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Compost applications vary in their plant growth promotion and plant disease suppression, likely due to differences in physico-chemical and biological parameters. Our hypothesis was that bacteria are important for plant growth promotion and disease suppression of composts and, therefore, composts having these traits would contain similar sets of indicative bacterial taxa. Seventeen composts prepared from five different commercial providers and different starting materials were classified accordingly with bioassays using cress plants and the pathogen Pythium ultimum. Using a metabarcoding approach, bacterial communities were assessed in bulk composts and cress rhizoplanes. Six and nine composts showed significant disease suppression or growth promotion, respectively, but these traits did not correlate. Growth promotion correlated positively with nitrate content of composts, whereas disease suppression correlated negatively with factors representing compost age. Growth promotion and disease suppression explained significant portions of variation in bacterial community structures, i.e. 11.5% and 14.7%, respectively. Among the sequence variants (SVs) associated with growth promotion, Microvirga, Acinetobacter, Streptomyces, Bradyrhizobium and Bacillus were highly promising, while in suppressive composts, Ureibacillus, Thermogutta and Sphingopyxis were most promising. Associated SVs represent the basis for developing prediction tools for growth promotion and disease suppression, a highly desired goal for targeted compost production and application.

Background and aimsNeutralization of adverse environmental effects of agriculture intensification to sustain population growth, requires ecologically sound alternatives for plant growth. We used as biostimulants towards germination of basil seeds and early growth of maize, two different humic materials: a potassium humate from leonardite (KH), and compost tea (CT) from a green compost made of coffee husks, and a 1:1 combination of the two (MIX). After their thorough chemical, molecular and conformational characterization, a relation between structure and bioactivity was investigated.ResultsCT showed the largest bioactivity on either seed germination or maize plantlets growth due to its great content of polar bioactive molecules including oxidized lignin fragment, saccharides and peptides. The more hydrophobic KH, rich of alkyl and aromatic moieties, also exerted a significant bioactivity on maize, though to a lesser extent. The application of MIX to hydroponically grown maize plantlets showed a smaller bioactivity of polar CT molecules due to their entrapment into new suprastructures stabilized by hydrogen bonds formed with complementary functions of KH hydrophobic components. However, while the KH hydrophobicity in MIX ensured adhesion to roots, its conformational flexibility was still sufficient to provide a greater bioactivity than control, by releasing bioactive CT components capable to enhance both biomass yield and root elongation.ConclusionsOur study suggests that a combination of humic materials with diverse and well-characterized molecular properties may become a new tool to produce innovative and ecologically viable plant growth promoters, whose bioactivity may be modulated.

Background and aimsBiostimulants of natural origin represent a growing ecological strategy to increase crops productivity, especially when applied in combination with microbial bioeffectors. We studied the effect of biostimulants such as Potassium Humates (KH) from Leonardite and Compost Tea (CT) from green compost on both productivity and nutritional status of lettuce plants, as well as on the primary and secondary metabolism of treated plants, when amended either alone or in combination with a commercial microbial inoculum (M+), mainly based on arbuscular mycorrhizal fungi (Micosat TabPlus).ResultsThe biomass production as well as the uptake of both macro- and micronutrients by lettuce plants significantly increased when amended by the mixture of both humic materials (MIX) combined with the microbial inoculum. Similarly, the synergic MIX_M+ treatment significantly affected both the primary and secondary metabolism of lettuce more than their individual applications, by increasing, respectively, the biosynthesis of essential amino acids and carbohydrates, and that of antioxidant polyphenolic compounds, such as hydroxycinnamic acids, flavonols and coumarins.ConclusionsOur findings suggest that a calibrated mixture of humic bioactive molecules in combination with microbial consortia represents a potential tool to improve crop productivity and its nutritional and metabolic status.

Composting has become a preferable option to treat organic wastes to obtain a final stable sanitized product that can be used as an organic amendment. From home composting to big municipal waste treatment plants, composting is one of the few technologies that can be practically implemented at any scale. This review explores some of the essential issues in the field of composting/compost research: on one hand, the main parameters related to composting performance are compiled, with especial emphasis on the maturity and stability of compost; on the other hand, the main rules of applying compost on crops and other applications are explored in detail, including all the effects that compost can have on agricultural land. Especial attention is paid to aspects such as the improvement of the fertility of soils once compost is applied, the suppressor effect of compost and some negative experiences of massive compost application.

Organic matter controls aggregate stability in loam soils. Intensive farming can lead to a decrease in soil organic matter content. In areas where livestock have disappeared, the recycling of composted urban organic wastes on agricultural soils may represent a valuable source of organic matter for restoring soil organic matter content. The effects on the aggregate stability of a silt loam soil of three urban composts (a municipal solid waste compost, a co-compost of sewage sludge and green waste, and a biowaste compost) sampled at two different stages of maturity (immature and mature composts) were studied during laboratory incubations. The results were related to (i) compost organic matter biodegradability, biochemical fractions, and humic substance content, (ii) microbial activity evaluated through organic C mineralization and microbial and fungal biomass evolution, (iii) hot-water-extractable polysaccharides, and (iv) aggregate hydrophobicity as revealed by the water drop penetration time test. Both immature and mature composts increased aggregate stability via different mechanisms. After immature compost addition, the enhanced microbial activity mainly improved aggregate stability by increasing water repellency. The fungal biomass was particularly involved in aggregate stabilization. The municipal solid waste compost was more efficient at improving resistance to slaking, probably because of its larger labile organic pool that enhanced microbial activity. The addition of mature composts immediately improved aggregate stability with similar efficiency for all composts but to a lesser extent than improvements from immature composts. The observed increase of interparticular cohesion could be due to the inward diffusion of binding organic substances within the aggregates.

Nowadays, modern agriculture looks for valid, sustainable, and green alternatives that are able to improve and maintain soil quality and fertility over time. Recycling organic waste as fertilizer is one of the strategies for sustainable production. Recently, the use of new products derived from compost, such as compost tea (CT), is increasing due to their positive effects on crops. This perspective wants to give an updated shot at the effect of compost tea in horticulture. In addition to the classification of compost tea, with a focus on production procedures and composition, the possible effects they have both on the control of phytopathogens in horticulture and the influence they can have on the content of bioactive molecules and nutrients were highlighted. It is interesting to note that compost teas can have an effect on the final content of micro and macronutrients, thus improving the nutritional qualities and also increasing the content of bioactive compounds that may play a role in maintaining and improving human health. The combined use of compost tea with other treatments is being explored as a promising and innovative direction.

Background and aims Besides general effect of organic residues on soil quality and plant crop, hormonal direct effect on plant growth by extracted humic acids of organic materials is interesting and profitable theme. In the present work, we studied on direct interaction between humic acid and root growth, depending on different origin of organic materials. Methods All extracted humic acids of four organic materials (sewage sludge, compost sewage sludge, municipal solid waste, compost municipal solid waste) were characterized chemically by elemental analyses, ion pair chromatography (ICP), size exclusion chromatography (HPSEC), solid-state nuclear magnetic resonance (13C-CPMAS-NMR) and quantification of IAA. Later, different morphological effects on maize (principal root growth, lateral root growth, root area, root mitosic site, root dry weight and H+-ATPase activity of plasma membrane) were analyzed. Results All humic acids samples promoted root growth and proton pump activity in maize vesicles, especially those composted samples, which contained more carboxylic groups and had a more hydrophobic character, produced preferentially morphological and biochemical effects.

In this study, the suitability of biowaste and green waste composts in organic farming is presented based on quality assurance data of approximately 21,000 compost analyses from 2015 to 2020. The evaluation of compost suitability was based on both the legal regulations of the EU 2021/1165 and the requirements of the two largest German organic farming associations Bioland and Naturland. In 2020, 70.1% of the composts agreed with the above-mentioned regulations, 21.6% exceeded the limits for heavy metals and 7.3% exceeded the limits for foreign matter. The negative influence of the single elements regarding the suitability of composts for organic agriculture declined in the order Zn > Pb > Cd > Ni > Cu. In the bio-waste composts, the impurity content subsequently decreased by more than 50% from 2015 to 2020. In 2019 and 2020, approximately 2.5 million Mg fresh mass (FM) of the analyzed composts were suitable for organic farming. With an average compost application of 5 Mg FM per hectare (ha) and year, about 500,000 ha of arable land could have been supplied in 2020.

Mushrooms are an important food crop for many millions of people worldwide. The most important edible mushroom is the button mushroom (Agaricus bisporus), an excellent example of sustainable food production which is cultivated on a selective compost produced from recycled agricultural waste products. A diverse population of bacteria and fungi are involved throughout the production of Agaricus. A range of successional taxa convert the wheat straw into compost in the thermophilic composting process. These initially break down readily accessible compounds and release ammonia, and then assimilate cellulose and hemicellulose into compost microbial biomass that forms the primary source of nutrition for the Agaricus mycelium. This key process in composting is performed by a microbial consortium consisting of the thermophilic fungus Mycothermus thermophilus (Scytalidium thermophilum) and a range of thermophilic proteobacteria and actinobacteria, many of which have only recently been identified. Certain bacterial taxa have been shown to promote elongation of the Agaricus hyphae, and bacterial activity is required to induce production of the mushroom fruiting bodies during cropping. Attempts to isolate mushroom growth-promoting bacteria for commercial mushroom production have not yet been successful. Compost bacteria and fungi also cause economically important losses in the cropping process, causing a range of destructive diseases of mushroom hyphae and fruiting bodies. Recent advances in our understanding of the key bacteria and fungi in mushroom compost provide the potential to improve productivity of mushroom compost and to reduce the impact of crop disease.

Rainfall regimes are expected to shift on a regional scale as the water cycle intensifies in a warmer climate, resulting in greater extremes in dry versus wet conditions. Such changes are having a strong impact on the agro-physiological functioning of plants that scale up to influence interactions between plants and microorganisms and hence ecosystems. In (semi)-arid ecosystems, the date palm ( Phoenix dactylifera L.) -an irreplaceable tree- plays important socio-economic roles. In the current study, we implemeted an adapted management program to improve date palm development and its tolerance to water deficit by using single or multiple combinations of exotic and native arbuscular mycorrhizal fungi (AMF1 and AMF2 respectively), and/or selected consortia of plant growth-promoting rhizobacteria (PGPR: B1 and B2), and/or composts from grasses and green waste (C1 and C2, respectively). We analyzed the potential for physiological functioning (photosynthesis, water status, osmolytes, mineral nutrition) to evolve in response to drought since this will be a key indicator of plant resilience in future environments. As result, under water deficit, the selected biofertilizers enhanced plant growth, leaf water potential, and electrical conductivity parameters. Further, the dual-inoculation of AMF/PGPR amended with composts alone or in combination boosted the biomass under water deficit conditions to a greater extent than in non-inoculated and/or non-amended plants. Both single and dual biofertilizers improved physiological parameters by elevating stomatal conductance, photosynthetic pigments (chlorophyll and carotenoids content), and photosynthetic efficiency. The dual inoculation and compost significantly enhanced, especially under drought stress, the concentrations of sugar and protein content, and antioxidant enzymes (polyphenoloxidase and peroxidase) activities as a defense strategy as compared with controls. Under water stress, we demonstrated that phosphorus was improved in the inoculated and amended plants alone or in combination in leaves (AMF2: 807%, AMF1+B2: 657%, AMF2+C1+B2: 500%, AMF2+C2: 478%, AMF1: 423%) and soil (AMF2: 397%, AMF1+B2: 322%, AMF2+C1+B2: 303%, AMF1: 190%, C1: 188%) in comparison with controls under severe water stress conditions. We summarize the extent to which the dual and multiple combinations of microorganisms can overcome challenges related to drought by enhancing plant physiological responses.