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Burkholderia vietnamiensis B418 is a multifunctional plant growth-promoting rhizobacteria (PGPR) strain with nitrogen-fixing and phosphate-solubilizing capability which can be employed for root-knot nematode (RKN) management on various crops and vegetables. Here we investigated the control efficacy of B. vietnamiensis B418 inoculation against RKN on watermelon, applied either alone or combined with nematicides fosthiazate or avermectin, and their effects on bacterial and fungal microbiomes in rhizosphere soil. The results of field experiments showed individual application of B418 displayed the highest control efficacy against RKN by 71.15%. The combinations with fosthiazate and avermectin exhibited slight incompatibility with lower inhibitory effects of 62.71% and 67.87%, respectively, which were still notably higher than these nematicides applied separately. Analysis of microbiome assemblages revealed B418 inoculation resulted in a slight reduction for bacterial community and a significant increment for fungal community, suggesting that B418 could compete with other bacteria and stimulate fungal diversity in rhizosphere. The relative abundance of Xanthomonadales, Gemmatimonadales and Sphingomonadales increased while that of Actinomycetales reduced with B418 inoculation. The predominate Sordariomycetes of fungal community decreased dramatically in control treatment with B418 inoculation whereas there were increments in fosthiazate and avermectin treatments. Additionally, nitrogen (N) cycling by soil microbes was estimated by quantifying the abundance of microbial functional genes involved in N-transformation processes as B418 has the capability of N-fixation. The copy number of N-fixing gene nifH increased with B418 inoculation, and the highest increment reached 35.66% in control treatment. Our results demonstrate that B. vietnamiensis B418 is an effective biological nematicide for nematode management, which acts through the modulation of rhizosphere microbial community.

Background The soil environment is responsible for sustaining most terrestrial plant life, yet we know surprisingly little about the important functions carried out by diverse microbial communities in soil. Soil microbes that inhabit the channels of decaying root systems, the detritusphere, are likely to be essential for plant growth and health, as these channels are the preferred locations of new root growth. Understanding the microbial metagenome of the detritusphere, and how it responds to agricultural management such as crop rotations and soil tillage, is vital for improving global food production. Results This study establishes an in-depth soil microbial gene catalogue based on the living-decaying rhizosphere niches in a cropping soil. The detritusphere microbiome regulates the composition and function of the rhizosphere microbiome to a greater extent than plant type: rhizosphere microbiomes of wheat and chickpea were homogenous (65–87% similarity) in the presence of decaying root (DR) systems but were heterogeneous (3–24% similarity) where DR was disrupted by tillage. When the microbiomes of the rhizosphere and the detritusphere interact in the presence of DR, there is significant degradation of plant root exudates by the rhizosphere microbiome, and genes associated with membrane transporters, carbohydrate and amino acid metabolism are enriched. Conclusions The study describes the diversity and functional capacity of a high-quality soil microbial metagenome. The results demonstrate the contribution of the detritusphere microbiome in determining the metagenome of developing root systems. Modifications in root microbial function through soil management can ultimately govern plant health, productivity and food security.

Chickpea ( Cicer arietinum ) establishes symbiotic relationships with several Mesorhizobium species and the three-way interaction between chickpea variety, M esorhizobium strain, and environment, drives plant growth and nitrogen fixation. Here we quantified the phenotypic plasticity for shoot dry weight, nodule dry weight, nodules per plant, nodule colour, symbiotic effectiveness, and nitrogen cost in a factorial experiment combining five chickpea varieties, seven Mesorhizobium strains and three photothermal regimes. Plant growth and nitrogen fixation traits varied with variety, Mesorhizobium strain, photothermal environment and their interaction. Phenotypic plasticity was larger for nodules per plant (7.3-fold) than for shoot dry weight (2.7-fold), verifying a hierarchy of plasticities between these traits. Strain-driven plasticity of plant growth and nitrogen fixation traits was larger than variety-driven plasticity for our combination of varieties, strains, and photothermal environments, with strain-driven phenotypic plasticity being 2.7-fold vs 1.4-fold for shoot dry matter, 2.5-fold vs 1.7-fold for nodule dry weight, 7.3-fold vs 2.1-fold for nodules per plant, 3.7-fold vs 1.7-fold for nodule color, 2.9-fold vs 1.6-fold for symbiotic effectiveness, and 2.3-fold vs 1.6-fold for nitrogen cost. Our study provides insights on the phenotypic plasticity of the legume-rhizobia interaction by considering the plants as part of the rhizobia environment and vice-versa.

Background Unveiling the diversity of plant strategies to acquire and use phosphorus (P) is crucial to understand factors promoting their coexistence in hyperdiverse P-impoverished communities within fire-prone landscapes such as in cerrado (South America), fynbos (South Africa) and kwongan (Australia). Scope We explore the diversity of P-acquisition strategies, highlighting one that has received little attention: acquisition of P following fires that temporarily enrich soil with P. This strategy is expressed by fire ephemerals as well as fast-resprouting perennial shrubs. A plant’s leaf manganese concentration ([Mn]) provides significant clues on P-acquisition strategies. High leaf [Mn] indicates carboxylate-releasing P-acquisition strategies, but other exudates may play the same role as carboxylates in P acquisition. Intermediate leaf [Mn] suggests facilitation of P acquisition by P-mobilising neighbours, through release of carboxylates or functionally similar compounds. Very low leaf [Mn] indicates that carboxylates play no immediate role in P acquisition. Release of phosphatases also represents a P-mining strategy, mobilising organic P. Some species may express multiple strategies, depending on time since germination or since fire, or on position in the landscape. In severely P-impoverished landscapes, photosynthetic P-use efficiency converges among species. Efficient species exhibit rapid rates of photosynthesis at low leaf P concentrations. A high P-remobilisation efficiency from senescing organs is another way to use P efficiently, as is extended longevity of plant organs. Conclusions Many P-acquisition strategies coexist in P-impoverished landscapes, but P-use strategies tend to converge. Common strategies of which we know little are those expressed by ephemeral or perennial species that are the first to respond after a fire. We surmise that carboxylate-releasing P-mobilising strategies are far more widespread than envisaged so far, and likely expressed by species that accumulate metals, exemplified by Mn, metalloids, such as selenium, fluorine, in the form of fluoroacetate, or silicon. Some carboxylate-releasing strategies are likely important to consider when restoring sites in biodiverse regions as well as in cropping systems on P-impoverished or strongly P-sorbing soils, because some species may only be able to establish themselves next to neighbours that mobilise P.

Fusarium crown rot and wheat sharp eyespot are major soil-borne diseases of wheat, causing serious losses to wheat yield in China. We applied high-throughput sequencing combined with qPCR to determine the effect of winter wheat seed dressing, with either Trichoderma atroviride HB20111 spore suspension or a chemical fungicide consisting of 6% tebuconazole, on the fungal community composition and absolute content of pathogens Fusarium pseudograminearum and Rhizoctonia cerealis in the rhizosphere at 180 days after planting. The results showed that the Trichoderma and chemical fungicide significantly reduced the amount of F. pseudograminearum in the rhizosphere soil ( p  < 0.05), and also changed the composition and structure of the fungal community. In addition, field disease investigation and yield measurement showed that T. atroviride HB20111 treatment reduced the whiteheads with an average control effect of 60.1%, 14.9% higher than the chemical treatment; T. atroviride HB20111 increased yield by 7.7%, which was slightly more than the chemical treatment. Therefore, T. atroviride HB20111 was found to have the potential to replace chemical fungicides to control an extended range of soil-borne diseases of wheat and to improve wheat yield.

PurposeThe responses of nitrification and denitrification are not well characterised at temperatures above 35 °C, which is the focus of our study.Materials and methodsSoils collected from two dairy pastures (Victoria, Australia) were incubated at 10 to 45 °C in the dark for 5 to 10 days following amendment with 100 μg N g−1 either as NH4NO3, 14NH415NO3 or 15NH415NO3 (10 atom% 15N excess) at 50% water-filled pore space. To detect N2O from heterotrophic nitrification, acetylene (0.01% v/v) was used in a subset of samples amended with 15NH415NO3. Atom% 15N enrichments of NO3ˉ, N2O and N2 were measured during the experiment to evaluate the responses of nitrification and denitrification to temperature.Results and discussionN2O production from the two soils increased with rising temperature and peaked between 35 and 40 °C. N2O production from nitrification and denitrification both had similar thermal responses, which were different to N2 production. The N2O/N2 ratio decreased from > 4 at 35–40 °C to 0.5 at 45 °C, due to greater N2 than N2O production in the Dermosol. Heterotrophic nitrifiers oxidised NH4+ and released N2O at 35–40 °C, suggesting a role for heterotrophs in N cycling under warm climates. Topt for nitrification was between 35 and 40 °C, which is higher than reported previously. A short-term effect of high temperatures could provide NH4+ for the growth of crops but may also decrease soil C pools.ConclusionsIncreasing temperature above 35 °C altered the rates of nitrification, denitrification associated N2O and N2 production. Nitrification and denitrification peaked at 35–40 °C in the Chromosol and Dermosol. The production of N2 increased rapidly above 40 °C, which may be related to high soil respiration rates that likely decreased O2 availability, thus expanding the anaerobic microsites; such circumstances increased the reduction of N2O to N2 production from the Dermosol.

Aims The relationships between nitrogen (N) and phosphorus (P) acquisition strategies among herbaceous legume species remain poorly understood, particularly in relation to how they are altered by N availability. This study aimed to investigate the relationships between N 2 fixation, plant N concentration and P acquisition through two main strategies in temperate herbaceous legumes, and to demonstrate the influences of soil N availability on these relationships. Methods In a field pot experiment, eight temperate herbaceous legumes were grown with or without N addition. Plant growth, plant N and P concentrations, N 2 fixation, arbuscular mycorrhizal (AM) fungal colonization and root phosphatase activity (RPA) were measured, and the relationships between N 2 fixation, plant N concentration and each P acquisition strategy were assessed under contrasting N availability. Results N addition increased RPA. However, AM fungal colonization showed species-specific responses to N addition, and the ratio of AM fungal colonization to root phosphatase activity was decreased by N addition. Among eight legume species, AM fungal colonization increased with N 2 fixation rate in the absence of N addition, but no relationship was observed with N addition. RPA increased with plant N concentration among legume species, regardless of N addition. Conclusions As two key P acquisition strategies, neither RPA nor AM fungal colonization of temperate herbaceous legumes were species-specific traits, since both were positively correlated with N 2 fixation rate and plant N concentration. In addition, the correlation between N 2 fixation and AM fungal colonization was regulated by N availability. While N enrichment intensifies the legume reliance on RPA for acquiring more P, it weakens the association of N 2 fixation in driving across species AM colonization.

Symbiotic N fixation inhibition induced by N supply to legumes is potentially regulated by the relative N and P availability in soil. However, the specific responses of different legume species to changes in N:P availability remain unclear, and must be better understood to optimize symbiotic N fixation inputs under N enrichment. This study investigated mechanisms by which soil N and P supply influence the symbiotic N fixation of eight legume species, to quantify the inter-specific differences, and to demonstrate how these differences can be determined by the stoichiometric homeostasis in N:P ratios (H ). Eight herbaceous legume species were grown separately in outdoor pots and treated with either no fertilizer (control), N fertilizer (14 g N m ), P fertilizer (3.5 g P m ) or both N and P fertilizer. Plant nutrients, stoichiometric characteristics, root biomass, non-structural carbohydrates (NSC), rhizosphere chemistry, P mobilization, root nodulation and symbiotic N fixation were measured. N addition enhanced rhizosphere P mobilization but drove a loss of root biomass and root NSC exudation of P mobilization compound (organic acid), especially so in treatments without P addition. N addition also induced a 2-14% or 14-36% decline in symbiotic N fixation per plant biomass by legumes in treatments with or without P addition, as a result of decreasing root biomass and root NSC. The changes in symbiotic N fixation were positively correlated with stoichiometric homeostasis of N:P ratios in intact plants without root nodules, regardless of P additions. This study indicates that N addition can induce relative P limitations for growth, which can stimulate rhizosphere P mobilization at the expense of root biomass and carbohydrate concentrations, reducing symbiotic N fixation in legumes. Legume species that had less changes in plant N:P ratio, such as maintained symbiotic N fixation to a greater extent under N addition.

Indole acetic acid (IAA) can upregulate genes encoding enzymes responsible for the synthesis of carboxylates involved in phosphorus (P) solubilisation. Here, we investigated whether IAA and its precursor affect the P-solubilising activity of rhizobacteria. A total of 841 rhizobacteria were obtained using taxonomically selective and enrichment isolation methods. Phylogenetic analysis revealed 15 genera of phosphate solubilising bacteria (PSB) capable of producing a wide range of IAA concentrations between 4.1 and 67.2 µg mL−1 in vitro. Addition of l-tryptophan to growth media improved the P-solubilising activity of PSB that were able to produce IAA greater than 20 µg mL−1. This effect was connected to the drop of pH and release of a high concentration of carboxylates, comprising α-ketoglutarate, cis-aconitate, citrate, malate and succinate. An increase in production of organic acids rather than IAA production per se appears to result in the improved P solubilisation in PSB.

This study applied the multi-group structural equation modeling technique to identify differences in farmer motivations to adopting agroforestry practices in the Mt. Elgon region of Uganda. Data were collected from interviews with 400 smallholder coffee farmers belonging to four categories which included: (1) those actively participating in an Australian-funded trees for food security (T4FS) project from phase 1 (2014); (2) farmers neighbouring those actively participating in the T4FS project; (3) farmers actively participating in the T4FS project from phase 2 (2017) and; (4) farmers living distant and unaware of the T4FS project. We used the theory of planned behaviour framework to assess the adoption behaviour of these farmer categories resulting from project interventions. About 40% of the variation in farmer motivation to integrate trees in their coffee plantations was explained by the significant variables of ‘attitude’ and ‘perceived behavioural control’ among farmers actively participating in the T4FS project from phase 1. However, the neighbors of participating farmers and farmers who had never interacted with the project were only motivated by ‘attitude’ and ‘social norms’ respectively. Farmer motivation resulting from social pressure was strongest among farmers who had never interacted with the project, and in the absence of project interventions, rely on existing social structures to drive change in their community. Farmers’ perceived behavioural control to overcome tree planting barriers and their attitude to the economic benefits of shaded coffee were significantly different among the four farmer categories (p < 0.05). The findings indicate that psychological factors are key drivers to the farmers’ internal decision-making process in agroforestry technology adoption and can be context-specific. The adoption behaviour of smallholder farmers is mainly shaped by existing community social norms and beliefs that tend to promote knowledge exchange, as opposed to the conventional knowledge transfer extension approaches. Norms are therefore an inherent part of social systems and can create distinct farming practices, habits and standards within a social group. Researchers and extension agents can act upon these identified positive attitudes, norms and perceived behavioural controls to guarantee adoption and sustainability of agricultural technologies.