Soil C dynamics after deforestation and subsequent conversion of arable cropland to grassland in humid temperate areas.

Land use and plant-soil management influence soil organic C stocks and soil properties. This study aimed to identify the main mechanisms by which these factors alter soil organic matter (SOM) dynamics and stocks. Changes in the organic C pools and biochemical quality in different OM compartments were assessed: a) after deforestation and intensive cultivation (SOM loss) and then, b) after the conversion of cropland to grassland (SOM replenishment) in a chronosequence of recovery (1-45 years). Topsoil samples were subjected to physical fractionation to assess the distribution of free particulate OM (POM) and mineral associated OM (MAOM). SOM quality was characterized by 13C NMR spectroscopy, thermal analysis (DSC/TG), and microbial activity was monitored by isothermal microcalorimetry. Deforestation and intensive cultivation led to the loss of 80 % of the C stored in the upper mineral soil (up to 30-35 cm). The POM was almost depleted, MAOM underwent significant losses (>40 %) and all OM compounds, including the aromatic C, were affected. The large and unexpected loss of MAOM can be attributed to the low specific surface soil area and also to the labile (biodegradable) nature of the OM in this fraction. After 45 years, conversion of cropland to grassland recovered 68 % of the C lost in the mineral soil (mainly as MAOM), at an annual rate of 1.25 Mg C ha-1. The present findings showed that the persistence of long-term OM depends on how strongly organic compounds are adsorbed onto mineral surfaces (i.e., the specific surface area) and the biochemical nature of OM compounds. Adequate plant-soil management favoured the replenishment of the MAOM under these experimental conditions, and this fraction was an active pool in terms of C storage and biochemical quality. This study served to test current theories about changes in soil C fractions due to land use changes and soil-plant management.

Encapsulation of Pseudomonas libanensis in alginate beads to sustain bacterial viability and inoculation of Vigna unguiculata under drought stress.

Conventional agricultural practices based on the application of synthetic fertilizers are increasingly considered as unsustainable. Under a forecasted scenario of drought for the next decades, there is a global demand for innovative and sustainable approaches to ameliorate plant performance. Here, encapsulating beneficial microbes (BMs) to promote plant growth is gaining attention. This study evaluates bacterial encapsulation using polymeric beads of alginate, testing the survival of Pseudomonas libanensis TR1 stored up to 90 days. Produced beads were subjected to different treatments (fresh, air-dried and pulverized), which resulted in a variable size range (1200-860 µm). After storage, bacterial viability was maintained, and air-dried beads displayed a higher number of colony-forming units (2 × 107). Then, a glasshouse experiment investigated the drought resistance (plant growth, biomass, and photosynthetic responses) of Vigna unguiculata plants inoculated with these alginate beads. After 10 days of complete water restriction, turgidity and relative water content of V. unguiculata were still high under drought stress (> 80%). Leaf and root growth and biomass did not evidence significant changes after water restriction even after P. libanensis inoculation. Plant photosynthetic parameters (stomatal conductance, net photosynthetic rate, leaf CO2 concentration, or F v'/F m') were slightly affected due to inoculation but the level of stress-induced minimal plant responses. In our experiment, water restriction might have been insufficient to downregulate photosynthetic efficiency and reduce plant growth, limiting our understanding of the role of P. libanensis inoculation in alleviating drought stress in V. unguiculata, but highlighting the important relationship between the stress level and agricultural benefits of using encapsulated BMs.

The Phytotoxic Potential of the Flowering Foliage of Gorse (Ulex europaeus) and Scotch Broom (Cytisus scoparius), as Pre-Emergent Weed Control in Maize in a Glasshouse Pot Experiment.

In our previous studies, the phytotoxicity of Ulex europaeus (gorse) and Cytisus scoparius (Scotch broom) was demonstrated in vitro and argued to be caused by the release of volatile and water-soluble compounds from fresh plant foliage. In light of these positive results, there was a need to test the effects ex vitro. In this work, gorse and Scotch broom were used as soil amendments in pot experiments in a glasshouse by incorporating slashed plant material into the soil at a ratio of 1% w/w on a dry mass basis. The phytotoxic effects on the emergence and early growth of maize and five accompanying weed species were analyzed, as were the effect on soil fertility and soil community-level physiological profiles. Thirty days after incorporation, significant decreases in weed density of 32.2% and 59.5% were found for gorse and Scotch broom soil amendments, respectively. Gorse soil amendment was notably effective impairing the establishment of Amaranthus retroflexus and diminishing the plant height of Digitaria sanguinalis and Portulaca oleracea. Scotch broom soil amendment was capable of significantly inhibiting the emergence of D. sanguinalis, Convolvulus arvensis, P. oleracea, and A. retroflexus, with a notable reduction of weed biomass. No undesirable side effects on maize crop or soil quality, including microbial activity, were detected. Our results suggest that the incorporation of gorse and Scotch broom foliage is promising for pre-emergent weed control in maize; however, field trials that support and expand these glasshouse results are essential.

Drifting away. Seawater survival and stochastic transport of the invasive Carpobrotus edulis.

Coastal areas are vulnerable and fluctuating habitats that include highly valuable spaces for habitat and species conservation and, at the same time, they are among the most invaded ecosystems worldwide. Occupying large areas within Mediterranean-climate coastlines, the "ecosystem engineer" Carpobrotus edulis appears as a menace for coastal biodiversity and ecosystem services. By combining the observation, current distribution, glasshouse experiment, and dispersion modeling, we aim to achieve a better understanding of the successful invasion process and potential dispersion patterns of C. edulis. We analyzed the response of plant propagules (seeds and plant fragments) to seawater immersion during increasing periods of time (up to 144 h). After 2 months of growth, plant fragments showed a total survival rate (100%) indicating high tolerance to salinity. During this time, fragment length was increased (up to 60%) and root length was higher than control in all cases. Also, immersed fragments consistently accumulated more biomass than control fragments. After two months of growth, photosynthetic parameters (Fv'/Fm', ΦNO, and ΦII) remained stable compared to control fragments. Physiologically, osmolyte and pigment content did not evidence significant changes regardless of immersion time. Based on the capacity of propagules to survive seawater immersion, we modeled the potential transport of C. edulis by combining an oceanic model (ROMS-AGRIF) with a particle-tracking model. Results indicated that propagules may travel variable distances maintaining physiological viability. Our model suggested that short-scale circulation would be the dominant process, however, long-scale circulation of propagules may be successfully accomplished in <6 days. Furthermore, under optimal conditions (southerly winds dominance), propagules may even travel large distances (250 km alongshore). Modeling transport processes, in combination with the dynamics of introduction and expansion, will contribute to a better understanding of the invasive mechanisms of C. edulis and, consequently, to design preventive strategies to reduce the impact of plant invasion.

Using microbial seed coating for improving cowpea productivity under a low-input agricultural system.

BACKGROUND: Plant-growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal (AM) fungi have the ability to enhance the growth, fitness, and quality of various agricultural crops, including cowpea. However, field trials confirming the benefits of microbes in large-scale applications using economically viable and efficient inoculation methods are still scarce. Microbial seed coating has a great potential for large-scale agriculture through the application of reduced amounts of PGPR and AM fungi inocula. Thus, in this study, the impact of seed coating with PGPR, Pseudomonas libanensis TR1 and AM fungus, Rhizophagus irregularis (single or multiple isolates) on grain yield and nutrient content of cowpea under low-input field conditions was evaluated. RESULTS: Seed coating with P. libanensis + multiple isolates of R. irregularis (coatPMR) resulted in significant increases in shoot dry weight (76%), and in the number of pods and seeds per plant (52% and 56%, respectively) and grain yield (56%), when compared with non-inoculated control plants. However, seed coating with P. libanensis + R. irregularis single-isolate (coatPR) did not influence cowpea grain yield. Grain lipid content was significantly higher (25%) in coatPMR plants in comparison with control. Higher soil organic matter and lower pH were observed in the coatPMR treatment. CONCLUSIONS: Our findings indicate that cowpea field productivity can be improved by seed coating with PGPR and multiple AM fungal isolates under low-input agricultural systems. © 2019 Society of Chemical Industry.

Seed Coating: A Tool for Delivering Beneficial Microbes to Agricultural Crops.

Plant beneficial microbes (PBMs), such as plant growth-promoting bacteria, rhizobia, arbuscular mycorrhizal fungi, and Trichoderma, can reduce the use of agrochemicals and increase plant yield, nutrition, and tolerance to biotic-abiotic stresses. Yet, large-scale applications of PBM have been hampered by the high amounts of inoculum per plant or per cultivation area needed for successful colonization and consequently the economic feasibility. Seed coating, a process that consists in covering seeds with low amounts of exogenous materials, is gaining attention as an efficient delivery system for PBM. Microbial seed coating comprises the use of a binder, in some cases a filler, mixed with inocula, and can be done using simple mixing equipment (e.g., cement mixer) or more specialized/sophisticated apparatus (e.g., fluidized bed). Binders/fillers can be used to extend microbial survival. The most reported types of seed coating are seed dressing, film coating, and pelleting. Tested in more than 50 plant species with seeds of different dimensions, forms, textures, and germination types (e.g., cereals, vegetables, fruits, pulses, and other legumes), seed coating has been studied using various species of plant growth-promoting bacteria, rhizobia, Trichoderma, and to a lesser extent mycorrhizal fungi. Most of the studies regarding PBM applied via seed coating are aimed at promoting crop growth, yield, and crop protection against pathogens. Studies have shown that coating seeds with PBM can assist crops in improving seedling establishment and germination or achieving high yields and food quality, under reduced chemical fertilization. The right combination of biological control agents applied via seed coating can be a powerful tool against a wide number of diseases and pathogens. Less frequently, studies report seed coating being used for adaptation and protection of crops under abiotic stresses. Notwithstanding the promising results, there are still challenges mainly related with the scaling up from the laboratory to the field and proper formulation, including efficient microbial combinations and coating materials that can result in extended shelf-life of both seeds and coated PBM. These limitations need to be addressed and overcome in order to allow a wider use of seed coating as a cost-effective delivery method for PBM in sustainable agricultural systems.

Influence of Acacia dealbata Link bark extracts on the growth of Allium cepa L. plants under high salinity conditions.

BACKGROUND: Acacia dealbata Link is an invasive plant worldwide. Finding potential uses for its waste that contribute to controling its spread and result in benefits for agriculture has recently become a new topic of research. This study aimed to evaluate the potential biostimulant effect of bark extract derived from the management of A. dealbata (0, 450 or 900 ppm) on onion plants growing under stressful conditions, such as in soils treated with saline solutions (0, 60 or 120 mmol L-1 NaCl) and with reduced irrigation (100%, 50% or 25%). RESULTS: A. dealbata Link bark extract significantly increased height as well as leaf, root and total biomass of plants in soils irrigated with NaCl solution (120 mmol L-1 ). These plants also had a higher content of Cl in roots, but a lower content of P in leaves and of K in bulbs. The 450 ppm bark treatment additionally increased the protein content in leaves and decreased the Na and Mg content in bulbs and bulbs and roots, respectively. The bark extract also increased the sugar content in plants under saline conditions. However, the effect of bark extract was negligible on plants that grew under drought stress. CONCLUSION: Results revealed that the bark extract might attenuate stress effects in plants growing at high salinity levels, probably by increasing their sugar and protein content and via the accumulation of ions in the roots. Although additional experiments are required, we suggest that the bark extract of A. dealbata has potential applications in agriculture concerned with biostimulant formulations. © 2019 Society of Chemical Industry.

Impact of an invasive nitrogen-fixing tree on arbuscular mycorrhizal fungi and the development of native species.

Arbuscular mycorrhizal fungi (AMF) are obligate soil biotrophs that establish intimate relationships with 80 % of terrestrial plant families. Arbuscular mycorrhizal fungi obtain carbon from host plants and contribute to the acquisition of mineral nutrients, mainly phosphorus. The presence of invasive plants has been identified as a soil disturbance factor, often conditioning the structure and function of soil microorganisms. Despite the investigation of many aspects related to the invasion ofAcacia dealbata, the effect produced on the structure of AMF communities has never been assessed. We hypothesize thatA. dealbatamodifies the structure of AMF community, influencing the establishment and growth of plants that are dependent on these mutualisms. To validate our hypothesis, we carried out denaturing gradient gel electrophoresis (DGGE) analysis and also grew plants ofPlantago lanceolatain pots using roots of native shrublands or fromA. dealbata, as inoculum of AMF. Cluster analyses from DGGE indicated an alteration in the structure of AMF communities in invaded soils. After 15 weeks, we found that plants grown in pots containing native roots presented higher stem and root growth and also produced higher biomass in comparison with plants grown withA. dealbatainoculum. Furthermore, plants that presented the highest biomass and growth exhibited the maximum mycorrhizal colonization and phosphorus content. Moreover, fluorescence measurements indicated that plants grown withA. dealbatainoculum even presented higher photosynthetic damage. Our results indicate that the presence of the invaderA. dealbatamodify the composition of the arbuscular fungal community, conditioning the establishment of native plants.

Structural changes in soil communities after triclopyr application in soils invaded by Acacia dealbata Link.

Triclopyr is a commonly used herbicide in the control of woody plants and can exhibit toxic effects to soil microorganisms. However, the impact on soils invaded by plant exotics has not yet been addressed. Here, we present the results of an 18-month field study conducted to evaluate the impact of triclopyr on the structure of fungal and bacterial communities in soils invaded by Acacia dealbata Link, through the use of denature gradient gel electrophoresis. After triclopyr application, analyses of bacterial fingerprints suggested a change in the structure of the soil bacterial community, whereas the structure of the soil fungal community remained unaltered. Bacterial density and F:B ratio values changed across the year but were not altered due to herbicide spraying. On the contrary, fungal diversity was increased in plots sprayed with triclopyr 5 months after the first application. Richness and diversity (H') of both bacteria and fungi were not modified after triclopyr application.

Ambient has become strained. Identification of Acacia dealbata Link volatiles interfering with germination and early growth of native species.

Acacia dealbata Link is a widespread invader in Mediterranean type ecosystems, and traits promoting its invasiveness are currently under investigation. Due to the dense atmosphere below its canopy, we hypothesized that volatile organic compounds (VOCs) released from flowers, leaves, litter, or a mixture of treatments exert inhibitory effects on the natives Trifolium subterraneum, Lolium multiflorum, Medicago sativa, and also on its own seeds. We reported that VOCs from flowers significantly reduced germination in L. multiflorum and A. dealbata; moreover, root length, stem length, aboveground and belowground biomass were also reduced in all species studied. Volatile organic compounds from flowers and the mixture also increased significantly malondialdehyde content in T. subterraneum and L. multiflorum. The effects of VOCs on antioxidant enzymatic activities were species dependent. Flowers enhanced peroxidase but decreased superoxide dismutase activity in T. subterraneum. In contrast, VOCs released from leaves increased the activity of superoxide dismutase in L. multiflorum. GC/MS analyses revealed 27 VOCs in the volatile fraction from flowers, 12 of which were exclusive to this fraction. Within them, heptadecadiene, n-nonadecane, n-tricosane, and octadecene represent 62% of the fraction. We present evidence that the VOCs released from A. dealbata flowers strongly inhibited germination and seedling growth of selected species, and mainly on its own seedlings. As far as we know, this is the first evidence of phytotoxicity induced by VOCs in invasive species belonging to the Acacia genus.