Root-soil Interface Research Articles

Physically-based reduction function to model unsaturated flow associated with plant transpiration

Vegetation plays an important ‘hydrological’ role in stabilising geostructures. Soil water is extracted by the roots due to transpiration, this increases soil suction and, hence, soil shear strength. Transpiration occurs in two different regimes, energy-limited and the water-limited regimes respectively. These two regimes are reflected in the two branches of the transpiration reduction function used to model the hydraulic boundary conditions for vegetated ground. The water-limited branch accounts for the reduced transmissivity of the soil-root system when the degree of saturation and, hence, the hydraulic conductivity declines. The water-limited branch of existing reduction functions (e.g., Feddes function) is defined in purely phenomenological fashion with parameters that have no clear link with the complex interaction between soil hydraulic properties and root architecture. A paradigm shift can be achieved through physically-based reduction functions. These require analytical closed-form solutions of radial water flow at the soil-root interface that, in turn require introducing simplifying assumptions, i.e., steady-state flow and a simplified hydraulic conductivity function. This paper explores the implications of these assumptions by i) benchmarking the water-limited branch of the reduction function derived analytically against the one derived numerically for more realistic hydraulic behaviour and ii) assessing the steady-state assumption.

Large cell triaxial tests of a partially saturated soil with vegetation

The use of vegetation roots as a nature-based solution against landslides and erosion requires the definition of sample preparation protocols and adoption of equipment that allows testing representative elementary volumes of the whole soil-root system. For this purpose, large cell triaxial compression tests were carried out on fallow and vegetated samples at different degrees of saturation. Samples were prepared by static compaction of a silty sand and seeded with Cynodon dactylon. The hydraulic state during plants growth was controlled and reproduced on bare soil samples. After isotropic compressions, the shearing phase was carried out at very low confining stresses (i.e., below 50 kPa). Tests were deemed to be comparable by assessing the normalised volume of roots with respect to soil, after shearing. For a given confining stress, soil samples with higher matric suction exhibited higher shear strength, furtherly increased by roots. The stress-strain behaviour observed in the vegetated soil systematically changed, when comparing tests at low and high matric suction values, due to the different mechanisms of vegetation reinforcement depending on the hydraulic state at the soil-root interface. The results were successfully interpreted within a failure criterion and skeleton stress framework for partially saturated soils, considering soil suction, degree of saturation, soil microstructure and the normalised volume of roots.

Grass rather than legume species decreases soil organic matter decomposition with nutrient addition

Nutrient addition to soil can strongly affect interactions at the root-soil interface, which play a central role in terrestrial ecosystem functions. Controversies, however, remain on whether or not soils sequester more carbon (C) with nutrient addition (for example in the context of increasing atmospheric CO2 concentrations), and what is the role of plant traits and growth strategies in these impacts. In this study, we conducted a laboratory manipulation experiment focused on the effects of a grass (Lolium perenne L.) species, a legume (Lotus corniculatus L.) species, and their mixture with or without nutrient addition on plant biomass, root-derived respiration, soil organic matter (SOM) decomposition, and microbial community structure.L. perenne biomass and root-derived respiration were more responsive than L. corniculatus biomass and root-derived respiration to nutrient addition. The decomposition of SOM decreased, and the priming effect (PE) was negative, both with and without nutrient addition, in both plant species. Nutrient addition, however, impacted the magnitude of PE to a lesser extent in L. corniculatus than in L. perenne and in monocultures than in the mixture. With nutrient addition, fungi were more abundant and utilized a greater proportion of root-derived C than bacteria. In conclusion, the current study suggests that nutrient addition may promote a positive soil C-balance through reduced SOM decomposition, and that plant species mixtures with increased root-derived C-flow (representing root and rhizomicrobial respiration) suppressed SOM decomposition to a greater extent.

Above and belowground traits impacting transpiration decline during soil drying in 48 maize (Zea mays) genotypes.

Stomatal regulation allows plants to promptly respond to water stress. However, our understanding of the impact of above and belowground hydraulic traits on stomatal regulation remains incomplete. The objective of this study was to investigate how key plant hydraulic traits impact transpiration of maize during soil drying. We hypothesize that the stomatal response to soil drying is related to a loss in soil hydraulic conductivity at the root-soil interface, which in turn depends on plant hydraulic traits. We investigate the response of 48 contrasting maize (Zea mays) genotypes to soil drying, utilizing a novel phenotyping facility. In this context, we measure the relationship between leaf water potential, soil water potential, soil water content and transpiration, as well as root, rhizosphere and aboveground plant traits. Genotypes differed in their responsiveness to soil drying. The critical soil water potential at which plants started decreasing transpiration was related to a combination of above and belowground traits: genotypes with a higher maximum transpiration and plant hydraulic conductance as well as a smaller root and rhizosphere system closed stomata at less negative soil water potentials. Our results demonstrate the importance of belowground hydraulics for stomatal regulation and hence drought responsiveness during soil drying. Furthermore, this finding supports the hypothesis that stomata start to close when soil hydraulic conductivity drops at the root-soil interface.

Iron supply confers tolerance in rice (Oryza sativa L.) to NaCl stress due to up-regulation of antioxidative enzymatic activity

• NaCl inhibited the growth and increased Na and Cl concentrations in rice leaves. • Application of Fe mitigated adverse effects of NaCl on rice growth. • Fe-supply enahnced plant antioxidant enzymatic activity in rice plants. • Fe-supply improved CEC of the soil and limited Na uptake by rice. Soil salinity affects plant growth and results in decreased agriculture productivity throughout the world. The present study was carried out to elucidate underlying mechanisms responsible for soil salinity tolerance in rice under the application of iron (Fe) as Fe(NH 4 ) 2 (SO 4 ) 2 ·6H 2 O. Rice seedlings were grown in pots containing soils (pH >8.0) and exposed to 0 and 150 mM (EC of 5.25 dS m −1 ) NaCl stress both with and without 0.5 mM Fe(NH 4 ) 2 (SO 4 ) 2 ·6H 2 O. Plant growth attributes like root length, shoot length, roots and shoots biomass, leaf chlorophyll contents, salt ions, plant mineral concentrations, plant antioxidant activities and cation exchange of soil were determined. Application of NaCl at the rate of 150 mM reduced the leaf chlorophyll contents (34%), root fresh and dry weights (39% and 45%), shoot fresh and dry weights (35% and 53%), grain yield (39%), potassium (K) (59% and 48%), calcium (Ca) (39% and 24%), and magnesium (Mg) (59% and 50%) concentrations in roots and leaves, whereas enhanced the sodium (Na) (537% and 500%) and chloride (Cl) (425% and 156%) accumulation both in roots and leaves as compared to control. Interestingly, Fe-supply enhanced the Fe uptake by rice plants and mitigated the adverse effects of salinity on rice growth and yield by 30%. Moreover, plant antioxidant enzymatic activity and cation exchange capacity (CEC) of soil (cmol Na + kg −1 ) were enhanced by Fe-supply. In conclusion, Fe-application enhanced tolerance in rice due to down regulation of Na and up-regulation of Fe and antioxidant enzymatic activity in leaves. In addition, increased Na adsorption onto the surface of soils suggests that Fe competes with Na at soil-root interface, thereby limiting Na uptake by the plants.

Microbial nutrient limitation in rhizosphere soils of different food crop families: Evidence from ecoenzymatic stoichiometry

AbstractMicrobial metabolism directly participates in soil nutrient recycling and ecosystem stability. Although the rhizosphere soil is one of the most active habitats, information on how microorganisms acquire nutrients based on ecoenzymatic stoichiometry is lacking, especially for major food crops under field conditions. Thus, this study aimed to explore the soil nutrient properties and microbial biomass concentrations as well as extracellular enzymatic activities and focused on carbon (C), nitrogen (N), and phosphorus (P) cycling of different crop types, including potato (Solanaceae), maize (cereal), soybean (Leguminosae), and buckwheat (Polygonaceae) in the arid area of northern Shaanxi, China. Microorganisms in the rhizosphere soil of four crops were subjected to relative C and N limitation tests based on ecoenzymatic activities. Among the crops, potato and soybean exhibited the maximum limitations. Linear regression analysis of soil nutrients, microorganisms (stoichiometric homeostasis), and threshold elemental ratio synthetically supported microbial metabolic limitations. This finding coincided with the highest and lowest microbial carbon use efficiencies in soybean (0.58) and potato (0.51), illustrating the distinguishing physiological responses of microbes. Partial least squares path modeling further confirmed that the soil microbial nutrient limitations were significantly regulated by soil nutrient properties and extracellular enzyme activities. Taken together, soil microbial metabolism in agricultural ecosystems was co‐limited by relative C and N regardless of the main food crop families in this region in China (northern Loess Plateau). These observations clarified the nutrient cycling driven by soil elemental stoichiometry at the root–soil interface in arid and oligotrophic ecosystems.

Root phenotypes as modulators of microbial microhabitats.

Plant roots are colonized by a multitude of microbial taxa that dynamically influence plant health. Plant-microbe interactions at the root-soil interface occur at the micro-scale and are affected by variation in root phenotypes. Different root phenotypes can have distinct impacts on physical and chemical gradients at the root-soil interface, leading to heterogeneous microhabitats for microbial colonization. Microbes that influence plant physiology will establish across these heterogeneous microhabitats, and, therefore, exploiting variation in root phenotypes can allow for targeted manipulation of plant-associated microbes. In this mini-review, we discuss how changes in root anatomy and architecture can influence resource availability and the spatial configuration of microbial microhabitats. We then propose research priorities that integrate root phenotypes and microbial microhabitats for advancing the manipulation of root-associated microbiomes. We foresee the yet-unexplored potential to harness diverse root phenotypes as a new level of precision in microbiome management in plant-root systems.

Back to the roots: Understanding banana below‐ground interactions is crucial for effective management of Fusarium wilt

AbstractGlobal banana production is affected by Fusarium wilt, a devastating disease caused by the soilborne root‐infecting fungus, Fusarium oxysporum f. sp. cubense (Foc). Fusarium wilt is notoriously difficult to manage because infection arises through complex below‐ground interactions between Foc, the plant, and the soil microbiome in the root–soil interface, defined as the rhizosphere. Interactions in the rhizosphere play a pivotal role in processes associated with pathogen development and plant health. Modulation of these processes through manipulation and management of the banana rhizosphere provides an auspicious prospect for management of Fusarium wilt. Yet, a fundamental understanding of interactions in the banana rhizosphere is still lacking. The objective of this review is to discuss the state‐of‐the‐art of the relatively scant data available on banana below‐ground interactions in relation to Fusarium wilt and, as a result, to highlight key research gaps. Specifically, we seek to understand (a) the biology of Foc and its interaction with banana; (b) the ecology of Foc, including the role of root‐exuded metabolites in rhizosphere interactions; and (c) soil management practices and how they modulate Fusarium wilt. A better understanding of molecular and ecological factors influencing banana below‐ground interactions has implications for the development of targeted interventions in the management of Fusarium wilt through manipulation of the banana rhizosphere.

A comprehensive review on coupled processes and mechanisms of soil-vegetation-hydrology, and recent research advances

Research on the coupling of soil, vegetation, and hydrological processes is not only a research hotspot in disciplines such as pedology, ecohydrology and Earth system science but also important for achieving sustainable development. However, scientists from different disciplines usually study the coupling mechanism of soil-vegetation-hydrological processes at very different space and time scales, and the mechanistic connections between different scales are quite few. This article reviewed research advances in coupled soil-vegetation-hydrological processes at different spatial scales—from leaf stomata to watershed and regional scales—and summarized the spatial upscaling methods and modeling approaches of coupled soil-vegetation-hydrological processes. We identify and summarize the following coupling processes: (1) carbon-water exchange in leaf stomata and root-soil interface; (2) changes in soil aggregates and profile hydraulic properties caused by plant roots and water movement; (3) precipitation and soil moisture redistribution by plant canopy and root; (4) interactions between vegetation patches and local hydrological process; (5) links between plant community succession and soil development; and (6) links between watershed/regional water budget and vegetation phenology and production. Meanwhile, the limitations and knowledge gaps in the observations, mechanisms, scaling methods, and modeling approaches of coupled soil-vegetation-hydrological processes were analyzed. To achieve a deep integration of various coupling processes across different spatiotemporal scales, future work should strengthen multiscale, multifactor and multiprocess soil-vegetation-hydrology coupling observations and mechanism studies, develop new scaling methods, identify different feedback pathways, and take time-variable plant behavior and soil hydraulic properties into account during modeling.

Metabolite Production in Alkanna tinctoria Links Plant Development with the Recruitment of Individual Members of Microbiome Thriving at the Root-Soil Interface.

ABSTRACTPlants are naturally associated with diverse microbial communities, which play significant roles in plant performance, such as growth promotion or fending off pathogens. The roots of Alkanna tinctoria L. are rich in naphthoquinones, particularly the medicinally used enantiomers alkannin and shikonin and their derivatives. Former studies already have shown that microorganisms may modulate plant metabolism. To further investigate the potential interaction between A. tinctoria and associated microorganisms, we performed a greenhouse experiment in which A. tinctoria plants were grown in the presence of three distinct soil microbiomes. At four defined plant developmental stages, we made an in-depth assessment of bacterial and fungal root-associated microbiomes as well as all extracted primary and secondary metabolite content of root material. Our results showed that the plant developmental stage was the most important driver influencing the plant metabolite content, revealing peak contents of alkannin/shikonin derivatives at the fruiting stage. Plant root microbial diversity was influenced both by bulk soil origin and to a small extent by the developmental stage. The performed correlation analyses and cooccurrence networks on the measured metabolite content and the abundance of individual bacterial and fungal taxa suggested a dynamic and at times positive or negative relationship between root-associated microorganisms and root metabolism. In particular, the bacterial genera Labrys and Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium as well as four species of the fungal genus Penicillium were found to be positively correlated with higher content of alkannins.IMPORTANCE Previous studies have shown that individual, isolated microorganisms may influence secondary metabolism of plants and induce or stimulate the production of medicinally relevant secondary metabolism. Here, we analyzed the microbiome-metabolome linkage of the medicinal plant Alkanna tinctoria, which is known to produce valuable compounds, particularly the naphthoquinones alkannin and shikonin and their derivatives. A detailed bacterial and fungal microbiome and metabolome analysis of A. tinctoria roots revealed that the plant developmental stage influenced root metabolite production, whereas soil inoculants from three different geographical origins in which plants were grown shaped root-associated microbiota. Metabolomes of plant roots of the same developmental stage across different soils were highly similar, pinpointing to plant maturity as the primary driver of secondary metabolite production. Correlation and network analyses identified bacterial and fungal taxa showing a positive relationship between root-associated microorganisms and root metabolism. In particular, the bacterial genera Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium and Labrys as well as the fungal species of genus Penicillium were found to be positively correlated with higher content of alkannins.

Root acid phosphatases and rhizobacteria synergistically enhance white lupin and rice phosphorus acquisition.

The rhizosheath is a belowground area that acts as a communication hub at the root-soil interface to promote water and nutrient acquisition. Certain crops, such as white lupin (Lupinus albus), acquire large amounts of phosphorus (P), owing partially to exudation of acid phosphatases (APases). Plant growth-promoting rhizobacteria also increase soil P availability. However, potential synergistic effects of root APases and rhizosheath-associated microbiota on P acquisition require further research. In this study, we investigated the roles of root purple APases (PAPs) and plant growth-promoting rhizobacteria in rhizosheath formation and P acquisition under conditions of soil drying (SD) and P treatment (+P: soil with P fertilizer; -P: soil without fertilizer). We expressed purple acid phosphatase12 (LaPAP12) in white lupin and rice (Oryza sativa) plants and analyzed the rhizosheath-associated microbiome. Increased or heterologous LaPAP12 expression promoted APase activity and rhizosheath formation, resulting in increased P acquisition mainly under SD-P conditions. It also increased the abundance of members of the genus Bacillus in the rhizosheath-associated microbial communities of white lupin and rice. We isolated a phosphate-solubilizing, auxin-producing Bacillus megaterium strain from the rhizosheath of white lupin and used this to inoculate white lupin and rice plants. Inoculation promoted rhizosheath formation and P acquisition, especially in plants with increased LaPAP12 expression and under SD-P conditions, suggesting a functional role of the bacteria in alleviating P deficit stress via rhizosheath formation. Together, our results suggest a synergistic enhancing effect of LaPAP12 and plant growth-promoting rhizobacteria on rhizosheath formation and P acquisition under SD-P conditions.

Experimental Study on Shear Strength of Root Composite Tailing Soil Based on Interfacial Bonding

At present, the root soil interface bonding is not considered in the root system of mechanical soil-fixing model. The typical restoration plant Amorpha fruticosa, utilizing the widely used Wu model (WWM), the tensile and tensile properties of single root, and the shear strength properties of root soil composite tailing, is analyzed by the tensile tests of plant roots, pullout tests, and shear tests based on the effect of interfacial bond strength; based on the failure mode of root system in root soil, the modified WWM model is used to calculate the increment of shear strength of composite tailing soil. The results showed that ① the relationship between root diameter of A. fruticosa and tensile strength was power function. ② The bond between root and soil becomes more tight, and the pullout strength of the root system increases significantly. ③ When root soil area ratio (RAR) is the same, shear deformation capacity of root soil composite tailing soil increases with the increase of interface bonding strength. Under the condition of the same interface bonding strength, the cohesion of root soil composite tailing soil is greater than that of tailing soil and increases with the increase of RAR, but the change of internal friction angle is not significant. When the pullout strength is added to the plant root prediction model, the soil consolidation effect of the plant root system can be better reflected. The range of the revised coefficient of the WWM model for the root soil composite tailing soil is 0.15~0.37. The research results will provide a theoretical basis and data support for quantifying the ecological restoration and reinforcement capacity of tailing pond shrubs and plants, slope stability, soil and water management, and other ecological soil consolidation capacity of mines.

Green manuring facilitates bacterial community dispersal across different compartments of subsequent tobacco

Green manure–crop rotation is a sustainable approach to protect crops against diseases and improve yield. However, the mechanism by which green manuring manipulates the crop-associated microbial community remains to be elucidated. In this study, we explored the horizontal processes of bacterial communities in different compartments of the soil–root interface (bulk soil, rhizosphere soil, rhizoplane and endosphere) of tobacco by performing a field experiment including four rotation practices, namely, tobacco rotated with smooth vetch, ryegrass, radish, and winter fallow (without green manure). Results showed that the co-occurrence networks constructed by adjacent compartments of the soil–root interface with green manuring had more edges than without green manuring, indicating dramatic microbial interactions. Green manuring increased the dispersal-niche continuum index between bulk soil and other compartments, indicating that it facilitated the horizontal dispersal of microbes. For the different green manuring practices, the neutral community model explained 24.6–27.6% of detection frequency for bacteria, and at least one compartment under each practice had a normalized stochasticity ratio higher than the 50% boundary point, suggesting that the deterministic and stochastic processes jointly shaped the tobacco microbiome. In conclusion, green manuring generally facilitates bacterial community dispersal across different compartments and enhances potential interactions among adjacent compartments. This study provides empirical evidence for understanding the microbiome assembly under green manure–crop rotation.

Rhizosheath: An adaptive root trait to improve plant tolerance to phosphorus and water deficits?

Drought and nutrient limitations adversely affect crop yields, with below‐ground traits enhancing crop production in these resource‐poor environments. This review explores the interacting biological, chemical and physical factors that determine rhizosheath (soil adhering to the root system) development, and its influence on plant water uptake and phosphorus acquisition in dry soils. Identification of quantitative trait loci for rhizosheath development indicate it is genetically determined, but the microbial community also directly (polysaccharide exudation) and indirectly (altered root hair development) affect its extent. Plants with longer and denser root hairs had greater rhizosheath development and increased P uptake efficiency. Moreover, enhanced rhizosheath formation maintains contact at the root‐soil interface thereby assisting water uptake from drying soil, consequently improving plant survival in droughted environments. Nevertheless, it can be difficult to determine if rhizosheath development is a cause or consequence of improved plant adaptation to dry and nutrient‐depleted soils. Does rhizosheath development directly enhance plant water and phosphorus use, or do other tolerance mechanisms allow plants to invest more resources in rhizosheath development? Much more work is required on the interacting genetic, physical, biochemical and microbial mechanisms that determine rhizosheath development, to demonstrate that selection for rhizosheath development is a viable crop improvement strategy.

Study on the cohesive shear characteristics and intrinsic modelling of the root–tailing soil interface of Amorpha fruticosa

To study the soil consolidation effect of shrub plant roots on tailings soil and to explore the frictional characteristics of plant roots on tailings soil, three experimental conditions of the root–soil interface were established by using a modified indoor direct shear instrument with binders such as liquid sodium silicate and cyanoacrylate to conduct direct shear frictional tests at the root–soil interface using the roots of the typical slope protection plant Amorpha fruticosa. The Gompertz improved curve model was established by using the relationship between shear stress and shear displacement and the trend of the root–soil interface parameter index. The results were compared between the improved Gompertz curve model and the Clough–Duncan hyperbolic model, and a two-factor coupled improved Gompertz interfacial intrinsic structure model with normal stress and cohesive strength factor was established. The results showed that the interface shear stress and shear displacement showed strain hardening characteristics at different normal pressures for cohesive strength ratios of 1.5 and 1.7 at the root–tailing soil interface. At a cohesive strength ratio of 1.6, strain-softening was observed from 100 to 300 kPa and strain hardening was observed at 400 kPa. The improved Gompertz curve model predicts the shear stress and shear displacement curves at the root–soil interface with different cohesive strengths more reasonably than the Clough–Duncan hyperbolic model, and the maximum accuracy can be improved by nearly 40%. The two-factor coupled improved Gompertz curve model can fit the shear stress versus shear displacement relationship at the A. fruticosa root–tailing soil interface.

Building soil sustainability from root-soil interface traits.

Great potential exists to harness plant traits at the root-soil interface, mainly rhizodeposition and root hairs, to 'build' soils with better structure that can trap more carbon and resources, resist climate stresses, and promote a healthy microbiome. These traits appear to have been preserved in modern crop varieties, but scope exists to improve them further because they vary considerably between genotypes and respond to environmental conditions. From emerging evidence, rhizodeposition can act as a disperser, aggregator, and/or hydrogel in soil, and root hairs expand rhizosheath size. Future research should explore impacts of selecting these traits on plants and soils concurrently, expanding from model plants to commercial genotypes, and observing whether impacts currently limited to glasshouse studies occur in the field.

Editorial: Special issue on silicon at the root-soil interface

Editorial: Special issue on silicon at the root-soil interface

Coupled effects of soil drying and salinity on soil-plant hydraulics.

Salinity and soil drying are expected to induce salt accumulation at the root-soil interface of transpiring plants. However, the consequences of this on the relationship between transpiration rate (E) and leaf xylem water potential (ψleaf-x) are yet to be quantified. Here, we used a noninvasive root pressure chamber to measure the E(ψleaf-x) relationship of tomato (Solanum lycopersicum L.) treated with (saline) or without 100-mM NaCl (nonsaline conditions). The results were reproduced and interpreted with a soil-plant hydraulic model. Under nonsaline conditions, the E(ψleaf-x) relationship became progressively more nonlinear as the soil dried (θ ≤ 0.13 cm3 cm-3, ψsoil = -0.08 MPa or less). Under saline conditions, plants exhibited an earlier nonlinearity in the E(ψleaf-x) relationship (θ ≤ 0.15 cm3 cm-3, ψsoil = -0.05 MPa or less). During soil drying, salinity induced a more negative ψleaf-x at predawn, reduced transpiration rate, and caused a reduction in root hydraulic conductance (from 1.48 × 10-6 to 1.30 × 10-6 cm3 s-1 hPa-1). The model suggested that the marked nonlinearity was caused by salt accumulation at the root surface and the consequential osmotic gradients. In dry soil, most water potential dissipation occurred in the bulk soil and rhizosphere rather than inside the plant. Under saline-dry conditions, the loss in osmotic potential at the root surface was the preeminent component of the total dissipation. The physical model of water flow and solute transport supports the hypothesis that a buildup of osmotic potential at the root-soil interface causes a large drop in ψleaf-x and limits transpiration rate under drought and salinity.

Roots and rhizospheric soil microbial community responses to tree species mixtures

Below-ground processes are crucial in determining the effects of plants on ecosystem function. The root-soil interface is a highly active zone due to root exudation and nutrient uptake. However, its role in determining effects of tree species and their interactions on the soil microbial community, ecosystem function and above-ground growth is less well known.We compared the effects of tree species monocultures and their mixture on rhizospheric microbial communities, specific functional genetic markers associated with processes in the nitrogen (N) cycle, and above-ground and below-ground growth and nutrient allocation. Two pairs of tree species were grown: Pseudotsuga menziesii and Alnus rubra; Acer pseudoplatanus and Quercus robur. Tree establishment altered soil microbial composition, but after 26 months differences amongst tree species and effects of species mixture were minor, suggesting functional redundancy in microbial communities. A greater abundance of fungi, bacteria, and specifically ammonia oxidising and denitrifying bacteria in the rhizospheric soil of the N-fixing A. rubra was the most notable trend. Mixing A. rubra with P. menziesii did produce overyielding: trees grown in mixture attained a two-fold greater (Relative Yield Total 2.03 ± 0.52) above-ground biomass than in a mixture predicted from trees grown in monoculture. We did not observe strong trends in overyielding for A. psuedoplatanus and Q. robur. Inclusion of the N-fixing species A. rubra in admixture with P. menziesii promoted N cycling, and decreased the C:N ratios of leaf, branch, and root tissues but not soil C:N ratio for P. menziesii. Given the observed overyielding in the A. rubra with P. menziesii mixtures, we explored potential mechanistic links between functional genetic markers for nitrification and ammonification, however we found no statistically significant effects attributable to these genetic markers. We found root area index was significantly lower in A. rubra monocultures than in admixture with P. menziesii. For both P. menziesii and A. rubra, the number of root tips was lower in mixture than monoculture, indicating physical partitioning of soil space as a result of growing in mixture. We documented additive and synergistic effects of tree species identity on above and belowground productivity, and rhizospheric microbial community development in these four tree species.

Wheat Straw Return Influences Soybean Root-Associated Bacterial and Fungal Microbiota in a Wheat-Soybean Rotation System.

Roots hold complex microbial communities at the soil–root interface, which can affect plant nutrition, growth, and health. Although the composition of plant microbiomes has been extensively described for various plant species and environments, little is known about the effect of wheat straw return (WSR) on the soybean root microbiota. We used Illumina-based 16S rRNA and ITS amplicon sequencing to track changes in bacterial and fungal microbiota in bulk soil and soybean rhizosphere, rhizoplane, s1and endosphere during the third and fourth years after implementing WSR in a wheat–soybean rotation system. The results revealed that WSR had a greater impact on fungal communities than bacterial communities, particularly in bulk soil, rhizosphere, and rhizoplane. WSR enriched the relative abundance of cellulose-degrading fungi (e.g., Acremonium, Trichoderma, and Myrmecridium, among which Trichoderma also had antimicrobial activity), saprotroph (e.g., Exophiala), and nitrogen cycling bacteria (e.g., Chryseolinea). Furthermore, WSR depleted the relative abundance of pathogenic fungi (e.g., Fusarium and Alternaria). These data revealed for the first time that WSR had diverse effects on soybean root-associated microbial community composition, not only in soil but also in the rhizosphere, rhizoplane, and endosphere.