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• Soil properties influence long-term P transformations in fertospheres. • P pools were examined in-situ using soil solution, isotopic dilution, and XANES. • P precipitation and sorption reactions were ongoing at 12 months. • Potentially-plant available P was controlled by soil type. Identifying the effects of soil properties on the transformations of phosphorus (P) in highly concentrated fertiliser bands is a critical component in improving P fertiliser use efficiency, but little is known regarding the kinetics of P transformation, including over longer periods of time that are relevant to multiple cropping seasons. In the present study, we utilised a 12-month soil-fertiliser incubation experiment to simulate deep-placed banded P fertilisers, investigating three contrasting soils from Australia, three P sources, and the co-application of potassium (K). Overall, concentrations of soil solution P in the fertosphere decreased substantially over time regardless of soil type and P form. In the Ultisol and Vertisol, this progressive decrease was due to the precipitation of aluminium-phosphates (and, to a lesser extent, magnesium-phosphates) as well as ongoing sorption reactions, whilst in the alkaline Alfisol this progressive decrease was due to precipitation of calcium-phosphates. Despite P precipitation and sorption reactions, concentrations of potentially-plant available P measured by isotopically exchangeable P within 24 h (Ee24h) remained relatively constant between 1 and 12 months for the Ultisol and Vertisol, with this indicating that the ongoing sorption reactions of P in these fertospheres of non-calcareous soils may not necessarily decrease plant availability over time. We also found that co-application of K resulted in further reduction in P availability in all three soils and was mediated by K displacement of other exchangeable cations, thereby increasing precipitation reactions with dominant minerals. To improve P use efficiency in semi-arid cropping environments, optimising P form as a function of soil type is critical, especially for systems that are reliant on subsoil moisture and where highly concentrated fertiliser bands are applied infrequently to supply P over multiple growing seasons. [ABSTRACT FROM AUTHOR]

The COVID-19 pandemic caused major disruptions worldwide to teaching and learning activities across the education sector. We investigated the impact of COVID-19 on student engagement and performance in a third-year undergraduate science course by comparing student activity during emergency online delivery and traditional mode delivery. We found that the rapid transition to fully online teaching without any physical face-to-face teaching caused by COVID-19 resulted in learning resources being accessed at a slower rate throughout the semester. Student engagement decreased as evidenced by lower attendance at tutorials, despite this being the only virtual face-to-face activity available to students in this course. Thus, despite the smooth transition to fully online mode, it was not sufficient to prevent a decrease in student activity and participation in the online environment, and we observed a downward spiral in student engagement and motivation. Results indicate the importance of providing structure and teacher-student-peer interaction in online delivery modes. [ABSTRACT FROM AUTHOR]

Iron (Fe) plays an important role within environmental systems. Synchrotron‐based X‐ray approaches, including X‐ray absorption spectroscopy (XAS), provide powerful tools for in situ analyses of Fe speciation, but beam damage during analysis may alter Fe speciation during its measurement. XAS was used to examine whether experimental conditions affect the analysis of Fe speciation in plant tissues. Even when analyzed in a cryostat at 12 K, it was found that FeIII can rapidly (within 0.5–1 min) photoreduce to FeII, although the magnitude of photoreduction varied depending upon the hydration of the sample, the coordination chemistry of the Fe, as well as other properties. For example, photoreduction of FeIII was considerably higher for aqueous standard compounds than for hydrated plant‐root tissues. The use of freeze‐dried samples in the cryostat (12 K) markedly reduced the magnitude of this FeIII photoreduction, and there was no evidence that the freeze‐drying process itself resulted in experimental artefacts under the current experimental conditions, such as through the oxidation of FeII, although some comparatively small differences were observed when comparing spectra of hydrated and freeze‐dried FeII compounds. The results of this study have demonstrated that FeIII photoreduction can occur during X‐ray analysis, and provides suitable conditions to preserve Fe speciation to minimize the extent of beam damage when analyzing environmental samples. All studies utilizing XAS are encouraged to include a preliminary experiment to determine if beam damage is occurring, and, where appropriate, to take the necessary steps (such as freeze drying) to overcome these issues. [ABSTRACT FROM AUTHOR]

Abstract Irrigation with alkaline and sodic groundwater, including water extracted from coal seams (CS-water), has the potential to cause environmental harm through the degradation of soil structure. This study evaluated the effectiveness of soil amendment prior to irrigation with CS-water (pH 9.39, sodium adsorption ratio [SAR] 83) on the soil chemical properties of two soils, a Red Ultisol and a Red-Brown Ultisol. Repacked columns of soil were amended with either gypsum alone or with gypsum combined with elemental sulfur (S). These amendments were either surface applied or incorporated to a depth of 20 cm, with columns irrigated with the equivalent of 36 ML ha−1 CS-water. In the Red Ultisol, soil solution SAR was maintained in the desired range of 10–25 in amended soils, which was generally similar to that achieved in control soils irrigated with pre-treated CS-water where the alkalinity had been neutralized with H 2 SO 4 and sodicity ameliorated by dosing with micronized gypsum prior to irrigation. In the Red-Brown Ultisol soil solution SAR values were higher. Surface-application of amendments (soil solution SAR values of 10–40) was more effective than incorporation (soil solution values of 80–120). Soil solution pH decreased with gypsum and S amendment of the Red Ultisol by up to ≤2.0 pH units, despite irrigation with alkaline CS-water. In contrast, soil solution pH was more difficult to maintain in the Red-Brown Ultisol, with pH increasing to ≈8.0 in most treatments. The results demonstrate that alkaline and sodic CS-water can be chemically ameliorated successfully within the soil profile by the addition of gypsum and S amendments, although the efficacy of chemical amendment depends upon the method of incorporation and soil properties. Highlights • Soil amendment with gypsum and sulfur can ameliorate irrigated coal seam-water. • Sodium adsorption ratios and pH was maintained under coal seam-water irrigation. • Surface-application of amendments was more effective than incorporation. • Efficacy of soil amendment also depends on the soil properties. [ABSTRACT FROM AUTHOR]

Abstract: Quantifying student engagement with online learning resources on virtual learning environments such as BlackBoard is important in understanding how these technologies enhance the student learning. In the present study, it was examined when, and how often, first‐year students accessed lecture recordings, lecture slides, and lecture notes via BlackBoard, in an introductory natural resource course taken by both on campus students and remote students. The findings demonstrated that lecture recordings were not well utilized by students—although only 58% of on campus students attended face‐to‐face lectures, less than 15% of absent students downloaded the missed lecture. Overall, more students downloaded lecture slides (an average of 63% per week) than notes (38%) or recordings (16%). Indeed, the average student downloaded only 1.1 types of the 3 online resources (recordings, slides, and notes) that were available each week, with 5.7% of students downloading all 3 types of resources, 23% downloading 2 types of resources, 42% downloading only 1 type of resource, and 29% downloading none of the 3 types of resources. Finally, remote students were more likely to download lecture notes and recordings than were on campus students. The information presented here is important in understanding student behaviour and engagement. [ABSTRACT FROM AUTHOR]

Summary: Soil acidity and waterlogging increase manganese (Mn) in leaf tissues to potentially toxic concentrations, an effect reportedly alleviated by increased silicon (Si) and phosphorus (P) supply. Effects of Si and P on Mn toxicity were studied in four plant species using synchrotron‐based micro X‐ray fluorescence (μ‐XRF) and nanoscale secondary ion mass spectrometry (NanoSIMS) to determine Mn distribution in leaf tissues and using synchrotron‐based X‐ray absorption spectroscopy (XAS) to measure Mn speciation in leaves, stems and roots. A concentration of 30 μM Mn in solution was toxic to cowpea and soybean, with 400 μM Mn toxic to sunflower but not white lupin. Unexpectedly, μ‐XRF analysis revealed that 1.4 mM Si in solution decreased Mn toxicity symptoms through increased Mn localization in leaf tissues. NanoSIMS showed Mn and Si co‐localized in the apoplast of soybean epidermal cells and basal cells of sunflower trichomes. Concomitantly, added Si decreased oxidation of Mn(II) to Mn(III) and Mn(IV). An increase from 5 to 50 μM P in solution changed some Mn toxicity symptoms but had little effect on Mn distribution or speciation. We conclude that Si increases localized apoplastic sorption of Mn in cowpea, soybean and sunflower leaves thereby decreasing free Mn2+ accumulation in the apoplast or cytoplasm. [ABSTRACT FROM AUTHOR]

Aluminium (Al) toxicity adversely impacts plant productivity in acid soils by restricting root growth and although several mechanisms are involved the physiological basis of decreased root elongation remains unclear. Understanding the primary mechanisms of Al rhizotoxicity is hindered due to the rapid effects of soluble Al on root growth and the close proximity of many cellular components within the cell wall, plasma membrane, cytosol and nucleus with which Al may react. To overcome some of these difficulties, we report on a novel method for investigating Al interactions with Komagataeibacter xylinus bacterial cellulose (BC)-pectin composites as cell wall analogues. The growth of K. xylinus in the presence of various plant cell wall polysaccharides, such as pectin, has provided a unique in vitro model system with which to investigate the interactions of Al with plant cell wall polysaccharides. The BC-pectin composites reacted in a similar way with Al as do plant cell walls, providing insights into the effects of Al on the mechanical properties of the BC-pectin composites as cell wall analogues. Our findings indicated that there were no significant effects of Al (4-160 μM) on the tensile stress, tensile strain or Young's modulus of the composites. This finding was consistent with cellulose, not pectin, being the major load bearing component in BC-pectin composites, as is also the case in plant cell walls. [ABSTRACT FROM AUTHOR]

Abstract: Rheology of Ca-pectate gels is widely studied, but the behaviour of pectate gels formed by Cu, Al and La is largely unknown. It is well known that gel strength increases with increasing Ca concentration, and it is hypothesised that this would also be the case for other cations. Pectins are a critical component of plant cell walls, imparting various physicochemical properties. Furthermore, the mechanism of metal toxicity in plants is hypothesised to be, in the short term, related to metal interactions with cell wall pectin. This study investigated the influence of Ca, Cu, Al and La ion concentrations at pH 4 on the storage modulus as a function of frequency for metal-pectin gels prepared from pectin (1%) with a degree of esterification of 30%. Gels were formed in situ over 6d in metal chloride solution adjusted daily to pH 4. Cation concentration was varied to develop a relationship between gel strength and cation concentration. At similar levels of cation saturation, gel strength increased in the order of La

Low concentrations of some trace metals markedly reduce root elongation rate and cause ruptures to root rhizodermal and outer cortical cells in the elongation zone. The interactions between the trace metals and plant components responsible for these effects are not well understood but may be linked to changes in water uptake, cell turgor and cell wall extensibility. An experiment was conducted to investigate the effects of Al, La, Cu, Gd, Sc and Ru on the saturated hydraulic conductivity of bacterial cellulose (BC)–pectin composites, used as plant cell wall analogs. Hydraulic conductivity was reduced to ≈30% of the initial flow rate by 39 µ M Al and 0.6 µ M Cu, ≈40% by 4.6 µ M La, 3 µ M Sc and 4.4 µ M Ru and ≈55% by 3.4 µ M Gd. Scanning electron microscopy (SEM) revealed changes in the ultrastructure of the composites. The results suggest that trace metal binding decreases the hydraulic conductivity through changes in pectin porosity. The experiment illustrates the importance of metal interactions with pectin, and the implications of such an interaction in plant metal toxicity and in normal cell wall processes. [ABSTRACT FROM AUTHOR]

The concentration, chemical speciation, and spatial distribution of essential and toxic mineral elements in cereal seeds have important implications for human health. To identify genes responsible for element uptake, translocation, and storage, high-throughput phenotyping methods are needed to visualize element distribution and concentration in seeds. Here, we used X-ray fluorescence microscopy (µ-XRF) as a method for rapid and high-throughput phenotyping of seed libraries and developed an ImageJ-based pipeline to analyze the spatial distribution of elements. Using this method, we nondestructively scanned 4,190 ethyl methanesulfonate (EMS)-mutagenized M1 rice (Oryza sativa) seeds and 533 diverse rice accessions in a genome-wide association study (GWAS) panel to simultaneously measure concentrations and spatial distribution of elements in the embryo, endosperm, and aleurone layer. A total of 692 putative mutants and 65 loci associated with the spatial distribution of elements in rice seed were identified. This powerful method provides a basis for investigating the genetics and molecular mechanisms controlling the accumulation and spatial variations of mineral elements in plant seeds. [ABSTRACT FROM AUTHOR]

Soil aggregation is critical for high-clay Vertisols to facilitate water movement, aeration, nutrient cycling, microbial activity, and hence soil productivity. Both abiotic and biotic factors affect aggregation, although the effect of biotic factors on aggregation remains unclear in subtropical Vertisols managed under no-till (NT), stubble retained (SR) and N fertilisation practice. Using a 54-years old cereal cropped NT experiment in subtropical semi-arid region, we examined the biotic effects on soil aggregate-size distribution in a smectitic Vertisol (65 % clay) at 0–0.1 m depth. The experiment consisted of NT versus conventional till (CT), SR versus stubble burned (SB), and N fertiliser rates (0, 30, and 90 kg N ha−1; N0, N30, and N90, respectively) arranged in a randomised complete block design, with four replications (48 plots). NT generally had no significant effect on biotic factors and aggregate mean weight diameter (MWD). SR had significantly higher microbial activity (as assessed by cumulative CO 2 production over 140 days), C-acquiring and N-acquiring enzyme activities (β-glucosidase, cellobiohydrolase, N -acetyl-β-D-glucosaminidase), easily extractable and total glomalin-related proteins, microbial biomass C, large- (>2 mm) and medium aggregates (1–2 mm), and MWD (1.23 ± 0.06 mm in SR and 1.02 ± 0.07 mm in SB (P < 0.02)). In addition, N fertiliser rates had significantly higher soil organic C and mineral associated organic N but lower C:N ratio. Thus, soil organic C and N, microbial biomass C, glomalin, and microbial and enzyme activities enhanced by SR, and lower C:N ratio of the organic matter following N fertilisation improved MWD in this subtropical Vertisol. • Stubble retention significantly increased microbial and enzyme activities, and aggregate MWD. • Stubble and N fertilisation significantly increased glomalin-related proteins. • Glomalin-related proteins were significantly correlated with MWD. • Soil organic C and N, and MBC were significantly correlated with MWD. • Vertisols require stubble retention and N fertilisation for improved aggregation. [ABSTRACT FROM AUTHOR]

Soil provides multiple, diverse functions, with these underpinning both planetary and human health. For planetary health, soil contributes to multiple critical processes, including through biomass production, by regulating the carbon pool, providing a habitat for 25% of global biodiversity, cycling the nutrients upon which terrestrial systems depend, and cycling water. Soil also underpins human health; humans use soil to provide 98.8% of our food and sustain our nutrition, regulate pathogens, and supply medicines. However, humans have tended to focus on soil almost solely for producing biomass (food, fiber, and energy) through intensive agriculture, and this narrow focus now causes rapid soil degradation, including through loss of soil organic matter, erosion, and salinization. This degradation directly harms planetary health and reduces the ability of soil to support health of future human generations. We argue that a healthy soil is a soil that is multifunctional and is capable of underpinning human and planetary health. Using this definition, a broad conceptual framework is provided for quantifying soil health, with such an approach enabling a shift in the way that we think about, plan, and manage systems to ensure ongoing planetary and human health. [ABSTRACT FROM AUTHOR]

Many oil and gas developments will soon be decommissioned and, knowledge on the accumulation of mercury (Hg), throughout offshore infrastructure is limited. Any release of Hg could have a detrimental impact on marine ecosystems. To bridge this knowledge gap, a fractionation approach was taken on steel samples exposed to Hg0 and H 2 S, separating Hg compounds removed from the surface into polar, non-polar and insoluble fractions. Hg0 reacted on corroded surfaces to form several compounds, over 50 % of which were removed by seawater. This suggests that pipelines on the seabed could release a dramatic amount of Hg into the sea if they are left in place. Furthermore, a Cu-Hg amalgam, was identified to be a dominant species, by a combination of XFM, XANES and LA-ICP-TOFMS. Seawater-soluble and amalgam-bound Hg were present regardless of co-exposure to H 2 S. When H 2 S was present Hg nanoparticles accounted for up to 1 % of the total Hg on the steel. This investigation has shown that the Hg speciation on the surfaces of pipelines is complex and future decommissioning strategies should consider a range of Hg species beyond only Hg0 and metacinnabar (β-HgS), all of which could interact with biota and impact Hg biomagnification through the marine the food web. [Display omitted] • Decommissioned pipelines, left in-place, will contain many Hg species which may enter the sea. • Seawater-soluble Hg species were found in high concentrations on Hg contaminated steels. • Hg nanoparticles can be formed when contaminated pipeline material encounters seawater. • Cu-Hg amalgams may act as a source for Hg0 in the deep sea if pipelines are left in-place. [ABSTRACT FROM AUTHOR]

Species' response to rapid climate change can be measured through shifts in timing of recurring biological events, known as phenology. The Gulf of Maine is one of the most rapidly warming regions of the ocean, and thus an ideal system to study phenological and biological responses to climate change. A better understanding of climate‐induced changes in phenology is needed to effectively and adaptively manage human‐wildlife conflicts. Using data from a 20+ year marine mammal observation program, we tested the hypothesis that the phenology of large whale habitat use in Cape Cod Bay has changed and is related to regional‐scale shifts in the thermal onset of spring. We used a multi‐season occupancy model to measure phenological shifts and evaluate trends in the date of peak habitat use for North Atlantic right (Eubalaena glacialis), humpback (Megaptera novaeangliae), and fin (Balaenoptera physalus) whales. The date of peak habitat use shifted by +18.1 days (0.90 days/year) for right whales and +19.1 days (0.96 days/year) for humpback whales. We then evaluated interannual variability in peak habitat use relative to thermal spring transition dates (STD), and hypothesized that right whales, as planktivorous specialist feeders, would exhibit a stronger response to thermal phenology than fin and humpback whales, which are more generalist piscivorous feeders. There was a significant negative effect of western region STD on right whale habitat use, and a significant positive effect of eastern region STD on fin whale habitat use indicating differential responses to spatial seasonal conditions. Protections for threatened and endangered whales have been designed to align with expected phenology of habitat use. Our results show that whales are becoming mismatched with static seasonal management measures through shifts in their timing of habitat use, and they suggest that effective management strategies may need to alter protections as species adapt to climate change. [ABSTRACT FROM AUTHOR]

The phytotoxicity of Mn is important globally due to its increased solubility in acid or waterlogged soils. Short-term (≤24 h) solution culture studies with 150 µ M Mn were conducted to investigate the in situ distribution and speciation of Mn in apical tissues of hydrated roots of cowpea [Vigna unguiculata (L.) Walp. cv. Red Caloona] using synchrotron-based techniques. Accumulation of Mn was rapid; exposure to 150 µ M Mn for only 5 min resulting in substantial Mn accumulation in the root cap and associated mucigel. The highest tissue concentrations of Mn were in the root cap, with linear combination fitting of the data suggesting that ≥80% of this Mn(II) was associated with citrate. Interestingly, although the primary site of Mn toxicity is typically the shoots, concentrations of Mn in the stele of the root were not noticeably higher than in the surrounding cortical tissues in the short-term (≤24 h). The data provided here from the in situ analyses of hydrated roots exposed to excess Mn are, to our knowledge, the first of this type to be reported for Mn and provide important information regarding plant responses to high Mn in the rooting environment. [ABSTRACT FROM AUTHOR]

Metal phytotoxicity is important in both environmental and agricultural systems. A solution culture study examined the toxicity of 26 metals to roots of cowpea (Vigna unguiculata (L.) Walp.); new data were collected for 15 metals and published data for 11 metals. Metal toxicity, calculated as causing a 50% reduction in root elongation rate, was determined based on either the measured concentration in the bulk solution (EC50b) or the calculated activity at the outer surface of the plasma membrane (EA50o0 ). The EC50b values ranged from 0.007 mM for Tl to 98,000mM for K, with the order of rhizotoxicity to cowpea, from most to least toxic, beingTl¼Ag>Cu>Hg¼Ni¼Ga¼Ru¼In Sc¼Cd¼Gd¼La¼Co¼Cs¼Pb>Zn¼Al¼H>Mn>Ba¼Sr>Li>Mg>Ca¼ b>Zn¼Al¼H>Mn>Ba¼Sr>Li>Mg>Ca¼ Na>K. The EA50o0 values suggest that the binding of metals to hard ligands is an important, general, nonspecific mechanism of toxicity, a hypothesis supported by the similar toxicity symptoms to roots of cowpea by many metals. However, additional mechanisms, such as strong binding to soft ligands, substantially increase rhizotoxicity of some metals, especially Tl, Ag, and Cs. Besides direct toxic effects, osmotic effects or reduced activity of Ca2þ at the outer surface of the root plasma membrane (and resultant Ca deficiency) may decrease short-term root growth. [ABSTRACT FROM AUTHOR]

Aluminium (Al) K‐ and L‐edge X‐ray absorption near‐edge structure (XANES) has been used to examine Al speciation in minerals but it remains unclear whether it is suitable for in situ analyses of Al speciation within plants. The XANES analyses for nine standard compounds and root tissues from soybean (Glycine max), buckwheat (Fagopyrum tataricum), and Arabidopsis (Arabidopsis thaliana) were conducted in situ. It was found that K‐edge XANES is suitable for differentiating between tetrahedral coordination (peak of 1566 eV) and octahedral coordination (peak of 1568 to 1571 eV) Al, but not suitable for separating Al binding to some of the common physiologically relevant compounds in plant tissues. The Al L‐edge XANES, which is more sensitive to changes in the chemical environment, was then examined. However, the poorer detection limit for analyses prevented differentiation of the Al forms in the plant tissues because of their comparatively low Al concentration. Where forms of Al differ markedly, K‐edge analyses are likely to be of value for the examination of Al speciation in plant tissues. However, the apparent inability of Al K‐edge XANES to differentiate between some of the physiologically relevant forms of Al may potentially limit its application within plant tissues, as does the poorer sensitivity at the L‐edge. [ABSTRACT FROM AUTHOR]

Highlights • A novel method using synchrotron-based micro X-ray fluorescence. • Provides laterally-resolved, multi-element, kinetic, in vivo analyses of plants. • Allows a wide range of studies for the examination of plant responses. Abstract The distribution and concentration of nutrients and contaminants affect almost every metabolic process in plants but analytical limitations have hindered the determination of microscopic changes over time within living plant tissues. We developed a novel method using synchrotron-based micro X-ray fluorescence (μ-XRF) that, for the first time, allows quantification of the spatial and temporal changes of multiple elements in the same area of living leaves. The utility of this approach was tested by examining changes over 48 h in unifoliate leaves of 7-d-old cowpea (Vigna unguiculata) plants simultaneously at 0.2 and 30 μM Mn in nutrient solution, with 30 μM Mn known to be toxic to cowpea and cause the formation of Mn-dense lesions. The fast X-ray fluorescence detector system reduced dwell on living leaf samples. This produced no evidence of tissue damage through repeated μ-XRF scanning, thereby overcoming previously noted experimental artifacts. This permitted, for the first time, visual and quantitative assessments of spatial and temporal changes in nutrient concentrations. By focusing on changes in Mn status, this study illustrated extension of two-dimensional μ-XRF scans to a three-dimensional geometry of Mn kinetics in the same area of leaves. The multi-element potential of this method was exemplified through the measurement of distributions and concentrations of K, Ca, Fe, Cu, and Zn within living plant leaves. This novel method and accompanying information on changes in Mn distribution showed the potential for microscopic, time-resolved, in vivo examination of changes in elemental distribution. We consider that this method will be of benefit for a wide range of studies, including functional characterization of molecular biology, examining changes in the distribution of nutrients, and understanding the movement and toxicity of contaminants. [ABSTRACT FROM AUTHOR]

Knowledge of the distribution of selenium (Se) species within plant tissues will assist in understanding the mechanisms of Se uptake and translocation, but in situ analysis of fresh and highly hydrated plant tissues is challenging. Using synchrotron-based fluorescence X-ray absorption near-edge spectroscopy (XANES) imaging to provide laterally resolved data, the speciation of Se in fresh roots and leaves of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) supplied with 1 nM of either selenate or selenite was investigated. For plant roots exposed to selenate, the majority of the Se was efficiently converted to C-Se-C compounds (i.e. methylselenocysteine or selenomethionine) as selenate was transported radially through the root cylinder. Indeed, even in the rhizodermis which is exposed directly to the bulk solution, only 12-31% of the Se was present as uncomplexed selenate. The C-Se-C compounds were probably sequestered within the roots, whilst much of the remaining uncomplexed Se was translocated to the leaves--selenate accounting for 52-56% of the total Se in the leaves. In a similar manner, for plants exposed to selenite, the Se was efficiently converted to C-Se-C compounds within the roots, with only a small proportion of uncomplexed selenite observed within the outer root tissues. This resulted in a substantial decrease in translocation of Se from the roots to leaves of selenite-exposed plants. This study provides important information for understanding the mechanisms responsible for the uptake and subsequent transformation of Se in plants. [ABSTRACT FROM AUTHOR]

Despite the rhizotoxicity of aluminum (Al) being identified over 100 years ago, there is still no consensus regarding the mechanisms whereby root elongation rate is initially reduced in the approximately 40% of arable soils worldwide that are acidic. We used high-resolution kinematic analyses, molecular biology, rheology, and advanced imaging techniques to examine soybean (Glycine max) roots exposed to Al. Using this multidisciplinary approach, we have conclusively shown that the primary lesion of Al is apoplastic. In particular, it was found that 75 mM Al reduced root growth after only 5 min (or 30 min at 30 mM Al), with Al being toxic by binding to the walls of outer cells, which directly inhibited their loosening in the elongation zone. An alteration in the biosynthesis and distribution of ethylene and auxin was a second, slower effect, causing both a transient decrease in the rate of cell elongation after 1.5 h but also a longer term gradual reduction in the length of the elongation zone. These findings show the importance of focusing on traits related to cell wall composition as well as mechanisms involved in wall loosening to overcome the deleterious effects of soluble Al. [ABSTRACT FROM AUTHOR]

Saline-sodic soils are often too saline and alkaline for plant survival. These soils are prone to dispersing and eroding after high rainfall events when salinity is reduced before the sodicity. Cost-effective and water efficient methods are needed to leach salts while maintaining sufficient ionic strength of the soil solution. We tested the ability of gypsum, both alone and combined with elemental sulfur and organic matter to remediate the upper 15 cm of a strongly saline-sodic alkaline Vertosol when leached with deionised water in repacked columns. Prior to leaching, all amendment combinations reduced soil alkalinity by 80% and dispersion by 47% by displacing exchangeable sodium (Na). After leaching with 600 mm of deionised water, electrical conductivity of the soil solution (EC ss) decreased from an average of 38–4.8 dS m−1 at 8 cm depth. Importantly, structure was maintained in all amended soils, despite this decrease in EC ss. In contrast, for the control treatment, there was a concomitant loss of soil structural stability as EC ss decreased. This decrease in stability also occurred in the subsoils of all treatments (which were not amended) because the applied calcium (Ca) precipitated before it could be leached to remediate the deeper layers. This study demonstrated that it was critical to first apply amendments as deep in the soil profile as possible to prevent the development of a non-saline sodic soil. Leaching the soil with low ionic strength water removed excess soluble salts from the plant root zone. We estimated that > 300 mm of water (rainfall or irrigation) was required to leach through the root zone to ensure a suitable soil profile for establishing of salt tolerant pioneer species such as Rhodes grass (Chloris gayana Kunth). • Little is known about remediating saline-sodic soils in subtropical climates. • Applying gypsum initially increased salinity but reduced alkalinity and sodicity. • Adding organic matter and sulfur with gypsum did not improve soil remediation. • Infiltrating 600 mm deionised water decreased salinity by 87% at 8 cm depth. • A growth zone for a pioneer plant species was created with 300 mm infiltration. [ABSTRACT FROM AUTHOR]

Accumulation of arsenic (As) within plant tissues represents a human health risk, but there remains much to learn regarding the speciation of As within plants., We developed synchrotron-based fluorescence-X-ray absorption near-edge spectroscopy (fluorescence- XANES) imaging in hydrated and fresh plant tissues to provide laterally resolved data on the in situ speciation of As in roots of wheat ( Triticum aestivum) and rice ( Oryza sativa) exposed to 2 μM As(V) or As( III)., When exposed to As(V), the As was rapidly reduced to As( III) within the root, with As(V) calculated to be present only in the rhizodermis. However, no uncomplexed As( III) was detected in any root tissues, because of the efficient formation of the As( III)-thiol complex - this As species was calculated to account for all of the As in the cortex and stele. The observation that uncomplexed As( III) was below the detection limit in all root tissues explains why the transport of As to the shoots is low, given that uncomplexed As( III) is the major As species transported within the xylem and phloem., Using fluorescence- XANES imaging, we have provided in situ data showing the accumulation and transformation of As within hydrated and fresh root tissues. [ABSTRACT FROM AUTHOR]

Mechanisms whereby metal cations are toxic to plant roots remain largely unknown. Aluminum, for example, has been recognized as rhizotoxic for approximately 100 yr, but there is no consensus on its mode of action. The authors contend that the primary mechanism of rhizotoxicity of many metal cations is nonspecific and that the magnitude of toxic effects is positively related to the strength with which they bind to hard ligands, especially carboxylate ligands of the cell-wall pectic matrix. Specifically, the authors propose that metal cations have a common toxic mechanism through inhibiting the controlled relaxation of the cell wall as required for elongation. Metal cations such as Al3+ and Hg2+, which bind strongly to hard ligands, are toxic at relatively low concentrations because they bind strongly to the walls of cells in the rhizodermis and outer cortex of the root elongation zone with little movement into the inner tissues. In contrast, metal cations such as Ca2+, Na+, Mn2+, and Zn2+, which bind weakly to hard ligands, bind only weakly to the cell wall and move farther into the root cylinder. Only at high concentrations is their weak binding sufficient to inhibit the relaxation of the cell wall. Finally, different mechanisms would explain why certain metal cations (for example, Tl+, Ag+, Cs+, and Cu2+) are sometimes more toxic than expected through binding to hard ligands. The data presented in the present study demonstrate the importance of strength of binding to hard ligands in influencing a range of important physiological processes within roots through nonspecific mechanisms. Environ Toxicol Chem 2014;33:268-277. © 2013 SETAC [ABSTRACT FROM AUTHOR]

The increasing use of zinc oxide nanoparticles (ZnO-NPs) in various commercial products is prompting detailed investigation regarding the fate of these materials in the environment. There is, however, a lack of information comparing the transformation of ZnO-NPs with soluble Zn2+ in both soils and plants. Synchrotron-based techniques were used to examine the uptake and transformation of Zn in various tissues of cowpea (Vigna unguiculata (L.) Walp.) exposed to ZnO-NPs or ZnCL2 following growth in either solution or soil culture. In solution culture, soluble Zn (ZnCL2) was more toxic than the ZnO-NPs, although there was substantial accumulation of ZnO-NPs on the root surface. When grown in soil, however, there was no significant difference in plant growth and accumulation or speciation of Zn between soluble Zn and ZnO-NP treatments, indicating that the added ZnO-NPs underwent rapid dissolution following their entry into the soil. This was conhrmed by an incubation experiment with two soils, in which ZnO-NPs could not be detected after incubation for 1 h. The speciation of Zn was similar in shoot tissues for both soluble Zn and ZnO-NPs treatments and no upward translocation of ZnO-NPs from roots to shoots was observed in either solution or soil culture. Under the current experimental conditions, the similarity in uptake and toxicity ot Zn from ZnO-NPs and soluble Zn in soils indicates that the ZnO-NPs used in this study did not constitute nanospecific risks. [ABSTRACT FROM AUTHOR]

Abstract: Many metals and metalloids, jointly termed metal(loid)s, are toxic to plants even at low levels. This has limited the study of their uptake, distribution, and modes of action in plant roots grown at physiologically relevant concentrations. Synchrotron-based X-ray fluorescence microscopy was used to examine metal(loid)s in hydrated cowpea (Vigna unguiculata L.) roots exposed to Zn(II), Ni(II), Mn(II), Cu(II), Hg(II), Se(IV), Se(VI), As(III), or As(V). Development of a mathematical model enabled in situ quantitative determination of their distribution in root tissues. The binding strength of metals influenced the extent of their movement through the root cylinder, which influenced the toxic effects exerted—metals (e.g. Cu, Hg) that bind more strongly to hard ligands had high concentrations in the rhizodermis and caused this tissue to rupture, while other metals (e.g. Ni, Zn) moved further into the root cylinder and did not cause ruptures. When longitudinal distributions were examined, the highest Se concentration in roots exposed to Se(VI) was in the more proximal root tissues, suggesting that Se(VI) is readily loaded into the stele. This contrasted with other metal(loid)s (e.g. Mn, As), which accumulated in the apex. These differences in metal(loid) spatial distribution provide valuable quantitative data on metal(loid) physiology, including uptake, transport, and toxicity in plant roots. [Copyright &y& Elsevier]

The speciation and spatial distribution of selenium (Se) in hydrated plant tissues is not well understood. Using synchrotron-based x-ray absorption spectroscopy and x-ray fluorescence microscopy (two-dimensional scanning [and associated mathematical model] and computed tomography), the speciation and distribution of toxic Se were examined within hydrated roots of cowpea (Vigna unguiculata) exposed to either 20 µM selenite or selenate. Based upon bulk solution concentrations, selenate was 9-fold more toxic to the roots than selenite, most likely due to increased accumulation of organoselenium (e.g. selenomethionine) in selenate-treated roots. Specifically, uptake of selenate (probably by sulfate transporters) occurred at a much higher rate than for selenite (apparently by both passive diffusion and phosphate transporters), with bulk root tissue Se concentrations approximately 18-fold higher in the selenate treatment. Although the proportion of Se converted to organic forms was higher for selenite (100%) than for selenate (26%), the absolute concentration of organoselenium was actually approximately 5-fold higher for selenate-treated roots. In addition, the longitudinal and radial distribution of Se in roots differed markedly: the highest tissue concentrations were in the endodermis and cortex approximately 4 mm or more behind the apex when exposed to selenate but in the meristem (approximately 1 mm from the apex) when exposed to selenite. The examination of the distribution and speciation of Se in hydrated roots provides valuable data in understanding Se uptake, transport, and toxicity. [ABSTRACT FROM AUTHOR]

Arsenic (As) is considered to be the environmental contaminant of greatest concern due to its potential accumulation in the food chain and in humans. Using novel synchrotron-based x-ray fluorescence techniques (including sequential computed tomography), short-term solution culture studies were used to examine the spatial distribution of As in hydrated and fresh roots of cowpea (Vigna unguiculata 'Red Caloona') seedlings exposed to 4 or 20 µm arsenate [As(V)] or 4 or 20 µm arsenite. For plants exposed to As(V), the highest concentrations were observed internally at the root apex (meristem), with As also accumulating in the root border cells and at the endodermis. When exposed to arsenite, the endodermis was again a site of accumulation, although no As was observed in border cells. For As(V), subsequent transfer of seedlings to an As-free solution resulted in a decrease in tissue As concentrations, but growth did not improve. These data suggest that, under our experimental conditions, the accumulation of As causes permanent damage to the meristem. In addition, we suggest that root border cells possibly contribute to the plant's ability to tolerate excess As(V) by accumulating high levels of As and Limiting its movement into the root. [ABSTRACT FROM AUTHOR]

The phytotoxicity of trace metals is of global concern due to contamination of the landscape by human activities. Using synchrotron-based x-ray fluorescence microscopy and x-ray absorption spectroscopy, the distribution and speciation of copper (Cu), nickel (Ni), and zinc (Zn) was examined in situ using hydrated roots of cowpea (Vigna unguiculata) exposed to 1.5 µM Cu, 5 µM Ni, or 40 µM Zn for 1 to 24 h. After 24 h of exposure, most Cu was bound to polygalacturonic acid of the rhizodermis and outer cortex, suggesting that binding of Cu to walls of cells in the rhizodermis possibly contributes to the toxic effects of Cu. When exposed to Zn, cortical concentrations remained comparatively low with much of the Zn accumulating in the meristematic region and moving into the stele; approximately 60% to 85% of the total Zn stored as Zn phytate within 3 h of exposure. While Ni concentrations were high in both the cortex and meristem, concentrations in the stele were comparatively low. To our knowledge, this is the first report of the in situ distribution and speciation of Cu, Ni, and Zn in hydrated (and fresh) plant tissues, providing valuable information on the potential mechanisms by which they are toxic. [ABSTRACT FROM AUTHOR]

In subtropical regions, we have an incomplete understanding of how long-term tillage, stubble, and nitrogen (N) fertilizer management affects soil biological functioning. We examined a subtropical site managed for 50 years using varying tillage (conventional till (CT) and no-till (NT)), stubble management (stubble burning (SB) and stubble retention (SR)), and N fertilization (0 (N0), 30 (N30), and 90 (N90) kg ha−1 y−1) to assess their impact on soil microbial respiration, easily extractable glomalin-related soil protein (EEGRSP), and N mineralization. A significant three-way tillage × stubble × N fertilizer interaction was observed for soil respiration, with NT+SB+N0 treatments generally releasing the highest amounts of CO2 over the incubation period (1135 mg/kg), and NT+SR+N0 treatments releasing the lowest (528 mg/kg). In contrast, a significant stubble × N interaction was observed for both EEGRSP and N mineralization, with the highest concentrations of both EEGRSP (2.66 ± 0.86 g kg−1) and N mineralization (30.7 mg/kg) observed in SR+N90 treatments. Furthermore, N mineralization was also positively correlated with EEGRSP (R2 = 0.76, p < 0.001), indicating that EEGRSP can potentially be used as an index of soil N availability. Overall, this study has shown that SR and N fertilization have a positive impact on soil biological functioning. [ABSTRACT FROM AUTHOR]

• Soils are under increasing stress to feed a rapidly growing human population. • We show that soils are a master variable regulating critical Earth-system processes. • Soils make major contributions to biogeochemical flows and land-system change. • Soils also contribute to climate change, ocean acidification, and ozone depletion. • Improving management of soils is critical to maintain Earth in a hospitable state. We use soils to provide 98.8% of our food, but we must ensure that the pressure we place on soils to provide this food in the short-term does not inadvertently push the Earth into a less hospitable state in the long-term. Using the planetary boundaries framework, we show that soils are a master variable for regulating critical Earth-system processes. Indeed, of the seven Earth-systems that have been quantified, soils play a critical and substantial role in changing the Earth-systems in at least two, either directly or indirectly, as well as smaller contributions for a further three. For the biogeochemical flows Earth-system process, soils contribute 66% of the total anthropogenic change for nitrogen and 38% for phosphorus, whilst for the land-system change Earth-system process, soils indirectly contribute 80% of global anthropogenic change. Furthermore, perturbations of soils contribute directly to 21% of climate change, 25% to ocean acidification, and 25% to stratospheric ozone depletion. We argue that urgent interventions are required to greatly improve soil management, especially for those Earth-system processes where the planetary boundary has already been exceeded and where soils make an important contribution, with this being for biogeochemical flows (both nitrogen and phosphorus), for climate change, and for land-system change. Of particular importance, it is noted that the highly inefficient use of N fertilizers results in release of excess N into the broader environment, contributes to climate change, and results in release of ozone-depleting substances. Furthermore, the use of soils for agricultural production results not only in land-system change, but also in the loss (mineralization) of organic matter with a concomitant release of CO 2 contributing to both climate change and ocean acidification. Thus, there is a need to markedly improve the efficiency of fertilizer applications and to intensify usage of our most fertile soils in order to allow the restoration of degraded soils and limit further areal expansion of agriculture. Understanding, and acting upon, the role of soils is critical in ensuring that planetary boundaries are not transgressed, with no other single variable playing such a strategic role across all of the planetary boundaries. [ABSTRACT FROM AUTHOR]

Soils are the most complex and diverse ecosystem in the world. In addition to providing humanity with 98.8% of its food, soils provide a broad range of other services, from carbon storage and greenhouse gas regulation, to flood mitigation and providing support for our sprawling cities. But soil is a finite resource, and rapid human population growth coupled with increasing consumption is placing unprecedented pressure on soils through the intensification of agricultural production – the increasing of crop yield per unit area of soil. Indeed, the human population has increased from ca. 250 million in the year 1000, to 6.1 billion in the year 2000, and is projected to reach 9.8 billion by the year 2050. The current intensification of agricultural practices is already resulting in the unsustainable degradation of soils. Major forms of this degradation include the loss of organic matter and the release of greenhouse gases, the over-application of fertilizers, erosion, contamination, acidification, salinization, and loss of genetic diversity. This ongoing soil degradation is decreasing the long-term ability of soils to provide humans with services, including future food production, and is causing environmental harm. It is imperative that the global society is not shortsighted by focusing solely on the near-immediate benefits of soils, such as food supply. A failure to identify the importance of soil within increasingly intensive agricultural systems will undoubtedly have serious consequences for humanity and represents a failure to consider intergenerational equity. Of utmost importance is the need to unequivocally recognize that the degradation of soils leads to a clear economic cost through the loss of services, with such principles needing to be explicitly considered in economic frameworks and decision-making processes at all levels of governance. We contend that the concept of the Water-Food-Energy nexus must be expanded, forming the Water-Soil-Food-Energy nexus. • Soils provide humans with 98.8% of our food. • Rapid population growth is unprecedented pressure on soils. • Agricultural intensification is already degrading soils unsustainably. • This degradation is decreasing the long-term ability of soils to produce food. • The Water-Food-Energy nexus must be expanded to the Water-Soil-Food-Energy nexus. [ABSTRACT FROM AUTHOR]