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.     

Amelioration of acid soil infertility by phosphogypsum

Amelioration of subsoil acidity requires an increase in Ca status along with a decrease in Al status in subsoil. In this study, effects of phosphogypsum (PG) on the amelioration of subsoil acidity have been evaluated, using cultivated and woodland subsoils representing Cecil, Wedowee (both Typic Hapludult) and Bladen (Typic Albaquult) series. Subsoil (0.6–0.8 m) samples were collected and treated with either PG (approximately 2 Mg ha^-1 rate), Ca(NO₃)₂ or Mg(NO₃)₂ along with an unamended control treatment. A fertile topsoil amended with NH₄NO₃ was placed on top of all treated subsoil. Top and root growth of alfalfa [Medicago sativa (L.) cv. Hunter River] and soybean [Glycine max (L.) Merr. cv. Lee] were significantly greater in PG-amended than in unamended pots of the Cecil and Wedowee soils, although most growth was observed with the Ca(NO₃)₂-amended treatment. In the Bladen soil, however, none of the amendments evoked a significant growth response in either alfalfa or soybean. The concentration of Ca in the displaced soil solution (in soils with no plants) as well as tissue levels of Ca suggest that the growth response was partly due to an improved Ca availability in both PG or Ca(NO₃)₂-treated soils. Exchangeable Al decreased in PG-amended soils. The self-liming effect of PG, which is a release of OH^- due to ligand exchange between SO₄ ^2- and OH^-, as well as a decrease in exchangeable Al in PG-amended soil is greater in predominantly kaolinitic Cecil and Wedowee soils than in smectitic Bladen soil. As a result, significant growth response to PG amendment was observed in the Cecil and Wedowee soils, but not in the Bladen soil.     

Growth,net photosynthesis and leaf nutrient status of <Emphasis Type="Italic">Fagus crenata</Emphasis> seedlings grown in brown forest soil acidified with H<Subscript>2</Subscript>SO<Subscript>4</Subscript> or HNO<Subscript>3</Subscript> solution

To obtain basic information for evaluating critical loads of acid deposition for protecting Japanese beech forests, growth, net photosynthesis and leaf nutrient status of Fagus crenata seedlings grown for two growing seasons in brown forest soil acidified with H₂SO₄ or HNO₃ solution were investigated. The whole-plant dry mass of the seedlings grown in the soil acidified by the addition of H₂SO₄ or HNO₃ solution was significantly less than that of the seedlings grown in the control soil not supplemented with H⁺ as H₂SO₄ or HNO₃ solution. However, the degrees of reduction in the whole-plant dry mass and net photosynthetic rate of the seedlings grown in the soil acidified by the addition of H⁺ as H₂SO₄ solution at 100 mg l^–1 on the basis of air-dried soil volume (S-100 treatment) were greater than those of the seedlings grown in the soil acidified by the addition of H⁺ as HNO₃ solution at 100 mg l^–1 (N-100 treatment). The concentrations of Al and Mn in the leaves of the seedlings grown in the S-100 treatment were significantly higher than those in the N-100 treatment. A positive correlation was obtained between the molar ratio of (Ca+Mg+K)/(Al+Mn) in the soil solution and the relative whole-plant dry mass of the seedlings grown in the acidified soils to that of the seedlings grown in the control soil. Based on the results, we concluded that the negative effects of soil acidification due to sulfate deposition are greater than those of soil acidification due to nitrate deposition on growth, net photosynthesis and leaf nutrient status of F. crenata, and that the molar ratio of (Ca+Mg+K)/(Al+Mn) in soil solution is a suitable soil parameter for evaluating critical loads of acid deposition in efforts to protect F. crenata forests in Japan.     

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.     

Rapid evaluation of sorghum for aluminum tolerance

Two screening methods tested in this study were based on the observation that roots of freshly germinated seeds exhibit their relative tolerance to Al during the first 48 hours of growth. Sorghum (Sorghum bicolor L. Moench) varieties were evaluated using toxic/nontoxic soil pairs in petri dishes and toxic/nontoxic solution pairs in a flowing aqueous system. The soils had 0, 61, and 72% Al saturation and the solutions had 0, 1.85 and 3.70 M l^–1 Al from AlCl₃ and 0.25 mM l^–1 Ca from Ca(NO₃)₂. Relative root lengths in both systems correctly differentiated 13 genotypes of known Al tolerance into tolerant and intolerant groups. Twenty six other sorghums genotypes were also screened using genotypes of known Al reactions as checks. The soil with 72% Al saturation and the 1.85 M l^–1 Al solution gave the highest correlation between the two systems and both effectively arrayed sorghums of known and unknown tolerance. For routine screening the authors prefer the soil system for its simplicity, efficiency, and use of a natural growth medium.Journal paper 11637. Purdue University Agricultural Experiment Station, Lafayette, Indiana. Contribution from the Agronomy Department.     

Is the Ca:Al ratio superior to pH,Ca or Al concentrations of soils in accounting for the distribution of plants in deciduous forest?

The distributions of vascular plants in south Swedish deciduous forests were related to exchangeable (exc) and soil solution concentrations of H⁺ (pH), Ca, Al and the Ca:Al ratios within these fractions. Topsoils (0–5 cm) of 172 sites with a pH_KCl of 3.2–3.9 (corresponding to 3.7–4.4 in soil solution) were used. In the soil solution both total Al_t and quickly reacting Al_r were determined. Exchangeable concentrations were generally well related to plant distributions, the highest correlation coefficients usually being given by pH_KCl>Ca_exc>Al_exc.>(Ca:Al)_exc. The (Ca:Al)_exc ratio was clearly inferior. Out of the soil solution variables studied, Ca concentration, followed by pH, was best correlated with plant distributions, Al_t, Al_r, and the Ca:Al ratios having similar and lower coefficients. It is concluded that the use of Ca:Al ratios as a general measure of Al toxicity in controlling plant distributions is rather problematic. It seems difficult to apply evidence for Ca-Al interactions from solution culture experiments to field conditions when measured as exchangeable or soil solution concentrations of the soil.     

Chemical composition of soil solutions extracted from New Zealand beech forests and West German beech and spruce forests

Concentrations of ions were measured in soil solutions from beech (Nothofagus) forests in remote areas of New Zealand and in solutions from beech (Fagus sylvatica) and Norway spruce (Picea abies) forests in North-East Bavaria, West Germany, to compare the chemistry of soil solutions which are unaffected by acid deposition (New Zealand) with those that are affected (West Germany). In New Zealand, soil solution SO₄ ^2– concentrations ranged between <2 and 58 mol L^–1, and NO₃ ^– concentrations ranged between <1 and 3 mol L^–1. In West Germany, SO₄ ^2– concentrations ranged between 80 and 700 mol L^–1, and NO₃ ^– concentrations at three of six sites ranged between 39 and 3750 mol L^–1, but was not detected at the remaining three sites. At all sites in New Zealand, and at sites where the soil base status was moderately high in West Germany, pH levels increased, and total Al (Al_t) and inorganic monomeric Al (Al_i) levels decreased rapidly with increasing soil depth. In contrast, at sites on soils of low base status in West Germany, pH levels increased only slightly, and Al levels did not decline with increasing soil depth.Under a high-elevation Norway spruce stand showing severe Mg deficiency and dieback symptoms in West Germany, soil solution Mg²⁺ levels ranged between 20 and 60 mol L^–, and were only half those under a healthy stand. Al_t and Al_i levels were substantially higher the healthy stand than under the unhealthy stand, indicating that Al toxicity was not the main cause of spruce decline.     

Calcium and temperature effects on seedling exudation and root rot infection of common bean on an acid sandy soil

Soil born fungi such as Phytium ultimum, Fusarium ssp., and Rhizoctonia solani (Kühn) severely restrict stand establishment of common bean (Phaseolus vulgaris L.) on acid soils of the Tropics. Calcium application is known to alleviate fungal infection in many legumes but the causes are still unclear. To investigate environmental factors and physiological mechanisms involved, growth chamber experiments were conducted with an acid sandy soil from Mexico. Treatments were soil liming at a rate of 0.67 g Ca(OH)₂ kg^-1, gypsum application at 0.49 g CaSO₄ 2H₂O kg^-1 soil placed around the seed, and an untreated control. Beans were grown under three temperature regimes with constant night and one constant day vs. two sinusoidal day temperatures. To examine patterns of seed and seedling exudation at regular intervals leachates of germinating seeds were collected on filter paper soaked with equilibrium solutions from soils of the three treatments. The severity of root rot in the control treatment was highest when plants were stressed by temperature extremes. At a sinusoidal day temperature peaking at 40°C soil liming and gypsum application to the seed increased the number of healthy seedlings similarly by over 60%. However, only liming which effectively eliminated growth constraints by low pH and high aluminum concentrations led to an increase in hypocotyl elongation by 22% and in total root length by 8%. Both calcium amendments increased the calcium and potassium contents in the hypocotyl tissue. From seeds exposed to the equilibrium solution of unlimed soil with pH 3.7, 1 mM Ca, and 0.6 mM Al considerable amounts of amino acids and carbohydrates were leached. In contrast, exposure to the equilibrium solution from limed soil with pH 4.3, 3 mM Ca, and negligible concentrations of Al led to a net uptake of amino acids and decreased leaching of carbohydrates. Exposure to the equilibrium solution of the gypsum treatment with pH 3.6, 20 mM Ca, and 1.2 mM Al resulted in a somewhat smaller net uptake of amino acids compared to liming. During germination pH around the seeds steeply increased in the untreated control but significantly less with both amendments. The results indicate that pH and the Ca/Al ratio in the soil solution around bean seeds determine their pattern of exudation and solute uptake. For bean germination and early growth on acid soils locally placed application of small amounts of gypsum as seed pelleting seems as effective as soil liming in reducing the incidence of root rot. The results indicate that this may be accomplished by decreasing the amount of leachates available for fungal development.     

Effects of inorganic nitrogen application on the dynamics of the soil solution composition in the root zone of maize

The effect of inorganic nitrogen (N) fertilizer on the ionic composition of the soil solution under maize (Zea mays L.) was studied. A pot experiment was carried out with two treatments combined factorially, with or without N application (Ca(NO₃)₂; +N and –N treatments, respectively), and with or without plants. Three looped hollow fiber samplers were installed in each pot to sample soil solutions nondestructively from the root zone, seven times during the 50-day growth period. Plants were harvested on the 50th day, and their nutrient contents determined.Effects of N fertilizer on the soil solutions were observed by the first sampling, 2 days after sowing. The concentrations of Ca and NO₃ ^– and electrical conductivity (EC) increased significantly in the +N treatments as direct effects of fertilizer application. In addition, the concentrations of Mg, K, Na and H⁺ also increased and that of P decreased significantly as indirect effects caused by the re-establishment of chemical equilibria. This suggested the greater supply as well as the greater possibility of leaching loss not only of NO₃ ^– but also of Ca, Mg and K. In the treatments with plants, the concentrations of NO₃ ^–, Ca, Mg and K decreased with time and pH increased significantly compared with the unplanted soil. The depletion of N in the soil solution roughly agreed with the amount of N taken up by the plant. The depletions of K from the soil solution amounted to less than 10% of the amount of the K taken up, suggesting intensive replenishment of K from exchange sites in the soil. Depletions of Ca and Mg were several times higher than the amounts taken up, indicating that the depletions resulted from the adsorption of the divalent cations by the soil rather than uptake by plants. Because NO₃ ^– is hardly absorbed by exchange sites in soil and was the dominant anion in solution, it was concluded that NO₃ ^– had a major role in controlling cation concentrations in the soil solution and, consequently, on their availability for uptake by plants as well as their possible leaching loss. ei]H Marschner     

Al avoidance and Al tolerance ofMucuna pruriens var.utilis: Effects of a heterogeneous root environment and the nitrogen form in the root environment

InMucuna pruriens var.utilis, grown with nitrate-N in a hydroponic split-root system, an Al avoidance reaction of root growth was observed, which was ascribed to local P stress in the Al containing compartment. The Al avoidance reaction was similar to the avoidance ofMucuna roots of acid subsoil in the field where roots grew preferentially in the topsoil. In the present paper the effect of different N forms (NO₃ ^– and NH₄ ⁺) on the reactions ofMucuna to Al were studied, since in acid soils N is present as a mixture of NO₃ ^– and NH₄ ⁺. No interaction between the N form and Al toxicity was found. A hydroponic split-root experiment with NH₄NO₃ nutrition, which is comparable to the situation in the field, showed that under these conditions Al avoidance did not occur. It is concluded that a relation between the Al avoidance reaction ofMucuna and P stress is still likely.Abbreviations D_r root diameter - L_pr total root length per plant - L_rw specific root length - NRA nitrate reductase activity - S/R shoot: root ratio     

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.     

Effects of elevated soil solution Al concentrations on fine roots in a middle-aged Norway spruce (Picea abies (L.) Karst.) stand

Aluminium (Al), mobilized by acidic deposition, has been claimed to be a major threat to forest vitality. Fine root mortality, decreased root growth and reduced nutrient uptake have been observed in controlled laboratory experiments where roots of tree seedlings were exposed to elevated concentrations of Al. Yet, evidence for Al-induced root damage from forest stands is scarcely reported. Nevertheless, Al dissolved in soil water has received a key role in the critical load concept for forests. Here, we present effects of artificially elevated concentrations of Al in the soil solution on fine roots in a middle-aged stand of Norway spruce (Picea abies (L.) Karst.). Although the inorganic Al concentrations about 200 µM and Ca:Al ratio about 0.7 that were established in the soil solution within this experiment have been associated with reduction of root growth and root mortality for spruce seedlings in hydroponic studies, no acute damage on fine roots was observed. Three years of treatment did not cause visual root damage, nor were effects on fine root necromass observed. Fine root necromass made up about 10% of fine root biomass for all treatments. However, significantly lower molar Ca:Al and Mg:Al ratios in living and dead fine roots were found in the plots where Al concentrations were highest and ratios of Ca to Al in the soil solution were lowest. The lack of response on fine root biomass suggests that forest stands tolerate higher Al levels than results from laboratory experiments indicate. We conclude that effect studies in the laboratory have limited value for field conditions. The key role of Al toxicity, expressed as the Ca/Al ratio, in critical load calculations for forests may have to be reconsidered.     

Rhizotoxicity of aluminate and polycationic aluminium at high pH

Although monomeric Al species are often toxic in acidic soils, the effects of the aluminate ion (Al(OH) ₄ ⁻ ) on roots grown in alkaline media are still unclear. Dilute, alkaline (pH 9.5) nutrient solutions were used to investigate the effects of Al(OH) ₄ ⁻ on root growth of mungbean (Vigna radiata L.). Root growth was reduced by 13% after 3 d growth in solutions with an Al(OH) ₄ ⁻ activity of 16 μM and no detectable polycationic Al (Al₁₃). This decrease in root growth was associated with the formation of lesions on the root tips (due to the rupturing of the epidermal and outer cortical cells) and a slight limitation to root hair growth (particularly on the lateral roots). When roots displaying these symptoms were transferred to fresh Al(OH) ₄ ⁻ solutions for a further 12 h, no root tip lesions were observed and root hair growth on the lateral roots improved. The symptoms were similar to those induced by Al₁₃ at concentrations as low as 0.50 μM Al which are below the detection limit of the ferron method. Thus, Al(OH) ₄ ⁻ is considered to be non-toxic, with the observed reduction in root growth in solutions containing Al(OH) ₄ ⁻ due to the gradual formation of toxic Al₁₃ in the bulk nutrient solution resulting from the acidification of the alkaline nutrient solution by the plant roots.     

Acid soil damage in sorghum genotypes: Role of magnesium deficiency and root impairment

The effects of liming and Mg fertilization on growth, specific root length (root length per unit of root dry weight; SRL) and nutrient uptake of twelve sorghum genotypes (Sorghum bicolor (L.) Moench) were studied in two pot experiments. Liming increased the pH of the sandy loam from pH 4.3 (unlimed) to 4.7 (with 0.5 g Ca(OH)₂ kg^-1 soil) and to 6.1 (with 2.5 g Ca(OH)₂ kg^-1 soil). Liming increased the dry matter yield of the genotypes by factors of 1.2 to 6.0 (between pH 4.3 and 4.7) and by 1.1 to 2.4 (between pH 4.7 and 6.1). In absence of Mg at soil pH of 4.3 and 4.7, all genotypes suffered from Mg deficiency, as indicated by low Mg concentrations in the shoots (26–94 mmol Mg kg^-1 DM) and visible Mg deficiency symptoms. At pH 4.7 several of the genotypes responded to Mg application and produced significantly more dry matter. At pH 4.3, however, none of the genotypes responded to Mg, even though the internal Mg concentrations were increased by applied Mg. The relative increase in dry matter yield between pH 4.3 and 4.7 was closely correlated to the relative change in specific root length in the same soil pH interval, especially when the soil was fertilized with Mg (r²=0.91^**). The group of genotypes where SRL and dry matter yield were reduced by soil acidity was not the same as the group that responded positively to Mg application at pH 4.7.It is concluded that the growth of sorghum genotypes on acid soils is determined by two independent characteristics: the sensitivity of root development to soil acidity and the efficiency of the uptake and utilization of Mg. The first characteristic is predminant at high soil acidity whilst the latter is dominant at moderate soil acidity.     

Immobilization and phytoavailability of cadmium in variable charge soils. II. Effect of lime addition

The effect of pH-increases due to Ca(OH)₂ and KOH addition on the adsorption of cadmium (Cd) was examined in two soils which varied in their variable-charge components. The effect of Ca(OH)₂ on immobilization and phytoavailability of Cd from one of the soils, treated with various levels of Cd (0–10 mg Cd kg^–1 soil), was further evaluated using mustard (Brassica juncea L.) plants. Cadmium immobilization in soil was evaluated by a chemical fractionation scheme. The addition of Ca(OH)₂ and KOH increased the soil pH, thereby increasing the adsorption of Cd, the effect being more pronounced in the soil dominated by variable charge components. There was a greater increase in Cd²⁺ adsorption in the KOH-treated than the Ca(OH)₂-treated soil, which is attributed to the greater competition of Ca²⁺ for adsorption. Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Although addition of Ca(OH)₂ effectively reduced Cd phytotoxicity, Cd uptake increased at the highest level, probably due to decreased Cd²⁺ adsorption resulting from increased Ca²⁺ competition. There was a significant inverse relationship between dry matter yield and Cd concentration in soil solution. Addition of Ca(OH)₂ decreased the concentration of the soluble + exchangeable Cd fraction but increased the concentration of inorganic-bound Cd fractions in soil. Since there was no direct evidence for CdCO₃ or Cd(OH)₂ precipitation in the variable charge soil used for the plant growth experiment, alleviation of phytotoxicity can be attributed primarily to immobilization of Cd by enhanced pH-induced increases in negative charge.     

Effects of water and nutrient applications in a Scots pine stand to tree growth and nutrient cycling

In a Scots pine forest stand, demineralized water and a complete set of nutrients with water were applied to the soil by means of frequent irrigation for four years in order to eliminate water and nutrient shortage of the trees. Apart from this optimization, dissolved (NH₄)₂SO₄ was irrigated at a rate of 120 kg N ha^-1 y^-1 to create a situation of N excess. Effect of treatments on tree growth and chemical composition of soil water and vegetation were monitored. From the first treatment year onwards basal area growth increased by ca. 35% as a result of the increased water supply. Nutrient applications increased K and P concentrations in pine needles immediately, but growth was enhanced only in the fourth treatment year and coincided with an improved K supply. Most of the applied P and K was retained in the soil, and only 6% was recovered in the vegetation. Tree nutrient status did not respond on Ca and Mg applications, whereas Ca and Mg seepage losses were increased with ca. 5 kg ha^-1 y^-1. The applied NH₄ was mostly retained in the 0–20 cm surface soil and caused a drastic increase of Al in soil solution. Tree growth was stimulated initially by extra NH₄, but was hampered after three years obviously because of a decreased P nutrition. The applied base cations were absorped to the soil and the accompanying anions were leached, thus temporarily increasing the acidification of the soil solution.     

Effects of soil pH and nitrogen fertility on the population dynamics of Thielaviopsis basicola

Black root rot of tobacco, caused by Thielaviopsis basicola, is generally severe at soil pH values >5.6 and suppressed under more acidic conditions (pH < 5.2). Soil acidifying fertilizers containing NH₄–N are generally recommended for burley tobacco production in North Carolina, but the effects of N form and application rate on development of black root rot and on the population dynamics of T. basicola have not been determined. Greenhouse and laboratory studies were conducted to evaluate the effects of N form (NH₄ ⁺ or NO₃ ^–) and rate on pathogen and disease parameters at several initial soil pH levels. A moderately-conducive field soil, initial pH 4.7, was adjusted to a pH of 5.5 or 6.5 by the addition of CaOH₂, then amended with the desired nitrogen form and rate. Pathogen populations were determined over time. In addition, spore production in extracts of roots from plants grown in the various nitrogen and pH treatments was determined. Finally, because tobacco responds to acidic soil conditions and exposure to NH₄–N by accumulating high concentrations of the polyamine putrescine, the toxicity of putrescine on vegetative growth and reproduction of T. basicola was investigated. Low soil pH and high levels of NH₄–N suppressed reproduction of T. basicola in soil and in root extract, while use of NO₃–N and depletion of NH₄–N resulted in rapid increases in populations of T. basicola. At 20 mM, putrescine inhibited hyphal growth by 60% and aleuriospore production by 98%. Fertilizers that reduced soil pH also reduced reproduction by T. basicola, and thus have potential for management of black root rot by suppressing populations of T. basicola over multiple years of crop production. The suppression of T. basicola and black root rot observed with NH₄–N amendments may partially be due to development of an inhibitory environment in the root and not solely to changes in rhizosphere pH.     

The significance of Cognettia sphagnetorum(Enchytraeidae) on nitrogen availability and plant growth in wood ash-treated humus soil

A laboratory microcosm experiment was established to study whether the role of Cognettia sphagnetorum (Enchytraeidae) in affecting Scots pine (Pinus sylvestris) seedling growth is influenced by wood ash-amendment, i.e., neutralisation of the raw humus soil. Coniferous forest soil, inoculated with soil microbes and nematodes, was either treated with wood ash or left as ash-free control. Wood ash (corresponding to an amount of 5000 kg ha^–1) was either spread on the soil surface or mixed into the soil. Enchytraeid and pine seedling biomass, abundance of nematodes, and water leachable NH₄ ⁺-N and NO₃ ^–-N were measured 26 and 51 weeks after initiation of the experiment and root length and N concentration of needles were measured 51 weeks after initiation of the experiment. Wood ash when mixed into the soil, reduced the biomass of C. sphagnetorum. Nematodes were unaffected by the treatments. In the ash-free soils C. sphagnetorum had little influence on pine growth, but it did decrease root length and root to shoot ratio. In the absence of enchytraeids wood ash decreased pine biomass production and root length. However, the presence of enchytraeids in the ash-treated soil compensated the ash-induced negative effects on the pine performance. Enchytraeids increased and wood ash decreased water leachable NH₄ ⁺-N in the presence but not in the absence of enchytraeids, while water leachable NO₃ ^–-N was not affected by the treatments. It was concluded that C. sphagnetorum can be important in ensuring nutrient cycling and plant growth in situations when an ecosystem encounters disturbances.     

Does low soil base saturation affect fine root properties of European beech (Fagus sylvatica L.)?

It is generally believed that high soil solution Al³⁺ in acidic soils with low base saturation (BS), negatively influences the properties of fine roots. Fine roots from European beech (Fagus sylvatica L.) trees growing in highly acidic soils with very low BS and potentially high Al³⁺ concentration in the soil solution were analysed and the dependency of fine root properties on soil BS was measured. The fine roots were sampled down to 1 m depth at seven forest sites located on the Swiss Plateau. These sites varied in their BS from 1.4 to 11.4% in the mineral layers. We evaluated relationships between the BS of these mineral layers and fine root properties, such as ratio between bio- and necromass (live/dead ratio), specific root length (SRL), root tip abundance (RTA), root branching abundance (RBA), O₂-consumption, and the Ca/Al molar ratio in the fine root tissue. The fine root properties were compared not only with the BS of the soil, but also with the Ca/Al molar ratio in the fine root tissues. Significant relations of fine root properties occurred when the soils of the seven sites were grouped into two BS groups (<5 and 5–10%). The live/dead ratio, the RTA, the RBA, the O₂-consumption, and Ca/Al molar ratio were lower in the group of BS <5% than in the group 5–10%. Decreases in the morphological properties and in the O₂-consumption were related to decrease in the Ca/Al molar ratio of the fine root tissues. There is evidence that the fine root properties are negatively influenced, nevertheless, fine root systems of mature European beech in their natural ecological environment seem to be able to compensate adverse effects of low BS. Responsible Editor: Philippe Hinsinger.     

Effects of sulphur dioxide fumigation on soil system and growth behaviour ofVicia faba plants

Summary To determine the possible changes in soil characteristics and plant growth resulting from SO₂-exposure, field grownVicia faba plants were exposed to 0.5 ppm SO₂ for 4 h daily from 40 to 100 days of their ages. The major effects of SO₂ on soil system includes increases in contents of total sulphur, organic carbon and exchangeable Al; lowering of pH and decreases in contents of N, P and exchangeable K and Ca. Besides, SO₂ interfered with nutrient uptake and plant growth leading to reductions in lengths of root and shoot, numbers of leaves, nodules and pods, phytomass accumulation and net primary productivity.