Coffee‐husk biochar application increased AMF root colonization,P accumulation,N2 fixation,and yield of soybean grown in a tropical Nitisol,southwest Ethiopia

Low soil fertility and soil acidity are among the major bottlenecks that limit agricultural productivity in the humid tropics. Soil management systems that enhance soil fertility and biological cycling of nutrients are crucial to sustain soil productivity. This study was, therefore, conducted to determine the effects of coffee‐husk biochar (0, 2.7, 5.4, and 16.2 g biochar kg^?1 soil), rhizobium inoculation (with and without), and P fertilizer application (0 and 9 mg P kg^?1 soil) on arbuscular mycorrhyzal fungi (AMF) root colonization, yield, P accumulation, and N₂ fixation of soybean [Glycine max (L.) Merrill cv. Clark 63‐K] grown in a tropical Nitisol in Ethiopia. ANOVA showed that integrated application of biochar and P fertilizer significantly improved soil chemical properties, P accumulation, and seed yield. Compared to the seed yield of the control (without inoculation, P, and biochar), inoculation, together with 9 and 16.2 g biochar kg^?1 soil gave more than two‐fold increment of seed yield and the highest total P accumulation (4.5 g plant^?1). However, the highest AMF root colonization (80%) was obtained at 16.2 g biochar kg^?1 soil without P and declined with application of 9 mg P kg^?1 soil. The highest total N content (4.2 g plant^?1) and N₂ fixed (4.6 g plant^?1) were obtained with inoculation, 9 mg P kg^?1, and 16.2 g biochar kg^?1 soil. However, the highest %N derived from the atmosphere (%Ndfa) (> 98%) did not significantly change between 5.4 and 16.2 g kg^?1 soil biochar treatments at each level of inoculation and P addition. The improved soil chemical properties, seed yield, P accumulation and N₂ fixation through combined use of biochar and P fertilizer suggest the importance of integrated use of biochar with P fertilizer to ensure that soybean crops are adequately supplied with P for nodulation and N₂‐fixation in tropical acid soils for sustainable soybean production in the long term.     

Assessing the Effect of Phosphorus Fertilizer Levels on Soil Phosphorus Fractionation in Rhizosphere and Non-Rhizosphere Soils of Wheat

This study evaluated the effects of phosphorus (P) fertilizer levels on inorganic P fractions. Wheat cultivars (Azadi and Marvdasht) were grown in the soils amended with the four rates of P fertilizer levels (no fertilizer, 10, 15, and 25 mg available P kg^?1 soil). Soils were sampled from rhizosphere and non-rhizosphere areas after 6 weeks. The mean of all P fractions was significantly different in various P fertilizer levels. The smallest and the largest amounts of all P fractions were observed in the soil with no P and in 25 mg kg^?1 soil P level, respectively. The Azadi cultivar, as P-efficient, showed the smallest increase in soil P fractions with increasing soil P levels. The means of all P fractions except Al-phosphates (Al-P) were significantly higher in non-rhizosphere soil. There were differences between these cultivars associated with the more inaccessible fractions at the 15 mg P kg^?1 soil level.     

Distribution of exudates of lupin roots in the rhizosphere under phosphorus deficient conditions

Abstract

The distribution of secretory acid phosphatase and organic acids enhanced by phosphorus deficiency in lupin rhizosphere was investigated using a rhizobox system which separated the rhizosphere soil into 0.5 mm fractions. In the soil fraction closest to the root surface, the lupin exudates displayed an acid phosphatase activity of 0.73 u g^?1 dry soil and citrate concentration of 85.2 μmol g^?1 dry soil, respectively. The increase of the acid phosphatase activity-induced an appreciable depletion of organic P in the rhizosphere, indicating that lupin efficiently utilized the organic P from soil through the enzyme activitye The sterile treatments demonstrated that the acid phosphatase in the rhizosphere was mainly derived from lupin root secretions. The secretory organic acids enhanced considerably the solubility of the inorganic P in three types of soil and a sludge. However, the secretory acid phosphatase and organic acids from lupin roots were only detected in a considerable amount in 0-2.5 mm soil fractions from root surface.     

Growth,P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability

The aim of the present study was to assess the role of soil type on growth, P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability. Two wheat (Goldmark and Janz) and two canola genotypes (Drum and Outback) were grown in a calcareous soil (pH 8.5) at two P levels [no P addition (0P) or addition of 200 mg kg⁻¹ P as Ca₃(PO₄)₂ (200P)] and harvested at flowering or maturity. Shoot and root dry weight, root length and shoot P content were greater in the two canola genotypes than in wheat. There were no consistent differences in available P, microbial P and phosphatase activity in the rhizosphere of the different genotypes. Shoot P content was significantly positively correlated with root length, pH and phosphatase activity in the rhizosphere. The microbial community composition, assessed by fatty acid methylester analysis, of the canola genotypes differed strongly from that of the wheat genotypes. The weight percentage bacterial fatty acids, the bacteria/fungi (b/f) ratio and the diversity of fatty acids were greater in the rhizosphere of the canolas than in the rhizosphere of the wheat genotypes. In contrast to the earlier studies in an acidic soil, only small differences in growth and P uptake between the genotypes of one crop were detected in the alkaline soil used here. The results confirmed the importance of root length for P uptake in soils with low P availability and suggest that the rhizosphere microbial community composition may play a role in the better growth of the canola compared to the wheat genotypes.     

Phosphorus acquisition and wheat growth are influenced by shoot phosphorus status and soil phosphorus distribution in a split‐root system

Root proliferation and greater uptake per unit of root in the nutrient‐rich zones are often considered to be compensatory responses. This study aimed to examine the influence of plant phosphorus (P) status and P distribution in the root zone on root P acquisition and root and shoot growth of wheat (Triticum aestivum L.) in a split‐root soil culture. One compartment (A) was supplied with either 4 or 14 mg P (kg soil)^–1, whereas the adjoining compartment (B) had 4 mg P kg^–1 with a vertical high‐P strip (44 mg kg^–1) at 90–110 mm from the plant. Three weeks after growing in the split‐root system, plants with 4 mg P kg^–1 (low‐P plants) started to show stimulatory root growth in the high‐P strip. Two weeks later, root dry weight and length density in the high‐P strip were significantly greater for the low‐P plants than for the plants with 14 mg P (kg soil)^–1. However, after 8 weeks of growth in the split‐root system, the two P treatments of compartment A had similar root growth in the high‐P strip of compartment B. The study also showed that shoot P concentrations in the low‐P plants were 0.6–0.8 mg g^–1 compared with 1.7–1.9 mg g^–1 in the 14 mg P kg^–1 plants after 3 and 5 weeks of growth, but were similar (1.1–1.4 mg g^–1) between the two plants by week 8. The low‐P plants had lower root P concentration in both compartments than those with 14 mg P kg^–1 throughout the three harvests. The findings may indicate that root proliferation and P acquisition under heterogeneous conditions are influenced by shoot P status (internal) and soil P distribution (external). There were no differences in the total root and shoot dry weight between the two P treatments at weeks 3 and 5 because enhanced root growth and P uptake in the high‐P strip by the low‐P plants were compensated by reduced root growth elsewhere. In contrast, total plant growth and total root and shoot P contents were greater in the 14 mg P kg^–1 soil than in the low‐P soil at week 8. The two P treatments did not affect the ratio of root to shoot dry weight with time. The results suggest that root proliferation and greater P uptake in the P‐enriched zone may meet the demand for P by P‐deficient plants only for a limited period of time.     

Effect of cadmium toxicity on micronutrient concentration,uptake and partitioning in seven rice cultivars

Heavy metal uptake, translocation and partitioning differ greatly among plant cultivars and plant parts. A pot experiment was conducted to determine the effect of cadmium (Cd) levels (0, 45 and 90 mg kg^?1 soil) on dry matter yield, and concentration, uptake and translocation of Cd, Fe, Zn, Mn and Cu in seven rice cultivars. Application of 45 mg Cd kg^?1 soil decreased root and shoot dry weight. On average, shoot and root Cd concentrations and uptake increased in all cultivars, but micronutrients uptake decreased following the application of 45 mg Cd kg^?1. No significant differences were observed between 45 and 90 mg kg^?1 Cd levels. On average, Cd treatments resulted in a decrease in Zn, Fe and Mn concentrations in shoots and Zn, Cu and Mn concentrations in roots. Differences were observed in Cd and micronutrient concentrations and uptake among rice cultivars. Translocation factor, defined as the shoot/root concentration ratio indicated that Cu and Fe contents in roots were higher than in shoots. The Mn concentration was much higher in shoots. Zinc concentrations were almost similar in the two organs of rice at 0 and 45 mg Cd kg^?1. A higher Cd level, however, led to a decrease in the Zn concentration in shoots.     

ROOT EXUDATION AND ZINC UPTAKE BY BARLEY GENOTYPES DIFFERING IN ZN EFFICIENCY

Two barley cultivars (‘Sahara’ = Zn-efficient and ‘Clipper’ = Zn-inefficient) were grown at different soil Zn fertilization (0, 0.2, 0.8, 1.6 and 3.2 mg Zn kg^?1 soil). Root exudates were collected 16 and 28 days after sowing. At Zn = 0, shoot dry matter was decreased in both genotypes, but more distinctly in ‘Clipper’. At 0.2 mg Zn kg^?1, the ‘Sahara’ shoot concentrations of Zn was 130% higher and shoot Zn content 44% greater compared with ‘Clipper’. Low-molecular-weight organic acid anions (=carboxylates) (malate, maleate, fumarate and cis-aconitate) and amino acids (alanine, valine, proline, aspartic acid and glutamic acid) were detected in root exudates, with the highest concentration at Zn = 0.2 mg kg^?1 soil. Higher concentrations of organic acid anions as well as amino acids were noted in the rhizosphere of ‘Sahara’ than ‘Clipper’. The genotypic differences in Zn acquisition from soil may be linked to differential carboxylate and amino acid composition of root exudates.     

Effects of zinc fertilizer amendments on yield and grain zinc concentration under controlled environment conditions

The application of zinc (Zn) fertilizer to lentil is an agronomic strategy that has the potential to improve yield and enhance grain Zn concentration. A pot study was conducted to determine if Zn fertilizer applied to three popular Saskatchewan lentil cultivars could increase yield and concentration of Zn in the grain. The effects of soil and foliar applied Zn forms, including ZnSO₄, Zn chelated with EDTA, Zn lignosulphonate, and a control were evaluated. Forms of Zn were not found to significantly increase yield (P = 0.828) or grain Zn concentration (P = 0.708) in any of the lentil cultivars tested. Fertilization with soil applied ZnSO₄ resulted in significantly (P < 0.0001) higher amounts of residual available Zn in the soil relative to other Zn treatments. Soil fertilized with ZnSO₄ had 1.13 mg kg^?1 diethylenetriaminepentaacetic acid (DTPA)-extractable Zn compared to 0.84 mg Zn kg^?1 and 0.77 mg Zn kg^?1 in the soil and foliar applied chelated Zn, respectively.     

Transformation of Lead Solid Fraction in the Rhizosphere of Elsholtzia splendens: The Importance of Organic Matter

The rhizosphere, enriched in organic matter, is the bottleneck of metal transfer in the soil–plant system. However, the transformation of metal fractions in the rhizosphere and the mechanisms that are involved, notably the role of organic matter, are poorly known. In this study, the solid-phase fractionation of lead (Pb) in the rhizosphere and non-rhizosphere soil of Elsholtzia splendens in a Pb-contaminated soil was investigated using a nine-step selective sequential extraction method in a pot experiment. Compared to the non-rhizosphere soil, there were measurable increases in Pb-fulvic complexes, Pb-humic complexes, organic Pb, and amorphous Pb but no significant changes in other forms of Pb in the rhizosphere soil. Pb-fulvic complexes and organic Pb, increasing from 397 to 438 mg kg^?1 and 229 to 258 mg kg^?1, respectively, showed a stronger accumulating trend than Pb-humic complexes and amorphous Pb, with an increase from 15.9 to 17.3 mg kg^?1 and 6.04 to 7.80 mg kg^?1 respectively, in the rhizosphere soil relative to non rhizosphere soil. These results may be mainly due to the enrichment of organic matter in the rhizosphere soil, resulting from root exudation and the enhanced turnover of microorganisms. The accumulation of Pb-fulvic complexes in the rhizosphere soil increases the potential phytoavailable pool, thus likely facilitating the phytoextraction of Pb in metal-contaminated soil.     

Effects of Soil Amended with Cadmium and Lead on Growth,Yield, and Metal Accumulation and Distribution in Parsley

A greenhouse experiment was designed to determine the cadmium (Cd) and lead (Pb) distribution and accumulation in parsley plants grown on soil amended with Cd and Pb. The soil was amended with 0, 5, 10 20, 40, 60, 80, and 100 mg Cd kg^?1 in the form of cadmium nitrate [Cd(NO₃)₂] and 0, 5, 10, 50 and 100 mg Pb kg^?1 in the form of lead nitrate [Pb(NO₃)₂]. The main soil properties; concentrations of the diethylenetriaminepentaacetic acid (DTPA)–extractable metals lead (Pb), Cd, copper (Cu), iron (Fe), zinc (Zn), and manganese (Mn) in soil; plant growth; and total contents of metals in shoots and roots were measured. The DTPA-extractable Cd was increased significantly by the addition of Cd. Despite the fact that Pb was not applied, its availability was significantly greater in treatments 40–100 mg Cd kg^?1 compared with the control. Fresh biomass was increased significantly in treatments of 5 and 10 mg Cd kg^?1 as compared to the control. Further addition of Cd reduced fresh weight but not significantly, although Cd concentration in shoots reached 26.5 mg kg^?1. Although Pb was not applied with Cd, its concentration in parsley increased significantly in treatments with 60, 80, and 100 mg Cd g^?1 compared with the others. Available soil Pb was increased significantly with Pb levels; nevertheless, the increase was small compared to the additions of Pb to soil. There were no significant differences in shoot and root fresh weights between treatments, although metal contents reached 20.0 mg Pb kg^?1 and 16.4 mg Pb kg^?1 respectively. Lead accumulation was enhanced by Pb treatments, but the positive effect on its uptake was not relative to the increase of Pb rates. Cadmium was not applied, and yet considerable uptake of Cd by control plants was evident. The interactive effects of Pb and Cd on their availability in soil and plants and their relation to other metals are also discussed.     

Phosphorus Efficiency in Sunflower Cultivars and Its Relationships with Phosphorus,Calcium, Iron,Zinc and Manganese Nutrition

ABSTRACT

Phosphorus (P) efficiency (shoot dry weight at low P/shoot dry weight at high P) of a cultivar is the ability to produce a high yield in a soil that is limited in that element for a standard genotype. The large variation in P efficiency of different crops provides opportunities for screening crop species that perform well on low phosphorus soil. To explain the differences in P efficiency of sunflower (Helianthus annuus L.) cultivars a glasshouse pot experiment was conducted by using P-deficient soil [0.5 M sodium bicarbonate (NaHCO₃)-extractable P 8.54 mg kg^?1] treated with 0 (low P) and 100 mg P kg^?1 soil (high P). The relationship between P efficiency and P, calcium (Ca), iron (Fe), zinc (Zn), and manganese (Mn) nutrition and anthocyanin accumulation was investigated in ten sunflower cultivars. Phosphorus deficiency resulted in significant decreases in the shoot and root yield. Phosphorus-efficient cultivars have the ability to produce higher yield than the inefficient cultivars in a limited P conditions. Our results showed that P-efficient cultivars had lower P concentrations, but higher P content in low P conditions. Phosphorus-efficient cultivars also have lower Ca and Fe concentrations in low P conditions but not in P-sufficient conditions. Applied P resulted in significant decreases in Zn concentrations in the shoots of the cultivars. Anthocyanin concentrations showed an accumulating pattern in all cultivars under P deficiency. The results demonstrated that phosphorus efficiency of the sunflower cultivars depends on their ability to produce higher yield and take up more P, and lower the concentration of Ca and Fe in shoots under low P conditions.     

Agronomic Efficiency of Feldspar,Quartz Silica,and Zeolite as Silicon (Si) Fertilizers in Sandy Soil

ABSTRACT

Partial replacement of synthetic chemical fertilizers by naturally occurring alternatives is environmentally recommended. Feldspars (F), quartz silica (S), and zeolites (Z) are silicon (Si)-rich minerals that may be utilized as Si fertilizers. This study aims to assess the agronomic efficiency (AE) of the mentioned minerals as Si fertilizers and to estimate Si-use efficiency (Si-UE) in sandy soil. A field experiment was carried out (summer seasons of 2016 and 2017) in which F, S, and Z were mixed with surface soil in an application rate 500 mg kg^?1 soil with and without potassium humate (K-H, 2 mg kg^?1 soil). Treatments were distributed in a complete randomized block design (CRBD) with three replicates including control before cultivation of soybean (Glycine max L.). Yield (kg ha^?1) of soybean, available Si (mg kg^?1) in soil and uptake (mg kg^?1) of N, P, K, Cu, Fe, Mn, Zn, and Si by soybean seeds and straw were estimated. The most significant increase was by 67.87% followed by 38.69% was recorded for the S and S K-H treatments, compared to the control. Same treatments showed nonsignificant decrease in the available Si (mg kg^?1) that may refer to partial replenishment of plant available Si (PAS) in soil and avoid significant deficiency. Silica treatments resulted in the most significant increase in the uptake (mg kg^?1 soil) of Si, N, P, K, Cu, Fe, Mn, and Zn by seeds and almost by straw. Silica was more efficient agronomically than feldspar and zeolite. Absorption of more biocompatible Si-organo species may depend on Si source.     

Effects of soil flooding and organic matter addition on plant accessible phosphorus in a tropical paddy soil: an isotope dilution study

Plant growth experiments were conducted to reveal the mechanism by which organic matter (OM) and soil flooding enhance phosphorus (P) bioavailability for rice. It was postulated that reductive dissolution of iron‐(III) [Fe(III)] oxyhydroxides in soil releases occluded phosphate ions (PO₄), i.e., PO₄ that is not isotopically exchangeable in the original soil prior to flooding. Rice was grown in P‐deficient soil treated with factorial combinations of addition of mineral P (0, 50 mg P kg^?1), OM (0, ≈ 20.5 g OM kg^?1 as cattle manure +/– rice straw) and water treatments (flooded vs. non‐flooded). The OM was either freshly added just before flooding or incubated moist in soil for 6 months prior to flooding; nitrogen and potassium were added in all treatments. The soil exchangeable P was labeled with ³³PO₄ prior to flooding. The plant accessible P in soil, the so‐called L‐value, was determined from the ³³P/³¹P ratio in the plants. The L‐values were inconsistently affected by flooding in contrast with the starting hypothesis. The OM and P addition to soil clearly increased the L‐value and, surprisingly, the increase due to OM application was larger than the total P addition to soil. An additional isotope exchange study in a soil extract (E‐value) at the end of the experiment showed that the E‐value increased less than the total P addition with OM. This suggests that plants preferentially take up unlabeled P from the OM in the rhizosphere compared to labeled labile inorganic P. The effects of soil flooding on P bioavailability is unlikely related to an increase of the quantity of bio‐accessible P in soil (L‐value) but is likely explained by differences in P mobility in soil.     

Cadmium Uptake by Winter Wheat Seedlings in Response to Interactions Between Phosphorus and Zinc Supply in Soils

ABSTRACT

Effects of application of zinc (Zn) (0, 1, 5, 10 mg kg^?1 soil) and phosphorus (P) (0, 10, 50, 100 mg kg^?1 soil) on growth and cadmium (Cd) accumulations in shoots and roots of winter wheat (Triticum aestivum L.) seedlings were investigated in a pot experiment. All soils were supplied with a constant concentration of Cd (6 mg kg^?1 soil). Phosphorus application resulted in a pronounced increase in shoot and root biomass. Effects of Zn on plant growth were not as marked as those of P. High Zn (10 mg kg^?1) decreased the biomass of both shoots and roots; this result may be ascribed to Zn toxicity. Phosphorus and Zn showed complicated interactions in uptake by plants within the ranges of P and Zn levels used. Cadmium in shoots decreased significantly with increasing Zn (P < 0.001) except at P addition of 10 mg kg^?1. In contrast, root Cd concentrations increased significantly except at Zn addition of 5 mg kg^?1 (P < 0.001). These results indicated that Zn might inhibit Cd translocation from roots to shoots. Cadmium concentrations increased in shoots (P < 0.001) but decreased in roots (P < 0.001) with increasing P supply. The interactions between Zn and P had a significant effect on Cd accumulation in both shoots (p = 0.002) and roots (P < 0.001).     

MANGANESE EFFICIENCY OF WHEAT CULTIVARS AS RELATED TO ROOT GROWTH AND INTERNAL MANGANESE REQUIREMENT

ABSTRACT

Under field conditions, wheat cultivar PBW 343 produced 1.5 times higher grain yield than PDW 233, when grown on low manganese (Mn) soil. To explain the differences in Mn efficiency a pot experiment was conducted using Mn deficient Typic ustochrept loamy sand soil treated with 0, 50, and 100?mg?Mn?kg^?1 soil. In no-Mn treatment, both the wheat cultivars showed Mn deficiency symptoms and cultivar PBW 343 produced 30% of the maximum dry matter yield (DMY) attained at high Mn supply, while PDW 233 produced only 18% of its maximum DMY after 40 days of growth. With application of 50?mg?Mn?kg^?1 soil, the DMY significantly increased to 87% and 50% of the maximum for PBW 343 and PDW 233, respectively. These results indicate that aestivum cultivar PBW 343 was more Mn efficient than durum cultivar PDW 233. Manganese efficient cultivar PBW 343 had a lower internal Mn requirement than PDW 233 because at the same shoot Mn concentration PBW 343 produced more DMY. The root growth of both wheat cultivars was similar at sufficient Mn supply, the root length (RL)?:?DMY ratio being equal. At decreasing Mn supply root growth was depressed more strongly than shoot growth, the inhibition being more severe in Mn inefficient cultivar PDW 233, indicating the importance of root system size for Mn efficiency between these two wheat cultivars. A nutrient uptake model closely described Mn influx in both the cultivars, indicating that calculated concentration profiles were realistic and that chemical mobilization of Mn in the rhizosphere was not responsible for higher Mn efficiency of PBW 343. Calculated concentration profiles showed that in soil not fertilized with Mn, initial soil solution Mn concentration of 0.23?µM decreased to only 0.21?µM at the root surface after 27 days of uptake. This 7.4% decrease in Mn concentration at the root surface indicated that roots could not decrease Mn concentration to a lower value which would have caused higher transport of Mn to root surface and hence resulted in higher Mn influx.     

Manganese availability and microbial populations in the rhizosphere of wheat genotypes differing in tolerance to Mn deficiency

Plant genotypes differ in their capacity to grow in soils with low manganese (Mn) availability. The physiological mechanisms underlying differential tolerance to Mn deficiency are poorly understood. To study the relationship between Mn content in soil, plant genotypes, and rhizosphere microorganisms in differential Mn efficiency, two wheat (Triticum aestivum L.) cultivars, RAC891 (tolerant to Mn deficiency) and Yanac (sensitive), were grown in a Mn‐deficient soil to which 5, 10, 20 or 40 mg Mn kg^–1 were added. The shoot dry matter of both cultivars increased with increasing Mn addition to the soil. At all soil Mn fertilizer levels, the tolerant RAC891 had a greater shoot dry matter and a higher total shoot Mn uptake than the sensitive Yanac. The concentration of DTPA‐extractable Mn in the rhizosphere soil of RAC891 at Mn20 and Mn40 was slightly lower than in the rhizosphere of Yanac. The population density of culturable microorganisms in the rhizosphere soil was low (log 6.8–6.9 cfu (g soil)^–1) in both cultivars and neither Mn oxidation nor reduction were observed in vitro. To assess the non‐culturable fraction of the soil microbial community, the ribosomal intergenetic spacer region of the bacterial DNA in the rhizosphere soil was amplified (RISA) and separated in agarose gels. The RISA banding patterns of the bacterial rhizosphere communities changed markedly with increasing soil Mn level, but there were no differences between the wheat cultivars. The bacterial community structure in the rhizosphere was significantly correlated with the concentration of DPTA‐extractable Mn in the rhizosphere, fertilizer Mn level, shoot dry matter, and total shoot Mn uptake. The results obtained by RISA indicate that differential tolerance to Mn deficiency in wheat may not be related to changes in the composition of the bacterial community in the rhizosphere.     

Assessment of arsenic availability to roots in contaminated Tuscany soils by a diffusion gradient in thin films (DGT) method and uptake by Pteris vittata and Agrostis capillaris

A laboratory study was conducted to evaluate the availability of arsenic (As) to roots in nine soils from five fields in Tuscany (Italy). Concentrations of As in soils range from 200 to 1200 mg kg^?1 as a result of human activities and natural deposition. In a first stage, potentially root‐available As and the risk of crop uptake were estimated using the diffusion gradient in thin films (DGT) technique. In a second stage, a glasshouse experiment was performed to compare As depletion in the rhizosphere by the hyperaccumulator Pteris vittata (ladder brake), and by the test plant Agrostis capillaris (colonial bentgrass). In this stage, DGT results were supported by a sequential extraction procedure. The main objectives were to study the root availability of As in old, contaminated soils and the modification of the available pool size over time. The phytoextraction efficiency of Pteris vittata as applied to this case study was also investigated. In all samples, concentrations of total and potentially root‐available As in soil solution were small compared with total As in soil (approximately 0.0003–0.03%, corresponding to 3.5–350 μg l^?1, respectively). There was also little re‐supply of As from the solid phase to the soil solution. In the rhizosphere, despite uptake by Pteris and Agrostis, total and root available As in solution were sustained over time. It appeared that plants induced a re‐supply of As from the solid phase. Despite the uptake and translocation of As by Pteris (50.7 mg kg^?1 and 6.6 mg As kg^?1 in fronds and roots, respectively) together with plant biomass (17.2 g per plant), the success of phytoextraction appears unlikely. The results obtained demonstrated the sensitivity of DGT to root‐induced changes in soil and the suitability of the technique as an easy‐to‐use tool to predict assimilation by plants.     

稻鸭生态种养系统直播水稻根表和根际土壤营养特性研究

采用田间小区试验对比分析了直播方式下稻鸭生态种养系统和水稻单一种植系统水稻根表和根际土壤的氮、磷、钾和有机碳含量及pH的变化。结果表明:与水稻单一种植系统相比,稻鸭生态种养系统水稻根表和根际pH分别升高4.41%、0.85%,全氮含量分别降低0.11g·kg-1、0.11g·kg-1,全磷和全钾含量变化不明显。水稻根表速效氮和速效钾含量分别增加30.80mg·kg-1、17.93mg·kg-1,速效磷含量降低8.66mg·kg-1;水稻根际速效氮和速效钾含量分别减少15.13mg·kg-1、7.61mg·kg-1,速效磷增加9.66mg·kg-1。稻鸭生态种养系统水稻根表活性有机碳和高活性有机碳分别增加2.17g·kg-1、0.56g·kg-1,全有机碳含量减少0.99g·kg-1;根际土壤全有机碳、活性有机碳和中活性有机碳含量分别减少2.39g·kg-1、2.64g·kg-1、0.72g·kg-1。稻鸭生态种养改变了速效磷、速效钾和活性有机碳在水稻根域土壤的相对富集部位,即速效钾和活性有机碳富集部位主要在根表土壤,速效磷则主要在根际土壤。表明稻鸭生态种养明显改善了水稻根表和根际土壤营养状况,这可能对水稻根系吸收和利用土壤养分具有积极意义。     

Potassium Status in Maize Rhizosphere of Smectitic Soils

This study has been taken up to generate information on potassium status in maize rhizosphere soils differing in their clay content at different levels of added potassium. Soils with larger amounts of clay showed greater amounts of water soluble and ammonium acetate extractable K (NH₄OAc K) in both the rhizosphere as well as non-rhizosphere. In the absence of added K (control), non-rhizosphere samples showed higher water soluble and NH₄OAc K ranging from 8.0 to 28.9 mg kg^?1 and from 132.5 to 294.0 mg kg^?1, respectively compared to rhizosphere samples where water soluble K varied between 6.0 and 26.5 mg kg^?1 and NH₄OAc K from 125.0 to 262.5 mg kg^?1. The difference in K content between rhizosphere and non-rhizosphere which could have been resulted from plant K uptake was significantly related with clay content (r = 0.98**) in control whereas at 150 mg kg^?1 K addition this relationship was found to be non significant (r = 0.64^NS). Electro ultra filtration (EUF) fractions also showed similar differences in K contents in soil between rhizosphere and non-rhizosphere.     

Mannitol Application Alleviates Boron Toxicity in Wheat Seedlings

Boron (B) toxicity is a considerable mineral nutritional problem for crop production in arid and semi-arid regions worldwide. The effect of mannitol (M) in wheat seedlings that are tolerant and sensitive to excessive B was studied to reduce B toxicity symptoms. Plants were grown in a peat with different concentrations of boric acid (0, 30, 45, 60 mg kg^?1) and treated additionally with M (0, 1, 5, 10 g kg^?1). Seedlings grown for 8 weeks were harvested for root length, shoot length, and dry-weight measurements and analyzed for B content of leaves. Compared with control groups (no boric acid treatment), B toxicity caused reductions in root length, shoot length, and dry weight of both wheat cultivars. Significant increases on growth parameters were observed under B treatments, the greatest with 1 g kg^?1 M application in a tolerant bread cultivar. On the other hand, 10 g kg^?1 M application under 60 mg kg^?1 B treatment gave also good results on root length in a sensitive durum cultivar. Significant decreases in leaf B content were observed under B treatments with all M applications in both wheat cultivars, the greatest with 5 g kg^?1 M application. The results suggest that M applications may have a possible role in overcoming in B toxicity in wheat grown in the field.