in the sandy-loamy soil. For both soils NaH₂PO₄ extracted significantly higher organic acid concentrations than water. Oven drying increased the extractability of organic acids in both soils. Field moist samples (i.e. where no storage occurred) of the sandy-loamy soil showed a similar behaviour than ?20° stored samples whereas the one of the sandy-loamy soil were more close to the air-dried samples. These results indicate that organic acid extraction strongly depends on soil storage as well as on the soil type. Sample storage seems thus to be a crucial issue for the determination of organic acids in rhizosphere soil and needs to be considered prior analysis.     

Rhizosphere priming effects on the decomposition of soil organic matter in C4 and C3 grassland soils

Using a natural abundance ¹³C method, soil organic matter (SOM) decomposition was studied in a C₃ plant – `C<sub>4</sub> soil' (C<sub>3</sub> plant grown in a soil obtained from a grassland dominated by C<sub>4</sub> grasses) system and a C<sub>4</sub> plant – `C₃ soil' (C₄ plant grown in a soil taken from a pasture dominated by C₃ grasses) system. In C₃ plant – `C<sub>4</sub> soil' system, cumulative soil-derived CO<sub>2</sub>–C were higher in the soils planted with soybean (5499 mg pot<sup>–1</sup>) and sunflower (4484 mg pot<sup>–1</sup>) than that in `C₄ soil' control (3237 mg pot^–1) without plants. In other words, the decomposition of SOM in soils planted with soybean and sunflower were 69.9% and 38.5% faster than `C<sub>4</sub> soil' control. In C<sub>4</sub> plant – `C₃ soil' system, there was an overall negative priming effect of live roots on the decomposition of SOM. The cumulative soil-derived CO₂–C were lower in the soils planted with sorghum (2308 mg pot^–1) and amaranthus (2413 mg pot^–1) than that in `C<sub>3</sub> soil' control (2541 mg pot<sup>–1</sup>). The decomposition of SOM in soils planted with sorghum and amaranthus were 9.2% and 5.1% slower than `C₃ soil' control. Our results also showed that rhizosphere priming effects on SOM decomposition were positive at all developmental stages in C₃ plant – `C<sub>4</sub> soil' system, but the direction of the rhizosphere priming effect changed at different developmental stages in the C<sub>4</sub> plant – `C₃ soil' system. Implications of rhizosphere priming effects on SOM decomposition were discussed.     

轮作模式对冬小麦土壤氨氧化微生物群落多样性和组成的影响

农田温室气体减排已成为农业绿色发展的重要内容,驱动温室气体氧化亚氮(N₂O)产生的氨氧化微生物受到了研究者们的广泛关注。为探究轮作模式对土壤氨氧化微生物群落的影响,基于田间定位试验,研究了夏红小豆-冬小麦、夏绿豆-冬小麦、夏花生-冬小麦、夏大豆-冬小麦和夏玉米-冬小麦5种轮作模式中冬小麦根际和非根际土壤氨氧化古菌(AOA)和氨氧化细菌(AOB)的群落组成和多样性变化特征。结果表明:与夏玉米-冬小麦模式相比,豆禾轮作模式增加了根际土中有机碳和硝态氮含量,以及非根际土中全氮和铵态氮含量。豆禾轮作模式降低了非根际土壤中AOA群落的ACE指数和Chao1指数,并显著降低根际土中AOB群落的ACE指数和Chao1指数(P<0.05)。豆禾轮作显著增加AOA群落中泉古菌门(Crenarchaeota)和AOB群落中变形菌门(Proteobacteria)某些类群的相对丰度(P<0.05)。根际土中豆禾轮作模式与麦玉模式的AOA群落结构发生明显分离,而非根际土中豆禾轮作模式与麦玉模式的AOB群落发生分离(P<0.05)。研究结果表明:豆禾轮作种植改变了AOA和AOB的群落结构和多样性,土壤pH值和速效氮含量是驱动AOA和AOB群落结构变化的重要因子,且根际与非根际土壤中氨氧化微生物存在生态位分离。     

The effect of phosphate rock dissolution on soil chemical properties and wheat seedling root elongation

Soils of the Appalachian region of the United States are acidic and deficient in P. North Carolina phosphate rock (PR), a highly substituted fluoroapatite, should be quite reactive in these soils, allowing it to serve both as a source of P and a potential ameliorant of soil acidity. An experiment was conducted to evaluate the influence of PR dissolution on soil chemical properties and wheat (Triticum aestivum cv. Hart) seedling root elongation. Ten treatments including nine rates of PR (0, 12.5, 25, 50, 100, 200, 400, 800, and 1600 mg P kg^-1) and a CaCO₃ (1000 mg kg^-1) control were mixed with two acidic soils, moistened to a level corresponding to 33 kPa moisture tension and incubated for 30 days. Pregerminated wheat seedlings were grown for three days in the PR treated soils and the CaCO₃ control. Root length was significantly (P<0.05) increased both by PR treatments and CaCO₃, indicating that PR dissolution was ameliorating soil acidity. The PR treatments increased soil pH, exchangeable Ca, and soil solution Ca while lowering exchangeable Al and 0.01 M CaCl₂ extractable soil Al. Root growth in PR treatments was best described by an exponential equation (P<0.01) containing 0.01 M CaCl₂ extractable Al. The PR dissolution did not reduce total soil solution Al, but did release Al complexing anions into soil solution, which along with increased pH, shifted Al speciation from toxic to nontoxic forms. These results suggest that North Carolina PR should contribute to amelioration of soil acidity in acidic, low CEC soils of the Appalachian region.     

Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review

In most soils, inorganic phosphorus occurs at fairly low concentrations in the soil solution whilst a large proportion of it is more or less strongly held by diverse soil minerals. Phosphate ions can indeed be adsorbed onto positively charged minerals such as Fe and Al oxides. Phosphate (P) ions can also form a range of minerals in combination with metals such as Ca, Fe and Al. These adsorption/desorption and precipitation/dissolution equilibria control the concentration of P in the soil solution and, thereby, both its chemical mobility and bioavailability. Apart from the concentration of P ions, the major factors that determine those equilibria as well as the speciation of soil P are (i) the pH, (ii) the concentrations of anions that compete with P ions for ligand exchange reactions and (iii) the concentrations of metals (Ca, Fe and Al) that can coprecipitate with P ions. The chemical conditions of the rhizosphere are known to considerably differ from those of the bulk soil, as a consequence of a range of processes that are induced either directly by the activity of plant roots or by the activity of rhizosphere microflora. The aim of this paper is to give an overview of those chemical processes that are directly induced by plant roots and which can affect the concentration of P in the soil solution and, ultimately, the bioavailability of soil inorganic P to plants. Amongst these, the uptake activity of plant roots should be taken into account in the first place. A second group of activities which is of major concern with respect to P bioavailability are those processes that can affect soil pH, such as proton/bicarbonate release (anion/cation balance) and gaseous (O₂/CO₂) exchanges. Thirdly, the release of root exudates such as organic ligands is another activity of the root that can alter the concentration of P in the soil solution. These various processes and their relative contributions to the changes in the bioavailability of soil inorganic P that can occur in the rhizosphere can considerably vary with (i) plant species, (ii) plant nutritional status and (iii) ambient soil conditions, as will be stressed in this paper. Their possible implications for the understanding and management of P nutrition of plants will be briefly addressed and discussed.