Showing papers in "Plant and Soil in 1991"
TL;DR: Mycorrhizal plants have been shown to increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates, however, studies in which the soil P has been labelled with radioactive 32P indicated that both mycor rhizal and non-mycorrhIZal plants utilized the similarly labelled P sources in soil.
Abstract: The beneficial effects of mycorrhizae on plant growth have often been related to the increase in the uptake of immobile nutrients, especially phosphorus (P). In this review the mechanisms for the increase in the uptake of P by mycorrhizae and the sources of soil P for mycorrhizal and non-mycorrhizal plants are examined. Various mechanisms have been suggested for the increase in the uptake of P by mycorrhizal plants. These include: exploration of larger soil volume; faster movement of P into mycorrhizal hyphae; and solubilization of soil phosphorus. Exploration of larger soil volume by mycorrhizal plants is achieved by decreasing the distance that P ions must diffuse to plant roots and by increasing the surface area for absorption. Faster movement of P into mycorrhizal hyphae is achieved by increasing the affinity for P ions and by decreasing the threshold concentration required for absorption of P. Solubilization of soil P is achieved by the release of organic acids and phosphatase enzymes. Mycorrhizal plants have been shown to increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates. However, studies in which the soil P has been labelled with radioactive 32P indicated that both mycorrhizal and non-mycorrhizal plants utilized the similarly labelled P sources in soil.
1,143 citations
TL;DR: In this article, the dissolution rate of Fe oxides in soil was investigated and it was shown that the dissolution features of the crystals appear to follow zones of weakness in the crystal.
Abstract: In most soils, FeIII oxides (group name) are the common source of Fe for plant nutrition. Since this Fe has to be supplied via solution, the solubility and the dissolution rate of the Fe oxides are essential for the Fe supply. Hydrolysis constants and solubility products (Ksp) describing the effect of pH on FeIII ion concentration in solution are available for the well-known Fe oxides occurring in soils such as goethite, hematite, ferrihydrite. Ksp values are usually extremely low ((Fe3+)·(OH)3=10−37−10−44). However, for each mineral type, Ksp may increase by several orders of magnitude with decreasing crystal size and it decreases with increasing Al substitution assuming ideal solid solution between the pure end-members. Based on such calculations a poorly crystalline goethite with a crystal size of 5 nm may well reach the solubility of ferrihydrite. The variations in Ksp are of relevance for soils because crystal size and Al substitution of soil Fe oxides vary considerably and can now be determined relatively easily. The concentration of Fe2+ in soil solutions is often much higher than that of Fe(III) ions. Therefore, redox potential strongly influences the activity of FeII. At a given pH and Eh, the activity of FeII is higher the higher Ksp of the FeIII oxide and thus also varies with the type of Fe oxide present. Besides the solubility, it is the dissolution rate which governs the supply of soluble Fe to the plant roots. Dissolution of Fe oxides takes place either by protonation, complexation or, most important, by reduction. Numerous dissolution rate studies with various FeIII oxides were conducted in strong mineral acids (protonation) and they have shown that besides the Fe oxide species, crystal size and/or crystal order and substitution are important determinative factors. For example, in soils, small amounts of a more highly soluble meta- or instable Fe oxide such as ferrihydrite with a large specific surface (several hundred m2g−1) may be essential for the Fe supply to the plant root. Its higher dissolution rate can also be used to quantify its amount in soils. Ferrihydrite can be an important component in soils with high amounts of organic matter and/or active redox dynamics, whereas highly aerated and strongly weathered soils are usually very low in ferrihydrite. On the other hand, dissolution rates of goethites decrease as their Al substitution increases. Much less information exists on the rate of reductive and chelative dissolution of Fe oxides which generally simulate soil conditions better than dissolution by protonation. Here again, type of oxide, crystal size and substitution are important factors. Organic anions such as oxalate, which are adsorbed at the surface, may weaken the Fe3+-O bonds and thereby increase reductive dissolution. As often observed in weathering, the dissolution features of the crystals appear to follow zones of weakness in the crystal.
689 citations
TL;DR: In order to have a better understanding of the principles of the mechanisms by which plants adapt to acid mineral soils more attention should be given to conditions at the root-soil interface.
Abstract: Major constraints for plant growth on acid mineral soils are toxic concentrations of mineral elements like Al, of H+, and/or low mineral nutrient availability either as a result of solubility (e.g. P and Mo), low reserves, and impaired uptake (e.g. Mg2+) at high H+ concentrations. Inhibition of root growth particularly by Al leads to more shallow root systems, which may affect the capacity for mineral nutrient acquisition and increase the risk of drought stress. Of the two principal strategies (tolerance and avoidance) of plants for adaptation to adverse soil conditions, the strategy of avoidance is more common for adaptation to acid mineral soils. Examples are (i) root-induced changes in the rhizosphere such as pH increase, (ii) release of chelators for Al, higher activity of ectoenzymes (acid phosphatases), and (iii) increase in root surface area via mycorrhizae. In order to have a better understanding of the principles of the mechanisms by which plants adapt to acid mineral soils more attention should thus be given to conditions at the root-soil interface.
429 citations
TL;DR: In this paper, the root hairs were included in mathematical model calculations, and the differences of P influx could be accounted for by root hairs' contribution to P uptake by root hair was up to 90% of total uptake.
Abstract: Fohse et al. (1988) have shown that P influx per unit root length in seven plant species growing in a low-P soil varied from 0.6×10-14 to 4.8×10-14 mol cm-1s-1. The objective of this work was to investigate the reasons for these differences. No correlation was found between P influx and root radius, root hairs, cation-anion balance and Ca uptake. However, when root hairs were included in mathematical model calculations, the differences of P influx could be accounted for. These calculations have shown that in soils low in available P, contribution to P uptake by root hairs was up to 90% of total uptake.
386 citations
TL;DR: In this article, a review of the processes of H+ ions release during N cycling and its effect on soil acidification is examined, which is mainly caused by the release of protons (H+) during the oxidation of carbon (C), sulphur (S) and nitrogen (N) compounds in soils.
Abstract: In areas that remain unaffected by industrial pollution soil acidification is mainly caused by the release of protons (H+) during the oxidation of carbon (C), sulphur (S) and nitrogen (N) compounds in soils. In this review the processes of H+ ions release during N cycling and its effect on soil acidification are examined. The major processes leading to acidification during N cycling in soils are: (i) the imbalance of cation over anion uptake in the rhizosphere of plants either actively fixing N2 gas or taking up NH 4 + ions as the major source of N, (ii) the net nitrification of N derived from fixation or from NH 4 + and R-NH2 based fertilizers, and (iii) the removal of plant and animal products containing N derived from the process described in (i) and losses of NO3-N by leaching when the N input form is N2,NH 4 + or R-NH2. The uptake of excess cations over anions by plants results in the acidification of the rhizosphere which is a “localized” effect and can be balanced by the release of hydroxyl (OH−) ions during subsequent plant decomposition. Nitrification of fixed N2 or NH 4 + and R-NH2 based fertilizers, and loss of N from the soil either by removal of products or by leaching of NO3-N with a companion basic cation, lead to ‘permanent’ acidification.
385 citations
TL;DR: Mycorrhizal inoculation doubled P concentration in shoot and root, and increased dry weight, especially of the shoot, irrespective of P levels, and almost all of this P was delivered by the hyphae from the outer compartment.
Abstract: To examine the influence of vesicular-arbuscular (VA) mycorrhizal fungi on phosphorus (P) depletion in the rhizosphere, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) were grown for seven weeks in a sterilized calcareous soil in pots with three compartments, a central one for root growth and two outer ones for hyphae growth. Compartmentation was accomplished by a 30-μm nylon net. The root compartment received a uniform level of P (50 mg kg−1 soil) in combination with low or high levels of P (50 or 150 mg kg−1 soil) in the hyphal compartments. Plants were inoculated withGlomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or remained uninfected.
342 citations
TL;DR: An increase in Cd solubility in the rhizosphere of apical root zones due to root exudates is likely to be an important cause of the relatively high Cd accumulation in Nicotiana spp.
Abstract: Soluble root exudates were collected from three plants (Nicotiana tabacum L., Nicotiana rustica L. and Zea mays L.), grown under axenic and hydroponic conditions, in order to study their metal-solubilizing ability for Cd and other cations (Cu, Fe, Mn, Ni, Zn). Nicotiana spp. and Zea mays L. root exudates differed markedly in C/N ratio, sugars vs. amino acids ratio and organic acids content. Metals from two soils were extracted with either root exudate solutions, containing equal amounts of organic carbon, or distilled water as control. In the presence or absence of root exudates, the solubility of Fe and Mn was much higher than of the four other metals tested. Root exudates increased the solubilities of Mn and Cu, whereas those of Ni and Zn were not affected. Root exudates of Nicotiana spp. enhanced the solubility of Cd. The extent of Cd extraction by root exudates (N. tabacum L. N. rustica L. Zea mays L.) was similar to the order of Cd bioavailability to these three plants when grown on soil. An increase in Cd solubility in the rhizosphere of apical root zones due to root exudates is likely to be an important cause of the relatively high Cd accumulation in Nicotiana spp.
313 citations
TL;DR: Concentrations of PS up to molar levels have been calculated under non-sterile conditions in the rhizosphere of Fe-deficient barley plants.
Abstract: Phytosiderophores (PS) are released in graminaceous species (Gramineae) under iron (Fe) and zinc (Zn) deficiency stress and are of great ecological significance for acquisition of Fe and presumably also of Zn. The potential for release of PS is much higher than reported up to now. Rapid microbial degradation during PS collection from nutrient solution-grown plants is the main cause of this underestimation. Due to spatial separation of PS release and microbial activity in the rhizosphere a much slower degradation of PS can be assumed in soil-grown plants. Concentrations of PS up to molar levels have been calculated under non-sterile conditions in the rhizosphere of Fe-deficient barley plants.
300 citations
TL;DR: It is shown that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).
Abstract: The abilities of seedling roots of twenty-two plant species to penetrate a strong growth medium were compared under controlled conditions. Seedlings were grown for 10 days in compression chambers filled with siliceous sandy soil at 0.2 kg kg−1 water content and mean penetrometer resistance of 4.2 MPa. Root elongation and thickening were measured after growth. The results show that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).
299 citations
TL;DR: In this article, it was shown that polycationic Al (charge > 2) is rhizotoxic as are other polyvalent cations, and that the identity and activities of the Al species contacting the cell surfaces are uncertain because of the H+ currents through the root surface and because of surface charges.
Abstract: The aluminium (III) released from soil minerals to the soil solution under acid conditions may appear as hexaaquaaluminium (Al(H2O) 6 3+ , or Al3+ for convenience) or may react with available ligands to form additional chemical species. That one or more of these species is rhizotoxic (inhibitory to root elongation) has been known for many decades, but the identity of the toxic species remains problematical for the following reasons. 1. Several Al species coexist in solution so individual species cannot be investigated in isolation, even in artificial culture media. 2. The activities of individual species must be calculated from equilibrium data that may be uncertain. 3. The unexpected or undetected appearance of the extremely toxic triskaidekaaluminium (AlO4Al12(OH)24(H2O) 12 7+ or Al13) may cause misattribution of toxicity to other species, especially to mononuclear hydroxy-Al. 4. If H+ ameliorates Al toxicity, or vice versa, then mononuclear hydroxy-Al may appear to be toxic when it is not. 5. The identity and activities of the Al species contacting the cell surfaces are uncertain because of the H+ currents through the root surface and because of surface charges. This article considers the implications of these problems for good experimental designs and critically evaluates current information regarding the relative toxicities of selected Al species. It is concluded that polycationic Al (charge >2) is rhizotoxic as are other polyvalent cations.
296 citations
TL;DR: In this article, the authors evaluated differences in plant and soil organic matter (SOM) between soil from under individual grass plants and from small openings in shortgrass steppe and found strong differentiation of surface soil at the scale of individual plants, with relative enrichment of soil under plants in total and active SOM.
Abstract: Small-scale spatial heterogeneity of soil organic matter (SOM) associated with patterns of plant cover can strongly influence population and ecosystem dynamics in dry regions but is not well characterized for semiarid grasslands. We evaluated differences in plant and soil N and C between soil from under individual grass plants and from small openings in shortgrass steppe. In samples from 0 to 5 cm depth, root biomass, root N, total and mineralizable soil N, total and respirable organic C, C:N ratio, fraction of organic C respired, and ratio of respiration to N mineralization were significantly greater for soil under plants than soil from openings. These differences, which were consistent for two sites with contrasting soil textures, indicate strong differentiation of surface soil at the scale of individual plants, with relative enrichment of soil under plants in total and active SOM. Between-microsite differences were substantial relative to previously reported differences associated with landscape position and grazing intensity in shortgrass steppe. We conclude that microscale heterogeneity in shortgrass steppe deserves attention in investigation of controls on ecosystem and population processes and when sampling to estimate properties at plot or site scales.
TL;DR: The results demonstrate that VA mycorrhizal hyphae uptake and translocation to the host is an important component of increased acquisition of P and Zn by mycorRHizal plants.
Abstract: An investigation was carried out to test whether the mechanism of increased zinc (Zn) uptake by mycorrhizal plants is similar to that of increased phosphorus (P) acquisition. Maize (Zea mays L.) was grown in pots containing sterilised calcareous soil either inoculated with a mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe or with a mixture of mycorrhizal fungi, or remaining non-inoculated as non-mycorrhizal control. The pots had three compartments, a central one for root growth and two outer ones for hyphal growth. The compartmentalization was done using a 30-μm nylon net. The root compartment received low or high levels of P (50 or 100 mg kg−1 soil) in combination with low or high levels of P and micronutrients (2 or 10 mg kg−1 Fe, Zn and Cu) in the hyphal compartments. Mycorrhizal fungus inoculation did not influence shoot dry weight, but reduced root dry weight when low P levels were supplied to the root compartment. Irrespective of the P levels in the root compartment, shoots and roots of mycorrhizal plants had on average 95 and 115% higher P concentrations, and 164 and 22% higher Zn concentrations, respectively, compared to non-mycorrhizal plants. These higher concentrations could be attributed to a substantial translocation of P and Zn from hyphal compartments to the plant via the mycorrhizal hyphae. Mycorrhizal inoculation also enhanced copper concentration in roots (135%) but not in shoots. In contrast, manganese (Mn) concentrations in shoots and roots of mycorrhizal plants were distinctly lower, especially in plants inoculated with the mixture of mycorrhizal fungi. The results demonstrate that VA mycorrhizal hyphae uptake and translocation to the host is an important component of increased acquisition of P and Zn by mycorrhizal plants. The minimal hyphae contribution (delivery by the hyphae from the outer compartments) to the total plant acquisition ranged from 13 to 20% for P and from 16 to 25% for Zn.
TL;DR: Taking up and transport by VA-mycorrhizal hyphae may contribute substantially not only to P nutrition, but also to Cu nutrition of the host.
Abstract: White clover (Trifolium repens L.) plants were grown in a calcareous soil in pots with three compartments, a central one for root growth and two outer ones for growth of vesicular-arbuscular (VA) mycorrhizal (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe) hyphae (hyphal compartments). Phosphorus (P) was applied at three levels (0, 20 and 50 mg kg−1 soil) in the outer compartments in mycorrhizal treatments. Root and shoot dry weight were increased in mycorrhizal plants with hyphal access to outer compartments. Growth of the mycorrhizal hyphae in the outer compartments was not significantly affected by variation in P level in these compartments. However, both concentration and amount of P in roots and shoots sharply increased with increasing P supply in the outer (hyphal) compartments. With increasing P levels the calculated delivery of P by the hyphae from the outer compartments increased from 34% to 90% of total P uptake.
TL;DR: Most bacteria, fungi, and some plants respond to Fe stress by the induction of high-affinity Fe transport systems that utilize biosynthetic chelates called siderophores, whereas other microbial species, such as Streptomyces pilosus, use a low specificity, high-Affinity transport system that recognizes more than one siderophile type.
Abstract: Most bacteria, fungi, and some plants respond to Fe stress by the induction of high-affinity Fe transport systems that utilize biosynthetic chelates called siderophores. To competitively acquire Fe, some microbes have transport systems that enable them to use other siderophore types in addition to their own. Bacteria such as Escherichia coli achieve this ability by using a combination of separate siderophore receptors and transporters, whereas other microbial species, such as Streptomyces pilosus, use a low specificity, high-affinity transport system that recognizes more than one siderophore type. By either strategy, such versatility may provide an advantage under Fe-limiting conditions; allowing use of siderophores produced at another organism’s expense, or Fe acquisition from siderophores that could otherwise sequester Fe in an unavailable form.
TL;DR: A recent15N dilution/N balance study confirmed that certain sugar cane varieties are capable of obtaining large contributions of nitrogen from plant-associated N2 fixation, and under good conditions of water and mineral nutrient supply, it may be possible to dispense with N fertilization of these varieties altogether.
Abstract: A recent 15N dilution/N balance study confirmed that certain sugar cane varieties are capable of obtaining large contributions of nitrogen from plant-associated N2 fixation. It was estimated that up to 60 to 80% of plant N could be derived from this source, and under good conditions of water and mineral nutrient supply, it may be possible to dispense with N fertilization of these varieties altogether. The recently discovered bacterium, Acetobacter diazotrophicus, apparently responsible for this N2 fixation associated with the plants, has unique physiological properties for a diazotroph, such as tolerance to low pH, and high sugar and salt concentrations, lack of nitrate reductase, and nitrogenase activity which tolerates short-term exposure to ammonium. Furthermore, it also behaves as an endophyte, in that it is unable to infect sugar cane plants unless through damaged tissue or by means of VA mycorrhizae and is propagated via the planting material (stem pieces).
TL;DR: Outputs from the model indicate that fertilizer N has a strong influence on ammonia volatilization, denitrification and leaching at a given site but that, over a range of sites with a given rate of fertilizer N, total loss and the proportions lost by the three processes are greatly influenced by the amount of N mineralized by the soil.
Abstract: The model simulates the cycling of N in grassland systems grazed by beef cattle and predicts the annual amount of N in liveweight gain, and the amounts lost through ammonia volatilization, denitrification and leaching, on the basis of fertilizer application and soil and site characteristics. It aims to provide a better understanding of the way in which these various factors interact in their influence on N transformations. The model has been programmed to run on IBM-compatible personal computers and responds rapidly to changes in input parameters.
TL;DR: Amino acids, carbohydrates and organic acids typically found in root exudates, were stimulatory to ethylene biosynthesis in soil and the classical ‘triple’ response of pea seedlings was exhibited.
Abstract: Our laboratory has focused on microbial production of plant hormones and precursor-inoculum interactions. Indole-3-acetic acid was detected in soils incubated with L-tryptophan (L-TRP). Inoculation with the ectomycorrhizae, Pisolithus tinctorius significantly stimulated the growth of Douglas fir when supplied with low concentrations of L-TRP to soil. Among three Azotobacter spp. and two Pseudomonas spp., the most prolific producer of cytokinins was A. chroococcum and among the precursors tested, adenine (ADE) and isopentyl alcohol (IA) were the most effective. Corn rhizosphere was found to be quite rich with microflora capable of producing ethylene from L-methionine (L-MET). Amino acids, carbohydrates and organic acids typically found in root exudates, were stimulatory to ethylene biosynthesis in soil. Etiolated pea seedlings exhibited the classical ‘triple’ response when L-MET and Acremonium falciforme were applied in combination to sterile soil or when L-MET was added to nonsterile soil.
TL;DR: The most FeDC resistant genotype had the slowest growth rate and this may be a mechanism for its greater resistance to FeDC.
Abstract: Iron deficiency chlorosis (FeDC) is a common disorder for sorghum [Sorghum bicolor (L.) Moench] grown on alkaline calcareous soils. Four sorghum genotypes were grown in growth chambers on a low Fe (1.3 μg/g DTPA-extractable), alkaline (pH 8.0), calcareous (3.87% CaCO3 equivalent) Aridic Haplustoll to determine effects of different soil temperatures (12, 17, 22 and 27°C at a constant 27°C air temperature) on various root and shoot growth traits and development of FeDC. As soil temperature increased, leaf chlorosis became more severe, and shoot and root dry weights, root lengths, and leaf areas increased markedly. Shoot/root ratios, shoot weight/root length, leaf area /shoot weight and leaf area /root weight and root length also increased while root length/root weight decreased as soil temperature increased. Severe FeDC developed in all genotypes even though genotypes had previously shown different degrees of resistance to FeDC. Genotypes differed in most growth traits, especially dry matter yields, root lengths, and leaf areas, but most traits did not appear to be related to genotype resistance to FeDC. The most FeDC resistant genotype had the slowest growth rate and this may be a mechanism for its greater resistance to FeDC.
TL;DR: The distribution of14C in components of the soil-plant system changed during the 24 h after labelling with the most rapid changes occurring generally during the first 7.5 h afterlabelling.
Abstract: Wheat (cv. Gutha) and barley (cv. O'Connor) were grown as field crops on a shallow duplex soil (sand over clay) in Western Australia with their root systems contained within pvc columns. At four stages during growth, the shoots were pulse-labelled for 1.5h with14CO2; immediately prior to labelling, the soil was isolated from the shoot atmosphere by pvc sheets. After labelling, the soil atmosphere was pumped through NaOH to trap respired CO2 and after 2.5, 5, 7.5 and 24 h from the start of labelling, columns were destructively sampled to recover14C from the roots, soil and shoot.
TL;DR: In this article, the emissions of CH4 from a flooded Louisiana, USA, rice field were measured over the first crop growing season, using a closed-chamber technique, and the results indicated that CH4 emissions were highly variable over the season and a significant urea fertilizer effect was observed.
Abstract: Methane emissions from a flooded Louisiana, USA, rice field were measured over the first cropgrowing season. Microplots contained the semidwarf Lemont rice cultivar drill seeded into a Crowley silt loam soil (Typic Albaqualfs). Urea fertilizer was applied preflood at rates of 0, 100, 200 and 300 kg N ha−1. Emissions of CH4 from the plots to the atmosphere were measured over a 86-d sampling period until harvest. Methane samples were collected in the morning hours (07∶30–09∶30) using a closed-chamber technique. Emissions of CH4 were highly variable over the first cropping season and a significant urea fertilizer effect was observed. Two peak CH4 emission periods were observed and occurred about 11 d after panicle differentiation and during the ripening stages. Maximum CH4 emmissions from the 0, 100, 200 and 300 urea-N treatments were 6.0, 8.9, 9.8 and 11.2 kg CH4 ha−1 d−1, respectively. These flux measurements corresponded to approximately 210, 300, 310 and 360 kg CH4 evolved ha−1 over the 86-d sampling period for the 4 treatments.
TL;DR: The depletion of FeII in the shoot was induced and severe chlorosis rapidly developed in the young rice plant under Fe-deficiency, and the activity of the transporter which absorb deoxy-MA-FeIII chelate and finally reduced the synthesis of de oxygen-MA from methionine.
Abstract: The reason why young rice plant is highly susceptible to Fe-deficiency was clarified as follows: Among Gramineae plants rice secreted a very low amount of deoxy-MA as a phytosiderophore even under Fe-deficiency, and the secretion by rice ceased within 10 days under Fe-deficiency although barley secreted MAs during a period of more than one month. When iron depletion continued, the rice root tips become chimeric and epidermal cells became necrotic. The mitochondrial membrane systems in the cortex cells were also severely damaged. Iron starvation occurred even in the mitochondria, and energy charge in the root decreased. This reduced energy charge has firstly diminished the secretion activity of deoxy-MA from the roots, secondly reduced the activity of the transporter which absorb deoxy-MA-FeIII chelate and finally reduced the synthesis of deoxy-MA from methionine. Consequently, the depletion of FeII in the shoot was induced and severe chlorosis rapidly developed in the young rice plant under Fe-deficiency.
TL;DR: In this paper, root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils, and it was shown that root elongation rate was negatively correlated with root penetration resistant, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa.
Abstract: Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.
TL;DR: In this article, an automated method for simultaneous determination of 15N and total N using an automated nitrogen analyser interfaced to a continuous flow isotope ratio mass spectrometer (ANA-MS) was evaluated.
Abstract: Simultaneous determination of 15N and total N using an automated nitrogen analyser interfaced to a continuous-flow isotope ratio mass spectrometer (ANA-MS method) was evaluated. The coefficient of variation (CV) of repeated analyses of homogeneous standards and samples at natural abundance was lower than 0.1%. The CV of repeated analyses of 15N-labelled plant material and soil samples varied between 0.3% and 1.1%. The reproductibility of repeated total N analyses using the automated method was comparable to results obtained with a semi-micro Kjeldahl procedure. However, the automated method gave results which were 3% to 5% higher than those obtained with the Kjeldahl procedure. Since only small samples can be analysed, careful sample homogenization and fine grinding are very important. Evaluation of a diffusion method for preparing nitrate and ammonium in solution for automated 15N analysis showed that the recovery of inorganic N in the NH3 trap was lower when the N was diffused from water than from 2 M KCl. The results also indicated that different proportions of the NO3- and the NH4+ in aqueous solution were recovered in the trap after combined diffusion. The method is most suited for diffusing either NO3- or NH4+ alone, but can be used for combined diffusion of the two ions.
TL;DR: In this article, the distribution of tree biomass and the allocation of organic matter production were measured in an 11-yr-old Pinus caribaea plantation and a paired broadleaf secondary forest growing under the same climatic conditions.
Abstract: The distribution of tree biomass and the allocation of organic matter production were measured in an 11-yr-old Pinus caribaea plantation and a paired broadleaf secondary forest growing under the same climatic conditions. The pine plantation had significantly more mass aboveground than the secondary forest (94.9 vs 35.6 t ha-1 for biomass and 10.5 vs 5.0 t ha-1 for litter), whereas the secondary forest had significantly more fine roots (⩽2 mm diameter) than the pine plantation (10.5 and 1.0 t ha-1, respectively). Standing stock of dead fine roots was higher than aboveground litter in the secondary forest. In contrast, aboveground litter in pine was more than ten times higher than the dead root fraction. Both pine and secondary forests had similar total organic matter productions (19.2 and 19.4 t ha-1 yr-1, respectively) but structural allocation of that production was significantly different between the two forests; 44% of total production was allocated belowground in the secondary forest, whereas 94% was allocated aboveground in pine. The growth strategies represented by fast growth and large structural allocation aboveground, as for pine, and almost half the production allocated belowground, as for the secondary forest, illustrate equally successful, but contrasting growth strategies under the same climate, regardless of soil characteristics. The patterns of accumulation of organic matter in the soil profile indicated contrasting nutrient immobilization and mineralization sites and sources for soil organic matter formation.
TL;DR: Iron uptake from ferrated (59Fe) pseudobactin (PSB), a Pseudomonas putida siderophore, by various plant species was studied in nutrient solution culture under short term and long term conditions.
Abstract: Iron uptake from ferrated (59Fe) pseudobactin (PSB), a Pseudomonas putida siderophore, by various plant species was studied in nutrient solution culture under short term (10 h) and long term (3 weeks) conditions. In the short term experiments, 59Fe uptake rate from 59FePSB by dicots (peanuts, cotton and sunflower) was relatively low when compared with 59Fe uptake rate from 59FeEDDHA. Iron uptake rate from 59FePSB was pH and concentration dependent, as was the Fe uptake rate from 59FeEDDHA. The rate was about 10 times lower than that of Fe uptake from the synthetic chelate. Results were similar for long term experiments.
TL;DR: In this article, root exudates were collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg−1 dry soil to silty clay and silt loam soils.
Abstract: This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg−1 dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting cycle after 21 d. Evolved CO2 was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil. In mucilage-amended soils CO2 evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase in evolved CO2 up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of these established organo-mineral bounds.
TL;DR: Pasture swards containing perennial ryegrass alone or with one of five different white clover cultivars were examined for production and transfer of fixed nitrogen to grass under dairy cow grazing, indicating that below-ground N transfer was largest during dry summer conditions.
Abstract: Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.
TL;DR: In this article, the availability of the free hydrated cation form of Fe3+, Cu2+, Zn2+, and Mn2+ versus their total metal concentrations (free ion plus complexes) in chelator-buffered solutions were estimated using the chemical equilibrium program GEOCHEM-PC.
Abstract: The form in which a micronutrient is found in the rhizosphere affects its availability to plants. We compared the availability to barley of the free hydrated cation form of Fe3+, Cu2+, Zn2+, and Mn2+ versus their total metal concentrations (free ion plus complexes) in chelator-buffered solutions. Free metal ion activities were estimated using the chemical equilibrium program GEOCHEM-PC with the corrected database. In experiment 1, barley was grown in nutrient solutions with different Fe3+ activities using chelators to control Fe levels. Chlorosis occurred at Fe3+ activities of 10−18 and 10−19M for barley grown in HEDTA and EDTA solutions, respectively. In experiment 2, barley was grown in nutrient solutions with the same calculated Fe3+ activity and the same chelator, but different total Fe concentrations. Leaf, root and shoot Fe concentrations were higher from CDTA buffered solutions which had the higher total Fe concentration indicating the importance of the total Fe concentration on Fe uptake. Results from treatments using EDTA or HEDTA, with one exception, were similar to the results from the CDTA treatment. This suggests differences in critical Fe3+ activities found in experiment 1 were due to differences in the total Fe concentration and not errors in chelate formation constants used to estimate the critical activities. Results for Cu, Zn, and Mn were similar to Fe; despite solutions with equal free Cu2+, Zn2+ and Mn2+ activities, plant concentrations of these metals were generally greater when grown in the solutions with the greater total amount of Cu, Zn, or Mn. When the free Zn2+ activity was kept constant while the total amount of Zn was increased from 4.4 to 49 μM, leaf Zn concentration increased from 77 to 146 μg g-1. In order to predict metal availability to barley and other species in chelator-buffered nutrient solutions, both free and total metal concentrations in solution must be considered. The critical Fe3+ activities required by barley in this study are much higher than those from tomato and soybean, 10-28M, which strongly supports the Strategy 2 model of Fe uptake for Poaceae. This is related to the importance of the Fe3+ (barley) and the Fe2+ (tomato and soybean) ions in Fe uptake. Fe-stressed barley is known to release phytosiderophores which compete for Fe3+ in the nutrient solution, while tomato and soybean reduce Fe3+ to Fe2+ at the epidermal cell membranes to allow uptake of Fe2+ from Fe3+ chelates in solution.
TL;DR: It was shown that the maximum concentration and penetration of soluble exudates differed markedly between different simulations and the implications for micronutrient acquisition by phytosiderophores and for colonisation of the rhizosphere by root pathogens were discussed.
Abstract: A mathematical model is outlined which is capable of simulating the radial and vertical distribution of soluble carbon, and of microbial biomass and necromass around a root growing through the soil. An alternating-direction implicit finite difference method is used to simulate the movement of soluble C by diffusion and convection in the rhizosphere cylinder surrounding the root. Results are presented which suggest that microbial populations in the rhizosphere of a growing root may vary considerably with distance along the root with the precise distribution of soluble C and biomass depending on the pattern of exudate release along the root length.
TL;DR: The solubility of Fe in soils is largely controlled by Fe oxides; ferrihydrite, amorphous ferric hydroxide, and soil-Fe are generally believed to exert the major control.
Abstract: The solubility of Fe in soils is largely controlled by Fe oxides; ferrihydrite, amorphous ferric hydroxide, and soil-Fe are generally believed to exert the major control. Fe(III) hydrolysis species constitute the major Fe species in solution. Other inorganic Fe complexes are present, but their concentrations are much less than the hydrolysis species. Organic complexes of Fe including those of organic acids like citrate, oxalate, and malate contribute slightly to increased Fe solubility in acid soils, but not in alkaline soils.