Bioavailability of phosphorus and zinc in calcareous soils of the Kurdistan region of Iraq
Publication Date
December 8, 2021
Creator
Abstract
The key objectives of the present project were to determine (i) the chemical form, solubility and bioavailability of zinc (Zn) and phosphorus (P) in the calcareous soils of Iraqi Kurdistan and their possible interactions, (ii) the uptake of Zn and P by crop plants and the viability of biofortification and (iii) the transfer of Zn and P into the human diet and their possible interactions through Zn binding to phytate.
A survey was conducted including a collection of 21 soil samples at seven different agricultural activity sites in Sulaimanyah province, Iraqi Kurdistan. The availability of both P and Zn nutrients for plant uptake was determined through their exchangeable and soluble amounts in the studied soils. Time-dependent changes in solubility were determined by incubation of selected soils up to 56 days at 20±20C and at half of their water holding capacity (WHC). The impacts of soil factors, including soil pH, inorganic carbon content (SIC) and soil organic carbon content (SOC), on the plant-available and soluble fractions of P and Zn and the solid⇌liquid partition coefficient (Log Kd) of both elements were determined. Total P (PT) and total zinc (ZnT) concentrations in all the studied soils were within the range of 622-1210 and 43-187 mg kg-1, respectively, which were in the range of P and Zn concentrations reported globally. The pH values were 7.33-8.27, soil inorganic carbon was in the range of 0.67-4.92% indicating the calcareous nature of the studied soils, and SOC was in the range 5-12%. Approximately 8% of PT was in the form of available P (sodium bicarbonate extractable P; POlsen) and 3% of ZnT was in the form of available Zn (extractable with 0.005 M DTPA; ZnDTPA). Furthermore, only about 2% of POlsen was in the form of soluble P (soluble in 0.01 M Ca(NO3)2; Psol) and 0.3% of ZnDTPA was in the form of soluble Zn (Znsol). Average values of log kd when available divided by soluble were 2.30 for P and 3.25 for Zn, indicating stronger adsorption of Zn than P. This is indicative of the lack of Zn fertilizer application to soils in Kurdistan because farmers only apply NPK fertilizers. A negative correlation was determined between PT and soil pH value whereas ZnT was positively correlated with loss on ignition. Values of POlsen, PSol, ZnDTPA and ZnSol were positively correlated with their total soil concentrations, PT and ZnT. Similarly, positive correlations were also observed between Psol and POlsen as well as between Znsol and ZnDTPA, while all four variables were negatively correlated with soil pH value. Log kd for Zn was negatively correlated with LOI but there was no correlation between soil pH value and Log Kd values possibly due to a narrow range of pH values.
To further examine P and Zn solubility, soils were incubated at half their water holding capacity (50% WHC) for 1, 7, 14, 28, and 56 days, without P and Zn application. P and Zn solubilities in soil suspended in 0.034 M Ca(NO3)2 were measured; the Ca concentration was determined from a Q/I relation as the natural equilibrium Ca concentration. Following solution speciation, using the geochemical speciation model WHAM-7, P and Zn solubility data were presented on stability diagrams comprising (i) phosphate potential (log H2PO4--pH) against lime potential (log Ca2++ 2pH) and (ii) the log10 activity of Zn (log Zn2+) against pH. Results indicated that P and Zn solubility decreased with increasing incubation time and increasing soil pH, and increased with increasing ‘available’ P and Zn (POlsen and ZnDTPA). Construction of solubility diagrams and determination of PSol and ZnSol fractions suggested that P release was controlled by a series of potential minerals, including dicalcium phosphate (DCP), dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), β-tricalcium phosphate (β-TCP) and hydroxyapatite (HP), depending on the phosphate status of the soil. By contrast, Zn was undersaturated with respect to Smithsonite.
A study was conducted to evaluate the effect of time of incubation on P and Zn adsorption and solubility in the calcareous soils. Two calcareous soils (pH 8.10 & 8.24, CaCO3 content 6.13 & 41.0%, and clay content 43.0 & 31.4%) from Sulaimanyah province, Iraqi Kurdistan, were treated with several levels of P (0, 50, 100, 200, and 300 mg kg-1) as KH2PO4 and the enriched stable isotope 70Zn (0, 2, 5, 10, and 20 mg kg-1) and incubated for 1, 7, 14, 28, 56, and 84 days at 50% of their water holding capacity under laboratory conditions with three replicates. The results indicated that adsorbed P and Zn, POlsen, ZnDTPA, Psol and Znsol, significantly increased with increase in the application rates but decreased with time of incubation. The ‘apparent’ E-value of 70Zn (ZnE) increased with increasing Zn application and incubation time in both soils as the added isotope gradually mixed with increasingly non-labile soil Zn pools.
At the end of the experiment (84 days), about 80 and 72% of applied P and 97.7 and 99.5% of applied 70Zn were adsorbed by soil components at 300 mg P kg-1 and 20 mg 70Zn kg-1 applied levels in Soils 1 and 2 respectively. The Langmuir-Freundlich model described the ‘available’ P and Zn solubility data (POlsen vs PSol and ZnDTPA vs ZnSol) well for both studied soils – regardless of incubation time. By contrast, adsorption isotherms constructed using total adsorbed P and Zn showed a systematic change with incubation time, indicating prolonged time-dependent P and Zn fixation in the calcareous soils. The lability (%ZnE) in the two selected soils was 70.4 and 90.5% of extracted 70Zn and 13.8 and 8.86% of total applied Zn recovered when the all the applied Zn levels were considered in Soils 1 and 2 respectively. At the end of the experiment, the solubility of P appeared to be controlled by OCP and β–TCP while the solubility of Zn was controlled by Smithsonite or was undersaturated. Application of Zn fertilizer directly to the calcareous soils of Kurdistan would appear to be non-viable due to their high capacity for continued time-dependent Zn adsorption and fixation.
To investigate the interaction between P and Zn and viability of Zn biofortification in a range of crops grown in the calcareous soil of Kurdistan region, a field experiment was carried out during the early season (March-August 2016) at the technical college of Halabja, Sulaimanyah, Iraqi Kurdistan. The trial was a completely randomized block design (CRBD), with twelve treatments and five replicates. The treatments included (i) four levels of P (0, 50, 100 and 150 kg ha-1) as triple super phosphate (TSP 20% P) and three levels of Zn (0, 0.8 and 1.2 kg ha-1) applied as foliar Zn-EDTA (5% Zn) to maize and (ii) four levels of P (0, 40, 80 and 120 kg ha-1) and three levels of Zn (0, 0.4 and 0.8 kg ha-1) applied to maize, cowpea and tomatoes. At harvest, both POlsen and Psol significantly increased with increasing rates of applied P, but the availability and solubility of P and Zn also decreased as time passed. Phosphorus solubility was undersaturated with respect to OCP and Zn solubility was undersaturated with respect to Smithsonite. The result of P fractionation showed that most of the applied phosphate had been re-distributed between P fractions in the same proportion as the native soil P.
The nutrient concentrations in shoots, grain, seed and fruits were determined, along with the phytic acid (PA) content of maize grain, cowpea seed and tomato fruit. The P and Zn concentrations in shoots, grains and fruits were increased by the application of P and Zn fertilizers compared with control, but the applications were more effective in shoots than in either grain, seed or fruit when compared with controls. The shoot P and Zn application were more effective in single P and Zn application than in a combined P+Zn treatment. The PA and PA/Zn molar ratio in the edible part of the crops increased with the increased rate of P applied and decreased with increasing Zn rates. Furthermore, the estimated human Zn bioavailability in grain, seed and fruit also increased with increasing Zn rates and decreased with P rates, indicating that foliar application of crops with Zn fertilizer may be a viable strategy for increasing bioavailable Zn in the crops grown in the calcareous soils of Kurdistan.
To evaluate the bioavailability of P and Zn in the soil, wheat, flour and bread and PA, PA to Zn molar ratio and estimated Zn bioavailability contents of wheat grain grown in the calcareous soils, a survey of flour and bread samples was undertaken in Sulaimanyah province, Kurdistan region, Iraq. Since wheat is the staple carbohydrate source in the region, a total of 120 farmers’ cereal fields were surveyed for soil and wheat grain samples along with 50 flour samples and corresponding bread samples which were collected in bakery markets.
At harvest, the soil samples were analysed for PT, ZnT, POlsen, ZnDTPA, PSol, and ZnSol. The values (mg kg-1) were within the range of 403 -3740 (PT), 42.0-203 (ZnT), 2.13-28.1 (POlsen), 0.14-5.23 (ZnDTPA), 0.07-9.91 mg P L-1 (PSol) and 0.21-9.46 µg Zn L-1 (ZnSol); pH was in the range 7.46-8.67, indicating that all the soils were calcareous. The values of Log Kd were in the range 1.65±0.47-0.40±0.28 for P and 3.00±0.30-2.32±0.29 for Zn. The results of soils analysis showed that P and Zn concentrations were in the range reported globally. However, about 83% and 53% of soil samples contained low concentrations of POlsen and PSol, below-recommended values of 10 mg kg-1 POlsen and 0.5 mg L-1 PSol. Furthermore, about 78% and 98% of soil samples contained a ZnDTPA and ZnSol below-recommended values of 0.5 mg kg-1 ZnDTPA and 4-270 µg L-1 Znsol. Again these figures reflect the high CaCO3 content and high pH values in the surveyed soils and the absence of Zn fertilization by farmers. Analysis of solubility indicated that TCP and OCP are probably the main calcium phosphate minerals controlling the solubility of P and Smithsonite is the main Zn mineral which controls Zn solubility in the studied soils or probably indicates adsorption of Zn on the surface of CaCO3, Fe oxides and on humus when undersaturated with respect to Smithsonite.
The concentrations of P, Zn, PA, PA/Zn molar ratio and estimated Zn bioavailability was also determined for wheat grain, flour and bread samples in various bread types including Hawrami, Tiry, Lawasha, Mashini and Samoon which are consumed in the Sulaimanyah community. The values of P, Zn and PA concentrations (mg kg-1) were 3131-3625, 1051-1872 and 1195-1631 (P), 26.3-35.0, 7.04-17.2 and 8.90-14.9 (Zn) and 3000-11000, 1600-3100 and 800-1800 (PA), giving a PA/Zn molar ratio of 15.7-30.6, 17.8-22.9 and 8.96-14.6 for whole wheat grain flours, flours and breads respectively. The results suggest that the concentrations of P and PA are high, about 40% of wheat grains contain higher amounts than the recommended value of 3570 mg kg-1 of P. This is probably due to extensive applications of phosphate fertilizers by farmers, typically 100-200 kg P2O5 ha-1 in every growing season. However, Zn concentration in 88% of wheat grain samples was lower than the recommended concentration of 41.6 mg kg-1 in whole wheat grains due to low available Zn in the surveyed soils.
A trivariate model was used to estimate intake of available Zn. Estimated bioavailable Zn (mg d-1) was estimated assuming an average wheat, flour and bread consumption of 300 g day-1 in the region. Consideration of the PA and Zn intake suggest only 21.5±2.9% of grain Zn, 43.6±6.52% of flour Zn, and 52.1±6.56% of bread Zn were bioavailable so that the effective Zn intake from wheat grain, flour and bread are only 1.84-2.63, 0.78-2.26 and 1.25-2.05 mg d-1 for the local population respectively. The results indicated low Zn absorption from wheat grain, flour and bread. However, the Zn absorption from a common flatbread, ‘Tiry’, was higher than Mashini. The bread making processing had a major influence on estimated bioavailable Zn in the final product, producing reductions of 15-47% in PA and 17.0-61.0% in the PA/Zn molar ratio. The estimated bioavailable Zn increased between 2-41% in white flours and bread depending on the amount of daily PA intake concentration. Overall results suggest that available dietary Zn is below recommended levels (11 mg d-1) which is partly due to low uptake by wheat but also due to the presence of large concentrations of PA in wheat grains and bread.
In general, the total concentration of P and Zn were found to be adequate in the Kurdistan soils. However, the bioavailability of both nutrients is low due to the calcareous nature of soils because of high calcium carbonate (CaCO3) content, high pH value, and high adsorption and ‘fixation’ capacities. The concentrations of P and PA in the crop plants and wheat grain grown in the soils of Kurdistan were high due to the application of phosphate fertilizers, while the Zn content was typically low, and under the recommended value in the case of wheat grain which is the staple daily food for the population of the Kurdistan region. The low concentration of Zn is due to some factors such as low soil Zn bioavailability, high application of phosphate fertilizers, P interaction with Zn in soils and plants, and absence of Zn biofortification as an agronomic strategy in the region. The daily Zn intake from crop plants is not adequate to meet the recommended Zn level in human populations due to a high level of PA and PA to Zn molar ratio. Biofortification of Zn is viable, but a key recommendation may be to reduce the application of phosphate fertilizer. The low intake of bioavailable Zn could cause health problems in the region. Given the current status of Zn in crop plants grown in Kurdistan, biofortification of Zn to increase Zn in staple crops is recommended. A crop breeding program combined with targeted agronomic management methods is also required to enhance both Zn uptake and its bioavailability in grains of cultivated wheat and crops.
Item Type
ethesis
Thesis Type
PhD
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Subjects (LC)
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School of Biosciences (UK)
eprints ID
56219
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