Rising sea levels due to climate change are causing increased salinisation of low-lying coastal and floodplain soils, and the impact of this process on the bioavailability of plant nutrients needs to be understood as mitigation strategies are adapted. Zinc (Zn) is an element of particular importance due to its function as a micronutrient for plants including rice and other staple foods. In the current study, our aim was to investigate the effects of salinisation on zinc adsorption onto soils representing at-risk coastal and floodplain environments, addressing in particular our knowledge gap concerning the roles that solution chemistry and soil composition play. To this end, we conducted batch adsorption experiments in the laboratory and ran geochemical models in saline solutions up to 0.7 mol L-1 ion strength incorporating both (i) a multi surface model (MSM) for surface reactions containing three phases, that is iron hydroxides, organic matter and phyllosilicate clays, and (ii) aqueous-phase complexation to dissolved organic and inorganic ligands. Surface reactions were modelled using the diffuse double layer model, the NICA-Donnan model and an ion exchange model using the Gaines-Thomas convention. We combined the experimentally determined mass composition of surface phases with generic modelling parameters taken from the literature. We first show that increasing salinity enhances the formation of aqueous Zn-chloride complexes in the presence of dissolved organic matter and bicarbonate, thereby decreasing the availability of free Zn2+ and supressing the partitioning of zinc to the adsorbed phase. We demonstrate using batch adsorption experiments with a calcareous hydraquent and a tropaquept, that salinity decreases zinc adsorption strongly in the pH range between 3 and 6. Satisfactory agreement between experiments and model calculations was achieved with root-mean-square errors ranging for different salinities between 2.88% and 2.92% for the hydraquent and between 4.59% and 2.74% for the tropaquept soil. Model predictions of adsorption were slightly inferior at low salinity for the hydraquent soil and at high salinity for the tropaquept soil, pointing possibly to an incomplete geochemical model or to a need to parametrise surface adsorption models at higher ionic strengths. Present surface models have been largely parametrised at lower ionic strength. We lastly apply the MSM to examine zinc adsorption in five endoaquepts soils, representing soil series from Bangladesh. We show that increasing salinity decreases zinc adsorption to the soil organic matter and the clay fractions. We conclude from our findings that increased soil salinity due to rising sea levels and climate change will have a significant impact on zinc cycling and possibly other micronutrients in areas where coastal soils and floodplain soils overlap, such as deltas and estuaries. In particular, we predict a decrease in zinc adsorption in acidic to neutral soils. The availability of zinc for biouptake through the roots of crop plants including rice will be significantly disturbed following salinisation, most likely affecting crop production. Our study demonstrates the potential that geochemical modelling combined with experimental data has to improve our capability to assess the effects of salinity due to rising seawater levels in vulnerable regions of the world. Adsorption of zinc in tropical wetland soils is studied using batch experiments and geochemical modelling. The constructed multi-surface model uses soil organic matter, iron oxides and clay. The binding modes and mathematical models include adsorption with the NICA-Donnan model, surface complexation with the DDL model, and cation exchange with anion exchange model. image

Citrate (Cit) and Deferoxamine B (DFOB) are two important organic ligands coexisting in soils with distinct different affinities for metal ions. It has been theorized that siderophores and weak organic ligands play a synergistic role during the transport of micronutrients in the rhizosphere, but the geochemical controls of this process remain unknown. Here we test the hypothesis that gradients in pH and ion strength regulate and enable the cooperation. To this end, first we use potentiometric titrations to identify the dominant Zn(II)–Cit and Zn(II)–DFOB complexes and to determine their ionic strength dependent stability constants between 0 and 1 mol dm −3 . We parametrise the Extended Debye-Hückel (EDH) equation and determine accurate intrinsic association constants (logβ 0 ) for the formation of the complexes present. The speciation model developed confirms the presence of [Zn(Cit)] − , [Zn(HCit)], [Zn 2 (Cit) 2 (OH) 2 ] 4− , and [Zn(Cit) 2 ] 4− , with [Zn(Cit)] − and [Zn 2 (Cit) 2 (OH) 2 ] 4− the dominant species in the pH range relevant to rhizosphere. We propose the existence of a new [Zn(Cit)(OH) 3 ] 4− complex above pH 10. We also verify the existence of two hexadentate Zn(II)–DFOB species, i.e., [Zn(DFOB)] − and [Zn(HDFOB)], and of one tetradentate species [Zn(H 2 DFOB)] + . Second, we identify the pH and ionic strength dependent ligand exchange points (LEP) of Zn with citrate and DFOB and the stability windows for Zn(II)–Cit and Zn(II)–DFOB complexes in NaCl and rice soil solutions. We find that the LEPs fall within the pH and ionic strength gradients expected in rhizospheres and that the stability windows for Zn(II)–citrate and Zn(II)–DFOB, i.e., low and high affinity ligands, can be distinctly set off. This suggests that pH and ion strength gradients allow for Zn(II) complexes with citrate and DFOB to dominate in different parts of the rhizosphere and this explains why mixtures of low and high affinity ligands increase leaching of micronutrients in soils. Speciation models of soil solutions using newly determined association constants demonstrate that the presence of dissolved organic matter and inorganic ligands (i.e., bicarbonate, phosphate, sulphate, or chlorides) do neither affect the position of the LEP nor the width of the stability windows significantly. In conclusion, we demonstrate that cooperative and synergistic ligand interaction between low and high affinity ligands is a valid mechanism for controlling zinc transport in the rhizosphere and possibly in other environmental reservoirs such as in the phycosphere. Multiple production of weak and strong ligands is therefore a valid strategy of plants and other soil organisms to improve access to micronutrients.

A wide range of organic ligands are found in the rhizosphere. Two important groups are low-molecular-weight organic acids (LMWOAs) and siderophores. We know that LMWOAs and siderophores coexist in the rhizosphere and it has been proposed that they interact, but it is not clear what controls this. Such knowledge gaps undermine biofortification efforts. In this study test the hypothesis that pH and ionic strength gradients make it possible for LMWOAs and siderophores to function synergistically during micronutrient cycling in the rhizosphere.

This study has been conducted to investigate the impact on different land-use on soil organic carbon stock and selected soil properties in soil profiles; along with a preponderance of SOC into different pools of oxidizability with depth increment; under different land-use systems in saline soil and to screen out which land practices are best for soil carbon storage. Soil samples were collected at 0–15, 15–30, 30–45, 45–60, 60–75, 75–90, and >90 cm depths by the opening pit in three different locations under different cropping patterns (Rice-vegetable, Rice-fallow, and Rice-shrimp) with three replicates, then analyzed. Results showed that the SOC content and SOC stock significantly (p ≤ 0.05) increased with depth increments in Rice-vegetables land-use system while it decreased with depth in the Rice-fallow system. At the greater depth of >90 cm, SOC stock has been found highest which is true for both Rice-vegetables (3375 t C/ha) and the Rice-shrimp system (2268 t C/ha). But the greater depth of the Rice-fallow system contains the least amount (296.4 t C/ha). Also, textural analysis of soil showed that the soils from each cropping system were dominant in silt particles. It is found that both clay and silt particles had accumulated at a lower depth in all of the cropping systems which was also significant (p ≤ 0.05) along soil profile may be due to the puddling and tillage operations for rice cultivation. Higher content of CaCO3 was obtained under both Rice-vegetable and Rice-fallow land-use systems (5.19%) at 75–90 cm and 60-75 cm depth, respectively which may be attributed to the process of calcium leaching and subsequently precipitated as CaCO3 at a lower profile. An increase in depth, active pools of carbon decreased significantly (p ≤ 0.05) in Rice-vegetables, Rice-shrimp and Rice-fallow land-use system till from 30 to 45 cm, 30 to 90 cm, and 45 to 90 cm respectively which is an early indicator of soil quality. The highest (5.1 g C/kg) and lowest (0.2 g C/kg) amount of very labile fraction was obtained under Rice-vegetable and Rice-shrimp system at 75–90 cm and 0–15 cm depth, respectively. By contrast, passive pools of carbon and its recalcitrant nature increased significantly (p ≤ 0.05) with depth for Rice-vegetable and Rice-shrimp land-use systems.

Non-traditional stable isotope systems are increasingly used to study micronutrient cycling and acquisition in terrestrial ecosystems. We previously proposed for zinc (Zn) a conceptual model linking observed isotope signatures and fractionations to biogeochemical processes occurring in the rice soil environment and we suggested that 2'deoxymugineic acid (DMA) could play an important role for rice during the acquisition of Zn when grown under Zn limiting conditions. This proposition was sustained by the extent and direction of isotope fractionation observed during the complexation of Zn with DMA synthesised in our laboratory. Here we report a new set of experimental data from field and laboratory studies designed to further elucidate the mechanisms controlling Zn isotope fractionation in the rice rhizosphere and the role of DMA. First, we present acidity (pK(a)) and complexation (logK) constants for DMA with H+ and Zn2+, respectively, using synthetic 2'deoxymugineic acid and show that they are significantly different from previously published data using isolates from plants. Our new set of thermodynamic data allows for a more accurate calculation of the formation of ZnDMA complexes over pH ranges typically found in the rhizosphere of flooded lowland rice soils and in rice plant compartments (xylem, phloem). We show that at pH > 6.5, Zn is fully complexed by DMA and at pH <4.5 fully dissociated. This has important implications, i.e. that in alkaline paddy soils, DMA can strip Zn from soil solids (organic and inorganic) and that ZnDMA complexes are stable at the root interface if the pH is alkaline and in the phloem and xylem of the rice shoot. Second, we present a new set of Zn isotope data in rice grown in alkaline soils with low Zn availability with and without Zn addition. We used two genotypes not tested to date, i.e. A69-1, tolerant to low Zn supply, and IR26, sensitive to low Zn supply. We confirm previous findings that, in contrast to observations with rice grown in hydroponic studies, no isotopically light Zn is taken up into the shoot irrespective of Zn fertilization and that isotopically heavier Zn is taken up by rice grown in soils with low Zn supply. Third, we determined the isotope fractionation isotherm for Zn during absorption on goethite, representing the iron phase (plaque) typically forming on rice roots, in acidic solution. The negative Zn isotope fractionation observed (i.e., Delta(66)Z(ngoethite-solution) < 0, where Delta(66)Z(ngoethite-solution) = (delta Zn-66 of goethite) - (delta Zn-66 of solution)) is in contrast to the positive isotope fractionation (Delta(66)Z(ngoethite-solution) > 0) detected during adsorption of Zn on goethite in alkaline solutions or between root and soil solution in rice grown in alkaline soils. We show that removal of different Zn species from solution and changes in the Zn coordination control extent and direction of isotope fraction during adsorption. Using the new set of results and combining it with recent findings from the literature, we present a refined conceptual model linking biogeochemical processes to Zn isotope fractionation in the rice soil system. Our results confirm the importance of root induced chemical changes in the rhizosphere of rice growing in soils with low Zn availability, demonstrate the unique ability of isotope signatures to deconvolve geochemical processes and conditions in the plant-soil environment and support the hypothesis of an important role of DMA in Zn acquisition under Zn limiting conditions.

Soil minerals study is vital in terms of investigating the major soil forming compounds and to find out the fate of minor and trace elements in soils. It is also essential for the soil-plant interaction purpose. To identify soil mineral phases especially clay minerals, X-ray diffraction (XRD) has been a popular technique. The clay mineralogical information of soils in Bangladesh is limited, especially in Ganges flood plain region (Agro Ecological Zone (AEZ) 12 and 13). Therefore, to overcome this limitation, in this study, we performed XRD analysis of <2 mm fractions soil samples of AEX 12 and 13. However, identifying mineralogical phases by XRD in <2 mm fractions soils is not so straight-forward due to many practical problems. We fully matched only two mineralogical phases in all the soil samples which is quartz and potassium-Aluminum-Silicate. However, the full XRD peaks indicate that more minerals are also present, but due to heterogeneity of soils samples, it is difficult to find other minerals phases by only XRD peak of <2 mm fractions. Therefore, to find more information about mineralogical phases, we performed XRF analysis that provides the elemental composition of minerals phase as oxide. XRF analysis indicated the presence of secondary minerals like illite and chlorite. The presence of high percentage Fe oxide not only indicated the iron mineral phase (goethite and ferrihydrite) but also indicated iron rich high charge smectite minerals (beidellite). The presence of iron rich smectite minerals in the Ganges sediments reported in several previous studies. Thus, we concluded that only XRD in <2 mm fractions of soils is not adequate to identify the mineralogical phases of soil samples. Others analyses like XRF, XRD in <2 μm fractions will be necessary to locate an entire image of soil mineralogical phases.

Understanding the controls of the oxidation rate of iron (Fe) in oxygenated aquatic systems is fundamental for students of the Earth and Environmental Sciences as it defines the bioavailability of Fe, a trace metal essential for life. The laboratory experiment presented here was successfully developed and used during a third-year undergraduate lab course at Imperial College London for several years. It employs ultraviolet–visible (UV–vis) spectroscopy calibrated externally with 0 to 50 μM Fe2+ standards created in a 492 μM ferrozine and 0.43 M acetate matrix. The students conducted the oxidation experiments in stirred batch reactors at equilibrium with atmospheric oxygen. The solution contained 40.5 μM initial Fe2+ concentration and a 5.1 mM imidazole buffer. The pH was adjusted to values between 7.22 and 7.77. The students observed a pseudo-first-order reaction with respect to Fe2+ concentration. Plotting the logarithms of the apparent rate constants (k′) at different pH values leads to a gradient of 2.2 ± 0.2 min–1 pH–1, indicating a second-order reaction with respect to OH– concentration, in agreement with published literature. The oxidation reaction occurred rapidly (tens of seconds to tens of minutes) indicating that in oxygenated aquatic systems, Fe3+ will be the dominant oxidation state, significantly reducing the bioavailability of Fe. The simple laboratory experiment presented here allows the students to learn about kinetic parameters for a fundamental chemical reaction. It allows the students to explore the significant implications this has for aquatic ecosystems.

Assessment of soil organic matter fractions can be instrumental in understanding the causes of limited nitrogen supply, and thus soil fertility restoration. A study was conducted in cultivated and uncultivated saline soil, in order to assay soil organic carbon (SOC), its particle-size fractions and their influence on cultivation and soil fertility at Sundarbans costal area in Bangladesh. Soil samples were taken from the 0 - 15 and 15 - 30 cm depths from four cultivated fields and from four nearby sites in a native mangrove forest as references. Soil samples were physically fractionated into sand (2000-50 μm), silt (50-2 μm) and clay ( silt > sand. The SOC pool and N in the clay-sized fraction were correlated to soil fertility indicators. More N was stored in the silt + clay size fractions, a generally more stable pool, than in the more labile sand-sized pool. The SOC pool in sand size fractions was far below in cultivated soils than in a reference uncultivated soil. Thus, the sand-sized pool emerged as the most likely cause of limited N supply in cultivated soils.