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.     

Soil solution chemistry in the rhizosphere of roots of sessile oak (Quercus petraea) as influenced by lime

This study describes the soil solution chemistry in the rhizosphere of fine roots of sessile oak ( Quercus petraea (M.) Liebl.) grown in rhizotrons. A control was compared with soils treated with an equivalent CaCO₃ of 1.4 t ha^-1 CaO. Solution samples were extracted from the B-horizon using micro suction cups with a suction of _∼40 kPa. Two series of experiments were carried out: one irrigated with rain water (age of seedling 2 to 4 months) and one irrigated with demineralized water (age of seedlings 1.5 to 2 months). Half of the sampling points were choosen close to the roots and half in the bulk soil. In both experiments there was generally no rhizospheric gradient after liming. In contrast, in the control, depletion in the rhizosphere occurred for most of the ions studied (Mg, Ca, Al, K, NO₃ ^-, NH₄ ⁺, Cl^-) in the demineralized water experiment, but this was different when rainwater was used. The latter effect is probably due to the higher solution concentrations in the rainwater experiment but could also be a result of root damage due to low Ca/Al ratios in the rhizosphere solution. It was concluded, that liming improved the chemical composition in the rhizosphere soil solution by increasing overall solute concentration to levels enabling sufficient and easier nutrient uptake by roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Depletion of Selenium in Soil Solution due to its Enhanced Sorption in the Rhizosphere of Soybean

The effect of plant roots on selenium (Se) mobility in soil was studied by a large-scale pot experiment in order to understand the environmental behavior of Se in agricultural soils under plant growth conditions. Soybean plants (Glycine max (L.) Merrill) were grown in a greenhouse for 84 d. The concentrations of Se and major elements (K, Ca, Mg, Na, and Al) in the soil solutions and in the plants were measured at different growth periods. Concentrations of Se and major cations in soil solution decreased as the soybean plants grew, while the concentrations of Al increased. It was assumed that the soybean roots released H⁺ with the uptake of cations; consequently, due to the acidification of the rhizosphere, Al³⁺ was released starting from the soil solid phase. The decreased Se concentration in the soil solution should be due to the enhancement of Se sorption onto the soil solid phase. The increase of Se sorption level in the rhizosphere was examined in a small-scale pot experiment. The soil–soil solution distribution coefficient of Se (K _d-Se) was observed as an index of Se sorption level. K _d-Se clearly increased in the rhizosphere soil after cultivation. The effects of pH and Al³⁺ in the rhizosphere on Se sorption were assessed by K _d-Se measurements at different levels of HCl and AlCl₃. In this third experiment, a decrease in pH increased K _d-Se values, but no specific effect was observed on Se sorption due to increased Al³⁺. These results show that the Se mobility in agricultural soil could be decreased by plant roots under plant growth conditions due to enhanced Se sorption in the rhizosphere.     

Microscale heterogeneity of acidity related stress-parameters in the soil solution of a forested cambic podzol

Acid related stress in soils might be caused by high concentrations of H⁺ and Al³⁺ in soil solution. Sampling of soil solution so far integrates over a relatively large soil volume, in the range of dm³. In order to study the microscale heterogeneity of acidity related stress-parameters the soil profile of a podzolic cambisol was covered by a 10×6 matrix of micro suction cups with a grid distance of 2 cm. The soil solution collected at 10 sampling events was analyzed for free cations and anions by capillary electrophoresis and for total metal content by a micro injection technique on ICP-OES. pH and UV absorption were also measured.There was a general trend of increasing pH and decreasing UV absorption with increasing soil depth, however without a clear correlation of concentration isolines to soil horizon borders. The latter was also true for total Al (Al_tot) and Al³⁺, with the exception of the soil horizon border A_he/B_h,which was very well reflected by Al³⁺ and also by the fraction of bound Al. In the A_he horizon less than 30%, in deeper mineral soil less than 50% of Al_tot were present as free Al³⁺. This fact is critical when calculating Ca/Al ratios as a stress parameter, because total metal content measured by ICP clearly overestimates the risk of root damage, even in deeper horizons of acid forest soils, where organic complexation of Al is of minor importance. The heterogeneity of soil solution chemistry and toxicity parameters on the cm-scale was found to be significant, for example with gradients of more than 0,5 pH-units within 2 cm. Because plant roots also experience soil on a microscale, high resolution investigations of soil solution chemistry offer a new approach for looking at the chemical environment relevant for root growth and plant nutrition.     

Equilibrium solution composition and exchange properties of disturbed and undisturbed soil samples from an acid forest soil

Microscalic heterogeneity of soil chemical properties caused by soil structure has been reported for several soils. We investigated exchange properties and soil solution composition of disturbed and undisturbed samples of an acid forest soil lacking visible structure. Cation concentrations in the soil solution resulting from two extraction procedures and two analytical methods were compared. The effective cation exchange capacity (CEC_e) of the undisturbed sample represented 56–69% of the bulk soil CEC_e. Base saturation of undisturbed samples equalled that of disturbed samples for EA, Bhs, and Bsh horizons, and was higher for the Bw horizon. Contradicting the results of other authors, soil pore solution obtained by percolating soil cores under conditions of low water tension offered more favourable conditions for plant roots when compared to the equilibrium soil solution of the bulk soil sample in all except the Bsh horizon. Ca²⁺/Al³⁺ molar ratios were higher and fractions of H⁺ + Al³⁺ on total cationic charge were lower in the soil pore solution. These results were obtained employing soil: solution ratios of about 1:0.5 during the extraction of soil pore solution, and by determination of free cations. Other authors used a water extraction with soil:solution ratios up to 1:2 and took total metal for ion concentrations. The combination of the latter extraction and analytical method in our study, too, led to unfavourable Ca²⁺/Al³⁺ ratios and high tractions of H⁺ + Al³⁺. The choice of analytical and extraction method are thus decisive for the valuation of the soil solution composition in view of plant nutrition.     

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。     

In-situ soil solution chemistry in an acid forest soil as influenced by growing roots of Norway spruce (Picea abies [L.] Karst.)

A new approach for non-destructive monitoring of soil solution chemistry in high spatial and temporal resolution for rhizosphere studies is presented. In a 5×10 mm grid, 30 micro suction cups (1mm) were installed in a rhizotron with Norway spruce (Picea abies [L.] Karst.) growing in low pH B-horizon soil. Roots grew through the grid, closely passing the suction cups. Soil solution composition before, during and after root passage was determined. For K⁺ and Mg²⁺ a significant decrease of soil solution concentration near root tips and elongation zones was observed, indicating a marked uptake of these elements. Mg²⁺ concentration was also significantly lowered when the root system aged, suggesting that this ion might also be taken up in older parts of the root system. No influence of growing roots was found on Na⁺-concentrations.     

增温和施氮对亚热带杉木人工林土壤溶液养分的影响

在增温和氮沉降等气候变化背景下,土壤溶液养分元素供应是否平衡对加速或减弱土壤养分循环起着至关重要的作用。为探究增温和施氮对亚热带杉木人工土壤溶液养分动态的影响,分别于各样地的0—15、15—30cm和30—60cm土层安装土壤溶液采集器。利用真空泵的负压原理采集土壤溶液,对其有机组分及无机组分进行了两年的动态监测。结果显示:增温及增温+施氮显著增加了各土层溶解性总氮(DTN)及硝态氮(NO-3)浓度,而施氮促进了植被对其的大量吸收而未呈现显著的促进作用。整体而言,短期增温和施氮处理显著降低了可溶性有机碳(DOC)浓度,对土壤溶液K~+、Ca~(2+)、Na~+、Mg~(2+)等离子含量影响较小。但相比于施氮,增温对土壤溶液中矿质元素的影响远大于施氮。增温导致的土壤孔径增大,通透性增强可极大地促进Fe~(3+)、Al~(3+)淋溶,同时导致表层Na~+、Mg~(2+)离子含量显著降低。增温+施氮交互作用对土壤溶液各养分的影响存在叠加效应,但并不是增温和施氮单因子的简单累加,要深入了解土壤养分对未来气候变迁的内部机制需进一步长期监测。     

Impact of faunal complexity on nutrient supply in field mesocosms from a spruce forest soil

A field study in an acidic spruce forest soil using soil mesocosms was conducted to investigate the effects of mesofauna and macrofauna on exchangeable cations, organic matter content, base saturation, and Ca-lactate extractable nutrients. In the field, intact soil monoliths were taken from the ground, defaunated by deep-freezing and wrapped in nets of various mesh-sizes to control immigration of different faunal size classes. The monoliths were then replanted in the field. Three types of treatments for the mesocosms were prepared: (1) microbiota only, (2) microbiota and mesofauna, (3) microbiota, mesofauna, and macrofauna (=complex fauna). After eight months the mesocosms and unmanipulated control plots (treatment 4) were destructively sampled and submitted to chemical analysis. Generally, the exchangeable base cations and Mn²⁺ showed higher contents with increasing faunal complexity, whereas the exchangeable acidic cations of Fe³⁺ and Al³⁺ decreased in the monoliths with complex fauna. These effects were significant for K⁺, Mg²⁺ and Mn²⁺ in the L/F-layer and for Ca²⁺, Mn²⁺, Al³⁺ and Fe³⁺ in the H-layer. As a possible explanation a rise of ion-binding sites in the course of enhanced humification processes is discussed.In the L/F-layer base cations showed higher concentrations in the monoliths with complex fauna as compared to the control plot, which contained intact roots. This might be due to nutrient uptake by roots in the control plot or enhanced mineralization in the monoliths with complex fauna, where roots had been cut.     

Effects of litter incorporation and nitrogen fertilization on the contents of extractable aluminium in the rhizosphere soil of tea plant (Camallia sinensis (L.) O. Kuntze)

The effects of litter incorporation and nitrogen application on the properties of rhizosphere and bulk soils of tea plants (Camellia sinensis (L.) O. Kuntze) were examined in a pot experiment. Total of 8 treatments included four levels of tea litter additions at 0, 4.9, 9.8, and 24.5 g kg^–1 in combination with two N levels (154.6 mg kg^–1 and without). After 18 months of growth the rhizosphere soil was collected by removing the soil adhering to plant roots and other soil was referred to as bulk soil. The dry matter productions of tea plants were significantly increased by N fertilization and litter incorporation. The effect of litter was time-depending and significantly decreased the content of exchangeable Al (Al_ex, by 1 mol L^–1 KCl) and Al saturation at 9 months after litter incorporation whereas soil pH was not affected, although the litter contained high Al content. After 18 months, the contents of extractable Al by dilute CaCl₂, CuCl₂ + KCl, NH₄OAC, ammonium oxalate and sodium citrate (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, Al_Oxal, and Al_Cit respectively) and Al_ex, were not affected by litter application, except that of Al_CaCl2 in the rhizosphere soil which was decreased following litter additions. Nitrogen fertilization with NH₄ ⁺ (urea and (NH₄)₂SO₄) significantly reduced soil pH, the contents of exchangeable Ca, K, Mg and base saturation while raised extractable Al levels (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, and Al_ex). In the rhizosphere soils exchangeable K accumulated in all treatments while exchangeable Ca and Mg depleted in treatments without litter application. The depletions of Ca and Mg were no longer observed following litter incorporation. This change of distribution gradients in rhizosphere was possibly due to the increase of nutrient supplies from litter decomposition and/or preferable root growth in soil microsites rich in organic matter. Lower pH and higher extractable Al (Al_CaCl2, Al_ex, and Al_NH4OAC) in the rhizosphere soils, regardless of N and litter treatments, were distinct and consistent in all treatments. Such enrichments of extractable Al in the rhizosphere soil might be of importance for tea plants capable of taking up large amounts of Al.     

Response of soil chemistry to forest dieback after bark beetle infestation

We evaluated changes in the chemistry of the uppermost soil horizons in an unmanaged spruce forest (National Park Bohemian Forest, Czech Republic) for 3 years after dieback caused by a bark beetle infestation, and compared these changes with a similar undisturbed forest area. The soils below the disturbed forest received 2–6 times more elements via litter fall compared to the unaffected plot. The subsequent decomposition of litter and reduced nutrient uptake by trees resulted in a steep increase in soil concentrations of soluble N (NH₄-N, organic-bound N) and P forms in the disturbed plot. The average concentrations of NH₄-N and soluble reactive P increased from 0.8 to 4.4 mmol kg^?1 and from 0.04 to 0.9 mmol kg^?1, respectively, in the uppermost soil horizon. Decomposition of litter at the disturbed plot elevated soil concentrations of Ca²⁺, Mg²⁺ and K⁺, which replaced Al³⁺ and H⁺ ions from the soil sorption complex. Consequently, soil concentrations of exchangeable base cations increased from 120 to 200 meq kg^?1, while exchangeable Al³⁺ and H⁺ decreased 66 and 50 %, respectively, and soil base saturation increased from 40 to 70 %. The Al³⁺ liberation did not elevate concentrations of ionic Al in the soil solution, because most of the liberated Al³⁺ was rapidly complexed by dissolved organic carbon (DOC) and transformed to DOC–Al complexes. The chemical parameters investigated at the unaffected plot remained stable during the study.     

Modified micro suction cup/rhizobox approach for the in-situ detection of organic acids in rhizosphere soil solution

Root–soil interactions can strongly influence the soil solution chemistry in the rhizosphere. In the present study we propose a modification of the classical rhizobox/micro suction cup system to make it suitable for the collection and analysis of organic acids in the rhizosphere. In order to show the potential of the method, we tested the modified system with Lupinus albus L. as a model plant known to exude large amounts of citrate. The suction cups were installed through the transparent front plate of the rhizoboxes just after the emergence of cluster roots in order to allow optimal localized collection of soil solution. A small dead-volume allowed almost immediate stabilisation with formaldehyde of the sampled soil solutions in the collection container to prevent microbial degradation. The concentrations of organic acids were significantly larger in the rhizosphere soil solution of active cluster roots of Lupinus albus L. than in the bulk soil solution (about 400 μM of citrate versus <0.05 μM). We were able to follow the exudation process in-situ, which occurred during 2–3 days. Also the concentrations of other organic acids and inorganic anions differed between the bulk soil and the rhizosphere of cluster roots, normal roots, and nodules.     

Effects of air pollutant-temperature interactions on mineral-N dynamics and cation leaching in reciplicate forest soil transplantation experiments

Increased emissions of nitrogen compounds have led to atmosphericdeposition to forest soils exceeding critical loads of N overlarge parts of Europe. To determine whether the chemistry offorest soils responds to changes in throughfall chemistry, intactsoil columns were reciprocally transplanted between sites, withdifferent physical conditions, across a gradient of N and Sdeposition in Europe.The transfer of a single soil to the various sites affected itsnet nitrification. This was not simply due to the nitrificationof different levels of N deposition but was explained bydifferences in physical climates which influenced mineralizationrates. Variation in the amount of net nitrification between soiltypes at a specific site were explained largely by soil pH.Within a site all soil types showed similar trends in netnitrification over time. Seasonal changes in net nitrificationcorresponds to oscillations in temperature but variable time lagshad to be introduced to explain the relationships. WithArrhenius law it was possible to approximate gross nitrificationas a function of temperature. Gross nitrification equalled netnitrification after adaptation of the microbial community oftransplanted soils to the new conditions. Time lags, andunderestimates of gross nitrification in autumn, were assumed tobe the result of increased NH ₄ ⁺ availability due either tochanges in the relative rates of gross and net N transformationsor to altered soil fauna-microbial interactions combined withimproved moisture conditions.Losses of NO ₃ ^- were associated with Ca²⁺and Mg²⁺ in non-acidified soil types and with losses ofAl³⁺ in the acidified soils. For single soils the ionequilibrium equation of Gaines-Thomas provided a useful approximationof Al³⁺ concentrations in the soil solution as a functionof the concentration of Ca²⁺. The between site deviationsfrom this predicted equilibrium, which existed for single soils, couldbe explained by differences in throughfall chemistry which affectedthe total ionic strength of the soil solution.The approach of reciprocally transferring soil columnshighlighted the importance of throughfall chemistry, interactingwith the effect of changes in physical climate on forest soilacidification through internal proton production, in determiningsoil solution chemistry. A framework outlining the etiology offorest die-back induced by nitrogen saturation is proposed.     

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.     

Effects of limestone and gypsum application to a Malaysian ultisol on soil solution composition and yields of maize and groundnut

A field experiment was conducted on an Ultisol in Malaysia to assess changes in soil solution composition and their effects on maize and groundnut yields, resulting from limestone and gypsum application. The results showed that soil solution Ca in the lime treatment remained mainly in the zone of incorporation, but in the gypsum treatment some Ca moved into 15–30 cm zone. Al³⁺ and AlSO₄ ⁺ were dominant Al species in the soil solution of nil treatment. Liming decreased Al³⁺ and AlSO₄ ⁺, but increased hydroxy-Al monomer activities. However, gypsum application resulted in an increase of AlSO₄ ⁺ activity and in a decrease of Al³⁺ activity. Relative maize and groundnut yields were negatively correlated with Al³⁺, Al(OH)²⁺ and Al_sum activities. Likewise, relative yields were negatively correlated with Al concentration and the Al concentration ratio and positively correlated with soil solution Mg concentration and Ca/Al ratio.     

Adsorption of Cd(II) by rhizosphere and non-rhizosphere soil originating from mulberry field under laboratory condition

In this study, the adsorption behavior of Cd ions by rhizosphere soil (RS) and non-rhizosphere soil (NS) originated from mulberry field was investigated. The Langmuir, Freundlich and the Dubinin–Radushkevich (D-R) equations were used to evaluate the type and efficiency of Cd adsorption. The RS was characterized by lower pH but the higher content of soil organic matter and cation exchange capacity (CEC) as compared to NS. Also, the maximum adsorption of Cd²⁺ for RS (5.87 mg/g) was slightly bigger than that for NS (5.36 mg/g). In Freundlich isotherm, the K_f of the adsorption of Cd²⁺ to surface of the RS components was higher than that of the NS, indicating stronger attraction between Cd²⁺ and components of the RS. According to the D-R model, the adsorption of Cd²⁺ by both soils was dominated by ion exchange phenomena. These results indicated that mulberry roots modified physical and chemical properties of the RS under field conditions, which also affected the Cd sorption efficiency by soil components during laboratory experiments. Current knowledge of the Cd²⁺ sorption processes in the rhizosphere of mulberry may be important if these trees are planted for use in phytoremediation of Cd contaminated soils.     

Role of Organic Acids in Sunflower Tolerance to Heavy Metals

Exposure of Helianthus annuus L. seedlings to Al³⁺, Cd²⁺ or Zn²⁺ resulted in a marked decrease of fresh and dry masses of the shoots and the roots. The increase of Al³⁺, Cd²⁺ or Zn²⁺ uptake was accompanied by a significant decrease of nitrate, phosphorus and K⁺ uptake. There was a significant increase of malic and citric acid contents in the shoots and roots of heavy metal-treated seedlings whereas the change in fumaric acid was insignificant. Al³⁺ and Zn²⁺ alone stimulated excretion of malic and citric acids to the rhizosphere. Addition of high concentrations of malic or citric acid alleviate to some extent the inhibitory effect of Al³⁺ and Zn²⁺ on plant growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

Root-induced modifications of the exchange of phosphate ion between soil solution and soil solid phase

The uptake of phosphorus (P) by roots results in a depletion of phosphate ions (PO₄) in the rhizosphere. The corresponding decrease in PO₄ concentration in the soil solution (C_P) gives rise to a replenishment of P from the solid phase which is time- and C_P-dependent. This PO₄ exchange which reflects the buffer power of the soil for PO₄ also varies with the composition and the physico-chemical conditions of the soil. As root activity can modify these physico-chemical conditions in the rhizosphere, the question arises whether these modifications affect the ability of PO₄ bound to the soil solid phase to exchange with PO₄ in soil solution. The aim of the present work was to measure and compare the parameters which describe the amount of PO₄ bound to soil solid phase that is capable to replenish solution P for both rhizosphere and bulk soils. The soil sample was a P-enriched, calcareous topsoil collected from a long-term fertiliser trial. Rhizosphere soil samples were obtained by growing dense mats of roots at the surface of 3 mm thick soil layer for one week. Three plant species were compared: oilseed rape (Brassica napus L., cv Goeland) pea (Pisum sativum L., cv. Solara) and maize ( Zea mays L., cv. Volga). The time- and C_P-dependence of the PO₄ exchange from soil to solution were described using an isotopic dilution method. The measured C_P values were 0.165 mg P L⁻¹ for bulk soil and 0.111, 0.101 and 0.081 mg P L⁻¹ for rhizosphere soils of maize, pea and rape, respectively. The kinetics of the PO₄ exchange between liquid and solid phases of soil were significantly different between rhizosphere and bulk soils. However, when changes in C_P were accounted for, the parameters describing the PO₄ exchange with time and C_P between soil solution and soil solid phase were found to be very close for bulk and rhizosphere soils. For this calcareous and P-enriched soil, plant species differed in their ability to deplete PO₄ in solution. The resulting changes in the ability of the soil solid phase to replenish solution PO₄ were almost fully explained by the depletion of soil solution P. This revised version was published online in June 2006 with corrections to the Cover Date.     

The influence of soil chemistry on fine root aluminum concentrations and root dynamics in a subalpine Spodosol,Washington State,USA

The relationship between root Al concentrations and Al fractions in the soil solution was examined in a mature Abies amabilis ecosystem in the Cascade Range of Washington State. The naturally acidic soils in these ecosystems lead to high concentrations of aqueous Al in soil solutions and contribute to the biocycling of Al by the A. amabilis/T. mertensiana stand. Root concentrations of Al were very closely related to aqueous Al³⁺ activities, but poorly correlated with total aqueous Al concentrations. The solution Al/Ca molar ratios followed a seasonal cycle with low values during the fall and high values during the spring. Ratios remained <1 throughout the year in the Oa horizon while they varied between 2 and 14 in the E and Bhs horizons. The vertical distribution of roots and the mortality of fine roots may be linked to the soil solution Al/Ca ratio. Root cation exchange capacity ranged between 180 and 225 mol g^-1 and the exchangeable Al fraction represented from 12–17% of the total Al content in the root. Evidence for solid-phase co-precipitates of Al with PO₄ and oxalate was indicated from selective dissolution of the root tissue. Sufficient quantities of PO₄ and oxalate exist in the roots to tie up 20–40% of the Al present in the roots of the Oa and E horizons, but only 9% of that present in the Bhs horizon. Species differences in the distribution of Al between the above-ground and below-ground components may be dictated by these retention processes in the fine roots.     

Spatial and temporal variation of soil solution chemistry and ion fluxes through the soil in a mature Norway Spruce (Picea abies (L.) Karst.) stand

In this study we investigated the spatial and temporal variation in soil solution chemistry and of water and ion fluxes through the soil in a forest ecosystem. Our aim was to evaluate the relevance of these variations for the accuracy of average areal soil solution concentrations and ion fluxes with seepage at 90 cm depth.Twenty spatially distinct subcompartments of approximately 1 m² were established within a mature stand of Norway spruce and ceramic suction lysimeters were installed at depths of 20, 35 and 90 cm. A tensiometer was placed close to each suction lysimeter, and one throughfall sampler was established for each subcompartment.Soil solution samples were analysed for major ions (H⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe³⁺, Al³⁺, Cl^-, NO ₃ ^- , SO ₄ ^2- . We calculated water fluxes for each subcompartment separately by a numeric simulation of the soil water flux close to the lysimeters. The ion fluxes at each lysimeter were calculated by multiplying the simulated water fluxes with the ion concentrations on a fortnightly base. Averaging these 20 independent ion fluxes gave the areal average flux and an estimate of its statistical accuracy. The spatial variation of ion concentrations in the soil solution was high with coefficients of variance ranging from 5% to 128%. Part of the spatial variation was related to stem distance. Temporal variation of the concentrations was less than spatial for most ions. The spatial variation of water and ion fluxes with seepage was also substantial; for example the fluxes of SO ₄ ^2- -S calculated for each subcompartment ranged from 21 to 119 kg ha^-1 yr^-1, with an arithmetic average of 47 kg ha^-1 yr^-1. For H₂O, Mg²⁺, Cl^-, and SO ₄ ^2- , the spatial heterogeneity of seepage fluxes was largely explained by the heterogeneity of throughfall fluxes. No such relationship was found for nitrogen.Despite using 20 replicates, the 95% confidence intervals of the average annual areal fluxes with seepage were found to be 20–30% for most ions.