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Soil Syst., Volume 2, Issue 4 (December 2018)

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Open AccessArticle Constraints to Synergistic Fe Mobilization from Calcareous Soil by a Phytosiderophore and a Reductant
Received: 6 November 2018 / Revised: 3 December 2018 / Accepted: 7 December 2018 / Published: 16 December 2018
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Abstract
Synergistic effects between ligand- and reductant-based Fe acquisition strategies can enhance the mobilization of Fe, but also of competing metals from soil. For phytosiderophores, this may alter the time and concentration window of Fe uptake during which plants can benefit from elevated Fe
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Synergistic effects between ligand- and reductant-based Fe acquisition strategies can enhance the mobilization of Fe, but also of competing metals from soil. For phytosiderophores, this may alter the time and concentration window of Fe uptake during which plants can benefit from elevated Fe concentrations. We examined how the size of this window is affected by the ligand and reductant concentration and by non-simultaneous addition. To this end, a series of kinetic batch experiments was conducted with a calcareous clay soil to which the phytosiderophore 2′-deoxymugineic acid (DMA) and the reductant ascorbate were added at various concentrations, either simultaneously or with a one- or two-day lag time. Both simultaneous and non-simultaneous addition of the reductant and the phytosiderophore induced synergistic Fe mobilization. Furthermore, initial Fe mobilization rates increased with increasing reductant and phytosiderophore concentrations. However, the duration of the synergistic effect and the window of Fe uptake decreased with increasing reductant concentration due to enhanced competitive mobilization of other metals. Rate laws accurately describing synergistic mobilization of Fe and other metals from soil were parameterized. Synergistic Fe mobilization may be vital for the survival of plants and microorganisms in soils of low Fe availability. However, in order to optimally benefit from these synergistic effects, exudation of ligands and reductants in the rhizosphere need to be carefully matched. Full article
(This article belongs to the Special Issue Iron and Manganese Biogeochemical Cycling in Soils)
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Open AccessArticle The Ability of Soil Pore Network Metrics to Predict Redox Dynamics Is Scale Dependent
Received: 31 August 2018 / Revised: 5 November 2018 / Accepted: 17 November 2018 / Published: 5 December 2018
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Abstract
Variations in microbial community structure and metabolic efficiency are governed in part by oxygen availability, which is a function of water content, diffusion distance, and oxygen demand; for this reason, the volume, connectivity, and geometry of soil pores may exert primary controls on
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Variations in microbial community structure and metabolic efficiency are governed in part by oxygen availability, which is a function of water content, diffusion distance, and oxygen demand; for this reason, the volume, connectivity, and geometry of soil pores may exert primary controls on spatial metabolic diversity in soil. Here, we combine quantitative pore network metrics derived from X-ray computed tomography (XCT) with measurements of electromotive potentials to assess how the metabolic status of soil depends on variations of the overall pore network architecture. Contrasting pore network architectures were generated using a Mollisol—A horizon, and compared to intact control samples from the same soil. Mesocosms from each structural treatment were instrumented with Pt-electrodes to record available energy dynamics during a regimen of varying moisture conditions. We found that volume-based XCT-metrics were more frequently correlated with metrics describing changes in available energy than medial-axis XCT-metrics. An abundance of significant correlations between pore network metrics and available energy parameters was not only a function of pore architecture, but also of the dimensions of the sub-sample chosen for XCT analysis. Pore network metrics had the greatest power to statistically explain changes in available energy in the smallest volumes analyzed. Our work underscores the importance of scale in observations of natural systems. Full article
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Open AccessArticle Soil Oxygen Limits Microbial Phosphorus Utilization in Humid Tropical Forest Soils
Received: 9 October 2018 / Revised: 5 November 2018 / Accepted: 21 November 2018 / Published: 29 November 2018
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Abstract
Soil phosphorus (P) availability is of special interest in many humid tropical forests, especially those on highly weathered, iron (Fe)- and aluminum (Al)-rich soils where P often limits net primary productivity. Phosphorus cycling is partly dependent on the ability of microbes to compete
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Soil phosphorus (P) availability is of special interest in many humid tropical forests, especially those on highly weathered, iron (Fe)- and aluminum (Al)-rich soils where P often limits net primary productivity. Phosphorus cycling is partly dependent on the ability of microbes to compete for P with Fe and Al minerals, which strongly bind P. Soil P availability is also indirectly affected by soil redox conditions due to its effects on microbial activity and reductive dissolution of Fe oxides that may weaken Fe-O-P sorption strength. Here, we explored P sorption, soil Fe (II) concentrations, soil CO2 production, organic and inorganic P pools, and microbial biomass P in tropical soils that typically experience frequent low redox (valley soils), or fluctuating redox conditions (slope soils). Soils from both topographic positions were pre-incubated under oxic or anoxic headspaces and then amended with a mixture of P (as orthophosphate) and carbon (C, as acetate, to maintain microbial activity) and incubated in the dark for 24 h. Phosphorus sorption to the mineral phase occurred on a time scale of seconds to minutes in valley and slope soils, reflecting strong abiotic P sorption capacity. Valley soils were characterized by inherently higher Fe(II) concentrations and lower respiration rates. Under anoxic headspaces, Fe(II) concentrations increased 3-to 5-fold in the both soils. Soil respiration and microbial P utilization declined significantly in both soils under anoxic conditions, regardless of Fe(II) concentrations. Microbial P concentrations were highest when slope soils were incubated under an oxic headspace, despite the high P sorption under these conditions. Our results suggest that microbial P utilization is indirectly limited by low O2 availability and that microbes are able to effectively compete with minerals for P under Fe-oxidizing conditions. These results emphasize the central role of soil microorganisms in regulating P availability, even in the presence of strong abiotic sorption capacity. Full article
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Open AccessArticle Non-Flat Earth Recalibrated for Terrain and Topsoil
Received: 24 September 2018 / Revised: 16 November 2018 / Accepted: 19 November 2018 / Published: 26 November 2018
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Abstract
Earth’s land surface is raised from conventionally flat 15 Gha to >64 Gha accounting for hilly slope undulation and topsoil relief detail. Three main aspects are: topography, rugosity/tortuosity, and micro-relief/porosity of ice/vegetation-free ground. Recalibration arises from four approaches: First, direct empirical estimates of
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Earth’s land surface is raised from conventionally flat 15 Gha to >64 Gha accounting for hilly slope undulation and topsoil relief detail. Three main aspects are: topography, rugosity/tortuosity, and micro-relief/porosity of ice/vegetation-free ground. Recalibration arises from four approaches: First, direct empirical estimates of compiled satellite/LiDAR data means of +2.5–26% surface progressively overlain by +94% at cm2 scale for soil ruggedness then +108% for mm2 micro-relief; Second, from digital elevation models with thrice 1.6–2.0 times flat areas; Third, by ‘reverse engineering’ global soil bulk densities and carbon reserves requiring ×4–6 land. Finally, a Fermi estimation doubles the Earth’s surface—as exposed to Sun, air and rain—conveniently set at 100 Gha (with 64 Gha land:36 Gha ocean). Soil organic carbon (SOC) thereby grows to 8580 Gt mainly in SOM-humus with its biotic complexity plus roots, Vesicular-Arbuscular Mycorrhiza (VAM-fungi), leaf-litter and earthworms itself totaling 17,810 Gt. Although four to six times IPCC’s or NASA/NOAA’s calculated 1500–2300 Gt SOC, this is likely an underestimation. Global biomass and biodiversity are at least doubled (×2–3.5) and net primary productivity (NPP) increases to >270 Gt C yr−1 due to terrain. Rationale for a ‘Soil Ecology Institute’ gains ground. Full article
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Open AccessArticle Nitrogen Mineralization and Microbial Biomass Dynamics in Different Tropical Soils Amended with Contrasting Organic Resources
Received: 21 September 2018 / Revised: 15 November 2018 / Accepted: 20 November 2018 / Published: 23 November 2018
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Abstract
The use of location-specific and underutilized organic residues (OR) as soil amendments in small-holder agro-ecosystems is promising. Six ORs (Leucaena leucocephala, Centrosema pubescens, Gliricidia sepium, Pueraria phaseoloides, Azadirachta indica, and Theobroma cacao) were amended to three
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The use of location-specific and underutilized organic residues (OR) as soil amendments in small-holder agro-ecosystems is promising. Six ORs (Leucaena leucocephala, Centrosema pubescens, Gliricidia sepium, Pueraria phaseoloides, Azadirachta indica, and Theobroma cacao) were amended to three tropical soils each at 24 mg g−1 dry soil in 120-day incubation study to estimate their nitrogen (N) mineralization and microbial biomass carbon (C) dynamics. Inorganic N contents varied among ORs, soil type and incubation days. Regardless of soil type, Gliricidia had the highest inorganic N among the studied ORs. Mineralization rate of 1.4 to 1.5 mg N kg−1 soil day−1 was observed for Lego and Tec soils, respectively, and was twice higher than Nya soil. However, Nya soil released higher inorganic N than Tec and Lego soils, implying high N mineralization efficiency in the former. Consistent soil pH increase was respectively observed for Theobroma and Pueraria treatments in all soils. Moreover, Theobroma and Pueraria amendments showed the highest soil microbial biomass C (MBC) at the end of the incubation. The assessed soil properties likely affected by the dominant edaphic factors and management influenced differences in MBC and dissolved organic carbon (DOC) while OR quality indices controlled N mineralization. Thus, we conclude that soil properties and OR type are important factors for optimal utilization of organic resources. Full article
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Open AccessFeature PaperArticle Nitrogen Fertilization Reduces the Capacity of Soils to Take up Atmospheric Carbonyl Sulphide
Received: 28 August 2018 / Revised: 10 November 2018 / Accepted: 13 November 2018 / Published: 15 November 2018
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Abstract
Soils are an important carbonyl sulphide (COS) sink. However, they can also act as sources of COS to the atmosphere. Here we demonstrate that variability in the soil COS sink and source strength is strongly linked to the available soil inorganic nitrogen (N)
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Soils are an important carbonyl sulphide (COS) sink. However, they can also act as sources of COS to the atmosphere. Here we demonstrate that variability in the soil COS sink and source strength is strongly linked to the available soil inorganic nitrogen (N) content across a diverse range of biomes in Europe. We revealed in controlled laboratory experiments that a one-off addition of ammonium nitrate systematically decreased the COS uptake rate whilst simultaneously increasing the COS production rate of soils from boreal and temperate sites in Europe. Furthermore, we found strong links between variations in the two gross COS fluxes, microbial biomass, and nitrate and ammonium contents, providing new insights into the mechanisms involved. Our findings provide evidence for how the soil–atmosphere exchange of COS is likely to vary spatially and temporally, a necessary step for constraining the role of soils and land use in the COS mass budget. Full article
(This article belongs to the Special Issue Formation and Fluxes of Soil Trace Gases)
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Open AccessArticle Salt Content of Dairy Farm Effluents as an Indicator of Salinization Risk to Soils
Received: 21 October 2018 / Revised: 4 November 2018 / Accepted: 6 November 2018 / Published: 8 November 2018
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Abstract
Water used for irrigation is a leading source of induced salinity in semiarid areas. Within the Irrigation District 005 in northern Mexico, there are more than 100 dairy farms housing over 72,000 dairy cows, 74% of which are concentrated in approximately 30 intensive-operation
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Water used for irrigation is a leading source of induced salinity in semiarid areas. Within the Irrigation District 005 in northern Mexico, there are more than 100 dairy farms housing over 72,000 dairy cows, 74% of which are concentrated in approximately 30 intensive-operation farms. Dairy farm effluents (DFE) and manure are collected and stored temporarily until they are applied to the land to fertilize pasture and other crops. DFE vary in salt content, depending on specific farm operations. The risk of soil salinization by DFE was estimated by measuring electrical conductivity (EC) of both well water and DFE, and comparing these values with 2.0 mS cm−1, a Mexican guideline for wastewater used in agriculture. Half of the effluents exceeded the EC limit, with values as high as 12.4 mS cm−1, whereas a few exceeded the EC limit in both well and effluent water. The generation of salt and its passing into soils expose a potential for soil salinization, if preventive measures are not taken. A salt load map was created that depicted the areas at higher risk of salinization. The simple technique utilized here can be applied in estimating salinization potential in areas where monitoring of soils, irrigation drains, and shallow groundwater is infrequent. Full article
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Open AccessArticle Microbial Population Dynamics and the Role of Sulfate Reducing Bacteria Genes in Stabilizing Pb, Zn, and Cd in the Terrestrial Subsurface
Received: 20 August 2018 / Revised: 16 October 2018 / Accepted: 31 October 2018 / Published: 3 November 2018
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Abstract
Milling and mining metal ores are major sources of toxic metals contamination. The Spring River and its tributaries in southeast Kansas are contaminated with Pb, Zn, and Cd because of 120 years of mining activities. Trace metal transformations and cycling in mine waste
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Milling and mining metal ores are major sources of toxic metals contamination. The Spring River and its tributaries in southeast Kansas are contaminated with Pb, Zn, and Cd because of 120 years of mining activities. Trace metal transformations and cycling in mine waste materials greatly influence their mobility and toxicity and they affect both plant productivity and human health. It has been hypothesized that under reduced conditions in sulfate-rich environments, these metals can be transformed into their sulfide forms, thus limiting mobility and toxicity. We studied biogeochemical transformations of Pb, Zn, and Cd in flooded subsurface mine waste materials, natural or treated with organic carbon (OC), and/or sulfur (S), by combining advanced microbiological and X-ray spectroscopic techniques to determine the effects of treatments on the microbial community structure and identify the dominant functional genes that are involved in the biogeochemical transformations, especially metal sulfide formation over time. Samples collected from medium-, and long-term submerged columns were used for microarray analysis via functional gene array (GeoChip 4.2). The total number of detected gene abundance decreased under long-term submergence, but major functional genes abundance was enhanced with OC-plus-S treatment. The microbial community exhibited a substantial change in structure in response to OC and S addition. Sulfate-reducing bacteria genes dsrA/B were identified as key players in metal sulfide formation via dissimilatory sulfate reduction. Uniqueness of this study is that microbial analyses presented here in detail are in agreement with molecular-scale synchrotron-based X-ray data, supporting that OC-plus-S treatment would be a promising strategy for reducing metal toxicity in mine waste materials in the subsurface environment. Full article
(This article belongs to the Special Issue Soil Processes Controlling Contaminant Dynamics)
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Open AccessArticle Hot Spots and Hot Moments of Soil Moisture Explain Fluctuations in Iron and Carbon Cycling in a Humid Tropical Forest Soil
Received: 30 September 2018 / Revised: 26 October 2018 / Accepted: 27 October 2018 / Published: 1 November 2018
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Abstract
Soils from humid forests undergo spatial and temporal variations in moisture and oxygen (O2) in response to rainfall, and induce changes in iron (Fe) and carbon (C) biogeochemistry. We hypothesized that high rainfall periods stimulate Fe and C cycling, with the
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Soils from humid forests undergo spatial and temporal variations in moisture and oxygen (O2) in response to rainfall, and induce changes in iron (Fe) and carbon (C) biogeochemistry. We hypothesized that high rainfall periods stimulate Fe and C cycling, with the greatest effects in areas of high soil moisture. To test this, we measured Fe and C cycling across three catenas at valley, slope, and ridge positions every two days for a two-month period in a rainforest in Puerto Rico. Over 12 days without rain, soil moisture, FeII, rapidly reducible Fe oxides (FeIIIRR), and dissolved organic C (DOC) declined, but Eh and O2 increased; conversely, during a 10-day period of intense rain (290 mm), we observed the opposite trends. Mixed-effects models suggest precipitation predicted soil moisture, soil redox potential (Eh), and O2, which in turn influenced Fe reduction/oxidation, C dissolution, and mineralization processes. The approximate turnover time for HCl-extractable FeII was four days for both production and consumption, and may be driven by fluctuations in FeIIIRR, which ranged from 42% to 100% of citrate–ascorbate-extractable FeIII (short-range order (SRO)-FeIII) at a given site. Our results demonstrated that periods of high precipitation (hot moments) influenced Fe and C-cycling within day-to-week timescales, and were more pronounced in humid valleys (hot spots). Full article
(This article belongs to the Special Issue Iron and Manganese Biogeochemical Cycling in Soils)
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Open AccessArticle Products of Hexavalent Chromium Reduction by Green Rust Sodium Sulfate and Associated Reaction Mechanisms
Received: 28 September 2018 / Revised: 12 October 2018 / Accepted: 25 October 2018 / Published: 29 October 2018
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Abstract
The efficacy of in vitro Cr(VI) reduction by green rust sulfate suggests that this mineral is potentially useful for remediation of Cr-contaminated groundwater. Previous investigations studied this reaction but did not sufficiently characterize the intermediates and end products at chromate (CrO42−
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The efficacy of in vitro Cr(VI) reduction by green rust sulfate suggests that this mineral is potentially useful for remediation of Cr-contaminated groundwater. Previous investigations studied this reaction but did not sufficiently characterize the intermediates and end products at chromate (CrO42−) concentrations typical of contaminant plumes, hindering identification of the dominant reaction mechanisms under these conditions. In this study, batch reactions at varying chromate concentrations and suspension densities were performed and the intermediate and final products of this reaction were analyzed using X-ray absorption spectroscopy and electron microscopy. This reaction produces particles that maintain the initial hexagonal morphology of green rust but have been topotactically transformed into a poorly crystalline Fe(III) oxyhydroxysulfate and are coated by a Cr (oxy) hydroxide layer that results from chromate reduction at the surface. Recent studies of the behavior of Cr(III) (oxy) hydroxides in soils have revealed that reductive transformation of CrO42− is reversible in the presence of Mn(IV) oxides, limiting the applicability of green rust for Cr remediation in some soils. The linkage of Cr redox speciation to existing Fe and Mn biogeochemical cycles in soils implies that modification of green rust particles to produce an insoluble, Cr(III)-bearing Fe oxide product may increase the efficacy of this technique. Full article
(This article belongs to the Special Issue Iron and Manganese Biogeochemical Cycling in Soils)
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Open AccessArticle Assessment of Soil Fertility under Different Land-Use Systems in Dhading District of Nepal
Received: 25 September 2018 / Revised: 16 October 2018 / Accepted: 24 October 2018 / Published: 29 October 2018
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Abstract
Unscientific land use and cropping techniques have led high soil erosion and degradation of soil quality in the mid-hills of Nepal. To understand the effects of land use systems for selected soil chemical properties in mid-hills, composite soil samples at 0 cm to
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Unscientific land use and cropping techniques have led high soil erosion and degradation of soil quality in the mid-hills of Nepal. To understand the effects of land use systems for selected soil chemical properties in mid-hills, composite soil samples at 0 cm to 20 cm depth were collected from five different land-use systems: Grassland, forest land, upland, lowland, and vegetable farms from Dhading district of Nepal in 2017. Soil samples were analyzed for soil fertility parameters: Soil pH, organic matter (OM), total nitrogen (N), available phosphorus (P), available potassium (K) and its effect due to different land use systems were compared. Results showed that soil pH was neutral in vegetable farms (6.61), whereas the rest of the land-use systems had acidic soils. Soil OM (3.55%) and N (0.18%) content was significantly higher in forest, but the lowest soil OM (1.26%) and N (0.06%) contents were recorded from upland and lowland farms, respectively. Available P was the highest in the vegetable farm (41.07 mg kg−1) and was the lowest in grazing land (2.89 mg kg−1). The upland farm had significantly higher P levels (39.89 mg kg−1) than the lowland farm (9.02 mg kg−1). Available K was the highest in the vegetable farm (130.2 mg kg−1) and lowest in grazing land (36.8 mg kg−1). These results indicated that the land under traditional mixed cereal-based farming had poor soil health compared with adjacent vegetable, grazing, and forest lands among the study area. The variations in soil fertility parameters suggest the immediate need for improvement in soil health of traditional farmlands. Full article
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Open AccessArticle Biologically Available Phosphorus in Biocrust-Dominated Soils of the Chihuahuan Desert
Received: 10 September 2018 / Revised: 27 September 2018 / Accepted: 3 October 2018 / Published: 10 October 2018
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Abstract
In desert soils, phosphorus (P) cycling is controlled by both geochemical and biological factors and remains less studied than nitrogen and carbon. We examined these P cycling factors in the context of biological soil crusts (biocrusts), which are important drivers of nutrient cycling
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In desert soils, phosphorus (P) cycling is controlled by both geochemical and biological factors and remains less studied than nitrogen and carbon. We examined these P cycling factors in the context of biological soil crusts (biocrusts), which are important drivers of nutrient cycling in drylands and have the potential to release bound labile P. We adopted the biologically-based P (BBP) method, which allows examination of biologically relevant P fractions. The BBP method incorporates four extractions: dilute calcium chloride (CaCl2), citric acid, phosphatase enzymes, and hydrochloric acid (HCl). We coupled the extractions with a 33P-labeled orthophosphate addition and incubation to assess the fate of freshly available phosphate (PO43−). Low P concentrations in the dilute CaCl2 extractions suggest that drylands lack accessible P in the soil solution, while higher amounts in the citric acid- and enzyme-extractable pools suggest that dryland microbes may acquire P through the release of organic acids and phosphatases. The addition of 33PO43− was, within 24 h, quickly adsorbed onto mineral surfaces or incorporated into hydrolysable organic compounds. Areas with biocrusts showed overall lower P concentrations across all four extractable pools. This suggests that biocrust organisms may prevent P adsorption onto mineral surfaces by incorporating P into their biomass. Overall, our results indicate that organisms may have to employ several viable strategies, including organic acid and enzyme production, to access P in dryland soils. Full article
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Open AccessArticle Estimating Soil Water Retention Curve by Inverse Modelling from Combination of In Situ Dynamic Soil Water Content and Soil Potential Data
Received: 11 August 2018 / Revised: 26 September 2018 / Accepted: 29 September 2018 / Published: 2 October 2018
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Abstract
Soil water retention curves (SWRCs) are crucial for characterizing soil moisture dynamics, and are particularly relevant in the context of irrigation management. Inverse modelling is one of the methods used to parameterize models representing these curves, which are closest to the field reality.
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Soil water retention curves (SWRCs) are crucial for characterizing soil moisture dynamics, and are particularly relevant in the context of irrigation management. Inverse modelling is one of the methods used to parameterize models representing these curves, which are closest to the field reality. The objective of this study is to estimate the soil hydraulic properties through inverse modelling using the HYDRUS-1D code based on soil moisture and potential data acquired in the field. The in situ SWRCs acquired every 30 min are based on simultaneous soil water content and soil water potential measurements with 10HS and MPS-2 sensors, respectively, in five experimental fields. The fields were planted with drip-irrigated lettuces from February to March 2016 in the Chrey Bak catchment located in the Tonlé Sap Lake region, Cambodia. After calibration of the van Genuchten soil water retention model parameters, we used them to evaluate the performance of HYDRUS-1D to predict soil moisture dynamics in the studied fields. Water flow was reasonably well reproduced in all sites covering a range of soil types (loamy sand and loamy soil) with root mean square errors ranging from 0.02 to 0.03 cm3 cm−3. Full article
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Open AccessArticle Quantitative Estimation of Fougerite Green Rust in Soils and Sediments by Citrate—Bicarbonate Kinetic Extractions
Received: 15 August 2018 / Revised: 17 September 2018 / Accepted: 27 September 2018 / Published: 1 October 2018
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Abstract
Fougerite (IMA 203-057), from green rust (GR) group, is difficult to quantify due to its reactivity and its small concentration in soils and sediments. Chemical extractions with citrate-bicarbonate (CB) reagent, in kinetic mode, can be used for a pre-diagnosis. Performed by steps (0,
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Fougerite (IMA 203-057), from green rust (GR) group, is difficult to quantify due to its reactivity and its small concentration in soils and sediments. Chemical extractions with citrate-bicarbonate (CB) reagent, in kinetic mode, can be used for a pre-diagnosis. Performed by steps (0, 1, 6, 48, 168 and 504 h), the proposed protocol was applied on samples from Gleysol of Fougère’s forest with mineralogical controls by Mössbauer and XRD (X-ray diffraction) after each step of extraction. In less than 6 h, the first fraction extracted is composed of 70% Si, 80% Al, 23% Fe and 80% Mg of total element extractable by CB and is ascribed to the “indefinable mineral mixture Si-Al-Fe” named by Tamm. Between 6 and 168 h, the second fraction extracted is composed of Fe and Mg with a constant mole ratio Fe/Mg equal to 10 and is ascribed to the fougerite-GR phase. Analysis of XRD pattern and of Mössbauer spectra confirms: (i) all the other mineral phases containing Al, Mg, Si were not dissolved by CB after 6 h; (ii) the CB treatment extracts fougerite-GR completely. The residual fraction is composed of components not dissolved by CB extraction. Thus, the selectivity of CB can be used to quantitatively estimate the presence of fougerite-GRs in soils and sediments. Full article
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