Showing papers in "Plant and Soil in 2015"

Ecological interpretations of nitrogen isotope ratios of terrestrial plants and soils

Abstract: Background Knowledge of biological and climatic controls in terrestrial nitrogen (N) cycling within and across ecosystems iscentral tounderstandingglobalpatternsof keyecosystemprocesses.Theratiosof 15 N: 14 Ninplants and soils have been used as indirect indices of N cycling parameters, yet our understanding of controls over N isotope ratios in plants and soils is still developing. Scope In this review, we provide background on the mainprocessesthataffectplantandsoilNisotoperatios. In a similar manner to partitioning the roles of state factors and interactive controls in determining ecosystem traits, we review N isotopes patterns in plants and soils across a number of proximal factors that influence ecosystem properties as well as mechanisms that affect these patterns. Lastly, some remaining questions that would improve our understanding of N isotopes in terrestrial ecosystems are highlighted. Conclusion Compared to a decade ago, the global patterns of plant and soil N isotope ratios are more resolved. Additionally, we better understand how plant and soil N isotope ratios are affected by such factors as mycorrhizal fungi, climate, and microbial processing. A comprehensive understanding of the N cycle that ascribes different degrees of isotopic fractionation for each step under different conditions is closer to being realized, but a number of process-level questions still remain.

Arbuscular mycorrhiza and nitrogen: implications for individual plants through to ecosystems

Abstract: Arbuscular mycorrhizal fungi (AMF) form mutualistic symbioses with c. two-thirds of all land plants. Traditionally, it was thought that they played no role in nitrogen (N) acquisition for their host, despite early evidence to the contrary. More recently, this perception has changed radically, with the demonstration that AMF can acquire N from both inorganic and organic N sources and transfer some of this N to their host plant. This review discusses the current evidence for AMF N uptake, transport and plant transfer under different experimental conditions and highlights key questions that remain to be resolved. The relevance of this AMF N acquisition pathway is discussed both in relation to host plant and fungal N nutrition. The importance of interactions with the soil community and subsequent implications for soil N cycling are also highlighted. Reported AMF contribution to plant N varies widely, but the reasons for this variability are unclear. In low N systems even small amounts of ‘extra’ N may confer the plant with a competitive advantage, but it is also likely that competition for N between symbionts occurs. To advance this area, a more mechanistic approach is required that treats the fungus as a Darwinian organism rather than a mere extension of the plant. Application of genomics and metabolomics technologies to this topic should enable resolution of some of the key questions outlined in this review.

Linkages of plant and soil C:N:P stoichiometry and their relationships to forest growth in subtropical plantations

Abstract: Ecological stoichiometry plays important roles in ecosystem dynamics and functioning, but relationships between above- and belowground stoichiometry and stoichiometric effects on the growth of different plant functional groups in forests remain poorly understood. In an age sequence of 2-, 4- and 6-year-old Eucalyptus plantations in subtropical China, we examined C, N and P concentrations and their ratios in the soil and leaves. Each plantation was divided into overstory and understory plant functional groups. The relationships between stoichiometric characteristics and forest growth were analyzed. Soil C and P decreased in the Eucalyptus age sequence, which led to changes in soil stoichiometric characteristics. Leaf C:P and N:P ratios were higher for Eucalyptus trees than for understory plants because of the low P concentrations in Eucalyptus leaves. Soil and plant N:P ratios were strongly related. Understory biomass was positively related to N:P ratios, while overstory growth was negatively related to N:P ratios. Our results suggest that nutrient concentrations in soil and plants are tightly linked in Eucalyptus plantations and that P limitation increases with stand age. Stoichiometric characteristics appear to mediate forest properties and functions under nutrient limitation in subtropical regions.

Soil C:N ratio is the major determinant of soil microbial community structure in subtropical coniferous and broadleaf forest plantations

Abstract: This study aimed to determine the influence of tree species on soil microbial community structure. We conducted a litter and root manipulation and a short-term nitrogen (N) addition experiment in 19-year-old broadleaf Mytilaria laosensis (Hamamelidaceae) and coniferous Chinese fir (Cunninghamia lanceolata) plantations in subtropical China. Phospholipid fatty acid (PLFA) analysis was used to examine treatment effects on soil microbial community structure. Redundancy analysis (RDA) was performed to determine the relationships between individual PLFAs and soil properties (soil pH, carbon (C) and N concentration and C:N ratio). Soil C:N ratio was significantly greater in M. laosensis (17.9) than in C. lanceolata (16.2). Soil C:N ratio was the key factor affecting the soil microbial community regardless of tree species and the litter, root and N treatments at our study site. The fungal biomarkers, 18:1ω9 and 18:2ω6,9 were significantly and positively related to soil C:N ratio and the abundance of bacterial lipid biomarkers was negatively related to soil C:N ratio. N addition for 8 months did not change the biomass and structure of the microbial community in M. laosensis and C. lanceolata soils. Soil nutrient availability before N addition was an important factor in determining the effect of N fertilization on soil microbial biomass and activity. PLFA analysis showed that root exclusion significantly decreased the abundance of the fungal biomarkers and increased the abundance of the Gram-positive bacteria. Rootless plots had a relatively lower Gram-positive to Gram-negative bacteria ratio and a higher fungi to bacteria ratio compared to the plots with roots under both M. laosensis and C. lanceolata. The response of arbuscular mycorrhizal fungi (16:1ω5) to root exclusion was species-specific. These observations suggest that soil C:N ratio was an important factor in influencing soil microbial community structure. Further studies are required to confirm the long-term effect of tree species on soil microbial community structure.

Drivers of soil organic carbon storage and vertical distribution in Eastern Australia

Abstract: Drivers of soil organic carbon (SOC) storage are likely to vary in importance in different regions and at different depths due to local factors influencing SOC dynamics. This paper explores the factors influencing SOC to a depth of 30 cm in eastern Australia. We used a machine learning approach to identify the key drivers of SOC storage and vertical distribution at 1401 sites from New South Wales, Australia. We then assessed the influence of the identified factors using traditional statistical approaches. Precipitation was important to and positively associated with SOC content, whereas temperature was important to and negatively associated with SOC vertical distribution. The importance of geology to SOC content increased with increasing soil depth. Land-use was important to both SOC content and its vertical distribution. We attribute these results to the influence of precipitation on primary production controlling SOC content, and the stronger influence of temperature on microbial activity affecting SOC degradation and vertical distribution. Geology affects SOC retention below the surface. Land-use controls SOC via production, removal and vertical mixing. The factors driving SOC storage are not identical to those driving SOC vertical distribution. Changes to these drivers will have differential effects on SOC storage and depth distribution.

Changes in nitrogen and phosphorus limitation during secondary succession in a karst region in southwest China

Abstract: Nitrogen (N) and/or phosphorus (P) limitation to primary productivity and other biological processes can change in a variety of ways as ecosystems develop. How N limitation and P limitation change from the early to the late stages of a secondary succession following farmland abandonment remains unclear in karst ecosystems in southwest China. We used community foliar N:P ratio, soil alkaline phosphatase activity (APA) and other indicators of nutrient status (soil organic carbon [SOC], total soil N [TN], and total soil P [TP], Alkali-hydrolyzable N [AN], and available soil phosphorus [AP] concentrations) to examine changes in N and P status during secondary vegetation succession. Four types of plant communities (grasslands, shrublands, secondary forest, and primary forest) represented the early, middle, late, and very late successional stages, respectively. Community foliar N:P ratio, APA, and APA per unit SOC increased as succession proceeded from the grassland to the secondary and primary forest communities. Moreover, community foliar N:P ratios in the grassland were positively correlated with soil TN, while community foliar N:P ratios in the secondary forest and primary forest were negatively correlated with soil TP, but were not correlated with soil TN. Community foliar N:P ratios in the shrubland were not correlated with either soil TN or TP. Our results suggest that the grassland in the karst region of southwest China is N limited, that the secondary and primary forests are P limited, and that the shrubland is constrained by N and P together or by other nutrients.

Rhizosphere bacteria and fungi associated with plant growth in soils of three replanted apple orchards

Abstract: High-throughput 454 pyrosequencing was applied to investigate differences in bacterial and fungal communities between replant and closely situated control non-replant (fallow) soils. The V1-V3 region of the bacterial 16S rRNA gene and the ITS1 region of fungi from the different soils were sequenced using 454 pyrosequencing (Titanium chemistry), and data were analysed using the MOTHUR pipeline. The bacterial phyla Proteobacteria, Actinobacteria and Acidobacteria dominated in both fallow and replant apple orchard soils, and community composition at both phylum and genus level did not significantly differ according to NP-MANOVA. The fungal phyla Ascomycota, Zygomycota and Basidiomycota were dominant, and communities also did not differ in composition at either phylum or genus level. High positive Pearson correlations with plant growth in a plant growth assay performed with apple rootstocks plantlets were detected for the bacterial genera Gp16 and Solirubrobacter (r: >0.82) and fungal genera Scutellinia, Penicillium, Lecythophora and Paecilomyces (r: >0.65). Strong negative correlations with plant growth were detected for the bacterial genera Chitinophaga and Hyphomicrobium (r: <−0.78) and the fungal genera Acremonium, Fusarium and Cylindrocarpon (r: <−0.81). Study findings are in part consistent with those of previous research, but also highlight associations between apple plants and certain microbial genera. The functional role of these genera in affecting soil health and fertility should be further investigated.

Biochar but not humic acid product amendment affected maize yields via improving plant-soil moisture relations

Abstract: Biochar (BC) and humic acid product (HAP) soil amendments may improve plant performance under water-limited conditions. Our aim was to investigate if BC and HAP amendments, alone or in combination, will have positive and synergistic effects. A three-factorial fully randomized study was carried out in the greenhouse for 66 days, including the factors ‘BC’, ‘HAP’ and ‘water regime’. Maize (Zea mays var. ‘Amadeo’ DKC-3399) was grown in pots (6 kg sandy soil pot−1) amended with/without BC (0, 1.5 and 3 %; w/w) and with/without HAP (0 or an equivalent of 8 kg ha−1). Two water regimes, limited and frequent (H2O limit , H2O frequ ), were applied after day 28 following seedling establishment at 60 % water holding capacity (WHC). In the H2O limit treatment, the soil water content was allowed to drop until wilting symptoms became visible (25–30 % WHC) while in H2O frequ the WHC was brought to 60 % of the maximum on a daily basis BC but not HAP, added alone or in combination with BC, significantly increased the biomass yield and the water and N use efficiency of plants at both water regimes. The BC-mediated relative increase in the yield was equal with both watering regimes, refuting initial hypotheses. BC had generally a stimulating effect on water relations and photosynthesis, it increased the relative water content and the leaf osmotic potential, decreased the stomatal resistance and stimulated the leaf gas exchange (transpiration). Both, BC and pure HAP addition, stimulated photosynthesis by increasing the electron transport rate (ETR) of photosystem II (PSII) and of the ratio between effective photochemical quantum yield to non-photochemical quenching (Y(II)/Y(NPQ), revealing reduced heat dissipation. Biochar use in poor sandy soils can improve plant growth by improving soil-plant water relations and photosynthesis under both H2O frequ and H2O limit conditions. HAP loading, however, did not improve the effect of biochar or vice versa.

Biochar amendment increases maize root surface areas and branching: a shovelomics study in Zambia

Abstract: Positive crop yield effects from biochar are likely explained by chemical, physical and/or biological factors. However, studies describing plant allometric changes are scarcer, but may be crucial to understand the biochar effect. The main aim of the present study is to investigate the effect of biochar on root architecture under field conditions in a tropical setting. The presented work describes a shovelomics (i.e., description of root traits in the field) study on the effect of biochar on maize root architecture. Four field experiments we carried out at two different locations in Zambia, exhibiting non-fertile to relatively fertile soils. Roots of maize crop (Zea mays L.) were sampled from treatments with fertilizer (control) and with a combination of fertilizer and 4 t.ha−1 maize biochar application incorporated in the soil. For the four sites, the average grain yield increase upon biochar addition was 45 ± 14 % relative to the fertilized control (from 2.1–6.0 to 3.1–9.1 ton ha−1). The root biomass was approximately twice as large for biochar-amended plots. More extensive root systems (especially characterized by a larger root opening angle (+14 ± 11 %) and wider root systems (+20 ± 15 %)) were observed at all biochar-amended sites. Root systems exhibited significantly higher specific surface areas (+54 ± 14 %), branching and fine roots: +70 ± 56 %) in the presence of biochar. Biochar amendment resulted in more developed root systems and larger yields. The more extensive root systems may have contributed to the observed yield increases, e.g., by improving immobile nutrients uptake in soils that are unfertile or in areas with prolonged dry spells.

Biochars and the plant-soil interface

Abstract: Over the past decade, biochar soil management has seen a surge in activities related to both research and development (Lehmann and Joseph 2015; Ok et al. 2015). Even though our knowledge has considerably advanced, the effects of biochars on crop growth still appear unpredictable, with in some instances increasing while in others decreasing yield responses (Liu et al. 2013; Jeffery et al. 2015a). To a large extent, this is a result of widely varying biochar properties (Enders et al. 2012; Schimmelpfennig and Glaser 2012) as well as of variable soil properties and environmental plant requirements. Some biochars may increase crop yield, whereas others may decrease yield for reasons that are readily explainable using known responses of crops to for example altered pH or salt contents (Van Zwieten et al. 2010; Rajkovich et al. 2012) and short-termN limitation in N deficient soil (Clough et al. 2013). However, we also observe a distinct lack of mechanistic insight into how properties that are shared by many biochars affect plant growth. This special issue focuses on identifying and explaining the mechanisms by which certain biochar properties affect plant performance through rhizosphere interactions.

Induced systemic resistance in tomato ( Solanum lycopersicum ) against Botrytis cinerea by biochar amendment involves jasmonic acid signaling

Abstract: The goal of this study was to identify the induced resistance pathway mediated by biochar in the tomato – Botrytis cinerea pathosystem. Tomato wild types and mutants modified in their salicylic acid (SA), ethylene (ET) or jasmonic acid (JA) metabolism were grown in a potting medium amended with biochar produced at 450 oC from greenhouse wastes, to identify the possible pathway(s) involved in biochar-mediated resistance to B. cinerea. Early cellular response of H2O2 accumulation was biochemically tested, and the transcriptional changes of 12 defense-related genes upon B. cinerea challenge of detached leaflets were analyzed. Biochar amendment resulted in about 50 % reduction in B. cinerea disease severity in all tested genotypes with the exception of a JA deficient mutant, def1. Biochar amendment induced priming of early as well as late-acting defense responses particularly in the genes Pti5 (ET-related) and Pi2 (JA-related), which are known to be crucial in resistance against B. cinerea. Stronger and earlier H2O2 accumulation subsequent to B. cinerea inoculation in all genotypes was observed as a result of biochar amendment, with the exception of the def1 mutation. Biochar-mediated IR in the B. cinerea-tomato pathosystem involves the JA pathway.

Competition with winter crops induces deeper rooting of walnut trees in a Mediterranean alley cropping agroforestry system

Abstract: Background and Aims: Characterising the spatial distribution of tree fine roots (diameter ≤ 2 mm) is fundamental for a better understanding of tree functioning in agroforestry ecosystems. The absorptive function of fine roots is closely associated to aboveground tree performance and the amount of competition experienced by a tree partly depends on how root spatial distribution varies with soil depth in mixed systems. We therefore compared root interception densities (RID) and fine root orientation of trees grown in an alley cropping agroforestry stand (AF) with those in a monocropped (M) tree stand. Methods: Fieldwork was conducted in two 17 year old hybrid walnut (Juglans regia × nigra cv. NG23) stands in southern France: one was an alley cropping agroforestry stand with durum wheat (Triticum turgidum L. subsp. durum) intercropped in strips between tree rows and the second was a monocropped stand with a natural understorey. One pit was dug in each stand to a depth of 1.6 m, and an additional pit was dug in the agroforestry stand to a depth of 4.0 m to examine the tree root distribution below the crop maximum rooting depth. Root intercepts were mapped on trench walls and soil cubes were sampled to assess 3D root orientation, and to establish a predictive model of root length densities (RLD). Results: In the monocropped stand, root mapping evidenced a very high RID in the top 0.5 m and a slight decrease with increasing soil depth, whereas in the agroforestry stand, RID was much lower for the same depth, but roots occupied a higher volume of soil. In the agroforestry stand, RID and RLD were higher within the tree row than in the inter-row at deeper depths. Tree fine roots in both stands were not preferentially oriented from the soil surface until a depth of 1.5 m, beneath which fine roots in the agroforestry stand were significantly more vertically oriented. Conclusions: Fine roots of intercropped walnut trees went significantly deeper indicating strong plasticity of root distribution. Deeper root systems would reduce direct competition from the crop and enable trees to access deeper water tables not available to the crop.

Zinc-enriched fertilisers as a potential public health intervention in Africa

Abstract: In this review, we examine the potential of Zn-enriched fertilisers to alleviate human dietary Zn deficiency The focus is on ten African countries where dietary Zn supply is low and where fertiliser subsidies are routinely deployed on cereal crops Dietary Zn supply and deficiency prevalence were quantified from food supply and composition data Typical effects of soil (granular) and foliar Zn applications on Zn concentrations in maize (Zea mays L), rice (Oryza sativa L) and wheat (Triticum aestivum L) grains were based on a systematic literature review Reductions in disease burdens attributable to Zn deficiency and cost-effectiveness were estimated using a disability-adjusted life years (DALYs) approach Baseline Zn supply in 2009 ranged from 71 (Zambia) to 119 (Mali) mg capita −1 day−1; prevalence of Zn deficiency ranged from 24 (Nigeria) to 66 % (Zambia) In reviewed studies, soil Zn application led to an increase in median Zn concentration in maize, rice and wheat grains of 23, 7 and 19 %; foliar application led to increases of 30, 25 and 63 % Enriching granular fertilisers within current subsidy schemes would be most effective in Malawi, reducing DALYs lost due to Zn deficiency by 10 % The cost per DALY saved ranged from US$ 624 to 5893 via granular fertilisers and from US$ 46 to 347 via foliar fertilisers Foliar applications are likely to be more cost effective than soil applications due to fixation of Zn in the soil but may be more difficult to deploy Zinc fertilisation is likely to be less cost-effective than breeding in the longer term although other micronutrients such as selenium could be incorporated

Heavy metal translocation and accumulation in iron plaques and plant tissues for 32 hybrid rice (Oryza sativa L.) cultivars

Abstract: Rice (Oryza sativa L.) planted on heavy metal contaminated soil is a source of toxic elements entering the food chain and thereby posing a threat to human health. The main objective of this study was to investigate heavy metals uptake, translocation, and accumulation differences in iron plaque and rice tissues among the rice cultivars. In the present study, 32 hybrid rice cultivars were cultured on heavy metal contaminated paddy soil. Pb, Cd, Cu, and Zn concentrations in rice tissues (root, straw, husk, and brown rice) and in the iron plaques on rice root surfaces were measured. Significant differences of Pb, Cd, Cu, and Zn tolerance, concentrations and accumulation in 32 rice cultivars were observed. However, no significant difference was observed in concentrations of Pb, Cd, Cu, and Zn in two-line hybrid rice and three-line hybrid rice. The concentrations of Pb in iron plaques were significantly positive linearly correlated with concentrations of Pb in rice roots, and concentrations of Cd and Cu were significantly negative linearly correlated with Cd and Cu in rice roots. The concentrations of heavy metal in brown rice were significantly positively correlated with the translocation factors TFhb (from husk to brown rice) and TFsh (from straw to husk). It indicated that iron plaque plays an important role in mediating heavy metal entering into rice roots, and heavy metal accumulation in brown rice was related to its ability to transfer in the husk and straw rather than the root.

Stable oxygen isotopes reveal distinct water use patterns of two Haloxylon species in the Gurbantonggut Desert

Abstract: Haloxylon ammodendron and Haloxylon persicum are the dominant species in the Gurbantonggut Desert, China, with the former grows at inter-dune lowland and the later grows at the sand dune. This study aims to investigate the dynamics of water usage of the two species and their responses to the soil water fluctuations resulting from summer precipitation. Oxygen isotope ratios (δ 18O) were measured for xylem water, soil water in different soil layers (0–300 cm), precipitation water and groundwater. Four potential water sources were identified: shallow (0–40 cm), middle (40–100 cm) and deep soil water (100–300 cm), as well as groundwater. The water sources used by the two species were calculated using the IsoSource model. When the upper soil water was abundant in early spring, H. ammodendron mainly used shallow soil water while H. persicum mainly used middle soil water; when the upper soil water was depleted in summer, H. ammodendron mainly used groundwater while H. persicum mainly used deep soil water. Both species were conservative in using shallow soil water following the relatively large summer precipitations. The two Haloxylon species had distinct water use patterns during the growing season, which reflects their adaptations to their specific habitats in this water-limited desert environment. Large precipitations in dry summer hardly altered their water use patterns, which mean that these species are rather conservative in water use strategy.

Plant species diversity affects infiltration capacity in an experimental grassland through changes in soil properties

Abstract: Soil hydraulic properties drive water distribution and availability in soil. There exists limited knowledge of how plant species diversity might influence soil hydraulic properties. We quantified the change in infiltration capacity affected by soil structural variables (soil bulk density, porosity and organic carbon content) along a gradient of soil texture, plant species richness (1, 2, 4, 8, 16 and 60) and functional group composition (grasses, legumes, small herbs, tall herbs). We conducted two infiltration measurement campaigns (May and October 2012) using a hood infiltrometer. Plant species richness significantly increased infiltration capacity in the studied grasslands. Both soil porosity (or inversely bulk density) and organic carbon played an important role in mediating the plant species richness effect. Soil texture did not correlate with infiltration capacity. In spring 2012, earthworm biomass increased infiltration capacity, but this effect could not be attributed to changes in soil structural variables. We experimentally identified important ecological drivers of infiltration capacity, suggesting complex interactions between plant species richness, earthworms, and soil structural variables, while showing little impact of soil texture. Changes in plant species richness may thus have significant effects on soil hydraulic properties with potential consequences for surface run-off and soil erosion.

High phosphorus supply reduced zinc concentration of wheat in native soil but not in autoclaved soil or nutrient solution

Abstract: Phosphorus (P)-induced zinc (Zn) deficiency is one of the most commonly studied antagonistic interactions in plant nutrition. However, there are many controversial reports about P–Zn interaction, possibly related to growth conditions. In this study, the effects of P supply on the root uptake and tissue concentrations of Zn as well as the development of Zn deficiency were investigated in wheat (Triticum aestivum) grown in different media. Plants were grown under greenhouse and growth chamber conditions in native soil, autoclaved soil and nutrient solution with different P and Zn supplies. In the soil experiment, the shoot biomass and grain yield were measured whereas in the nutrient solution experiment, the root and shoot biomass were determined. Development of Zn deficiency symptoms was examined. Concentrations of Zn, P and other elements were measured in harvested tissues. Mycorrhizal colonization of roots was scored in soil-grown plants. Root uptake of stable Zn isotope (70Zn) was investigated at different P rates in a separate nutrient solution experiment. Higher P rates caused substantial decreases in shoot and grain Zn concentrations in native soil but not in autoclaved soil. Treatment of native soil with increasing P significantly reduced mycorrhizal colonization. At low Zn, P applications aggravated Zn deficiency symptoms in both soil and solution culture. In solution culture, root and shoot Zn concentrations were not lowered by higher P rates. Root uptake of 70Zn from nutrient solution was even depressed at low P. The negative effect of increasing P supply on root Zn uptake and tissue Zn concentrations in wheat is mycorrhiza-dependent and may completely disappear in a mycorrhiza-free environment.

Next generation shovelomics: set up a tent and REST

Abstract: Root system architecture traits (RSAT) are crucial for crop productivity, especially under drought and low soil fertility. The “shovelomics” method of field excavation of mature root crowns followed by manual phenotyping enables a relatively high throughput as needed for breeding and quantitative genetics. We aimed to develop a new sampling protocol in combination with digital imaging and new software. Sampled rootstocks were split lengthwise, photographed under controlled illumination in an imaging tent and analysed using Root Estimator for Shovelomics Traits (REST). A set of 33 diverse maize hybrids, grown at 46 and 192 kg N ha−1, was used to evaluate the method and software. Splitting of the crowns enhanced soil removal and enabled access to occluded traits: REST-derived median gap size correlated negatively (r = −0.62) with lateral root density based on counting. The manually measured root angle correlated with the image-derived root angle (r = 0.89) and the horizontal extension of the root system (r = 0.91). The heritabilities of RSAT ranged from 0.45 to 0.81, comparable to heritabilities of plant height and leaf biomass. The combination of the novel crown splitting method, combined with imaging under controlled illumination followed by automatic analysis with REST, allowed for higher throughput while maintaining precision. The REST Software is available as supplement.

Impacts of bulk soil microbial community structure on rhizosphere microbiomes of Zea mays

Abstract: It has frequently been shown that plants interact with soils to shape rhizosphere microbiomes. However, previous work has not distinguished between effects of soil properties per se, and effects attributable to the resident microbial communities of those soils. We aimed to test whether differences in the structure of bulk soil microbial communities, within a given soil type, would carry over to impact the structure of the rhizosphere microbial community. We used repeated chemical amendments to develop divergent bulk soil microbial community starting points from which rhizosphere development proceeded. Additionally, we contrasted rhizosphere microbiomes associated with two different cultivars of corn (Zea mays). A wide range of bacterial and archaeal taxa responded to chemical resource amendments, which reduced bulk soil microbiome diversities. Corn genotypes P9714XR and 35F40 had largely similar impacts on rhizosphere microbiome development, although significant differences were evident in select treatments. Notably, in cases where resource amendments altered bulk soil microbial community composition, legacy effects persisted into the rhizosphere. Our results suggest that rhizosphere microbial communities may develop into different states depending on site history and prior selective events. This work advances our understanding of soil microbiome dynamics and responsiveness to change in the form of simple resource amendments and the development of the rhizosphere.

Soils naturally suppressive to banana Fusarium wilt disease harbor unique bacterial communities

Abstract: Banana Fusarium wilt disease is caused by the Fusarium oxysporum f. sp. cubense race 4 fungus and is a vast problem for global banana production. Suppressive and conducive soils were analyzed to characterize important microbial populations and soil chemical properties that contribute to disease suppressiveness. Soil bacteria communities from the two banana orchards with excellent Fusarium disease suppression (suppressive soil) after long-term monoculture and two adjacent banana orchards with serious Fusarium wilt disease (conducive soils) were compared using deep 16S RNA barcode pyrosequencing. Compared to the conducive soils within the same field site, higher (P < 0.05) richness and diversity indices were observed in both suppressive soils. Moreover, more operational taxonomic units (OTUs) were observed in the two suppressive soils. Hierarchical cluster analyses showed that bacterial community membership and structure in disease-suppressive soils differed from disease-conducive soils. The Acidobacteria phylum was significantly (P < 0.05) elevated, but Bacteroidetes was significantly (P < 0.05) reduced in suppressive soils. The Gp4, Gp5, Chthonomonas, Pseudomonas, and Tumebacillus genera were significantly (P < 0.05) enriched in suppressive soils, but Gp2 was significantly (P < 0.05) reduced in suppressive soils. Furthermore, the enrichment of Gp5 and Pseudomonas as well as the soil physicochemical properties of available phosphorus were significantly (P < 0.05) correlated with disease suppression. Naturally disease suppressive soils to banana Fusarium wilt disease harbor unique bacterial communities.

Dynamics of soil aggregate-associated organic carbon along an afforestation chronosequence

Abstract: The objectives of this study were to determine the dynamics of aggregate-associated organic carbon (OC) along an afforestation chronosequence on abandoned farmland of China, and to examine the contributions of changes in aggregate-associated OC to changes in total soil OC. We investigated the dynamics of OC associated with aggregates along an afforestation chronosequence. Water-stable aggregates were isolated, and the OC concentrations in total soil and the aggregates were measured. Averaged across the entire chronosequence, afforestation led to 116, 128 and 108 % average increases in OC concentrations in macroaggregates, microaggregates and the <0.053 mm size class, respectively, in the top 20 cm of soil. The OC stocks in macroaggregates increased by averages of 651 and 473 % at 0–10 and 10–20 cm depths, respectively, mostly within the first 24 years. The OC stocks in microaggregates decreased during the first 35 years and then increased during 48–200 years of afforestation. Averaged across the entire chronosequence, the increases in OC stocks in macroaggregates accounted for 83 and 100 % of the total increase in OC stocks in soils at 0–10 and 10–20 cm depths, respectively. Our results indicated that the accumulation of OC in soils after afforestation on abandoned farmland was mainly due to the accumulation of OC in macroaggregates.

Enhanced biological N2 fixation and yield of faba bean (Vicia faba L.) in an acid soil following biochar addition: dissection of causal mechanisms

Abstract: Acid soils constrain legume growth and biochars have been shown to address these constraints and enhance biological N2 fixation in glasshouse studies. A dissection of causal mechanisms from multiple crop field studies is lacking. In a sub-tropical field study, faba bean (Vicia faba L.) was cultivated in rotation with corn (Zea mays) following amendment of two contrasting biochars, compost and lime in a rhodic ferralsol. Key soil parameters and plant nutrient uptake were investigated alongside stable 15 N isotope methodologies to elucidate the causal mechanisms for enhanced biological N2 fixation and crop productivity. Biological N2 fixation was associated with plant Mo uptake, which was driven by reductions in soil acidity following lime and papermill (PM) biochar amendment. In contrast, crop yield was associated with plant P and B uptake, and amelioration of soil pH constraints. These were most effectively ameliorated by PM biochar as it addressed both pH constraints and low soil nutrient status. While liming resulted in the highest biological N2 fixation, biochars provided greater benefits to faba bean yield by addressing P nutrition and ameliorating Al toxicity.

Root cell wall polysaccharides are involved in cadmium hyperaccumulation in Sedum alfredii

Abstract: The role of polysaccharide modification in metal accumulation in hyperaccumulators is still unknown. Our aim was to compare the differences in the role of root cell-wall polysaccharides in cadmium (Cd) accumulation between hyperaccumulating (HE) and non-hyperaccumulating ecotype (NHE) of Sedm alfredii. Hydroponic experiments were performed to characterize root-to-shoot Cd translocation, cadmium species and polysaccharide modification in root cell-wall of S. alfredii using stable isotope tracing, X-ray absorption near edge structure and immunofluorescence localization techniques. Cd absorbed was more readily available for transport to the shoots by the HE roots than by the NHE roots, which is confirmed by a 6-fold higher 113Cd concentration in xylem sap. Root Cd efflux originated mainly from the cell walls. The concentration of cell-wall polysaccharides and activity of pectin methylesterase were higher in the NHE than in the HE in the absence of Cd, and even higher in the presence of Cd. More pectins were methylated in the HE than in the NHE, indicating more free pectic acid residues in the NHE. The cell-wall-bound Cd was retained more tightly in the NHE than in the HE. Cadmium hyperaccumulation by HE of S. alfredii is associated with its enhanced Cd flux into the xylem, which is partly regulated by cell-wall polysaccharide modification in roots.

Plant growth and metal uptake by a non-hyperaccumulating species (Lolium perenne) and a Cd-Zn hyperaccumulator (Noccaea caerulescens) in contaminated soils amended with biochar

Abstract: Biochar could be used as a soil amendment in metal contaminated soils, for safe crop production or soil remediation purposes This work was conducted to study the effects of biochar amendments on metal uptake by two contrasted plants grown on metal-contaminated soils A non-hyperaccumulating plant (Lolium perenne) and a Cd- and Zn-hyperaccumulator (Noccea caerulescens) were grown in pots on acidic (A) and alkaline (B) soil contaminated by Cd, Pb and Zn, both amended by a wood-derived biochar Biochar amendments decreased the availability of metals by increasing soil pH, but also decreased Ca, P and N availability Growth of L perenne was increased and shoot metal uptake decreased by biochar addition in both soils, although increasing biochar dose above 05 % resulted in a progressive decrease of shoot production on soil B Growth of N caerulescens was not significantly affected by biochar But an increase of Cd uptake with 5 % biochar was recorded on both soils, and of Zn uptake on soil B Beside immobilizing metals, biochar may decrease the availability of nutrients, leading either to plant deficiency or to a decreased competition with cations for metal uptake, thus enhancing extraction of metals by hyperaccumulators

Biological nitrogen fixation in soybean in Argentina: relationships with crop, soil, and meteorological factors

Abstract: This study aims to (i) asses the contribution of biological N fixation (%Ndfa) in the soybean production area of Argentina, (ii) build a model for predicting %Ndfa using crop, soil, and meteorological variables, and (iii) estimate %Ndfa at the country level using values obtained in this study. %Ndfa was assessed in paddocks and experimental plots located in an area 22–39° S and 56–66° W. %Ndfa was determined using the natural 15N abundance method. A complete data set of soil and meteorological variables (n–47) was used to develop a model for predicting %Ndfa. A median value of %Ndfa in aboveground biomass of 60 % (interquartile range 46–71 %) was estimated. Larger %Ndfa values were observed in areas with high crops yields. When seed yield was above 3.7 Mg ha−1, effective rainfall during fallow and mean temperature in the seed-filling period explained %Ndfa. Below 3.7 Mg ha−1, soil phosphorus content, pH, and effective rainfall in the vegetative period explicated %Ndfa. Soybean production systems in Argentina showed larger %Ndfa than reported values in literature that may affect global N balances. Identified soil and meteorological variables may be useful for predicting %Ndfa in future studies, taking into account their spatial variation in the soil-plant system.

Effects of arbuscular mycorrhizal fungi on plant growth depend on root system: a meta-analysis

Abstract: Our aim was to explore the way that root system type affects mycorrhizal growth response of plants. An extensive meta-analysis with 943 peer-review publications was conducted to test the difference in mycorrhizal responses between taproot plants and plants with a fibrous root system. We found that taproot plants showed greater growth response (biomass, P and N uptake) to colonization by arbuscular mycorrhizal fungi (AMF) than do plant species with fibrous root systems. This response pattern was dependent on stress types, AMF identity and species richness, and particularly the type of stress (abiotic vs. biotic). Taproot plants respond more to AMF than plants with a fibrous root system; but no difference was shown under biotic stress. The interaction effect seen for AMF and biotic stress was significantly higher for plants with fibrous root system, but was not significant between taproot plants and abiotic stress. Difference in biomass response was only found for Glomeraceae and Gigasporaceae between the two types of plants, while difference was found in P uptake response for Glomeraceae and Claroideoglomeraceae. However, plants with fibrous root system showed higher growth response than taproot plants under nematode stress. Taproot plants might be more dependent on mycorrhiza than plants with fibrous root system. This indicates that environmental conditions can modify the relative abundance of taproot plants and plants with fibrous root system through mycorrhizal functioning, which will regulate plant community dynamics and processes.

Colonization of Trichoderma harzianum strain SQR-T037 on tomato roots and its relationship to plant growth, nutrient availability and soil microflora

Abstract: This research was to study the capacity of Trichoderma harzianum strain SQR-T037 to colonize tomato roots and how this strain and its bio-organic fertilizer stimulate plant growth and increase yields under field conditions. Field trials were conducted with a reduced application of chemical fertilizer (75 % of the recommended application) plus Trichoderma-enriched bio-organic fertilizer (BF) or organic fertilizer (OF) or Trichoderma spore suspension (SS), while 100 % of the recommended chemical fertilizer (CF) was used as control. Trichoderma SQR-T037 could efficiently colonize tomato roots and soils based on the reverse transcription-quantitative PCR analysis, and significantly stimulate biomass accumulation at an early stage. The 75 % rates of chemical fertilizer coupled with bio-organic fertilizer (BF) produced tomato yields equivalent to those obtained using the 100 % rates of chemical fertilizer (CF), while inoculation with the Trichoderma alone (SS) or supplement with organic fertilizer alone (OF) would cause 11 and 13 % decreases in yield over the control (CF). The efficacy of BF for maintaining a stable tomato yield may be due to the enhanced soil nutrients availability and the increased abundance of soil microflora, including bacteria, fungi, actinomycetes and Trichoderma communities, which had a positive linear correlation in most of the cases revealed by the Pearson correlation analysis. The results of this study imply that the reason by which the bio-organic fertilizer promotes plant growth and increases the tomato yields can be attributed to the enhancement of the rhizospheric microflora, which promotes nutrient activation. Therefore, T. harzianum could be employed in a combination with composts. In this way, the application rate of chemical fertilizers can be practically decreased by 25 % to obtain maximum benefits.

Contribution of greenhouse gas emissions during cropping and fallow seasons on total global warming potential in mono-rice paddy soils

Abstract: Background and aims Temperate rice paddy fields are generally flooded for less than 100 days a year during the rice cropping season and are kept under dried soil conditions during the fallow season of over 200 days. The impacts of rice paddy soil on the global warming potential (GWP) are generally analysed during rice cultivation, without consideration of the fallow season, using only methane (CH4) and nitrous oxide (N2O) fluxes. To compare the impact of greenhouse gas (GHG) emissions during the flooded rice cultivation and the dried fallow seasons on the annual GWP in a mono-rice cultivation system, the emission fluxes of CH4, N2O and carbon dioxide (CO2) were evaluated under two different fertilization systems (NPK and NPK + Cover crop) for two consecutive years.

Reviving of the endophytic bacterial community as a putative mechanism of plant resistance

Abstract: From the point of view of systems biology, the plant is considered a super organism that consists of the plant per se and numerous populations of pro- and eukaryotic microbial organisms. Each plant species hosts a genotype-specific core microbiome, dynamically responding to environmental cues, such as soil quality. In the plant endosphere, microbial organisms are an indispensable part of the information processing system, and the plant-endophyte interrelationships result in mutual adjustments through this system. Within the plant tissue, part of the microbiota resides in state called “viable but nonculturable”. Pathogen attack or environmental stress can provoke the dormant forms. The link between reviving of endophytic bacterial populations and plant disease resistance in the endosphere is discussed in this paper. The innate endophytic communities possess resistance-competent members, which can be in a latent state. The latent populations can be revived by an incoming microbial organism (e.g. a biocontrol agent or a pathogen), or other environmental factors. Reviving endophytic bacterial populations can be a putative mechanism of the endophyte-mediated plant resistance. Based on the published results, we suggest that the endophyte-mediated stress tolerance or disease resistance can develop, if the plant hosts a sufficient diversity of ‘protective’ endophytes. Alternatively, the plant can become susceptible upon loss of strategic members from endophytic microbial cohorts. Resident endophytes can be envisaged as a hidden reserve of plant protection to be used in green plant biotechnology. Selection of plant genotypes and soil type hosting beneficial microbiomes should become a common practice for improving plant resistance to complement advanced genetic technologies applied in plant biotechnology.

An asexual Epichloë endophyte modifies the nutrient stoichiometry of wild barley (Hordeum brevisubulatum) under salt stress

Abstract: Background and aims Salinization of soils causes severe problems to plant growth. With the important role of stoichiometry in many ecological processes, this study investigated the effect of an asexual Epichloe endophyte on the nutrient stoichiometry of wild barley (Hordeum brevisubulatum) under salt stress.