Bacterial community shifts occur primarily through rhizosphere expansion in response to subsoil amendments.

To comprehensively evaluate the impact of agricultural management practices on soil productivity, it is imperative to conduct a thorough analysis of soil bacterial ecology. Deep-banding nutrient-rich amendments is a soil management practice that aims to improve plant growth and soil structure by addressing the plant-growth constraints posed by dense-clay subsoils. However, the response of bacterial communities to deep-banded amendments has not been thoroughly studied. To address this knowledge gap, we conducted a controlled-environment column experiment to examine the effects of different types of soil amendments (poultry litter, wheat straw + chemical fertiliser and chemical fertiliser alone) on bacterial taxonomic composition in simulated dense-clay subsoils. We evaluated the bacterial taxonomic and ecological group composition in soils beside and below the amendment using 16S rRNA amplicon sequencing and robust statistical methods. Our results indicate that deep-banded amendments alter bacterial communities through direct and indirect mechanisms. All amendments directly facilitated a shift in bacterial communities in the absence of growing wheat. However, a combination of amendments with growing wheat led to a more pronounced bacterial community shift which was distinct from and eclipsed the direct impact of the amendments and plants alone. This indirect mechanism was evidenced to be mediated primarily by plant growth and hypothesised to result from an enhancement in wheat root distribution, density and rhizodeposition changes. Therefore, we propose that subsoil amendments regardless of type facilitated an expansion in the rhizosphere which engineered a substantial plant-mediated bacterial community response within the simulated dense-clay subsoils. Overall, our findings highlight the importance of considering the complex and synergistic interactions between soil physicochemical properties, plant growth and bacterial communities when assessing agricultural management strategies for improving soil and plant productivity.

Shining a Light on How Soil Organic Carbon Behaves at Fine Scales under Long-Term Elevated CO2: An 8 Year Free-Air Carbon Dioxide Enrichment Study.

Building and protecting soil organic carbon (SOC) are critical to agricultural productivity, soil health, and climate change mitigation. We aim to understand how mechanisms at the organo-mineral interfaces influence SOC persistence in three contrasting soils (Luvisol, Vertisol, and Calcisol) under long-term free air CO2 enrichment conditions. A continuous wheat-field pea-canola rotation was maintained. For the first time, we provided evidence to a novel notion that persistent SOC is molecularly simple even under elevated CO2 conditions. We found that the elevated CO2 condition did not change the total SOC content or C forms compared with the soils under ambient CO2 as identified by synchrotron-based soft X-ray analyses. Furthermore, synchrotron-based infrared microspectroscopy confirmed a two-dimensional microscale distribution of similar and less diverse C forms in intact microaggregates under long-term elevated CO2 conditions. Strong correlations between the distribution of C forms and O-H groups of clays can explain the steady state of the total SOC content. However, the correlations between C forms and clay minerals were weakened in the coarse-textured Calcisol under long-term elevated CO2. Our findings suggested that we should emphasize identifying management practices that increase the physical protection of SOC instead of increasing complexity of C. Such information is valuable in developing more accurate C prediction models under elevated CO2 conditions and shift our thinking in developing management practices for maintaining and building SOC for better soil fertility and future environmental sustainability.

Changes in the extractability and fractionation of cadmium and copper in a contaminated soil amended with various sugarcane bagasse-based materials.

Heavy-metal contamination in soil has long been a persistent challenge and the utilization of agricultural waste for in-situ stabilization remediation presents a promising approach to tackle this problem. Agricultural wastes exhibit promising potential in the remediation of contaminated land and modification could improve the adsorption performance markedly. Citric acid and Fe3O4 treated sugarcane bagasse adsorbed more heavy metals than raw materials in the aqueous system, employing these materials for heavy metal remediation in soil holds significant implications for broadening the raw material source of passivators and enhancing waste utilization efficiency. In this paper, a 120-day soil incubation study was conducted to compare the effects of pristine sugarcane bagasse (SB), citric-acid modified (SSB1, SSB2 and SSB3 with increasing proportion of citric acid) and citric-acid/Fe3O4 modified (MSB1, MSB4 and MSB7 with increasing proportion of Fe3O4) sugarcane bagasse at 1 % addition rate on cadmium (Cd) and copper (Cu) passivation. The SB, SSB1 and MSB1 did not always decrease the content of CaCl2-extractable Cd while all the seven amendments decreased the CaCl2-extractable Cu during the experiment period. Among all materials, SSB3 and MSB7 exhibited the highest efficiency in reducing the concentrations of CaCl2-extractable Cd and Cu. At Day 120, SB, SSB3 and MSB7 reduced the content of CaCl2-extractable Cd by 8 %, 18 % and 24 %, and of CaCl2-extractable Cu by 25 %, 50 % and 61 %, respectively. The efficiency of Cd and Cu immobilization was associated positively with the pH, functional groups and H-bonds of the amendments. The results suggest that the efficiency of sugarcane bagasse in heavy-metal passivation can be largely enhanced through chemical modifications using high proportions of citric acid and Fe3O4.

Strip clear-cutting transformations increase soil N2O emissions in abandoned Moso bamboo forests.

Forest transformation can markedly impact soil greenhouse gas emissions and soil environmental factors. Due to increasing labor costs and declining bamboo prices, the abandonment of Moso bamboo forests is sharply escalating in recent years, which weakens the carbon sequestration capacity and decreases the ecological function of forests. To improve the ecological quality of abandoned Moso bamboo forests, transformations of abandoned bamboo forests have occurred. However, the impact of such transformations on N2O emissions remains elusive. To bridge the knowledge gap, we conducted a 23-month field experiment to compare the effects of various forest management practices on soil N2O emissions and soil environmental factors in abandoned Moso bamboo forests in subtropical China. These practices included uncut abandonment as a control, intensive management, three intensities (light, moderate, and heavy) of strip clear-cutting with replanting local tree species, and clear-cutting with replanting transformation. During the experimental period, the mean soil N2O flux in abandoned Moso bamboo forests was 13.2 ± 0.1 µg m-2 h-1, representing a 44% reduction compared to intensive management forests. In comparison to the uncut control, light, moderate, and heavy strip clear-cutting and clear-cutting transformations increased soil N2O emission rates by 20%, 43%, 64%, and 94%, respectively. Soil temperature (69-71%), labile C (2-6%) and N (3-8%) were the main factors that explain N2O emissions following the transformation of abandoned Moso bamboo forests. Additionally, replanting could decrease soil N2O emissions by increasing the contribution of soil moisture. Overall, the light strip clear-cutting transformation is suggested to convert abandoned Moso bamboo forests to mitigate N2O emissions.

Elevated CO2 improves phosphorus nutrition and growth of citrate-secreting wheat when grown under adequate phosphorus supply on an Al3+ -toxic soil.

BACKGROUND: Understanding how climate change affects the phosphorus (P) nutrition of crops grown on acid soils is important in optimizing the management of P, and to secure future food production on these soils. This study assessed the impact of elevated CO2 (eCO2 ) on the P nutrition of wheat (Triticum aestivum) grown on Al3+ -toxic and P-deficient soils or in hydroponics. The aluminium-resistant near-isogenic wheat lines EGA-Burke (malate efflux only) and EGA-Burke TaMATE1B (malate and citrate efflux) were grown under ambient (400 µmol mol-1 ) and elevated CO2 (800 µmol mol-1 ) in growth chambers for 4-6 weeks. RESULTS: Elevated CO2 enhanced shoot growth and total P uptake of both lines at P rates >250 mg kg-1 , which was associated with improved root biomass allocation and thus increased root growth, but these effects were not apparent at lower P rates. Elevated CO2 decreased specific P uptake (P uptake per unit root length) at P supply >250 mg kg-1 , but did not significantly affect external or internal P requirements. This effect on the specific P uptake was less for EGA-Burke TaMATE1B than for EGA-Burke, possibly due to the increased citrate efflux and decreased Al concentration in root tips of EGA-Burke TaMATE1B. Compared to EGA-Burke, citrate-exuding EGA-Burke TaMATE1B had greater shoot P concentration and greater specific P uptake. CONCLUSION: Elevated CO2 improved root growth, and thus total P uptake and plant production of both lines when high P alleviated Al3+ toxicity and improved P nutrition in acid soils. The decreased P uptake efficiency under eCO2 was less for EGA-Burke TaMATE1B than EGA-Burke. © 2022 Society of Chemical Industry.

Differential responses of the sunn4 and rdn1-1 super-nodulation mutants of Medicago truncatula to elevated atmospheric CO2.

BACKGROUND AND AIMS: Nitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. 'Autoregulation of nodulation' mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants. METHODS: We compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO2 levels (300-850 µmol mol-1). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing. KEY RESULTS: Shoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO2 concentration and reached a maximum at 700 µmol mol-1. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO2 increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant. CONCLUSIONS: These results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO2 for plant growth and N2 fixation.

Attapulgite and processed oyster shell powder effectively reduce cadmium accumulation in grains of rice growing in a contaminated acidic paddy field.

Heavy-metal contamination is widespread in agricultural soils worldwide, especially paddy soils contaminated by Cd. Amendment-induced immobilization of heavy metals is an attractive and effective technique, provided that cost-effective materials are used. This field experiment compared three alkaline passivators (attapulgite, processed oyster shell powder, and mixed soil conditioner) at a rate of 2.25 t ha-1 for their effectiveness in decreasing Cd bioavailability in soils and accumulation in rice plants in a paddy field contaminated by Cd (0.38 Cd mg kg-1). The utilization of attapulgite and processed oyster shell powder decreased labile fractions but increased stable fractions of Cd in soils through ion exchange, precipitation and complexation. The addition of attapulgite decreased the concentration of bioavailable Cd in both bulk and rhizosphere soils, whereas the amendment of processed oyster shell powder decreased it only in bulk soil. The Cd accumulation in rice plants correlated significantly with acid-soluble and residual Cd fractions in the rhizosphere soil but not in the bulk soil. The addition of attapulgite and processed oyster shell powder decreased Cd accumulation in rice grains from 0.26 mg kg-1 to 0.14 and 0.19 mg kg-1, respectively, meeting the National Food Safety Standard (< 0.20 mg kg-1). However, the mixed soil conditioner did not decrease the Cd accumulation in rice shoots or grains. This study demonstrated that attapulgite and processed oyster shell powder were economic agents in reducing Cd accumulation in rice grains.

Rhizosphere priming of two near-isogenic wheat lines varying in citrate efflux under different levels of phosphorus supply.

BACKGROUNDS AND AIMS: The rhizosphere priming effect (RPE) has been explained from the perspective of microbial responses to root exudates and nutrient availability. This study introduced a chemical process that could also contribute to RPE: root exudates (organic acid ligands) could liberate mineral-protected carbon (C) in soil for microbial degradation. METHODS: Wheat (Triticum aestivum L.) near-isogenic lines varying in citrate efflux were grown for 6 weeks in a C4 soil supplied with either low (10 µg g-1) or high P (40 µg g-1). Total below-ground CO2 was trapped and partitioned for determination of soil organic C decomposition and RPE using a stable isotopic tracing technique. Mineral dissolution was examined by incubating soil with citric ligand at a series of concentrations. KEY RESULTS: High P increased RPE (81 %), shoot (32 %) and root biomass (57 %), root-derived CO2-C (20 %), microbial biomass C (28 %) and N (100%), soil respiration (20 %) and concentrations of water-extractable P (30 %), Fe (43 %) and Al (190 %), but decreased inorganic N in the rhizosphere. Compared with Egret-Burke, wheat line Egret-Burke TaMATE1B with citrate efflux had lower inorganic N, microbial biomass C (16 %) and N (30 %) in the rhizosphere but greater RPE (18 %), shoot biomass (12 %) and root-derived CO2-C (low P 36 %, high P 13 %). Egret-Burke TaMATE1B also had higher concentrations of water-extractable P, Fe and Al in the rhizosphere, indicating the release of mineral-protected C. In addition, citrate ligand facilitated Fe and Al release from soil, with their concentrations rising with increasing ligand concentration and incubation time. CONCLUSIONS: While high P supply increased microbial growth and RPE possibly due to higher total root exudation, citrate efflux from the root might have facilitated the liberation of mineral-bound C, leading to the higher RPE under Egret-Burke TaMATE1B. Mineral dissolution may be an important process that regulates RPE and should be considered in future RPE research.

Salinity decreases Cd translocation by altering Cd speciation in the halophytic Cd-accumulator Carpobrotus rossii.

Background and Aims: Salt has been shown to affect Cd translocation and accumulation in plants but the associated mechanisms are unclear. This study examined the effects of salt type and concentration on Cd uptake, translocation and accumulation in Carpobrotus rossii. Methods: Plants were grown in nutrient solution with the same Cd concentration or Cd2+ activity in the presence of 25 mm NaNO3, 12.5 mm Na2SO4 or 25 mm NaCl for ≤10 d. Plant growth and Cd uptake were measured and the accumulation of peptides and organic acids, and Cd speciation in plant tissues were analysed. Key Results: Salt addition decreased shoot Cd accumulation by >50 % due to decreased root-to-shoot translocation, irrespective of salt type. Synchrotron-based X-ray absorption spectroscopy revealed that, after 10 d, 61-94 % Cd was bound to S-containing ligands (Cd-S) in both roots and shoots, but its speciation was not affected by salt. In contrast, Cd in the xylem sap was present either as free Cd2+ or complexes with carboxyl groups (Cd-OH). When plants were exposed to Cd for ≤24 h, 70 % of the Cd in the roots was present as Cd-OH rather than Cd-S. However, NaCl addition decreased the proportion of Cd-OH in the roots within 24 h by forming Cd-Cl complexes and increasing the proportion of Cd-S. This increase in Cd-S complexes by salt was not due to changes in glutathione and phytochelatin synthesis. Conclusions: Salt addition decreased shoot Cd accumulation by decreasing Cd root-to-shoot translocation due to the rapid formation of Cd-S complexes (low mobility) within the root, without changing the concentrations of glutathione and phytochelatins.

Elevated CO2 (free-air CO2 enrichment) increases grain yield of aluminium-resistant but not aluminium-sensitive wheat (Triticum aestivum) grown in an acid soil.

BACKGROUND AND AIMS: Soil acidity currently limits root growth and crop production in many regions, and climate change is leading to uncertainties regarding future food supply. However, it is unknown how elevated CO2 (eCO2) affects the performance of wheat crops in acid soils under field conditions. We investigated the effects of eCO2 on plant growth and yield of three pairs of near-isogenic hexaploid wheat lines differing in alleles of aluminium-resistant genes TaALMT1 (conferring root malate efflux) and TaMATE1B (conferring citrate efflux). METHODS: Plants were grown until maturity in an acid soil under ambient CO2 (aCO2; 400 µmol mol-1) and eCO2 (550 µmol mol-1) in a soil free-air CO2 enrichment facility (SoilFACE). Growth parameters and grain yields were measured. KEY RESULTS: Elevated CO2 increased grain yield of lines carrying TaMATE1B by 22 % and lines carrying only TaALMT1 by 31 %, but did not increase the grain yield of Al3+-sensitive lines. Although eCO2 promoted tiller formation, coarse root length and root biomass of lines carrying TaMATE1B, it did not affect ear number, and it therefore limited yield potential. By contrast, eCO2 decreased or did not change these parameters for lines carrying only TaALMT1, and enhanced biomass allocation to grains thereby resulting in increased grain yield. Despite TaMATE1B being less effective than TaALMT1 at conferring Al3+ resistance based on root growth, the gene promoted grain yield to a similar level to TaALMT1 when the plants were grown in acid soil. Furthermore, TaALMT1 and TaMATE1B were not additive in their effects. CONCLUSIONS: As atmospheric CO2 increases, it is critical that both Al3+-resistance genes (particularly TaALMT1) should be maintained in hexaploid wheat germplasm in order for yield increases from CO2 fertilization to be realized in acid soils.

Absorption of foliar-applied Zn in sunflower (Helianthus annuus): importance of the cuticle, stomata and trichomes.

Background and Aims: The pathways whereby foliar-applied nutrients move across the leaf surface remain unclear. The aim of the present study was to examine the pathways by which foliar-applied Zn moves across the sunflower (Helianthus annuus) leaf surface, considering the potential importance of the cuticle, stomata and trichomes. Methods: Using synchrotron-based X-ray florescence microscopy and nanoscale secondary ion mass spectrometry (NanoSIMS), the absorption of foliar-applied ZnSO4 and nano-ZnO were studied in sunflower. The speciation of Zn was also examined using synchrotron-based X-ray absorption spectroscopy. Key Results: Non-glandular trichomes (NGTs) were particularly important for foliar Zn absorption, with Zn preferentially accumulating within trichomes in ≤15 min. The cuticle was also found to have a role, with Zn appearing to move across the cuticle before accumulating in the walls of the epidermal cells. After 6 h, the total Zn that accumulated in the NGTs was approx. 1.9 times higher than in the cuticular tissues. No marked accumulation of Zn was found within the stomatal cavity, probably indicating a limited contribution of the stomatal pathway. Once absorbed, the Zn accumulated in the walls of the epidermal and the vascular cells, and trichome bases of both leaf sides, with the bundle sheath extensions that connected to the trichomes seemingly facilitating this translocation. Finally, the absorption of nano-ZnO was substantially lower than for ZnSO4, with Zn probably moving across the leaf surface as soluble Zn rather than nanoparticles. Conclusions: In sunflower, both the trichomes and cuticle appear to be important for foliar Zn absorption.

Manganese distribution and speciation help to explain the effects of silicate and phosphate on manganese toxicity in four crop species.

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.

Absorption of foliar-applied Zn fertilizers by trichomes in soybean and tomato.

The present study investigated the role of trichomes in absorption of foliar-applied zinc fertilizers in soybean and tomato. Using synchrotron-based X-ray fluorescence microscopy for in situ analyses of hydrated leaves, we found that upon foliar application of ZnSO4, Zn accumulated within 15 min in some non-glandular trichomes in soybean, but not in tomato. However, analyses of cross-sections of soybean leaves did not show any marked accumulation of Zn in tissues surrounding trichomes. Furthermore, when near-isogenic lines of soybean differing 10-fold in trichome density were used to compare Zn absorption, it was found that foliar Zn absorption was not related to trichome density. Therefore, it is suggested that trichomes are not part of the primary pathway through which foliar-applied Zn moves across the leaf surface in soybean and tomato. However, this does not preclude trichomes being important in other plant species, as they are known to be highly diverse. We also compared the absorption of Zn when supplied as either ZnSO4, nano-ZnO, or bulk-ZnO, and found that absorption from ZnSO4 was about 10-fold higher than from nano- and bulk-ZnO, suggesting that it was mainly absorbed as soluble Zn. This study improves our understanding of the absorption of foliar-applied nutrients.

The role of rhizosphere pH in regulating the rhizosphere priming effect and implications for the availability of soil-derived nitrogen to plants.

Background and Aims: A comprehensive understanding of the rhizosphere priming effect (RPE) on the decomposition of soil organic carbon (SOC) requires an integration of many factors. It is unclear how N form-induced change in soil pH affects the RPE and SOC sequestration. Methods: This study compared the change in the RPE under supply of NO3-N and NH4-N. The effect of the RPE on the mineralization of soil N and hence its availability to plant and microbes was also examined using a 15N-labelled N source. Key Results: The supply of NH4-N decreased rhizosphere pH by 0.16-0.38 units, and resulted in a decreased or negative RPE. In contrast, NO3-N nutrition increased rhizosphere pH by 0.19-0.78 units, and led to a persistently positive RPE. The amounts of rhizosphere-primed C were positively correlated with rhizosphere pH. Rhizosphere pH affected the RPE mainly through influencing microbial biomass, activity and utilization of root exudates, and the availability of SOC to microbes. Furthermore, the amount of rhizosphere primed C correlated negatively with microbial biomass atom% 15N (R2 0.77-0.98, n = 12), suggesting that microbes in the rhizosphere acted as the immediate sink for N released from enhanced SOC decomposition via the RPE. Conclusion: N form was an important factor affecting the magnitude and direction of the RPE via its effect on rhizosphere pH. Rhizosphere pH needs to be considered in SOC and RPE modelling.

Sodium chloride decreases cadmium accumulation and changes the response of metabolites to cadmium stress in the halophyte Carpobrotus rossii.

Background and Aims: Salinity affects the bioavailability of cadmium (Cd) in soils and Cd accumulation in plants, but the associated mechanisms remain unclear. This study aimed to assess the metabolic response to NaCl and Cd and the relationship between metabolites and Cd accumulation in the halophyte Carpobrotus rossii, which has potential for Cd phytoextraction. Methods: Plants were grown in nutrient solution with 0-400 mm NaCl in the presence of 5 or 15 µm Cd, with varied or constant solution Cd2+ activity. Plant growth and Cd uptake were measured, and the accumulation of peptides, and organic and amino acids in plant tissues were assessed. Key Results: The addition of NaCl to Cd-containing solutions improved plant growth along with 70-87 % less shoot Cd accumulation, resulting from decreases in Cd root uptake and root-to-shoot translocation irrespective of Cd2+ activity in solutions. Moreover, Cd exposure increased the concentration of phytochelatins, which correlated positively with Cd concentrations in plants regardless of NaCl addition. In comparison, Cd inhibited the synthesis of organic acids in shoots and roots in the absence of NaCl, but increased it in shoots in the presence of NaCl. While Cd increased the concentrations of amino acids in plant shoots, the effect of NaCl on the synthesis of amino acids was inconsistent. Conclusions: Our data provide the first evidence that NaCl decreased Cd shoot accumulation in C. rossii by decreasing Cd root uptake and root-to-shoot translocation even under constant Cd2+ activity. The present study also supports the important role of peptides and organic acids, particular of phytochelatins, in Cd tolerance and accumulation although the changes of those metabolites was not the main reason for the decreased Cd accumulation.

Rhizosphere priming effect on soil organic carbon decomposition under plant species differing in soil acidification and root exudation.

Effects of rhizosphere properties on the rhizosphere priming effect (RPE) are unknown. This study aimed to link species variation in RPE with plant traits and rhizosphere properties. Four C3 species (chickpea, Cicer arietinum; field pea, Pisum sativum; wheat, Triticum aestivum; and white lupin, Lupinus albus) differing in soil acidification and root exudation, were grown in a C4 soil. The CO2 released from soil was trapped using a newly developed NaOH-trapping system. White lupin and wheat showed greater positive RPEs, in contrast to the negative RPE produced by chickpea. The greatest RPE of white lupin was in line with its capacity to release root exudates, whereas the negative RPE of chickpea was attributed to its great ability to acidify rhizosphere soil. The enhanced RPE of field pea at maturity might result from high nitrogen deposition and release of structural root carbon components following root senescence. Root biomass and length played a minor role in the species variation in RPE. Rhizosphere acidification was shown to be an important factor affecting the magnitude and direction of RPE. Future studies on RPE modelling and mechanistic understanding of the processes that regulate RPE should consider the effect of rhizosphere pH.

Synchrotron-Based Techniques Shed Light on Mechanisms of Plant Sensitivity and Tolerance to High Manganese in the Root Environment.

Plant species differ in response to high available manganese (Mn), but the mechanisms of sensitivity and tolerance are poorly understood. In solution culture, greater than or equal to 30 µm Mn decreased the growth of soybean (Glycine max), but white lupin (Lupinus albus), narrow-leafed lupin (Lupin angustifolius), and sunflower (Helianthus annuus) grew well at 100 µm Mn. Differences in species' tolerance to high Mn could not be explained simply by differences in root, stem, or leaf Mn status, being 8.6, 17.1, 6.8, and 9.5 mmol kg(-1) leaf fresh mass at 100 µm Mn. Furthermore, x-ray absorption near edge structure analyses identified the predominance of Mn(II), bound mostly to malate or citrate, in roots and stems of all four species. Rather, differences in tolerance were due to variations in Mn distribution and speciation within leaves. In Mn-sensitive soybean, in situ analysis of fresh leaves using x-ray fluorescence microscopy combined with x-ray absorption near edge structure showed high Mn in the veins, and manganite [Mn(III)] accumulated in necrotic lesions apparently through low Mn sequestration in vacuoles or other vesicles. In the two lupin species, most Mn accumulated in vacuoles as either soluble Mn(II) malate or citrate. In sunflower, Mn was sequestered as manganite at the base of nonglandular trichomes. Hence, tolerance to high Mn was ascribed to effective sinks for Mn in leaves, as Mn(II) within vacuoles or through oxidation of Mn(II) to Mn(III) in trichomes. These two mechanisms prevented Mn accumulation in the cytoplasm and apoplast, thereby ensuring tolerance to high Mn in the root environment.

Cadmium accumulation is enhanced by ammonium compared to nitrate in two hyperaccumulators, without affecting speciation.

Nitrogen fertilization could improve the efficiency of Cd phytoextraction in contaminated soil and thus shorten the remediation time. However, limited information is available on the effect of N form on Cd phytoextraction and associated mechanisms in plants. This study examined the effect of N form on Cd accumulation, translocation, and speciation in Carpobrotus rossii and Solanum nigrum Plants were grown in nutrient solution with 5-15 µM Cd in the presence of 1000 µM NH4 (+) or NO3 (-) Plant growth and Cd uptake were measured, and Cd speciation was analyzed using synchrotron-based X-ray absorption spectroscopy. Shoot Cd accumulation was 30% greater with NH4 (+) than NO3 (-) supply. Carpobrotus rossii accumulated three times more Cd than S. nigrum. However, Cd speciation in the plants was not influenced by N form, but it did vary with species and tissues. In C. rossii, up to 91% of Cd was bound to S-containing ligands in all tissues except the xylem sap where 87-95% were Cd-OH complexes. Furthermore, the proportion of Cd-S in shoots was substantially lower in S. nigrum (44-69%) than in C. rossii (60-91%). It is concluded that the application of NH4 (+) (instead of NO3 (-)) increased shoot Cd accumulation by increasing uptake and translocation, rather than changing Cd speciation, and is potentially an effective approach for increasing Cd phytoextraction.

Free-air CO2 enrichment (FACE) reduces the inhibitory effect of soil nitrate on N2 fixation of Pisum sativum.

BACKGROUND AND AIMS: Additional carbohydrate supply resulting from enhanced photosynthesis under predicted future elevated CO2 is likely to increase symbiotic nitrogen (N) fixation in legumes. This study examined the interactive effects of atmospheric CO2 and nitrate (NO3(-)) concentration on the growth, nodulation and N fixation of field pea (Pisum sativum) in a semi-arid cropping system. METHODS: Field pea was grown for 15 weeks in a Vertosol containing 5, 25, 50 or 90 mg NO3(-)-N kg(-1) under either ambient CO2 (aCO2; 390 ppm) or elevated CO2 (eCO2; 550 ppm) using free-air CO2 enrichment (SoilFACE). KEY RESULTS: Under aCO2, field pea biomass was significantly lower at 5 mg NO3(-)-N kg(-1) than at 90 mg NO3(-)-N kg(-1) soil. However, increasing the soil N level significantly reduced nodulation of lateral roots but not the primary root, and nodules were significantly smaller, with 85% less nodule mass in the 90 NO3(-)-N kg(-1) than in the 5 mg NO3(-)-N kg(-1) treatment, highlighting the inhibitory effects of NO3(-). Field pea grown under eCO2 had greater biomass (approx. 30%) than those grown under aCO2, and was not affected by N level. Overall, the inhibitory effects of NO3(-) on nodulation and nodule mass appeared to be reduced under eCO2 compared with aCO2, although the effects of CO2 on root growth were not significant. CONCLUSIONS: Elevated CO2 alleviated the inhibitory effect of soil NO3(-) on nodulation and N2 fixation and is likely to lead to greater total N content of field pea growing under future elevated CO2 environments.

Crop acquisition of phosphorus, iron and zinc from soil in cereal/legume intercropping systems: a critical review.

BACKGROUND: Phosphorus (P), iron (Fe) and zinc (Zn) are essential elements for plant growth and development, but their availability in soil is often limited. Intercropping contributes to increased P, Fe and Zn uptake and thereby increases yield and improves grain nutritional quality and ultimately human health. A better understanding of how intercropping leads to increased plant P, Fe and Zn availability will help to improve P-fertilizer-use efficiency and agronomic Fe and Zn biofortification. SCOPE: This review synthesizes the literature on how intercropping of legumes with cereals increases acquisition of P, Fe and Zn from soil and recapitulates what is known about root-to-shoot nutrient translocation, plant-internal nutrient remobilization and allocation to grains. CONCLUSIONS: Direct interspecific facilitation in intercropping involves below-ground processes in which cereals increase Fe and Zn bioavailability while companion legumes benefit. This has been demonstrated and verified using isotopic nutrient tracing and molecular analysis. The same methodological approaches and field studies should be used to explore direct interspecific P facilitation. Both niche complementarity and interspecific facilitation contribute to increased P acquisition in intercropping. Niche complementarity may also contribute to increased Fe and Zn acquisition, an aspect poorly understood. Interspecific mobilization and uptake facilitation of sparingly soluble P, Fe and Zn from soil, however, are not the only determinants of the concentrations of P, Fe and Zn in grains. Grain yield and nutrient translocation from roots to shoots further influence the concentrations of these nutrients in grains.