Yona Chen

Iron Nutrition of Plants in Calcareous Soils

Publisher Summary This chapter reviews iron nutrition of plants in calcareous soils. Many agricultural crops worldwide, especially in semiarid climates, suffer from iron deficiencies. Deficiencies are usually recognized by chlorotic, or yellowed, intervein areas in new leaves and are typically found among sensitive crops grown in calcareous soils. Iron deficiency in extreme cases may lead to complete crop failure. Two principal methods of treating iron deficiencies are accepted. Spraying foliage with inorganic salts has been shown to be of benefit, but often gives spotty results because of, limited penetration of iron into leaves. Also, repeated treatments are required during the course of canopy development. Soil treatment with synthetic chelates, principally FeEDDHA (ethylenediaminedi– o –hydroxyphenylacetic acid), has been found to be an unqualified success, but for the drawback of their high costs. Iron deficiency in plants causes chlorosis of leaf tissue because of inadequate chlorophyll synthesis. In a healthy plant, 60% of all leaf iron is concentrated in chloroplasts. The exact role of iron in chlorophyll synthesis is not certain but there is evidence of the involvement of ferrous iron in the condensation of succinic acid and glycine to form γ-aminolevulinic acid. Magnesium is incorporated into the molecule to form chlorophyll, possibly with the catalytic action of iron.

Scanning Electron Microscope Observations On Soil Crusts And Their Formation

Scanning electron micrographs (SEM) of crusts of loessial soils are presented. SEM observations were performed on crusts formed by raindrop impact at various stages of their formation. The crust structure was compared to the natural undisturbed soil. During the crust formation, a middle-term stage developed at which coarse particles, stripped of the fine ones, composed the surface layer of the soil. At the final stage of the crust formation, the coarse particles were washed away, and a thin seal skin, about 0.1 millimeter thick, formed the uppermost layer of the soil. A depositional crust, which was formed mainly by the translocation of fine particles, was marked by the presence of a thin skin also about 0.1 millimeter thick, suggesting involvement of similar secondary mechanisms of formation. This work illustrates the use of SEM for the study of soil crust formation and structure.

Siderophores of Pseudomonas putida as an iron source for dicot and monocot plants

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.

The Role of Ligand Exchange in the Uptake of Iron from Microbial Siderophores by Gramineous Plants

The siderophore rhizoferrin, produced by the fungus Rhizopus arrhizus, was previously found to be as an efficient Fe source as Fe-ethylenediamine-di(o-hydroxphenylacetic acid) to strategy I plants. The role of this microbial siderophore in Fe uptake by strategy II plants is the focus of this research. Fe-rhizoferrin was found to be an efficient Fe source for barley (Hordeum vulgare L.) and corn (Zea mays L.). The mechanisms by which these Gramineae utilize Fe from Fe-rhizoferrin and from other chelators were studied. Fe uptake from 59Fe-rhizoferrin, 59Fe-ferrioxamine B, 59Fe-ethylenediaminetetraacetic acid, and 59Fe-2[prime]-deoxymugineic acid by barley plants grown in nutrient solution at pH 6.0 was examined during periods of high (morning) and low (evening) phytosiderophore release. Uptake and translocation rates from Fe chelates paralleled the diurnal rhythm of phytosiderophore release. In corn, however, similar uptake and translocation rates were observed both in the morning and in the evening. A constant rate of the phytosiderophores release during 14 h of light was found in the corn cv Alice. The results presented support the hypothesis that Fe from Fe-rhizoferrin is taken up by strategy II plants via an indirect mechanism that involves ligand exchange between the ferrated microbial siderophore and phytosiderophores, which are then taken up by the plant. This hypothesis was verified by in vitro ligand-exchange experiments.

The influence of soluble organic matter from municipal solid waste compost on trace metal leaching in calcareous soils

The use of municipal solid waste (MSW) compost as fertilizer may cause increased leaching due to its high content of trace metals and thus pose a threat to groundwater quality. The effect of MSW compost application on trace metal leaching in calcareous soils has been studied in soil column experiments under laboratory conditions using three soils from the study area in the Gaza Strip and Israel. Higher levels of organic matter in solution (TOMS), nitrate, and the trace metals Cu, Ni and Zn were found in the leachates of a sandy soil and, to a lesser extent, a loamy soil, to which MSW compost had been applied at a rate of 65 Mg ha(-1) (dry weight basis). Nevertheless, the majority of water-soluble trace metal species from compost accumulated in the topsoil rather than washing out, with the exception of aqueous Ni species. Ni concentrations exceeded the maximum allowable limits for drinking water (in Germany: 50 microg l(-1)) at peak times in the leachates from sandy soil, while all other trace metals remained far below the corresponding limits. The highest absolute concentrations of trace metals were found for the leaching of Cu from compost-amended sandy soil (100 microg l(-1)). For Cd, Pb and Hg no evidence of downward movement was found in any assay. Gel filtration studies of the collected soil leachates showed that all trace metals encountered in the leachates existed mostly as organic complexes. In sandy soil most of the water-soluble organic matter added with the compost had leached from the rootzone after a years equivalent of rainfall, while TOMS mobility was greatly reduced in the loamy soil. The makeup of the TOMS in the sandy soil and its metal-binding capacity was strongly influenced by compost-derived dissolved organic matter (DOM) as observed by FTIR spectrometry. Hence the vertical displacement of trace metals (Cu, Ni, Zn) in these calcareous soils seemed to result primarily from the presence of mobile metal-organic complexes in the soil solution after compost addition. Further studies are required to validate these findings in the field, especially to assess the risk of Cu and Ni leaching in sandy soil.

Mechanisms of plant growth stimulation by humic substances: The role of organo-iron complexes

Abstract Stimulatory effects of humic substances (HS) on plant growth have been observed and widely documented. Studies have often shown positive effects on seed germination, root initiation and total plant biomass. The consistency of these observations has been uncertain, predominantly due to the lack of understanding of the plant growth promotion mechanism. Often these effects have been attributed to a direct effect of plant growth hormones; whereas in other instances the term “hormone-like activity” has been used to describe the plant growth stimulation (Chen and Aviad, Humic Substances in Soil and Crop Sciences: Selected Readings, American Society of Agronomy, Soil Science Society of America, 1990; Nardi et al., Humic Substances in Terrestrial Ecosystems, Elsevier Science B.V., 1996). Yet, investigators have been unable to prove that plant growth regulators are present in HS preparations, or the evidence provided remains unconvincing. An alternative hypothesis suggesting that growth enhancement of plants grown in nutrient solution (NS) containing HS is the result of improved micronutrient availability, Fe in particular, has been postulated and tested in the present study. Nutrient solutions containing N, P, K, Ca, Mg, S, B, Mo, Cu, Mn, Zn, and Fe at concentrations considered to be optimal for plant growth were tested for solubility of the Fe, Zn, and Mn, 7 days after preparation. In addition to control solutions at pH values of 5, 6, 7, and 7.5, 0 to 200 mg L−1 of leonardite humic acid (HA) were added to the solutions and they were tested for Fe and Zn solubility. The HA greatly enhanced the maintenance in solution of Fe, in all the tested solutions, and Zn at pH 7.5. Mn mostly remained in solution in its inorganic forms. Plant growth experiments were carried out on both dicotyledonous plants (melons and soybean) and monocotytedonous Poaceae plants (ryegrass), due to the major difference in their Fe uptake mechanism. Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts. The addition of HA or fulvic acid (FA) without addition of Fe, and Zn resulted in partial growth enhancement and correction of Fe deficiency, or none of the two, in the various experiments. This suggests that the growth enhancement effect observed in solutions containing Fe, Zn, and HS was related to the micronutrients rather than to phytohormones. However, the addition of Fe, Zn and either EDTA, HA or FA resulted in healthy, chlorophyll rich plants and enhanced growth, thereby providing evidence that improved Fe, and possibly Zn nutrition is a major mechanism of plant growth stimulation by HS. The use of the term hormone-like activity could be the result of the similarity of the physiological effects obtained in plants enjoying sufficient supply of Fe and Zn.

Effect of solar heating of soils by transparent polyethylene mulching on their chemical properties.

Solar heating of soils by mulching with transparent polyethylene during the hot season results in increased soil temperatures and the killing of certain pathogens. Mulching increases temperatures by up to 10 to 12°C in the upper soil layer. We studied the effect of this treatment on the chemical properties and hydraulic conductivity (HC) of soil. Field experiments showed enhanced plant growth in solar heated soils, even in the absence of known pathogens. Saturated extracts of the upper soil layers of eight different solar heated soils showed increased concentrations of soluble organic matter and minerals. The greatest increase was in NO3- concentration; NH4+, K+, Ca2+ + Mg2+, and Cl- were also found to increase. Changes in soil pH, total organic matter, NaHCCv extractable P, and HC were small or inconsistent. Tomato seedlings grown on extracts of heated soils showed enhanced growth in comparison with seedlings grown on extracts of unheated soils.

Dissolved Organic Carbon Fractions Formed during Composting of Municipal Solid Waste: Properties and Significance

The properties and transformation of dissolved organic matter (DOM) extracted (10 L water/kg compost) from municipal solid waste (MSW) compost at five stages (days 47, 77, 105, 126, and 187) of composting were investigated. The DOM was fractionated into hydrophobic or hydrophilic neutrals, acids, and bases. The unfractionated DOM, the hydrophobic acids and neutrals (HoA and HoN, respectively), and the hydrophilic neutrals (HiN) fractions were studied using solid-state 13 C-NMR, FTIR, and DRIFT spectroscopy. The HoA fraction was found to be the dominant (percentage of total DOM) hydrophobic fraction, exhibiting a moderate increase during composting. The HoN fraction increased sharply from less than 1 to 18% of the total DOM during 187 days of composting, while the hydrophobic bases (HoB) exhibited the opposite trend. The HiN represented the major fraction of the hydrophiles up to 120 days of composting, decreasing thereafter by 38 %. The relative concentration of the hydrophilic acids and bases (HiA and HiB, respectively) exhibited no consistent trend during composting. DRIFT spectra of the unfractionated DOM taken from the composting MSW revealed a decreasing level of polysaccharide structures with time. The 13 C-NMR and FTIR spectra of the HoA fraction exhibited a polyphenol-humic structure, whereas the HoN spectra exhibited strong aliphatic features. The spectra of the HiN fraction confirmed its polysaccharide nature. During the final stage of composting, the DOM concentration was steady, while a relative decrease of HiN concomitant with an increase of HoA and HoN fractions was observed. These indicate that the DOM contained a low concentration of biodegradable organic matter and a higher content of macromolecules related to humic substances. The biological significance and heavy metal binding of these fractions are being studied based on earlier observations showing enhanced plant growth in the presence of DOM extracted from mature as opposed to immature compost.

Iron Nutrition and Interactions in Plants

In most soils, FellI 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 _1044 ). 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(llI) ions. Therefore, redox potential strongly influences the activity of Fell. At a given pH and Eh , the activity of Fell 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 FellI 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 metaor 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+ -0 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.

Nature, formation and effects of soil crusts formed by water drop impact

Abstract Research was conducted to investigate: (1) the structure, formation and nature of soil crusts which develop as a result of raindrop impact under controlled conditions of simulated rain, and (2) the crust effect on infiltration rates. Scanning electron microscope (SEM) observations performed on sandy, sandy-loam and clayey soil samples show the crust to be composed of two layers: (1) a “skin”, 0.1 mm thick, and (2) a layer, 2–3 mm thick, with a higher bulk density in which aggregates have been destroyed. A “washing-in” zone described by earlier investigators could not be detected. The soil beneath the crust maintains its original structure and particle orientation. The chemical composition of the rain water was found to have a significant but not very large effect on the infiltration rate vs. cumulative rain curve of the clayey soil material only; final infiltration rate was about 20% lower when distilled water was applied compared to tap water (electrical conductivity ≈ 0.6 mmho cm−1). An increase of the bulk density from 1.35–1.48 g cm−3 in undisturbed soil to 1.74–1.88 g cm−3 in crusts was found. The amount of eroded material increased gradually with increase of runoff and decrease of infiltration. At steady state the erosion rates were 0.45, 1.25, and 0.76 g m−2 min−1 in the sandy, sandy-loam and clayey soil specimens, respectively. Percentages of clay and silt in the eroded material were higher than in the bulk samples. Three stages to describe the changes in nature and effects of crusts during their formation are suggested: (1) the rate of infiltration decreases to a point at which runoff beings, (2) the initiation of runoff until infiltration and runoff rates become stabilized, and (3) infiltration and runoff maintain steady rates.