Showing papers in "Plant and Soil in 2019"

Hidden miners – the roles of cover crops and soil microorganisms in phosphorus cycling through agroecosystems

Abstract: Phosphorus (P) is a limiting nutrient in many agroecosystems and costly fertilizer inputs can cause negative environmental impacts. Cover crops constitute a promising management option for sustainable intensification of agriculture. However, their interactions with the soil microbial community, which is a key driver of P cycling, and their effects on the following crop, have not yet been systematically assessed. We conducted a meta-analysis of published field studies on cover crops and P cycling, focusing on plant-microbe interactions. We describe several distinct, simultaneous mechanisms of P benefits for the main crop. Decomposition dynamics, governed by P concentration, are critical for the transfer of P from cover crop residues to the main crop. Cover crops may enhance the soil microbial community by providing a legacy of increased mycorrhizal abundance, microbial biomass P, and phosphatase activity. Cover crops are generally most effective in systems low in available P, and may access ‘unavailable’ P pools. However, their effects on P availability are difficult to detect by standard soil P tests, except for increases after the use of Lupinus sp. Agricultural management (i.e. cover crop species selection, tillage, fertilization) can improve cover crop effects. In summary, cover cropping has the potential to tighten nutrient cycling in agricultural systems under different conditions, increasing crop P nutrition and yield.

Preserving the nutritional quality of crop plants under a changing climate: importance and strategies

Abstract: Global climate is changing more rapidly than ever, threatening plant growth and productivity while exerting considerable direct and indirect effects on the quality and quantity of plant nutrients. This review focuses on the global impact of climate change on the nutritional value of plant foods. It showcases the existing evidence linking the effects of climate change factors on crop nutrition and the concentration of nutrients in edible plant parts. It focuses on the effect of elevated CO2 (eCO2), elevated temperature (eT), salinity, waterlogging and drought stresses, and what is known regarding their direct and indirect influence on nutrient availability. Furthermore, it provides possible strategies to preserve the nutritional composition of plant foods under changing climates. Climate change has an impact on the accumulation of minerals and protein in crop plants, with eCO2 being the underlying factor of most of the reported changes. The effects are clearly dependent on the type, intensity and duration of the imposed stress, plant genotype and developmental stage. Strong interactions (both positive and negative) can be found between individual climatic factors and soil availability of nitrogen (N), potassium (K), iron (Fe) and phosphorous (P). The development of future interventions to ensure that the world's population has access to plentiful, safe and nutritious food may need to rely on breeding for nutrients under the context of climate change, including legumes in cropping systems, better farm management practices and utilization of microbial inoculants that enhance nutrient availability.

Plant allelochemicals: agronomic, nutritional and ecological relevance in the soil system

Abstract: Allelopathy is an ecological phenomenon consisting of both positive and negative effects between organisms determined by the release of secondary metabolites into the environment. Root exudation represents the most important pathway of releasing allelochemicals. Once released into the soil, allelochemicals interact with the organic and inorganic soil phases, as well as with soil microorganisms. The set of these interactions fix allelochemicals bioavailability and phytotoxic level. Here we critically review the interactions between plant allelochemicals and physical, chemical and biological soil characteristics by reporting the literature available and pointing out both positive and negative relationships affecting allelochemicals phytotoxicity and nutrient availability. In addition, we have reported a qualitative balance of allelochemicals in the soil. Thirdly, we reviewed the exudation process of allelochemicals and the transport mechanisms across plasma membranes. A two-way relationship exists between soil characteristics and allelochemicals. The level of phytotoxicity is not affected only by a single soil characteristic, but they are closely linked to each other and exert a multiple-effect on retention, transport and transformation processes of allelochemicals in soil. Further efforts are needed to better understand the interactions involved in soil allelopathy and to create new opportunities for a sustainable control of agroecosystems.

Litter decomposition: effects of temperature driven by soil moisture and vegetation type

Abstract: We examined the importance of litter quality and microclimate on early-stage litter mass loss, analysed the importance of interactions among environmental factors in determining key decomposition parameters and compared the variation in decomposition rates in vegetation types and sites with similar climate. Following the Tea-Bag Index approach, 464 tea-bags were incubated in the soil in 79 sites, distributed across Italy, which included six vegetation types and a broad range of microclimatic conditions. Litter type exerted a stronger control on mass loss compared to climatic factors. The effects of soil moisture were not the same for high and lower quality litter. In addition, the effects of temperature on the decomposition rate depended on soil moisture. The stabilization factor was strongly temperature-dependent, but the influence of temperature differed among vegetation types: those dominated by small-size plants showed a strong decrease in the potential amount of plant material entering into the soil stock under warmer temperatures. The lowest variation in decomposition rate was found in sites characterised by low temperatures, and, among the vegetation types, in alpine snowbeds. The role of litter quality and of the interactions among environmental conditions can potentially determine significant shifts in the expected patterns of ecosystem carbon fluxes.

Isotope fractionation during root water uptake by Acacia caven is enhanced by arbuscular mycorrhizas

Abstract: A growing number of studies show a discrepancy between the isotopic composition of xylem water and plant water sources. We tested the effect of arbuscular mycorrhizal fungi (AMF) on the isotopic composition of Acacia caven xylem water. As the most common plant-fungal association, AMF might explain this isotopic mismatch. Seedlings were grown with and without AMF and irrigated with the same water. After 120 days, stem and soil samples were collected and following cryogenic distillation, H and O isotopic composition of xylem and soil water, as well as irrigation water, was measured. Xylem water of non-mycorrhizal seedlings was significantly depleted in 2H compared to soil water (differences up to −15.6‰). When AMF were present, the depletion was significantly higher and appeared for both H and O (differences up to −24.6‰ for δ2H and − 2.9‰ for δ18O between soil and xylem water). Results suggest that isotopic fractionation occurred during water uptake in this xerophytic species. To explain this, we propose an aquaporin-driven mechanism mediating water transport via transmembrane passage. Furthermore, we show for the first time, that AMF enhance the observed discrimination against heavy isotopes, probably by enforcing water passage through aquaporins. Given their ubiquity, AMF could question the fractionation-free assumption during root water uptake.

Nutrient acquisition strategies in agroforestry systems

Abstract: Disentangling nutrient acquisition strategies between trees and crops is central to understanding positive nutrient interactions in agroforestry systems for improved low-input agriculture. However, as plants are responsive to a complex soil matrix at multiple scales, generalizable diagnostics across diverse agroforests remains challenging. We synthesize research at various scales of the tree-crop interface that are cumulatively hypothesized to underpin nutrient acquisition strategies in agroforestry systems. These scales span the whole root system to fine-scale sites of acquisition actively engaged in biological and chemical interactions with soil. We target vertical and horizontal dimensions of acquisition patterns; localized root-soil dynamics including biological associations; root-scale plasticity for higher acquisition; and nutrient additions via biological nitrogen fixation and deep soil nutrient uplift. We consolidate methodological advances and the effects of environmental change on well-established nutrient interactions. Root distribution patterns remain one of the most universal indicators of nutrient acquisition strategies in a range of agroforestry systems, while root functional traits are emerging as an effective root-scale indicator of nutrient acquisition strategy. We validate that in agroforestry systems crop root functional traits reveal bivariate trade-offs similar to, but weaker than, crops in monoculture, with mechanistic links to nutrient acquisition strategies. While interspecific root overlap may be associated with nutrient competition, clear cases of enhanced chemically and microbially meditated processes result in species- and management-specific nutrient facilitation. We argue for agroforestry science to use distinct and standardized nutrient acquisition indicators and processes at multiple scales to generate more nuanced, while also generalizable, diagnostics of tree-crop interactions. And extensive research is needed on how agroforestry practices stabilize key nutrient acquisition patterns in the face of environmental change.

Effects of phytolithic rice-straw biochar, soil buffering capacity and pH on silicon bioavailability

Abstract: Supplying phytolith-rich biochar in agrosystems increases soil pH, CEC and nutrient availability, adding to the impact of Si uptake on plant growth. Here we studied this specific impact as influenced by soil properties, and assessed the role of phytoliths to provide plant available Si. We used a young Cambisol and a highly weathered, poorly buffered, desilicated Nitisol. The biochars were produced from rice plants respectively enriched (Si+) and depleted (Si-) in Si. They had identical pH and nutrient contents, but largely differed in Si content (51.3 g Si kg−1 in Si + vs 0.3 g Si kg−1 in Si-). We compared their effects to that of wollastonite (CaSiO3) on the biomass and mineralomass of wheat plants in a soil:solution:plant device. The contents of soil bioavailable Si and biogenic Si were assessed through an original CaCl2 kinetic extraction and the DeMaster Na2CO3 alkaline dissolution, respectively. The DeMaster technique dissolved Si from phytolith as well as from wollastonite. The soil buffering capacity (cmolc kg−1) was 31 in the Cambisol and 0.2 in the Nitisol. An identical supply of phytolithic biochar increased pH from 4.5 to 4.8 in the Cambisol, and from 4.8 to 7.4 in NI. It further increased the content of bioavailable Si (from 55 to 97 mg kg−1 in the Cambisol, and 36 to 209 mg kg−1 in the Nitisol), as well as plant Si uptake, biomass and Si mineralomass. That increase was largest in the Nitisol. The DeMaster technique did not specifically quantify the phytolith pool. This pool was the main source of plant available Si in both the Cambisol and Nitisol amended with phytolithic biochar. At identical phytolithic Si supply, however, soil pH and soil buffering capacity controlled the transfer of Si in the soil-plant system, which was largest in the poorly buffered Nitisol. The effect of phytolithic biochar on Si bioavailability was depending on soil constituents and properties, and thus on soil type.

Quantification of the contribution of nitrogen fertilization and crop harvesting to soil acidification in a wheat-maize double cropping system

Abstract: Over fertilization with nitrogen (N) is considered the main driver of agricultural soil acidification in China. However, the contribution of this driver compared to other causes of soil acidification on intensive croplands has seldom been quantified under field conditions. We measured the fate of major nutrients, and calculated the related H+ production, based on the difference between inputs and leaching losses of those nutrients for a wheat-maize rotation system on a moderate acid silty clay loam soil in a two-year field experiment. Topsoil pH decreased 0.3 units in the plots with conventional (current farmer practice) high N fertilization after two years, with a proton production of 13.1 keq H+ ha−1 yr.−1. No apparent changes in topsoil pH were observed in the plots without N application, in spite of a proton production of 4.7 keq H+ ha−1 yr.−1. Crop uptake was the primary driver of H+ production, followed by N transformation processes and HCO3− leaching in both plots. Nitrogen fertilization had a relative small direct impact on soil acidification due to a very limited nitrate leaching, induced by large N losses to air by denitrification in this specific moderately acid soil, whereas elevated base cation uptake by crops induced by N fertilization indirectly had a relative large impact.

Heterotrophic nitrification and denitrification are the main sources of nitrous oxide in two paddy soils

Abstract: Paddy soil is one of the main sources of global nitrous oxide (N2O) emissions via multiple pathways regulated by different microbes. However, the relative contributions of N2O production pathways with the addition of organic carbon (C) in different paddy soils are poorly understood. 15N-stable isotope and acetylene (C2H2) inhibition were used to differentiate the relative contributions of autotrophic and heterotrophic nitrification (ANF and HNF) and denitrification (DNF) to N2O emissions in two paddy soils (acid vs. neutral soil) with glucose addition. HNF and DNF were the main N2O pathways which contributed between 85% to 100% of the total N2O production at 70% water filled pore space. Low soil pH inhibited soil nitrification and the activity of ammonia oxidizers compared with neutral paddy soil. Glucose reduced nitrification rate and stimulated N2O production significantly, mainly via DNF in the two paddy soils. Moreover, glucose increased the relative contribution of DNF to total N2O production in the first 7 days and total N2O amounts from HNF over the 14-day incubation. HNF and DNF rather than ANF dominated the N2O emissions regardless of soil pH. Glucose had a positive effect on N2O emissions by influencing HNF and DNF.

A shape-based method for automatic and rapid segmentation of roots in soil from X-ray computed tomography images: Rootine

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

Soil N/P and C/P ratio regulate the responses of soil microbial community composition and enzyme activities in a long-term nitrogen loaded Chinese fir forest

Abstract: Long-term nitrogen (N) fertilization has been shown to profoundly affect the soil microorganisms and strongly result in several imbalances in element concentrations. The objective of this study was to examine links among the soil microorganisms, enzyme activities, and soil carbon (C), N, and phosphorus (P) stoichiometry in a subtropical Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation after continuous N fertilization for 13 years. This study was performed in 25-year-old fir plantation along a fertilization gradient (0, 60, 120, and 240 kg N ha−1 yr.−1), designated as N0, N1, N2, and N3, respectively. Soil microbial properties, including the microbial community composition, as revealed by phospholipid fatty acids (PLFAs), and soil enzyme activities (i.e., sucrase, urease and catalase) were measured, and soil elemental stoichiometry was calculated based on soil C, N, and P concentrations. A redundancy analysis (RDA) was conducted to determine the relationship between soil C:N:P stoichiometry and soil microbial properties. Compared with the control (N0), N fertilization decreased the total PLFAs (−12.20%), bacteria (−14.33%), fungi (−12.97%), and actinomycetes (−17.11%) on average. Sucrase, urease and catalase activities were enhanced by low and middle levels of N (N1 and N2), but not with high level of N (N3). Long-term N fertilization decreased soil pH, C to N ratio (C/N), and C to P ratio (C/P), while increased soil C, N and N to P ratio (N/P). The RDA identified the first two axes of soil stoichiometry variation that explained 20.4% of the variation at the soil depth of 0–20 cm, 28.6% at 20–40 cm and 49.9% at 40–60 cm in PLFAs biomarkers and enzymes, respectively. Significant correlations between soil stoichiometry (soil N/P and C/P ratio) and soil microbial properties were found in this study. These observations suggested that long-term N fertilization influenced soil microbial community composition and enzyme activities by changing the soil C/P and N/P ratios. Future studies are needed to consider the coupling relationships between soil microbial community composition, enzyme activities and elemental stoichiometry in different ecosystems under future climatic change.

Imposing and maintaining soil water deficits in drought studies in pots

Abstract: Pot studies are frequently used to study the influence of water deficits on plants and to screen genotypes for drought-resistance traits. Limited space and the need to screen large numbers of plants in rapid phenotyping platforms has led to the use of small pots for water-deficit studies. This paper reviews the influence of pot size, pot shape, soil medium, and the method of imposing water deficits on the development of soil water deficits, plant growth and function. Small pot size limits plant growth as the small soil volume limits root extension and proliferation. High-frequency deficit irrigation results in uneven distribution of water in the soil with consequent effects on plant growth, root distribution, water and nutrient uptake, and root-shoot interactions. Cycles of slow drying followed by fully rewetting the soil result in a more even distribution of water and roots throughout the pot and responses to water deficits more similar to those in the field. Small shallow pots and high-frequency deficit irrigation are inappropriate for inducing and maintaining water deficits, particularly when studying roots and root-shoot interactions. Large tall pots and cycles of drying and wetting better simulate water deficits encountered in the field and for identifying drought-resistant traits.

Phosphorus uptake of rice plants is affected by phosphorus forms and physicochemical properties of tropical weathered soils

Abstract: Phosphorus (P) deficiency is a major constraint for rice production in the tropics. Field-specific P management is key for resource-limited farmers to increase yields with minimal inputs. We used soil P fractionation analysis to identify the relevant factors controlling P uptake and the responses to P fertilization of rice in flooded and highly weathered soils. Phytometric pot-based experiments and a modified Hedley fractionation analysis were repeated for soils from extensive regions and from geographically adjacent fields in Madagascar. Large field-to-field variations in indigenous P supply from soils (total P uptake of rice when P is omitted) and fertilizer-P recovery efficiencies (increased P uptake when P is applied) were observed not only for soils with various geological backgrounds but also for soils from adjacent fields. Regression models indicated that the indigenous P supply in soils was largely controlled by readily available inorganic and organic P pools (r2 = 0.72), whereas fertilizer-P recovery efficiencies were controlled by the abundance of oxalate-extractable aluminum and iron in soils (r2 = 0.81). Spatial heterogeneity even within adjacent fields leads to benefits from field-specific fertilizer management based on indigenous P supply from soils and fertilizer-P recovery efficiencies evaluated by different soil properties.

The rhizosheath: a potential root trait helping plants to tolerate drought stress

Abstract: Rhizosheath is known as a layer of adhering soil particle to the root surface. Despite several speculations, the positive function of rhizosheath in acquisition of water and nutrients from drying soil has not yet been experimentally proven. The objective of this study was to experimentally show whether an enhanced rhizosheath formation could help plants to better access water from drying soil. Eight wheat cultivars were grown in a sandy-loam soil. When plants were 35 days old let dry soil to a water content at which evident wilting symptoms appeared on the plant leaves. During this drying cycle, soil water content and transpiration rate of plants were gravimetrically measured by weighing the plant pots. At the end of this drying cycle, the roots were excavated out of the soil and the rhizosheath formation was gravimetrically quantified by weighing the soil attached to the root system. The results showed that plant cultivars with greater rhizosheath formation could sustain higher transpiration rates at dry condition (water content of 0.07 cm3 cm−3) while the plant cultivars with lower rhizosheath formation suffered from drought stress and reached their permanent wilting points at the same water content. The findings of this study gathered evidence that under severe drought condition plant cultivars with an enhanced rhizosheath formation could better survive by sustaining their transpirational and nutritional demands.

Vermicompost can suppress Fusarium oxysporum f. sp. lycopersici via generation of beneficial bacteria in a long-term tomato monoculture soil

Abstract: Fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici (Fol) has severely decreased global tomato production. Organic amendments are widely applied to suppress Fol all over the world. However, the ways in which different amendments alter soil bulk microflora and thereby induce the suppression of Fol remain unclear. In this study, the effects of three organic amendments on the suppression of Fol in soil and the underlying mechanisms of those effects were studied. The organic amendments included in this study are rice straw, chicken manure compost, and vermicompost. High-throughput HiSeq sequencing and Real-Time PCR were used to determine the effect of the soil microbiota on the abundance of Fol. The abundance of Fol increased significantly with the duration of tomato cultivation. Vermicompost was the most effective organic fertilizer to suppress Fol in long-term continuous tomato cropping soil. Partial Least Squares Path Modeling revealed a strong positive relationship between the relative abundance of bacterial groups (including the genera Nocardioides, Ilumatobacter and Gaiella) and Fol inhibition. Soil chemical properties (pH, NH4+-N, soil organic matter and dissolved organic carbon) were positively associated with the genera Nocardioides, Ilumatobacter and Gaiella. Compared with chemical fertilizer and rice straw, vermicompost addition significantly increased soil pH, NH4+-N, soil organic matter and dissolved organic carbon concentrations in the soil with 20 years of tomato cultivation. Most importantly, the genera Nocardioides, Ilumatobacter and Gaiella were enriched in vermicompost, which may contribute to the propagation of these bacteria in the soil when vermicompost is added. This study provides a mechanistic framework that permits the exploration of specific functions at lower taxonomic levels. This may represent a novel approach in the management of crop pathogens via promotion of beneficial organisms.

Paecilomyces variotii extracts (ZNC) enhance plant immunity and promote plant growth

Abstract: The crude extract of the endophyte Paecilomyces variotii known as ZhiNengCong (ZNC) has function of promoting plant growth and enhancing disease resistance and is widely used in China. Our study aims to evaluate the molecular mechanisms of plant growth promotion and disease protection. We generated transcriptome profiles from ZNC-treated seedlings using RNA sequencing. The function of salicylic acid (SA) in ZNC-mediated immunity was examined using SA biosynthesis and signaling pathway mutants. The concentrations of nitrogen (N) and phosphorus (P) in seedlings under ZNC treatment were measured. The effect of ZNC on the level of the hormone auxin in roots was tested using transgenic plants containing DR5::GFP. ZNC exhibited ultrahigh activity in promoting plant growth and enhancing disease resistance, even at concentrations as low as 1–10 ng/ml. ZNC induced ROS accumulation, callose deposition, and expression of PR genes. SA biosynthesis and signaling pathways were required for the ZNC-mediated defense response. Moreover, in improving plant growth, ZNC increased the level of auxin in root tips and regulated the absorption of N and P. According to these results, ZNC is a highly effective plant elicitor that promotes plant growth by inducing auxin accumulation at the root tip at low concentrations and enhances plant disease resistance by activating the SA signaling pathway at high concentrations.

Role of plant growth promoting Bacteria (PGPRs) as biocontrol agents of Meloidogyne incognita through improved plant defense of Lycopersicon esculentum

Abstract: Root-knot nematodes are major constraints among different pathogens with wide host range and cause severe agricultural loss worldwide. The present study was designed to understand the role of plant growth promoting bacteria (Pseudomonas aeruginosa & Burkholderia gladioli) on growth and antioxidative potential in nematode infected Lycopersicon esculentum seedlings. An experiment was conducted to assess the levels of superoxide anions, H2O2 and MDA contents generated during nematode infection. Moreover, the contribution of antioxidative enzymes, non-enzymatic antioxidants, total antioxidants and gene expression profiling was also carried out in nematode infected Lycopersicon esculentum seedlings. The results of present study revealed that nematode infection reduced the growth of seedlings which upon inoculation of microbes was improved. Moreover, number of galls were reduced upon supplementation of these strains. Nematode infection also caused accumulation of superoxide anion, H2O2, and malondialdehyde contents along with nuclear damage and loss of cell viability which was reduced upon supplementation of microbes. The oxidative burst generated enhanced various antioxidant enzymes such as SOD (30.6%), POD (3.6%), CAT (18.1%), GPOX (65.9%), APOX (24.8%), GST (5.6%), DHAR (13.9%), GR (11%) and PPO (2.5%) which were further elevated upon application of P. aeruginosa (23.9%, 7.2%, 7%, 66%, 28.9%, 71.3%, 14.5%, 10.6% and 38.3%) and B. gladioli (5.1%, 30.6%, 16.2%, 92.1%, 78.5%, 97.5%, 15.5%, 65.7% and 23.2%). The non-enzymatic antioxidants (glutathione, ascorbic acid and tocopherol) and total antioxidants contents (both water soluble and lipid soluble) were also enhanced upon inoculation of microbes. Confocal microscopy revealed the improvement in nuclear damage and cell viability in microbe inoculated roots. Gene expression profiling revealed the enhanced expression levels of SOD, POD, CAT, GR, GPOX, APOX, PPOgenes in P.aeruginosa inoculated nematode infected seedlings by 53%, 2.7%, 64.1%, 10.4%,19.7%, 29.2%, 38.4% and B. gladioli inoculated seedlings by 18.3%,144%, 67%, 43%, 308%, 151% respectively. The results therefore suggest the favourable aspects of micro-organisms in modulating growth characteristics and antioxidative defense expression of Lycopersicon esculentum to encounter oxidative stress generated under nematode infection.

Interactive effects of nitrogen and phosphorus additions on plant growth vary with ecosystem type

Abstract: Co-limitation of ecosystem productivity by nitrogen (N) and phosphorus (P) is gaining increasing recognition, but how co-limitation through N and P interactions differs among different terrestrial ecosystems remains unclear. We performed a meta-analysis of 133 independent studies conducted in four natural terrestrial ecosystems to examine the interactive effects of N and P additions on ten plant growth-related variables. Adding N and P individually or in combination significantly increased aboveground biomass (AGB), and the interactions were uniformly synergistic for AGB, and additive for belowground biomass (BGB), but variable for other eight growth-related variables among four different ecosystems. The interaction was synergistic for leaf P and soil NO3-N only in tropical forests, and antagonistic for soil available P (AP) in tropical forests, leaf N in grasslands, root P in wetlands, and leaf P and soil NH4-N in tundra. The interaction for leaf N: P ratios was additive only in tropical forests, and synergistic in the other three ecosystems. Our results highlighted the interactions of N and P additions can promote uptake of both nutrients by plants, and plants tend to maintain the optimal nutrient balance for growth and reproduction through regulating biomass production and tissue nutrient concentrations.

Root-induced soil acidification and cadmium mobilization in the rhizosphere of Sedum plumbizincicola: evidence from a high-resolution imaging study

Abstract: Plant roots can significantly alter soil pH and the chemical concentration and distribution of different elements in the rhizosphere environment. Here we ask whether cadmium (Cd) bioavailability in the rhizosphere of Cd-hyperaccumulator Sedum plumbizincicola can be influenced by root-induced effects on soil pH. The Cd-hyperaccumulator S. plumbizincicola and the Cd non-hyperaccumulator ecotype Sedum alfredii were both grown in four different Cd-contaminated soils. We used the planar optode imaging technique to produce two-dimensional and high-resolution measurements of soil pH. Shoot excess cation concentration, root architecture and Cd concentrations ([Cd]) in soil pore water were also measured. Spatial analyses based on kernel density estimate of roots (KDE) and a Moran’s I correlogram were performed to assess spatial patterns and potential relationships among root distribution, soil pH and [Cd]. Both Sedum species showed root-induced increases in soil acidification (i.e. soil pH decreases of 0.1 to 0.62 units), which were clearly associated with greater root density of these plants. Remarkable excess cation uptakes by both Sedum species were detected and likely a driving factor for the root-induced acidification. The presence of the roots of S. plumbizincicola were then related to higher [Cd] in the rhizosphere than in bulk soil in Orthic Acrisol (+342%) and in Hydragric Antrosol soils (+296%). The hyperaccumulator S. plumbizincicola had larger root systems, higher acidification ability, and was associated with greater soil [Cd] than S. alfredii. Spatial patterns of root distribution and soil pH were similar between Sedum plants, however, spatial patterns of [Cd] differed across polluted soils. Rhizosphere acidification induced by S. plumbizincicola plants can play an important role on soil Cd mobilization, but overall effects on soil Cd bioavailability will depend on intrinsic soil biogeochemical properties.

Priming of soil organic carbon decomposition induced by exogenous organic carbon input: a meta-analysis

Abstract: Priming effect (PE) of soil organic carbon (SOC) decomposition induced by exogenous organic C is an important ecological process in regulating the soil C cycle. The objective of this study was to evaluate how the PE varied among different ecosystems at the global scale and explore factors that drive the direction and magnitude of the PE. Using 2048 experimental comparisons compiled from 94 incubation studies with stable (13C) or radioactive (14C) carbon isotopic techniques, we performed a meta-analysis on the effect of exogenous organic C input on native SOC decomposition (i.e., PE) across multiple terrestrial ecosystems. In particular, the linear mixed-effect model was used to examine the relationship between the PE and potential influencing factors. The addition of exogenous organic C significantly enhanced native SOC decomposition by 47.5% (i.e., positive PE), with the highest value in cropland soils (60.9%) and the lowest value in forest soils (26.2%). The intensity of the PE decreased with increasing SOC content, soil total nitrogen content, soil C/N, incubation duration, and incubation temperature, but increased with increasing exogenous organic C addition rate and soil pH. Soil PE was not affected by the complexity of exogenous organic C. Our results indicate that positive PE is a widespread phenomenon in terrestrial ecosystems, and that the magnitude is closely related to soil properties and experimental conditions. These findings may be useful for understanding soil C priming and the effect on soil C balance under climate change scenarios.

Mapping and cloning of quantitative trait loci for phosphorus efficiency in crops: opportunities and challenges

Abstract: Phosphorus (P) is an essential mineral element required in large quantities by plants. Globally, the availability of P in many soils is poor. Breeding crops that can acquire and utilise this limited resource with high efficiency is an important goal for agricultural sustainability in the future. The mapping and cloning of quantitative trait loci (QTLs) provides an effective tool in analyzing the genetic mechanisms underlying P efficiency and breeding P-efficient varieties. This paper describes the QTL mapping of traits related to P efficiency which impact on shoot biomass or yield of crops in the past 20 years. It summarises the progress of studies on crop P-efficiency related QTLs and discusses the challenges for the cloning of QTLs. It proposes a scheme to develop crop genotypes with improved P efficiency. It also describes emerging methods, such as QTL-seq, genome-wide association analysis, and RNA-seq, that aid the rapid identification of P-efficiency related genes in crops. Traits conferring P efficiency are heritable. Thus, it is feasible to incorporate phenotyping and selection for P efficiency in crop breeding programs. Identification of QTLs for target traits is a key step to enhancing the P efficiency of crops. Numerous QTLs have been identified that affect P efficiency in key crops, but few causal genes have been identified and breeding P-efficient crop varieties using marker-assisted selection (MAS) has not progressed far. The challenge now is to identify the specific genes controlling P-efficiency related traits. The availability of complete genome sequences for more crops, and the combination of conventional linkage mapping, association mapping, QTL-seq, transcriptomics and gene editing technologies can accelerate the cloning and confirmation of genes underlying QTL affecting P-efficiency related traits. Knowledge of these genes will be helpful in revealing the molecular mechanisms underlying P efficiency in crops, as well as providing the opportunity to improve crop P efficiency through MAS or gene manipulation.

Plant-assisted selection: a promising alternative for in vivo identification of wheat (Triticum turgidum L. subsp. Durum) growth promoting bacteria

Abstract: In this work we present the development of an easy and feasible in vivo alternative to identify promising Plant Growth Promoting Bacteria (PGPB), using wheat -as a model plant- growing under variable soil and climate conditions. The identification of promising strains was carried out by Plant-Assistant Selection (PAS) (compared with the conventional PGPB selection, named in this work as Metabolic Traits Selection or MTS). We validated the ability of the obtained strains by PAS to promote wheat growth, by analyzing biometric and nutrimental parameters, as well as the relative expressions of NRT1.4, GluTR, and 6-SFT1 genes. Twenty strains were obtained by PAS (170 bacterial strains were originally co-inoculated to plants), of which, twelve strains showed the ability to promote wheat growth mainly by the stem development and the number of leaves. Moreover, thirteen strains up-regulated the 6-SFT1 gene, and three strains up-regulated the GluTR gen. Thus, the strains Enterobacter cloacae TS3, Microbacterium foliorum TS9, Bacillus cereus TS10, Paenibacillus lautus TE8, and Paenibacillus lautus TE10 were identified as promising PGPB, showing strong wheat growth promotion events compared with those strains obtained by MTS. PAS is an easy and feasible alternative for identification of PGPB. However, ecological and economic factors need to be investigated to use the obtained strains by PAS for commercial microbial inoculants formulations.

Dark septate endophytes improve the growth of host and non-host plants under drought stress through altered root development

Abstract: This study aimed to investigate how dark septate endophytes (DSE) from arid habitats affect host growth and their application to crops and medicinal plants in drought-prone soils. First, the osmotic-stress tolerance of Paraphoma sp., Embellisia chlamydospora, and Cladosporium oxysporum, isolated from Hedysarum scoparium, was tested using osmotically adjusted pure culture. Second, we examined the performance of host (H. scoparium) and non-host (Glycyrrhiza uralensis and Zea mays) plants inoculated with these fungi under mild (MD) and extreme drought (ED) conditions in a growth chamber. All the DSE showed high tolerance to osmotic stress in vitro and could colonise the roots of all the plants. For H. scoparium, DSE improved the root biomass and length depending on DSE species, with Paraphoma sp. and C. oxysporum exhibiting positive effects under all the drought treatments. For G. uralensis and Z. mays, DSE inoculation enhanced the root development of plants under MD condition and was dependent on the plant–fungus species. However, this positive effect was weakened under extreme drought stress. DSE isolated from H. scoparium enhanced the root growth of the host plant under drought conditions and may also be used to promote the cultivation of agricultural and medicinal plants.

High aluminum stress drives different rhizosphere soil enzyme activities and bacterial community structure between aluminum-tolerant and aluminum-sensitive soybean genotypes

Abstract: Aluminum is a major deleterious factor for soybean productivity in acidic soils. Soil enzyme activities and bacteria play important roles in improving the stress tolerance of soybean. We assessed soil enzyme activities and bacterial structure and functions in Al-tolerant (Al-T) and Al-sensitive (Al-S) soybean genotypes subjected to different Al stress. We used a pot experiment system and determined plant biomass, urease, acid phosphatase, catalase, sucrase, and amylase activities, soil chemical properties, and the rhizosphere bacterial community diversity and structure of Al-T and Al-S soybean genotypes under different Al concentrations (0, 0.2, and 0.4 Al3+ g kg−1). Significant differences in soil enzyme activities and bacterial community structure were only observed between Al-T and Al-S soybeans under high Al stress. We identified 23 operational taxonomic units (OTUs), including OTU46 (Tumebacillus), OTU253 (Granulicella), and OTU180 (Burkholderia), which may improve soybean tolerance to Al toxicity. The results of canonical correspondence analysis (CCA) indicated that NH4+-N was an important factor that drove bacterial community structure differences between the two genotypes. Al stress simplified the network structure in the Al-T soybean, which may cause the soil bacterial community to be easily influenced by biotic and abiotic factors. High Al stress drove different soil enzyme activities and bacterial community structures between Al-T and Al-S soybean genotypes, and NH4+-N was the most important factor that drove the bacterial community structure between the two genotypes. Al-T soybeans recruited Al-tolerant microorganisms, such as Tumebacillus, Granulicella, and Burkholderia, to improve the resistance to Al stress. Nevertheless, Al stress simplified the network structure in the Al-T soybeans, which may allow for the soil bacterial community to be easily influenced by other biotic and abiotic factors.

H2S is involved in ABA-mediated stomatal movement through MPK4 to alleviate drought stress in Arabidopsis thaliana

Abstract: Hydrogen sulfide (H2S) is a gaseous signaling molecule that participates in multiple physiological processes in both animals and plants. Mitogen-activated protein kinase (MAPK) is important signaling molecule that links the growth and developmental signals and environment stimuli to cellular responses. In the current study we explored the relationship between H2S and MAPK in drought stress resistance in Arabidopsis. The quantitative real-time (qRT)-PCR, root tip bending experiment and stomatal aperture assay were used in this paper. Drought stress activated both H2S biosynthesis and gene expression of MAPKs. The increase in MAPK expression was depressed in lcd/des1, a double mutant of H2S synthesis. Then we selected MPK4 as our target and used mpk4 mutants for further studies. H2S was able to alleviate the drought stress in wild-type (WT) Arabidopsis but not in mpk4 mutants. Meanwhile, H2S-induced stomatal movement was impaired in mpk4 mutants. We then examined the role of H2S and MPK4 in stomatal movements in response to abscisic acid (ABA) and hydrogen peroxide (H2O2). ABA- and H2O2- mediated stomatal movements were impaired in lcd/des1 and mpk4 mutants, and H2S-induced stomatal closure was impaired in slac1–3 mutants. Our results suggested that MPK4 is important downstream of H2S in the drought stress response and in stomatal movement, and that the H2S-MPK4 cascade is involved in ABA-mediated stomatal movement to regulate the drought stress.

Carbon and nitrogen availability in paddy soil affects rice photosynthate allocation, microbial community composition, and priming: combining continuous 13C labeling with PLFA analysis

Abstract: Carbon (C) and nitrogen (N) availability in soil change microbial community composition and activity and so, might affect soil organic matter (SOM) decomposition as well as allocation of plant assimilates. The study was focused on interactions between C and N availability and consequences for rhizodeposition and microbial community structure in paddy soil. Rice continuously labeled in a 13CO2 atmosphere was fertilized with either carboxymethyl cellulose (CMC) (+C), ammonium sulfate (+N), or their combination (+CN), and unfertilized soil was used as a control. 13C was traced in aboveground and belowground plant biomass, soil organic matter, and microbial biomass. Microbial community composition was analyzed by phospholipid fatty acids (PLFAs). +CN application led to a higher yield and lower root C and N content: 13C assimilated in shoots increased by 1.39-fold and that in roots decreased by 0.75-fold. Correspondingly, after +CN addition, 13C from rhizodeposits incorporated into SOM and microorganisms decreased by 0.68-fold and 0.53-fold, respectively, as compared with that in the unfertilized soil. The application of +C or + N alone resulted in smaller changes. CMC led to a 3% of total N mobilized from SOM and resulted in a positive priming effect. Both fertilizations (+C, +N, or + CN) and plant growth stages affected soil microbial community composition. With decreasing microbial biomass C and N, and PLFA content under +CN amendment, +CN fertilization decreased Gram-positive (G+)/ Gram-negative (G-) ratios, and resulted in lower G+ bacteria and fungi abundance, whereas G- and actinomycetes were stimulated by N fertilization. Organic C fertilization led to a positive N priming effect. Organic C and mineral N application decreased C input by rhizodeposition associated with lower 13C recovery in SOM and microbial incorporation. C and N addition also altered microbial community composition, as +CN decreased content of microbial groups, such as G+ bacteria and fungi, but +N stimulated G- bacteria and actinomycetes.

Surface tension, rheology and hydrophobicity of rhizodeposits and seed mucilage influence soil water retention and hysteresis

Abstract: Rhizodeposits collected from hydroponic solutions with roots of maize and barley, and seed mucilage washed from chia, were added to soil to measure their impact on water retention and hysteresis in a sandy loam soil at a range of concentrations. We test the hypothesis that the effect of plant exudates and mucilages on hydraulic properties of soils depends on their physicochemical characteristics and origin. Surface tension and viscosity of the exudate solutions were measured using the Du Nouy ring method and a cone-plate rheometer, respectively. The contact angle of water on exudate treated soil was measured with the sessile drop method. Water retention and hysteresis were measured by equilibrating soil samples, treated with exudates and mucilages at 0.46 and 4.6 mg g−1 concentration, on dialysis tubing filled with polyethylene glycol (PEG) solution of known osmotic potential. Surface tension decreased and viscosity increased with increasing concentration of the exudates and mucilage in solutions. Change in surface tension and viscosity was greatest for chia seed exudate and least for barley root exudate. Contact angle increased with increasing maize root and chia seed exudate concentration in soil, but not barley root. Chia seed mucilage and maize root rhizodeposits enhanced soil water retention and increased hysteresis index, whereas barley root rhizodeposits decreased soil water retention and the hysteresis effect. The impact of exudates and mucilages on soil water retention almost ceased when approaching wilting point at −1500 kPa matric potential. Barley rhizodeposits behaved as surfactants, drying the rhizosphere at smaller suctions. Chia seed mucilage and maize root rhizodeposits behaved as hydrogels that hold more water in the rhizosphere, but with slower rewetting and greater hysteresis.

Nitrogen fertilization increases rice rhizodeposition and its stabilization in soil aggregates and the humus fraction

Abstract: Rhizodeposited-carbon (C) plays an important role in regulating soil C concentrations and turnover, however, the distribution of rhizodeposited-C into different soil organic carbon (SOC) pools and how regulated by nitrogen (N) fertilization still remains elusive. We applied five N fertilization rates (0, 10, 20, 40, and 60 mg N kg−1 soil) to rice (Oryza sativa L.) with continuously labeled 13CO2 for 18 days, to measure 13C allocation into plant tissues and soil C fractions. Relative to the unfertilized controls, the ratio of 13C in plant aboveground shoot /belowground root increased as a result of N fertilization, and the contribution of rhizodeposited-C to SOC was increased by N fertilization, presumably due to the relatively high root biomass and exudates. Also, N fertilization increased 13C incorporation into large aggregates (0.25–2.0 mm) and the humic acid fraction. Biological C immobilization might occur and preserve rhizodeposition following high rates of N addition, which regulates rhizodeposits and C cycling, thus determining the stabilization of rhizodeposits in the different SOC pools. Rhizodeposited-C from rice plants and its distribution within SOC pools strongly depend upon N fertilization, thus determines C sequestration potential from the rice plants.

Pre-colonization of PGPR triggers rhizosphere microbiota succession associated with crop yield enhancement

Abstract: Plant growth-promoting rhizobacteria (PGPR) substantially improve plant growth and health, but their effects on the succession of rhizosphere microbiota throughout the growth period triggered by pre-inoculation have not yet been considered. Pepper seedlings cultured from a bio-nursery substrate containing Bacillus velezensis NJAU-Z9 and ordinary nursery substrate were used in this study to evaluate the effects of pre-colonization of a PGPR strain at the seedling stage on yield enhancement. To elucidate the underlying mechanisms involved in the rhizosphere microbiota succession during the whole growth period and their association with yield enhancement, high-throughput sequencing combined with qPCR was conducted. The results showed that, compared to the control without inoculation, pre-inoculation led to a steady yield enhancement in two-season field trials, as well as higher rhizosphere bacterial richness (Chao1) and diversity (Shannon-Wiener). The plant growth stage as the first driving factor, followed by pre-colonization drove the variations of the rhizosphere microbial community composition according to multivariate regression tree analysis and principal coordinate analysis. Variance partitioning analysis (VPA) and Mantel test results showed that the previous plant growth period induced variations in the fungal and bacterial communities at the next stage. Compared to the seedling and flowering stages, the mature-stage microbial community showed a higher degree of explanation of yield enhancement. Additionally, pre-inoculation led to distinctive rhizosphere microbiota succession compared to the control, due to alteration of the initial community. The heat map analysis showed that the rhizosphere microbiota was related to crop yield. In addition to Bacillus velezensis NJAU-Z9, which showed stable abundance in the pepper rhizosphere, stable higher relative abundance of the bacterial genera Sphingomonas, Sphingopyxis, Bradyrhizobium, Chitinophaga, Dyadobacter, Streptomyces, Lysobacter, Pseudomonas and Rhizomicrobium, and the fungal genera Cladorrhinum, Cladosporium and Aspergillus throughout the growth period induced by pre-colonization was associated with yield enhancement. Overall, we conclude that pre-colonization with PGPR changed the initial rhizosphere microbiota and that the plant was triggered to further select distinctive microbes to form unique rhizosphere microbial consortia at the later growth stages, which resulted in plant growth promotion.

Harvesting forage of the perennial grain crop kernza ( Thinopyrum intermedium ) increases root biomass and soil nitrogen cycling

Abstract: Emerging perennial grain crops yield less grain than annual crops, but the economic viability of these perennial systems could be improved if both forage and grain are harvested. However, the belowground consequences of forage removal in perennial grain systems are unknown. This study aimed to determine the effect of the additional harvest of forage biomass on overall plant biomass allocation and labile soil C and N dynamics within a perennial grain dual-use system. Plant biomass and associated soil samples of a perennial grain [Kernza (Thinopyrum intermedium)] were taken monthly over the first three growing seasons under three harvest regiments: No Cut (0x), Summer Cut (1x), and Summer and Fall Cut (2x). The harvesting of forage biomass significantly increased both above- and belowground biomass. The once and twice forage-harvested treatments averaged 39% and 73% greater root biomass in 2016 and 39% and 49% greater root biomass in 2017 relative to the treatment not harvested for forage. Soil indicators of carbon and nitrogen storage were not affected by forage harvest but mineralizable carbon, an indicator of nutrient cycling, was greater under the forage harvested treatments. The harvest of forage and grain promoted nutrient availability and overall productivity (forage, root and grain biomass) relative to harvesting for grain only. Our findings suggest dual-use management of Kernza can provide a productive and profitable pathway for perennial grain adoption.