The quantitative effect of chemical phosphate mobilization by carboxylate anions on P uptake by a single root. II. The importance of soil and plant parameters for uptake of mobilized P

Model calculations were made in order to quantify the effect of carboxylate excretion on phosphate (P) uptake by a single root. The uptake of chemically mobilized P increased exponentially with increasing concentration of adsorbed citrate or oxalate in soil because of the exponential relationship between adsorbed carboxylate and the solubilizing effect of carboxylate on P. The effect of local citrate excretion compared with uniform citrate excretion along the whole root was also calculated. Local exudation increased the uptake of chemically mobilized P because the higher concentration of citrate increases the solubilization of P. Additionally the effect of citrate excretion by root clusters e.g. proteoid roots was evaluated. Uptake of chemically mobilized P by root clusters was much higher than that of single roots, especially if the ratio of P buffering to citrate buffering was high. This is often the case in P fixing soils where by definition P buffering is high and citrate buffering is low because of the short time of reaction between root excreted citrate and rhizosphere soil. The reason for the superiority of cluster roots lies in the fact that most of the mobilized P is transported away from a single root to be absorbed by neighbouring roots in the clusters. This phenomenon demonstrates the strong ecological significance of cluster roots in relation to nutrient mobilization. The calculations on the effect of oxalate excretion by sugar beet roots on the uptake of mobilized P show that under P fixing conditions the influx of mobilized P will exceed that of P transported by diffusion to the root surface by a factor of 1.5—6.0.     

Phosphate aquisition by red clover and black mustard on a humic podzol

Recent investigations have shown that phosphate (P) mobilization by root exudates is an important feature of genotypes to acquire P even in soils of low‐P availability. We, therefore, investigated P mobilization processes in the rhizosphere of red clover (Trifolium pratense L.) and black mustard (Brassica nigra L.) on a humic podzol. As measured by the Kuchenbuch‐Jungk method (Kuchenbuch and Jungk, 1982), both species accumulated similar quantities of citrate (12 μmol/g soil) in the rhizosphere in about 1 mm distance from the soil‐root interface. Despite of similar concentrations of P‐mobilizing citrate in the rhizosphere of both species, red clover took up nearly the two‐fold of P compared to black mustard. Differences in rhizosphere pH were determined between both species. Black mustard did not acidify the rhizosphere, whereas red clover decreased the pH in the rhizosphere from 5.8 to about 4.0 (in 0.01M CaCl₂). The simultaneous acidification and excretion of citrate compared to citrate excretion alone had consequences for P mobilization processes in the rhizosphere. Phosphate mobilization from the soil solid phase was higher at higher pH. Thus, the citrate‐induced P desorption was not the limiting step in P acquisition by red clover and black mustard. Calculations of P distribution in the soil solution between free ortho‐P and humic‐associated P showed that at higher pH most of the P was associated with dissolved humic substances, whereas at pH < 5, most of the P was present as free ortho‐P. These P species can readily be taken up by the roots whereas humic‐associated P must probably be desorbed from the humic surface before uptake. Phosphate species calculations, therefore, explained the higher P uptake of red clover compared to black mustard. Aluminum species distribution calculations in the soil solution further show that even at pH < 5.0 in the soil solution, citrate strongly complex Al and thereby reduce the activity of monomeric Al species. The excretion of citrate can, therefore, counteract the root induced acidification of the rhizosphere with respect to Al toxicity.     

P and N deficiency change the relative abundance and function of rhizosphere microorganisms during cluster root development of white lupin (Lupinus albus L.)

We studied microbe-plant interactions of white lupin, a cluster root-forming plant, under low P and N conditions to examine increased nutrient acquisition by plants either by a shift to a more specialized microbial community or changes in microbial enzyme production. White lupin plants were grown in rhizoboxes filled with either P- or N-deficient soil; fertilized soil was used as control. After cultivation of plants in a glasshouse for 41 d, plant growth (shoot and roots) and P and N accumulation in shoots were measured. Microbial functions were analyzed by P- and N-cycling enzymes. The microbial community structure was estimated by fingerprinting (denaturing gradient gel electrophoresis) and sequencing techniques. P deficiency induced the released citrate and acid phosphomonoesterases from cluster roots and stimulated the production of microbe-derived alkaline phosphomonoesterase in the rhizosphere. P deficiency decreased microbial diversity in the cluster root rhizosphere. Increased relative abundance of Burkholderiales in the rhizosphere of P deficient plants might be responsible for the degradation of different organic P fractions such as phytates. N deficiency induced an increase of the number of nodules and P concentration in shoot as well as roots of white lupin. We clarified that high release of citrate from cluster roots might be the preferred mechanisms to meet the P demand of nodulated plants under N deficiency. In addition, the high abundance of Rhizobiales and Rhodospirillales in the rhizosphere of cluster roots showed that the importance of N-fixing microorganisms under N deficiency. The contribution of rhizosphere microorganisms due to similar activities of N-cycling enzymes under the two different N treatments is less important for N nutrition of plants. Further understanding of the regulation of cluster roots under N-deficiency will provide new information on the interactions between P and N nutrition.     

Biodegradation of low molecular weight organic acids in rhizosphere soils from a tropical montane rain forest

Root exudation of organic acids could be an important strategy for plant acquisition of phosphorus (P) from P-deficient soils in tropical rain forests. However, the efficacy of organic acids on P mobilization in the rhizosphere could be reduced due to their rapid biodegradation by rhizosphere microorganisms. To assess the dynamics and function of organic acids in the rhizosphere soils in tropical rain forests, we examined the concentrations of oxalate, citrate, and malate in soil solution and the mineralization kinetics of ¹⁴C-radiolabelled oxalate and citrate in the rhizosphere and bulk soil fractions. We compared two tropical montane rain forests from Mt. Kinabalu, Borneo that share similar parent material (i.e., sedimentary rocks) and climate but differ in terms of soil age. The older soil (Tertiary age materials) was affected by podzolization and had less inorganic labile P compared to the younger soil (Quaternary colluvial deposits). In the P-deficient older soil, the rhizosphere soil solution contained markedly higher concentrations of oxalate, citrate, and malate than did the bulk soil, whereas in the P-rich younger soil, the levels of organic acids in the rhizosphere were lower. The higher levels of organic acids in the rhizosphere of P-deficient soils are caused by greater root exudation and the lower sorption capacity for organic acids. The results of mineralization kinetics showed that oxalate and citrate in soil solution were rapidly mineralized in both rhizosphere and bulk fractions of both P-rich and P-deficient soils, having short mean residence times (2.3–13.1 h for oxalate and 0.8–1.6 h for citrate). The mineralization rates of oxalate and citrate were highest in the rhizosphere fraction of the P-deficient soil, where the pool of organic acids was largest and rapidly replenished by root exudation. Our data indicate that consumption as well as production of organic acids in the rhizosphere could be enhanced in P-deficient soil. The efficacy of organic acids on P mobilization in the rhizosphere in tropical montane rain forests appears to vary depending on the level of soil P availability and the anion sorption capacity, attributable to soil aging with podzolization.     

Flavonoids of white lupin roots participate in phosphorus mobilization from soil

The impact of flavonoids released by phosphorus-deficient white lupin roots on inorganic P and soil microorganisms is largely unknown. We report that flavonoids isolated from white lupin roots mobilized inorganic phosphorus and decreased soil microbial respiration, citrate mineralization, and soil phosphohydrolase activities, but did not reduce the soil ATP content. The results suggest that white lupin's release of flavonoids into the rhizosphere plays a significant role in its efficient P-acquisition strategy by solubilizing Fe-bound P and by limiting the microbial mineralization of citrate.     

Root-induced changes in the rhizosphere: Importance for the mineral nutrition of plants

Root-induced changes in the rhizosphere may affect mineral nutrition of plants in various ways. Examples for this are changes in rhizosphere pH in response to the source of nitrogen (NH₄-N versus NO₃-N), and iron and phosphorus deficiency. These pH changes can readily be demonstrated by infiltration of the soil with agar containing a pH indicator. The rhizosphere pH may be as much as 2 units higher or lower than the pH of the bulk soil. Also along the roots distinct differences in rhizosphere pH exist. In response to iron deficiency most plant species in their apical root zones increase the rate of H⁺ net excretion (acidification), the reducing capacity, the rate of Fe^III reduction and iron uptake. Also manganese reduction and uptake is increased several-fold, leading to high manganese concentrations in iron deficient plants. Low-molecular-weight root exudates may enhance mobilization of mineral nutrients in the rhizosphere. In response to iron deficiency, roots of grass species release non-proteinogenic amino acids (?phytosiderophores”?) which dissolve inorganic iron compounds by chelation of Fe^III and also mediate the plasma membrane transport of this chelated iron into the roots. A particular mechanism of mobilization of phosphorus in the rhizosphere exists in white lupin (Lupinus albus L.). In this species, phosphorus deficiency induces the formation of so-called proteoid roots. In these root zones sparingly soluble iron and aluminium phosphates are mobilized by the exudation of chelating substances (probably citrate), net excretion of H⁺ and increase in the reducing capacity. In mixed culture with white lupin, phosphorus uptake per unit root length of wheat (Triticum aestivum L.) plants from a soil low in available P is increased, indicating that wheat can take up phosphorus mobilized in the proteoid root zones of lupin. At the rhizoplane and in the root (root homogenates) of several plant species grown in different soils, of the total number of bacteria less than 1 % are N₂-fixing (diazotrophe) bacteria, mainly Enterobacter and Klebsiella. The proportion of the diazotroph bacteria is higher in the rhizosphere soil. This discrimination of diazotroph bacteria in the rhizosphere is increased with foliar application of combined nitrogen. Inoculation with the diazotroph bacteria Azospirillum increases root length and enhances formation of lateral roots and root hairs similarly as does application of auxin (IAA). Thus rhizosphere bacteria such as Azospirillum may affect mineral nutrition and plant growth indirectly rather than by supply of nitrogen.     

Phosphate,Fe and Mn uptake of N2 fixing red clover and ryegrass from an Oxisol as affected by P and model humic substances application. 1. Plant parameters and soil solution composition

The effect of humic substances on P-availability in soil is still debated. Therefore, the effect of model humic substances synthesized from hydroquinone on P, Fe, and Al solubility in a strong P fixing Oxisol and on P acquisition by red clover and ryegrass was investigated. After 4 months of incubation, P concentration of soil solution had increased by a factor of > 10 at the highest humic level (50 g humics kg^?1 soil), accompanied by a similar increase in Fe and Al concentrations. Soil samples with 0, 10, 30, 50 g humics kg^?1 soil were planted with red clover and ryegrass. Red clover showed a small increase of shoot yield and a moderate increase of P uptake after humics addition. High humics levels increased slightly Fe concentration in the shoots but strongly that of Mn leading to Mn toxicity. Ryegrass showed a strong increase in shoot yield after humics addition of about 150 % at the highest humics level compared to the control without humics. At each humic level, P application (100 mg kg^?1 soil) had no effect on P uptake of red clover and a small effect on P uptake by ryegrass. The relatively small effect of humics and P application on shoot yield of clover compared to grass can be explained by chemical P mobilization of red clover via exudation of citrate (about 12 μmol citrate g^?1 soil). This agrees with the finding that P solubility increased in the soil under red clover but not under ryegrass from the first to the second harvest, indicating that red clover mobilized P.     

Subsoil rhizosphere modification by chickpea under a dry topsoil: implications for phosphorus acquisition

Chickpea (Cicer arietinum L.) roots exude carboxylates. While chickpea commonly grows where the topsoil dries out during crop growth, the importance of carboxylate exudation by the roots and mobilization of soil P from below the dry topsoil has not been examined. The study investigates the response of carboxylate exudation and soil P mobilization by this crop to subsoil P fertilizer rate. In constructed soil columns in the glasshouse, the P levels (high, low, and nil P) were varied in the well‐watered subsoil (10–30 cm), while a low level of P in the dry topsoil (0–10 cm) was maintained. At flowering, rhizosphere carboxylates and rhizosphere soil from topsoil and subsoil roots were collected separately and analyzed. The concentration of total carboxylates per unit rhizosphere mass in the subsoil was nearly double that of the topsoil. Plants depleted sparingly soluble inorganic P (P_i), NaOH‐P_i, and HCl‐P_i, along with the labile P_i (water soluble and NaHCO₃‐Pi). The P depletion by plants was greater from the subsoil than the topsoil. The study concluded that depletion of sparingly soluble P from the chickpea rhizosphere in the subsoil was linked with the greater levels of carboxylates in the rhizosphere. These findings indicate that chickpea, with its deep rooting pattern, can increase its access to subsoil P when the topsoil dries out during crop growth by subsoil rhizosphere modification.     

The excretion of citric and malic acid by proteoid roots of Lupinus albus L.; effects on soil solution concentrations of phosphate,iron, and aluminum in the proteoid rhizosphere in samples of an oxisol and a luvisol

Organic acid concentration in the proteoid rhizosphere of White Lupin in different soil samples (Oxisol-A_p = Ox, Luvisol-A_p and Luvisol-C = L_A and L_C) was determined in order to study the influence of root-released carboxylates on the mobilization of phosphate, aluminum, and iron in the rhizosphere. In the L_C, organic acids were accumulated as Casalts extractable with water. In the proteoid rhizosphere of this soil sample 55 μmol citrate and 8 μmol malate per g soil were found. In the Ox, no water extractable organic acids were present. However, determination of citrate in the solid phase of this soil by Diffuse Reflectance Infrared Fourier Transform Spectroscopy gave concentrations of 88 and 68 μmol citrate per g soil without and with P application, respectively. Displaced soil solution from the proteoid root rhizosphere of the Ox and the L_A increased in Fe and Al concentrations from <50 μmol/L (soil from reference pots without plants) to more than 600 μmol Fe+Al/L. The concentration of P was increased by a factor of 2 despite of P uptake by the proteoid roots. The mobilization of Al, Fe, and P is attributed to ligand exchange of phosphate against citrate and to the solubilization of Al and Fe as carboxylate complexes.     

Interactive effects of organic acids in the rhizosphere

Organic acids released into the rhizosphere may perform many beneficial functions to the plant including metal detoxification and enhancement of nutrient acquisition. Typically, these organic acids are studied in isolation; however, roots simultaneously exude a cocktail of organic acids and other substances, and their combined impact on rhizosphere processes may be quite different. It has been hypothesized that some exudates may play secondary roles (e.g. inhibitors of microbial activity, blockage of sorption sites), which might enhance the longevity and nutrient-mobilization capacity of others. Here we investigated how the decomposition, sorption and P-solubilizing effects of citrate, malate and oxalate are affected by the presence of malonate and shikimate. We found that in a range of agricultural soils the decomposition of citrate, malate and oxalate was rapid, but not influenced by the presence of large quantities of shikimate or malonate. This suggests that the individual organic acids are taken up by different transport mechanisms or components of the microbial community. At large concentrations, malonate decreased sorption of citrate, malate and oxalate on the soil, whilst shikimate had little effect. The capacity of citrate, malate and oxalate to desorb P was significantly greater in cocktails containing malonate compared with the single organic acid; no effect was seen with shikimate. We conclude that neither malonate nor shikimate at realistic concentrations will significantly affect the biodegradation of citrate, malate or oxalate in the rhizosphere, and while malonate did enhance P desorption, this effect is additive rather than synergistic. Overall, we found little evidence that malonate and shikimate act as secondary regulators of citrate, malate and oxalate behavior in soil.     

Cluster Root Formation by Lupinus Albus is Modified by Stratified Application of Phosphorus in a Split-Root System

ABSTRACT

Cluster root formation by white lupin (Lupinus albus L. cv. Kiev Mutant) in response to stratified application of hydroxyapatite was examined in a split-root system. The system consisted of two vertical compartments, each divided horizontally into five 60-mm layers. Hydroxyapatite was applied to different layers at 150 mg phosphorus(P) kg^?1 soil. The proportion of dry biomass of cluster roots in the whole root system was significantly reduced when P concentration was high in shoots due to P application, suggesting that cluster root formation was regulated by the shoot P status. However, the cluster root percentage increased in the soil layer supplemented with P, and decreased in other layers, especially when P was applied in a deep layer. The formation of cluster roots is regulated by internal plant P status, but is also greatly affected by localized P supply. Heterogeneous P supply can modify the distribution of cluster roots.     

Iron Mobilization and Mineralogical Alterations Induced by Iron-Deficient Cucumber Plants (Cucumis sativus L.) in a Calcareous Soil

Dicotyledons cope with ion (Fe) shortage by releasing low-molecular-weight organic compounds into the rhizosphere to mobilize Fe through reduction and complexation mechanisms. The effects induced by these root exudates on soil mineralogy and the connections between Fe mobilization and mineral weathering processes have not been completely clarified. In a batch experiment, we tested two different kinds of organic compounds commonly exuded by Fe-deficient plants, i.e., three organic acids (citrate, malate, and oxalate) and three flavonoids (rutin, quercetin, and genistein), alone or in combination, for their ability to mobilize Fe from a calcareous soil and modify its mineralogy. The effect of root exudates on soil mineralogy was assessed in vivo by cultivating Fe-deficient and Fe-sufficient cucumber plants (Cucumis sativus L.) in a RHIZOtest device. Mineralogical analyses were performed by X-ray powder diffraction. The batch experiment showed that citrate and, particularly, rutin (alone or combined with organic acids or genistein) promoted Fe mobilization from the soil. The combinations of rutin and organic acids modified the soil mineralogy by dissolving the amorphous fractions and promoting the formation of illite. These mineralogical alterations were significantly correlated with the amount of Fe mobilized from the soil. The RHIZOtest experiment revealed a drastic dissolution of amorphous components in the rhizosphere soil of Fe-deficient plants, possibly caused by the intense release of phenolics, amino acids, and organic acids, but without any formation of illite. Both batch and RHIZOtest experiments proved that exudates released by cucumber under Fe deficiency concurred to the rapid modification (on a day-scale) of the mineralogy of a calcareous soil.     

Solubilization and Acquisition of Phosphorus from Sparingly Soluble Phosphorus Sources and Differential Growth Response of Brassica Cultivars Exposed to Phosphorus-Stress Environment

Phosphate (Pi), the fully oxidized and assimilated form of phosphorus (P), influences virtually all developmental and biochemical processes in plants; however, its availability and distribution are widely heterogeneous. Paradoxically, although total P is abundant in lithosphere, elusive soil chemistry of Pi renders the element the most dilute and the least mobile in natural and agricultural ecosystems, resulting in P deprivation due to its low mobility and high fixation capacity in the soil. Nonmycorrhizal Brassica does not produce specialized cluster/dauciform roots but is an effective P user compared to other crops. Using a soil low in P (Mehlich 3–extractable P) with or without P fertilization, Brassica cultivars showed substantial genetic diversity in P-utilization efficiency (PUE), P efficiency (PE), P-efficiency ratio (PER), and P-stress factor (PSF). Cultivars producing greater root biomass accumulated greater total P contents, which in turn was related negatively to PSF and positively to shoot and total biomass. Plant survival and reproduction rely on efficient strategies in exploring culture media for P. Acquisition of orthophosphate from extracellular sparse P sources may be enhanced by biochemical rescue strategies such as copious H⁺ efflux and/or carboxylates exudation into rhizosphere by roots via plasmalemma H⁺-ATPase and anion channels triggered by P starvation. The P-starvation-induced solution pH changes due to H⁺ efflux, and carboxylates exudations were estimated by low-P-tolerant and low-P-sensitive cultivars in solution culture experiments. Low-P-tolerant cultivars showed more decrease in pH compared to low-P-sensitive cultivars when cultivars were grown under a P-stress environment induced by using sparingly soluble P sources (rock phosphate and tricalcium phosphate). The P contents of cultivars were inversely related to decrease in culture media pH. Low P-tolerant cultivars presented enhanced H⁺-efflux and total carboxylates exudations compared to low-P-sensitive cultivars, resulting in more rhizosphere acidification to scavenge Pi, evidencing their adaptability to P starvation. These elegant P-stress-induced rescue strategies by tested cultivars provided the basis of enhanced P solubilization and acquisition of P from sparingly soluble P sources to combat P-starved environments.     

碳对微生物–根系介导的蔬菜作物磷吸收的影响

  【目的】  碳是微生物代谢活动的能量来源,解析碳驱动的微生物磷周转对根系/根际属性以及作物磷吸收的影响,对探索提高磷利用效率的根际调控措施具有重要的指导意义。  【方法】  以绿叶蔬菜上海青(Brassica chinensis L., Xiaqing 3)为供试作物进行盆栽试验,供试碳源为葡萄糖。设置添加葡萄糖(+G)和不添加葡萄糖(?G,对照)两个处理,在添加葡萄糖后第7天和第21天,测定土壤微生物量磷与Olsen-P含量、根际酸性磷酸酶活性以及柠檬酸和苹果酸含量、根系形态(生物量、根冠比、根长、根系直径、比根长和根系组织密度)与根际生理(酸性磷酸酶、柠檬酸和苹果酸)指标和作物磷吸收量。  【结果】  添加葡萄糖后第7天,土壤微生物量磷增加,Olsen-P含量降低;上海青根系生物量和根冠比显著高于对照,另外,与不加葡萄糖处理相比,添加葡萄糖导致上海青总根长降低33%,根系平均直径增加27%,比根长降低46%,根际柠檬酸含量增加106%。从第7天到第21天,添加葡萄糖处理土壤微生物量磷降低,Olsen-P含量增加,上海青根系生长速率显著提高。葡萄糖添加后第21天,添加葡萄糖处理土壤Olsen-P含量高于对照土壤;与不加葡萄糖的处理相比,根际酸性磷酸酶和柠檬酸的分泌降低,上海青根系总根长增加,其相对增加量为31%。添加葡萄糖对第7天和第21天上海青地上部磷吸收没有显著影响。  【结论】  添加葡萄糖提高了前期(添加葡萄糖后第7天)根际微生物量磷,降低了Olsen-P含量,促进根际柠檬酸的分泌满足作物生长对磷的需求。后期(添加葡萄糖后第21天),微生物量磷的降低促进土壤有效磷含量的增加,刺激根系快速伸长。微生物介导磷周转诱导作物调节根系形态和根际分泌物响应土壤磷环境的变化,维持地上部磷营养。     

Cadmium and lead affect the status of mineral nutrients in alfalfa grown on a calcareous soil

ABSTRACT

Cadmium (Cd) and lead (Pb) are toxic trace elements which are not essential for plants but can be easily taken up by roots and accumulated in various organs, and cause irreversible damages to plants. A pot experiment was carried out to investigate the individual and combined effects of Cd (0, 10, 20 mg kg^?1) and Pb (0, 500, 1000 mg kg^?1) level in a calcareous soil on the status of mineral nutrients, including K, P, Ca, Mg, S, Fe, Mn, Cu, and Zn, in alfalfa (Medicago sativa L.) plants. Soil Pb level considerably (P ≤ 0.05) affected the concentrations of more elements in plants than soil Cd level did, and there were combined effects of soil Cd level and Pb level on the concentrations of some nutrients (Ca, Mg, and Cu) in plants. The effects of soil Cd level and Pb level on plant nutrient concentrations varied among plant parts. Cd and Pb contamination did not considerably affect the exudation of carboxylates in the rhizosphere. An increase in rhizosphere pH and exudation of significant amounts of carboxylates (especially oxalate) in the rhizosphere might contribute to the exclusion and detoxification of Cd and Pb. Neither shoot dry mass nor root dry mass was significantly influenced by soil Cd level, but both of them were considerably reduced (by up to 25% and 45% on average for shoot dry mass and root dry mass, respectively) by increasing soil Pb level. The interaction between soil Cd level and Pb level was significant for root dry mass, but not significant for shoot dry mass. The results indicate that alfalfa is tolerant to Cd and Pb stress, and it is promising to grow alfalfa for phytostabilization of Cd and Pb on calcareous soils contaminated with Cd and Pb.     

In situ gradient distribution of polycyclic aromatic hydrocarbons (PAHs) in contaminated rhizosphere soil: a field study

Purpose

Little information is available heretofore on the gradient distribution of persistent organic pollutants in rhizosphere on a field scale. In this field study, we seek to explore the in situ distribution gradient of polycyclic aromatic hydrocarbons (PAHs) in rhizosphere soil proximal to the roots.

Materials and methods

Clover (Trifolium pratense L.) and hyssop (Hyssopus officinalis L.) grew in situ in the contaminated field soil near a petrochemical plant and were harvested when about 30 cm tall with mature roots. Rhizosphere soils of the plants were sampled including the rhizoplane, strongly adhering soil, and loosely adhering soil. Eleven EPA-priority PAHs were detected in each layer of rhizosphere soils in proximity to the root surface.

Results and discussion

The PAH concentrations followed the descending order of bulk soil, loosely adhering soil, strongly adhering soil, and rhizoplane soil in proximity to the root surface of clover and hyssop. The rhizosphere effect (R, in percent) on PAH distribution clearly decreased with increasing distance from the root, and a more significant decrease was observed for hyssop compared to clover. R values were generally lower for three- and four-ringed PAHs in the rhizosphere, which were more significant in loosely and strongly adhering rhizosphere layers.

Conclusions

Our field observations combined with previous potted studies demonstrated that PAH concentrations in rhizosphere soils increased with distance from the root. Results of this work provide new information on the fate of PAHs in rhizosphere.     

Soil Aggregate Size and Mycorrhizal Colonization Effect on Root Growth and Phosphorus Accumulation by Berseem Clover

The effects of soil aggregate size and mycorrhizal colonization on phosphorus (P) accumulation and root growth of Berseem clover (Trifolium alexandrinum L.) were studied. Root length and dry weight decreased with increasing aggregate diameter. Colonization of clover plants by arbuscular mycorrhizae (Glomus intraradices Schenck and Smith) improved root growth and P accumulation in all aggregate‐size classes. Although total root length of either mycorrhizal or nonmycorrhizal plants decreased with increasing aggregate diameter, the length of living external hyphae was not affected by aggregate size. Thus, colonized root length was improved by 20% as soil aggregate diameter increased. Total P accumulation per plant decreased with increasing aggregate size. However, total P accumulation per unit root length improved as the size of soil aggregate increased. In our study, mycorrhizal colonization improved total P accumulation and root growth in soil with large aggregates and compensated, in part, for the effect of soil strength.     

The effect of root hairs on rhizosphere phosphatase activity

Background: Phosphatases in soil are of great importance for plant P acquisition. It is hypothesized that root hairs increase rhizosphere phosphatase activity as they release enzymes into soil and stimulate microbial activity. Methods: To test the effect of root hairs on soil phosphatase activity, we grew barley (Hordeum vulgare ‘Pallas') wild type and its root‐hairless mutant in rhizoboxes and determined phosphatase activity using soil zymography. Measurements were done at three moisture levels (30, 15, and 5% VWC). Rhizosphere phosphatase activity was estimated for the two genotypes and two locations along the root [root tip region (0–4 cm behind tip) and mature roots (> 7 cm behind tip)]. Results: Rhizosphere phosphatase activity was similar in the two locations along the root (root tip region vs. mature root parts). In contrast, rhizosphere phosphatase extension was two times larger for the root tip region of the wild type than for the mutant at 30% and 15% VWC. However, as phosphatase activities at the root surface of tips and mature root parts were slightly higher for the mutant than for the wild type, average enzyme activities were unaffected by the genotype. Conclusions: We conclude that the mutant seems to compensate for the lack of root hairs by increased phosphatase activity close to the root surface. However, the increased rhizosphere phosphatase extension for the wild type may be equally efficient as it allows P mobilization and uptake from large soil volumes.     

Mobilization and turnover of soil phosphorus in the rhizosphere

Recent progress in methods enables a better understanding of the turnover of P in the rhizosphere. Examples of this progress are the separation of soil layers differing in proximity to the roots, improved methods for extraction and fractionation of soil P, application of ³²P isotope dilution analysis to follow P fluxes between various fractions and direct determination of microbially bound P and of root phosphatases.

• These methods were combined to investigate the following aspects
• –labile P pools, the P fluxes between these pools and their contribution to the P supply to growing maize roots
• –the role of microbial biomass in these interactions and the partition of mobilized P between plants and microorganisms
• –modifications of sorption and transport of P in the rhizosphere
• –plant availability of native and added organic phosphates, and the relative significance of root and soil phosphatases.

There is a significant transformation of P in the rhizosphere with a corresponding redistribution among fractions of different plant availability. About 9% of the inorganic ³²P added to soil were incorporated within 2 weeks into microbial and organic fractions. The transfer of P from non-exchangeable forms exceeded the depletion of the exchangeable P by a factor of 5. About 53% of the mobilized P originated from inorganic, the remaining 47% from organic fractions. Of the mobilized P 80% was taken up by the plants and 20% was found in the microbial biomass. Up to 90% of the P in the rhizosphere soil solution was organic with a maximum just outside the root zone. Soluble inositol hexaphosphate modified the sorption of inorganic P, thus shifting its equilibrium solution concentration. The phosphatase activity of the roots is considerable. Both root phosphatase activity and the utilization of inositol hexaphosphate depend on the P supply and nutritional status of plants with regard to P. It is concluded that the rhizosphere is a key site of P transformation with a significant mobilization of P from the non-exchangeable inorganic and organic fractions. Organic P fractions not only play a significant role as a P source but also modify important soil parameters related to the sorption and transport of P in the rhizosphere.