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Background It is known that the soil near roots, the so-called rhizosphere, has physical and chemical properties different from those of the bulk soil. Rhizosphere properties are the result of several processes: root and soil shrinking/swelling during drying/wetting cycles, soil compaction by root growth, mucilage exuded by root caps, interaction of mucilage with soil particles, mucilage shrinking/swelling and mucilage biodegradation. These processes may lead to variable rhizosphere properties, i.e. the presence of air-filled gaps between soil and roots; water repellence in the rhizosphere caused by drying of mucilage around the soil particles; or water accumulation in the rhizosphere due to the high water-holding capacity of mucilage. The resulting properties are not constant in time but they change as a function of soil condition, root growth rate and mucilage age. Scope We consider such a variability as an expression of rhizosphere plasticity, which may be a strategy for plants to control which part of the root system will have a facilitated access to water and which roots will be disconnected from the soil, for instance by air-filled gaps or by rhizosphere hydrophobicity. To describe such a dualism, we suggest classifying rhizosphere into two categories: class A refers to a rhizosphere covered with hydrated mucilage that optimally connects roots to soil and facilitates water uptake from dry soils. Class B refers to the case of air-filled gaps and/or hydrophobic rhizosphere, which isolate roots from the soil and may limit water uptake from the soil as well water loss to the soil. The main function of roots covered by class B will be long-distance transport of water. Outlook This concept has implications for soil and plant water relations at the plant scale. Root water uptake in dry conditions is expected to shift to regions covered with rhizosphere class A. On the other hand, hydraulic lift may be limited in regions covered with rhizosphere class B. New experimental methods need to be developed and applied to different plant species and soil types, in order to understand whether such dualism in rhizosphere properties is an important mechanism for efficient utilization of scarce resources and drought tolerance. [ABSTRACT FROM AUTHOR]

Root growth responds to local differences in N-form and concentration. This is known for artificial systems and assumed to be valid in soil. The purpose of this study is to challenge this assumption for soil mesocosms locally supplied with urea with and without nitrification inhibitor. Soil column experiments with Vicia faba ('Fuego') and Hordeum vulgare ('Marthe') were performed to investigate soil solution chemistry and root growth response of these two species with contrasting root architectures to the different N-supply simultaneously. Root growth was analysed over time and separately for the fertiliser layer and the areas above and below with X-ray CT (via region growing) and WinRHIZO. Additionally, NO3− and NH4+ in soil and soil solution were analysed. In Vicia faba, no pronounced differences were observed, although CT analysis indicated different root soil exploration for high NH4+. In Hordeum vulgare, high NO3− inhibited lateral root growth while high NH4+ stimulated the formation of first order laterals. The growth response to locally distributed N-forms in soil is species specific and less pronounced than in artificial systems. The combination of soil solution studies and non-invasive imaging of root growth can substantially improve the mechanistic understanding of root responses to different N-forms in soil. [ABSTRACT FROM AUTHOR]

Rhizodeposition fuels carbon (C) and nutrient cycling in soil. However, changes in the dynamics of microbial growth on rhizodeposits with increasing distance from the root is not well studied. This study investigates microbial growth on individual organic components of rhizodeposits and maize root‐derived exudates and mucilage from agricultural soil. By creating a gradient of substrate concentrations, we simulated reduced microbial access to rhizosphere C with increasing distance to the root surface. We identified distinct C‐thresholds for the activation of microbial growth, and these were significantly higher for rhizodeposits than singular, simple sugars. In addition, testing for stoichiometric constraints of microbial growth by supplementing nitrogen (N) and phosphorus (P) showed accelerated and increased microbial growth by activating a larger proportion of the microbial biomass. Early and late season exudates triggered significantly different microbial growth responses. The mineralization of early‐season exudates was induced at a high C‐threshold. In contrast, the mineralization of late‐season exudates showed 'sugar‐like' properties, with a low C‐threshold, high substrate affinity, and a reduced maximum respiration rate of microorganisms growing on the added substrate. Mucilage exhibited the highest C‐threshold for the activation of microbial growth, although with a short lag‐period and with an efficient mucilage degradation comparable to that of sugars. By determining kinetic parameters and turnover times for different root‐derived substrates, our data enable the upscaling of micro‐scale processes to the whole root system, allowing more accurate predictions of how rhizodeposition drives microbial C and nutrient dynamics in the soil. [ABSTRACT FROM AUTHOR]

Contradictory evidence exists regarding whether and to which extend roots change soil structure in their vicinity. Here we attempt to reconcile disparate views allowing for the two-way interaction between soil structure and root traits, i.e. changes in soil structure due to plants and changes in root growth due to soil structure. Porosity gradients extending from the root/biopore surface into the bulk soil were investigated with X-ray µCT for undisturbed soil samples from a field chronosequence as well as for a laboratory experiment with Zea mays growing into three different bulk densities. An image analysis protocol was developed, which enabled a fast analysis of the large sample pool (n > 300) at a resolution of 19 µm. Lab experiment showed that growing roots only compact the surrounding soil if macroporosity is low and dominated by isolated pores. When roots can grow into a highly connected macropore system showing high connectivity the rhizosphere is more porous compared to the bulk soil. A compaction around roots/biopores in the field chronosequence was only observed in combination with high root/biopore length densities. We conclude that roots compact the rhizosphere only if the initial soil structure does not offer a sufficient volume of well-connected macropores. [ABSTRACT FROM AUTHOR]

Modulating the soil microbiome by applying microbial inoculants has gained increasing attention as eco-friendly option to improve soil disease suppressiveness. Currently, studies unraveling the interplay of inoculants, root-associated microbiome, and plant response are lacking for apple trees. Here, we provide insights into the ability of Bacillus velezensis FZB42 or Pseudomonas sp. RU47 to colonize apple root-associated microhabitats and to modulate their microbiome. We applied the two strains to apple plants grown in soils from the same site either affected by apple replant disease (ARD) or not (grass), screened their establishment by selective plating, and measured phytoalexins in roots 3, 16, and 28 days post inoculation (dpi). Sequencing of 16S rRNA gene and ITS fragments amplified from DNA extracted 28 dpi from different microhabitat samples revealed significant inoculation effects on fungal β-diversity in root-affected soil and rhizoplane. Interestingly, only in ARD soil, most abundant bacterial amplicon sequence variants (ASVs) changed significantly in relative abundance. Relative abundances of ASVs affiliated with Enterobacteriaceae were higher in rhizoplane of apple grown in ARD soil and reduced by both inoculants. Bacterial communities in the root endosphere were not affected by the inoculants but their presence was indicated. Interestingly and previously unobserved, apple plants responded to the inoculants with increased phytoalexin content in roots, more pronounced in grass than ARD soil. Altogether, our results indicate that FZB42 and RU47 were rhizosphere competent, modulated the root-associated microbiome, and were perceived by the apple plants, which could make them interesting candidates for an eco-friendly mitigation strategy of ARD. Key points: • Rhizosphere competent inoculants modulated the microbiome (mainly fungi) • Inoculants reduced relative abundance of Enterobacteriaceae in the ARD rhizoplane • Inoculants increased phytoalexin content in roots, stronger in grass than ARD soil [ABSTRACT FROM AUTHOR]

Background: X-ray computed tomography is acknowledged as a powerful tool for the study of root system architecture of plants growing in soil. In this paper, we improved the original root segmentation algorithm "Rootine" and present its succeeding version "Rootine v.2". In addition to gray value information, Rootine algorithms are based on shape detection of cylindrical roots. Both algorithms are macros for the ImageJ software and are made freely available to the public. New features in Rootine v.2 are (i) a pot wall detection and removal step to avoid segmentation artefacts for roots growing along the pot wall, (ii) a calculation of the root average gray value based on a histogram analysis, (iii) an automatic calculation of thresholds for hysteresis thresholding of the tubeness image to reduce the number of parameters and (iv) a false negatives recovery based on shape criteria to increase root recovery. We compare the segmentation results of Rootine v.1 and Rootine v.2 with the results of root washing and subsequent analysis with WinRhizo. We use a benchmark dataset of maize roots (Zea mays L. cv. B73) grown in repacked soil for two scenarios with differing soil heterogeneity and image quality. Results: We demonstrate that Rootine v.2 outperforms its preceding version in terms of root recovery and enables to match better the root diameter distribution data obtained with root washing. Despite a longer processing time, Rootine v.2 comprises less user-defined parameters and shows an overall greater usability. Conclusion: The proposed method facilitates higher root detection accuracy than its predecessor and has the potential for improving high-throughput root phenotyping procedures based on X-ray computed tomography data analysis. [ABSTRACT FROM AUTHOR]

Deciphering root exudate composition of soil-grown plants is considered a crucial step to better understand plant–soil–microbe interactions affecting plant growth performance. In this study, two genotypes of Zea mays L. (WT, rth3) differing in root hair elongation were grown in the field in two substrates (sand, loam) in custom-made, perforated columns inserted into the field plots. Root exudates were collected at different plant developmental stages (BBCH 14, 19, 59, 83) using a soil-hydroponic-hybrid exudation sampling approach. Exudates were characterized by LC-MS based non-targeted metabolomics, as well as by photometric assays targeting total dissolved organic carbon, soluble carbohydrates, proteins, amino acids, and phenolics. Results showed that plant developmental stage was the main driver shaping both the composition and quantity of exuded compounds. Carbon (C) exudation per plant increased with increasing biomass production over time, while C exudation rate per cm² root surface area h−1 decreased with plant maturity. Furthermore, exudation rates were higher in the substrate with lower nutrient mobility (i.e., loam). Surprisingly, we observed higher exudation rates in the root hairless rth3 mutant compared to the root hair-forming WT sibling, though exudate metabolite composition remained similar. Our results highlight the impact of plant developmental stage on the plant–soil–microbe interplay. • Root exudation profiling of field-grown maize plants across plant development. • Plant developmental stage was main driver of changes in exudate the quality and quantity. • Absence of root hairs enhanced exudation rate and caused changes in root morphology. • Exudation was influenced by the substrate sorption capacity and nutrient mobility. [ABSTRACT FROM AUTHOR]

Aims: Different drivers are known to shape rhizosphere microbiome assembly. How soil texture (Texture) and presence or lack of root hairs (Root Hair) of plants affect the rhizosphere microbiome assembly and soil potential extracellular enzyme activities (EEA) at defined rooting depth (Depth) is still a knowledge gap. We investigated effects of these drivers on microbial assembly in rhizosphere and on potential EEA in root-affected soil of maize. Methods: Samples were taken from three depths of root hair defective mutant rth3 and wild-type WT maize planted on loam and sand in soil columns after 22 days. Rhizosphere bacterial, archaeal, fungal and cercozoan communities were analysed by sequencing of 16S rRNA gene, ITS and 18S rRNA gene fragments. Soil potential EEA of ß-glucosidase, acid phosphatase and chitinase were estimated using fluorogenic substrates. Results: The bacterial, archaeal and cercozoan alpha- and beta-diversities were significantly and strongly altered by Texture, followed by Depth and Root Hair. Texture and Depth had a small impact on fungal assembly, and only fungal beta-diversity was significantly affected. Significant impacts by Depth and Root Hair on beta-diversity and relative abundances at taxonomic levels of bacteria, archaea, fungi and cercozoa were dependent on Texture. Likewise, the patterns of potential EEA followed the trends of microbial communities, and the potential EEA correlated with the relative abundances of several taxa. Conclusions: Texture was the strongest driver of rhizosphere microbiome and of soil potential EEA, followed by Depth and Root Hair, similarly to findings in maize root architecture and plant gene expression studies. [ABSTRACT FROM AUTHOR]

Background and aims: Impact of drought on crop growth depends on soil and root hydraulic properties that determine the access of plant roots to soil water. Root hairs may increase the accessible water pool but their effect depends on soil hydraulic properties and adaptions of root systems to drought. These adaptions are difficult to investigate in pot experiments that focus on juvenile plants. Methods: A wild-type and its root hairless mutant maize (Zea mays) were grown in the field in loam and sand substrates during two growing seasons with a large precipitation deficit. A comprehensive dataset of soil and plant properties and monitored variables were collected and interpreted using simulations with a mechanistic root water uptake model. Results: Total crop water use was similar in both soils and for both genotypes whereas shoot biomass was larger for the wild type than for the hairless mutant and did not differ between soils. Total final root length was larger in sand than in loam but did not differ between genotypes. Simulations showed that root systems of both genotypes and in both soils extracted all plant available soil water, which was similar for sand and loam, at a potential rate. Leaf water potentials were overestimated by the model, especially for the hairless mutant in sand substrate because the water potential drop in the rhizosphere was not considered. Conclusions: A direct effect of root hairs on water uptake was not observed but root hairs might influence leaf water potential dependent growth. [ABSTRACT FROM AUTHOR]

Purpose: Mucilage plays crucial roles in root-soil interactions. Collection systems for maize (Zea mays L.) use primary and seminal roots of aeroponically-grown seedlings (CSA), or brace roots of soil-grown plants (CSB). While each method represents specific plant developmental stages, and root types growing in specific (micro-)environments, these factors are rarely considered. It is unclear whether mucilage exhibits distinct physico-chemical properties related to collection system-inherent factors. Methods: Mucilage of maize genotype B73 was collected from systems CSA and CSB. Chemical composition was assessed by pH, nutrient contents, neutral sugar composition, and polysaccharide polymer length. Viscosity, surface tension and contact angle represented physical properties. Results: The share of hexoses among total polysaccharides was 11% higher in CSB than in CSA, whereas pentoses were predominant in CSA, together with higher nutrient concentrations and pH values. Mannose was detected only in CSB, which also exhibited higher surface tension, viscosity and contact angle compared to CSA. Conclusions: Physico-chemical differences between the two mucilages are related to root type functions, environmental root growth conditions, and plant developmental state. Higher fractions of pentoses in CSA mucilage seem related to semi-sterile system conditions. Higher viscosity of CSB mucilage might reflect the need for enhanced water holding capacity of brace roots growing in drier conditions. A strong influence of environmental factors on mucilage properties even for a single genotype might play additional roles e.g. in the attraction of microbiomes. These aspects are relevant when assessing the role of mucilage in the rhizosphere, or when developing models of rhizosphere processes. [ABSTRACT FROM AUTHOR]

Aims: Root hairs and lateral growth are root traits among many which enable plants to adapt to environmental conditions. How different traits are coordinated under local heterogeneity, especially when two or more environmental factors vary in space, is currently poorly understood. We investigated the effect of heterogeneity on root system architecture of maize in response to the presence of loamy macroaggregates, which come along with both, increased penetration resistance and nutrient availability, i.e., two important environmental factors shaping root system architecture. The comparison between a mutant with defective root hairs and the corresponding wild type made it possible to investigate the importance of root hairs in the adaptation strategies of plant roots to these factors. Methods: Changes in root growth and root distribution with respect to macroaggregates were investigated using X-ray computed tomography. The wild-type of Zea mays L. was compared with the root hair defective mutant (rth3) to investigate the importance of root hairs in addition to adaption of root architecture. Results: The presence of aggregates lead to increased root length and branch densities around aggregates, while only a few roots were able to grow into them. Thereby, wildtype and rth3 were influenced in the same way. Aboveground biomass, however, was not affected by the presence of macroaggregates, as compared to controls with homogenously distributed loam. Conclusions: Macroaggregation of loam in sandy soil shows little influence on maize growth, due to local adaptations of root architecture to the heterogeneity in nutrient availability and penetration resistance caused by the aggregates. [ABSTRACT FROM AUTHOR]

Roots and the associated soil directly affected by root activity, termed the rhizosphere, have both been extensively studied and recognized for their crucial role in soil functioning. The formation of the rhizosphere is primarily driven by the effect of roots on shaping the physical structure of the soil, which in turn has direct feedbacks on the interactions between physical, biological and chemical processes. As a result, the rhizosphere is a hot spot for microbial activity, cycling of nutrients and turnover of organic matter. Despite the pivotal role of soil structure in controlling rhizosphere processes, we still lack a quantitative description and understanding of the interrelationships of root-systems and soil in the creation and stabilization of soil structure. We provide a comprehensive review of current knowledge and novel insights into processes that drive the formation and stabilization of soil structure in the rhizosphere. These processes are regulated by multiple indirect and direct pathways, involving root growth, the production of rhizodeposits and root hairs, as well as the activity of soil microorganisms and fauna. Further, we highlight that rhizosphere processes may persist and evolve after root death to an extent currently largely unknown. Finally, we identify five pertinent challenges that should be addressed to fully apprehend rhizosphere processes and thus harness the potential resilience of plant-soil interactions. These challenges include refining structural assessment and sampling of rhizosheaths, examining the rhizosphere in-situ and bridging the gap between solid phase and pore scale research. In our view, overcoming these obstacles can be accomplished by combining the power of imaging and isotopic approaches, especially at the field scale, encompassing diverse soils and plant species. The ultimate objective of future research should be to upscale rhizosphere processes by conducting more field experiments in concert with modeling efforts, under the umbrella of collaborative interdisciplinary research. • Plant roots are inherently bound to soil structure dynamics through rhizosphere processes. • Rhizodeposits and root hairs trigger rhizosheath formation through multiple routes. • Roots foster formation of unique habitat by affecting soil porosity and aggregation. • Soil fauna and legacy of dead roots are overlooked factors for rhizosphere structure formation and stability. • Bridging solid phase and pore scale research needed to advance rhizosphere research. [ABSTRACT FROM AUTHOR]

Aims: Root hairs are one root trait among many which enables plants to adapt to environmental conditions. How different traits are coordinated and whether some are mutually exclusive is currently poorly understood. Comparing a root hair defective mutant with its corresponding wild-type, we explored if and how the mutant exhibited root growth adaptation strategies and how dependent this was on substrate. Methods: Zea mays root hair defective mutant (rth3) and the corresponding wild-type siblings were grown under well-watered conditions on two substrates with contrasting texture and hence nutrient mobility. Root system architecture was investigated over time using repeated X-ray computed tomography. Results: There was no plastic adaptation of root system architecture to the lack of root hairs, which resulted in lower uptake of nutrients especially in the substrate with high sorption capacity. The function of the root hairs for anchoring did not result in different root length density profiles between genotypes. Both maize genotypes showed a marked response to substrate. This was well reflected in the spatiotemporal development of rhizosphere volume fraction but especially in the highly significant response of root diameter to substrate, irrespective of genotype. Conclusions: The most salient root plasticity trait was root diameter in response to substrate. Coping mechanisms for missing root hairs were limited to a shift in root-shoot ratio in loam. Further experiments are required, to elucidate whether observed differences can be explained by mechanical properties beyond mechanical impedance, root or microbiome ethylene production or differences in diffusion processes within the root or the rhizosphere. [ABSTRACT FROM AUTHOR]

During soil formation, the interaction of different biota (plants, soil fauna, microbes) with weathered mineral material shapes unique structures depending on the parental material and the site specific climatic conditions. While many of these interactions are known, the relative importance of the different biota is difficult to unravel and therefore difficult to quantify. Biological soil structure formation is often superimposed by soil management and swell-shrink dynamics, making it even more difficult to derive mechanistic understanding. We here explore soil structure formation within a "space-for-time" chronosequence in the Rhenish lignite mining area. Loess material from a depth of 4–10 m has been used for reclamation in a standardized procedure for 24 years. Changes in soil pore system are characterized by properties such as connectivity (Euler number) and pore size distribution using undisturbed soil columns with a diameter of 10 cm. They were taken from two different depths (0–20 cm and 40–60 cm) at different sites ranging in age from 0 to 24 years. X-ray CT is used for scanning the original columns as well as undisturbed subsamples of 3 and 0.7 cm diameter. This hierarchical sampling scheme was developed to overcome the trade-off between sample size and resolution. For the first time also information on the development of biopores could be measured by separating them from other structural pores based on their unique shape. The data were complemented by destructive sampling and determination of root length with WinRHIZO to give an estimate of how many biopores are filled with roots. Furthermore HYPROP measurements of water retention curves were conducted and showed a general agreement with the image-derived pore size distribution merged across three scales. An increase in biopore density throughout year zero to year 12, in particular in 40–60 cm soil depth, was observed. The biopore length densities of approximately 17 cm/cm3 obtained in year 12 was similar to the one measured in year 24, suggesting that equilibrium was reached. Only about 10% of these biopores were filled with roots. In the topsoil (0–20 cm) the equilibrium value in biopore density is already reached after six years due to a higher root length density. Ploughing lead to higher mean pore size and to lower connectivity compared to the well-connected, very stable pore network in 40–60 cm depth. This study shows how fast plant roots create a stable and connected biopore system and how this is disrupted by soil tillage, which produces completely contrasting pore characteristics. Unlabelled Image • X-ray μCT disentangles effect of tillage and plant roots on soil structure over time. • A new segmentation method allowed for quantification of biopore-network • A maximum biopore density (18 cm cm−3) was already reached after 6 years 18 cm cm−3. • Tillage led to total different macropore characteristics. [ABSTRACT FROM AUTHOR]

Background and aims: Root morphological response to localised phosphorus (P) application plays a crucial role in P acquisition. However, detailed knowledge of when and where roots respond to P patch (localised P supply) in situ is still lacking. Methods: X-ray Computed Tomography (CT) was used to quantify the root response to localised and uniform triple superphosphate supply in maize (Zea mays) and faba bean (Vicia faba). Soil solution was extracted by micro suction cups. Results: On the spatial scale, X-ray CT results showed that maize roots proliferated in the P patch, resulting in higher root length density compared to the uniform P supply. On the temporal scale, the proliferation of maize roots into the patch occurred as early as 11 days after sowing. Faba bean showed no response to the P patch during the experimental period. In soil solution, localised P fertiliser supply not only increased the concentration of P and calcium, elements present in the fertiliser, but also potassium and magnesium. Conclusions: Our results indicate a fine-scale time pattern for P concentration in the local site to enhance root proliferation of maize but not faba bean, which suggests an evident difference in root response to local P supply between maize and faba bean at the seedling stage. [ABSTRACT FROM AUTHOR]

Aims: X-ray computed tomography (CT) is widely recognized as a powerful tool for in-situ quantification of root system architecture (RSA) in soil. However, employing X-ray CT to identify the spatio-temporal dynamics of RSA still remains a challenge due to non-automatic, time-consuming image processing protocols and their poor recovery of fine roots in soil. Methods: Here we present a new protocol (Rootine) to segment roots rapidly and precisely down to fine roots with two voxels in diameter (90 μm in pots with 70 mm in diameter). This is facilitated by feature detection of the tubular shape of roots, an approach that was originally developed for detecting blood vessels in medical imaging. Results: In comparison to established root segmentation methods, Rootine produced a more accurate root network, i.e. more roots and less over-segmentation. Root length quantified by X-ray CT showed high correlation with results by root washing combined with 2D light scanning (R2 = 0.92). Tests with different soil materials showed that the recovery of roots depends on signal-to-noise ratio but can be up to 99% for a favorable contrast between fine roots and background. Conclusions: This new protocol provides great efficiency to study RSA in undisturbed soil. As it is fully automated it has the potential for high-throughput root phenotyping and related modelling. [ABSTRACT FROM AUTHOR]

• CaCO 3 as cementing agent dominates soil aggregation in loess used for reclamation. • SOC is dominantly stored in large aggregates in reclaimed soils. • N is a limiting factor for development of the microbial community. • Crop residues are not sufficient to build up SOM content. After reclamation of open-cast mining pits, soil formation starts from the deposited calcareous loess characterised by its basic physical and chemical properties whereas soil biology and structure need to develop to achieve a fully functional soil. In this study we used a chronosequence approach to elucidate soil formation on agriculturally reclaimed loess soils in an open-cast lignite mining area in Garzweiler (Germany). We selected six fields aged 0, 1, 3, 6, 12, and 24 years after the first seeding in order to observe the initial stage of development of soil properties and assess the role of management with conventional crop rotation in soil structure formation and soil organic carbon (SOC) accumulation. Loess parent material had a strong impact on aggregation, as CaCO 3 acted as a strong cementing agent. Alfalfa cultivation in the pioneering phase was of high importance in the development of microbial biomass, as it protects microbes from N limitation. Soil macroporosity and pore connectivity increased only after compost application and ploughing during agricultural crop rotation. Soil organic matter (SOM) build-up was strongly dependent on the addition of compost, as crop residues from conventional crop rotation are not sufficient to maintain high SOC contents. [ABSTRACT FROM AUTHOR]

[Display omitted] • Penetration resistance and grayvalue are correlated. • The size of the zone of influence of a penetrometer tip depends on soil density. • X-ray CT can be used to generate 3D penetration resistance maps. • Penetration resistance maps are valuable tools for root-soil interactions research. Noninvasive imaging of soils with X-ray CT has proven to be a useful method to assess soil structure from a pore space perspective. In contrast, methods like cone penetration tests reflect soil structure from the perspective of the soil matrix as assessed by its mechanical strength. Because both the gray value (GV) obtained with X-ray CT and the penetration resistance (PR) obtained with a cone penetration test depend on soil density there should be a relationship between the two. To the best of our knowledge, no studies attempted so far to investigate the nature of the PR ∼ GV relationship and to understand how well PR and GV are correlated. We aimed at bridging that gap and carried out a combined analysis of local GV and PR with undisturbed soil cores sampled in two soil textures, i.e. , loam and sand. To carry out the GV measurements, we developed a new approach which considers an adaptive volume of the zone of influence of the penetrometer tip as a function of soil density. For sand and when looking at samples individually, the correlation between PR and GV was best when the soil microscale heterogeneity was high, i.e. , when dense and loose zones of soil were present on the course of the penetrometer tip. For loam, the correlation between PR and GV was not dependent on soil heterogeneity. When looking at the whole dataset, the agreement between PR and GV was better in loam than in sand, with a distance correlation metric of 0.66 for loam and 0.34 for sand, respectively. For loam, the relationship PR ∼ GV had a trend which was similar to that of a hyperbola, i.e. , with escalating PR values in a narrow GV range. For sand, no particular model could be recognized. In order to provide a proof-of-concept on how to generate 3D PR maps, the co-located measurements of GV and PR were used to establish an empirical relationship and X-ray CT was used to extrapolate it in 3D. This was carried out with the loam dataset by fitting a hyperbolic function to the PR ∼ GV data pairs. This model was then used to convert GVs into PR values, at a spatial resolution equal to that of the shaft diameter of the penetrometer tip we have used. Notwithstanding the fact that the suggested approach is dependent on numerous experimental conditions and edaphic factors, we advocate for the use of 3D PR maps. These maps could be used in root-soil interactions research, for which the study and breeding of cultivars that could show plastic response in their root systems under mechanical stress is becoming more and more important. This is particularly relevant in the context of mechanized modern agriculture. [ABSTRACT FROM AUTHOR]

Aims: Root shrinkage in drying soil has been shown repeatedly. The aim of this study was to investigate the dynamics of root-soil contact and its relationship with plant water status during soil drying.Methods: The development of root-soil contact of Vicia faba L. during a drying period was studied. Plants (N = 4) were grown in cylinders filled with a sandy soil. Samples were repeatedly scanned with an X-ray CT scanner to visualize root-soil contact. Soil matric potential, transpiration rate, and stomatal conductance were measured daily.Results: Root-soil contact was lower in taproots than in lateral roots at any time. Transpiration rate and stomatal conductance decreased before roots started to shrink. Root-soil contact decreased significantly over the course of the drying period, starting at soil matric potentials below −20 kPa. Root shrinkage did not differ significantly between taproots and laterals.Conclusions: This study confirms previous findings with Lupinus albus roots in that roots shrink after transpiration rate decreases. The dynamics of root shrinkage are governed by soil water availability and transpirational demand. [ABSTRACT FROM AUTHOR]

Soil texture, i.e. the fractions of different sized mineral particles, is critical to root growth and an important determinant of the occurrence and distribution of soil microbiota. More recently it was shown that individual plant species and even different cultivars harbor highly distinct rhizosphere associated microbiota, but it is still an open question how soil texture and its influence on root growth feeds back on root microbial assembly. We manipulated soil texture by stepwise additions of quartz sand to an agricultural loam. We grew maize (Zea mays L.) in these soils, measured changes in root traits and sampled bulk soil and rhizosphere to apply amplicon based high-throughput sequencing. We investigated changes in root morphology of maize and the concomitant shift in prokaryote (archaea and bacteria) and protist (Cercozoa and Endomyxa) diversity, community composition and co-occurrence in the maize rhizosphere along the soil texture gradient. A linear relationship between loam fraction and root morphology and a shift in microbial diversity along the soil texture gradient, as well as a stronger selection effect of the rhizosphere in soils with a high sand fraction (and high bulk density) were found. Co-occurrence network analysis revealed high modularity in fine textured soil, demonstrating that bulk density and texture are important factors affecting the recruitment of the core rhizosphere microbiome of maize. • Compensatory root growth in maize caused by soil texture and bulk density. • Linear shift of microbial community structure with changes in soil texture. • Amplified soil texture effects on community composition in the rhizosphere. • Less recruitment of bulk soil taxa into the rhizosphere microbiome with increasing bulk density. • Network analysis indicated the formation of small, isolated sub-communities in fine textured soil. [ABSTRACT FROM AUTHOR]

Biopore recycling is the process during which roots ingress into existing biopores instead of creating new ones. Previous studies investigated biopore recycling in rather artificial conditions, e.g., with artificially created vertical macropores, by neglecting the smaller biopore diameter classes or by focusing on high bulk density soil material only. To address these shortcomings, we designed a soil column experiment and characterized the degree of biopore recycling for two soil textures (sand, loam) and two bulk density treatments (loam: 1.26 vs 1.36 g cm−3, sand: 1.50 vs 1.60 g cm−3). We developed a novel method based on the analysis of X-ray CT 3D images which enabled us to characterize the degree of biopore recycling for root-induced biopores down to 60 µm of diameter. The degree of biopore recycling was two orders of magnitude lower than previously reported in the literature (on average 0.0036 centimeters of roots were found in 1 centimeter of biopores). Roots were crossing the biopores rather than colonizing them. Visual analysis of the images showed that the propensity of roots to grow into biopores was higher when the angle at which roots and biopores touched was inferior to 45 degrees and when the root diameter was approximately equal to or inferior to the biopore diameter. There were no statistical differences for the biopore recycling fraction between the two bulk density treatments in loam. In loam, roots degraded quickly (less than 78 days) and the biopores created were stable over time. In sand, some biopores were still filled with root residues after 216 days and many biopores had collapsed. We conclude that biopore recycling is less likely to occur in sand, as compared to in loam. We further used the model CPlantBox to simulate root system architectures with identical root length density as observed in the experiment but random arrangement with respect to biopores. By comparing the modeling results with the experimental results, we showed that roots had no preference for growing into biopores under the studied conditions. • Soil texture affects root decomposition. • Biopores are more prone to collapsing in sand than in loam. • Biopore recycling is more likely to occur when the roots and biopores meet with a contact angle inferior to 45 degrees. • Roots have no preference for biopores in well aerated, fertilized and repacked soil columns. [ABSTRACT FROM AUTHOR]

Understanding phosphorus (P) dynamics in the rhizosphere is crucial for sustainable crop production. P mobilization processes in the rhizosphere include the release of plant and microbially-derived protons and extracellular phosphatases. We investigated the effect of root hairs and soil texture on the spatial distribution and intensity of P mobilizing processes in the rhizosphere of Zea mays L. root-hair defective mutant (rth3) and wild-type (WT) grown in two substrates (loam, sand). We applied 2D-chemical imaging methods in custom-designed root windows installed in the field to visualize soil pH (optodes), acid phosphatase activity (zymography), and labile P and Mn fluxes (diffusive gradients in thin films, DGT). The average rhizosphere extent for phosphatase activity and pH was greater in sand than in loam, while the presence of root-hairs had no impact. Acidification was significantly stronger at young root tissue (<2 cm from root cap) than at older root segments (>4 cm from root cap) and stronger in WT than rth3. Accompanied with stronger acidification, higher P flux was observed mainly around young, actively growing root tissues for both genotypes. Our results indicate that acidification was linked to root growth and created a pH optimum for acid phosphatase activity, i.e., mineralization of organic P, especially at young root tissues which are major sites of P uptake. Both genotypes grew better in loam than in sand; however, the presence of root hairs generally resulted in higher shoot P concentrations and greater shoot biomass of WT compared to rth3. We conclude that soil substrate had a larger impact on the extent and intensity of P solubilization processes in the rhizosphere of maize than the presence of root hairs. For the first time, we combined 2D-imaging of soil pH, phosphatase activity, and nutrient gradients in the field and demonstrated a novel approach of stepwise data integration revealing the interplay of various P solubilizing processes in situ. [Display omitted] • In situ chemical imaging of pH, acid phosphatase activity, P and Mn flux in the rhizosphere of field grown maize. • Average rhizosphere extents were larger in sand than in loam. • Root hairs mostly did not increase rhizosphere extents of imaged parameters. • Young root tissue of wild-type maize was more acidic than of root hair-defective mutant rth3. • Acidification from young root tissue created pH optimum for acid phosphatase activity and P uptake. [ABSTRACT FROM AUTHOR]

Understanding the biological roles of root hairs is key to projecting their contributions to plant growth and to assess their relevance for plant breeding. The objective of this study was to assess the importance of root hairs for maize nutrition, carbon allocation and root gene expression in a field experiment. Applying wild type and root hairless rth3 maize grown on loam and sand, we examined the period of growth including 4-leaf, 9-leaf and tassel emergence stages, accompanied with a low precipitation rate. rth3 maize had lower shoot growth and lower total amounts of mineral nutrients than wild type, but the concentrations of mineral elements, root gene expression, or carbon allocation were largely unchanged. For these parameters, growth stage accounted for the main differences, followed by substrate. Substrate-related changes were pronounced during tassel emergence, where the concentrations of several elements in leaves as well as cell wall formation-related root gene expression and C allocation decreased. In conclusion, the presence of root hairs stimulated maize shoot growth and total nutrient uptake, but other parameters were more impacted by growth stage and soil texture. Further research should relate root hair functioning to the observed losses in maize productivity and growth efficiency.

The interplay of rhizosphere components such as root exudates, microbes, and minerals results in small-scale gradients of organic molecules in the soil around roots. The current methods for the direct chemical imaging of plant metabolites in the rhizosphere often lack molecular information or require labeling with fluorescent tags or isotopes. Here, we present a novel workflow using laser desorption ionization (LDI) combined with mass spectrometric imaging (MSI) to directly analyze plant metabolites in a complex soil matrix. Undisturbed samples of the roots and the surrounding soil of Zea mays L. plants from either field- or laboratory-scale experiments were embedded and cryosectioned to 100 μm thin sections. The target metabolites were detected with a spatial resolution of 25 μm in the root and the surrounding soil based on accurate masses using ultra-high mass resolution laser desorption ionization Fourier-transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS). Using this workflow, we could determine the rhizosphere gradients of a dihexose (e.g., sucrose) and other plant metabolites (e.g., coumaric acid, vanillic acid). The molecular gradients for the dihexose showed a high abundance of this metabolite in the root and a strong depletion of the signal intensity within 150 μm from the root surface. Analyzing several sections from the same undisturbed soil sample allowed us to follow molecular gradients along the root axis. Benefiting from the ultra-high mass resolution, isotopologues of the dihexose could be readily resolved to enable the detection of stable isotope labels on the compound level. Overall, the direct molecular imaging via LDI-FT-ICR-MS allows for the first time a non-targeted or targeted analysis of plant metabolites in undisturbed soil samples, paving the way to study the turnover of root-derived organic carbon in the rhizosphere with high chemical and spatial resolution., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Lohse, Haag, Lippold, Vetterlein, Reemtsma and Lechtenfeld.)