Fertilization Weakens the Ecological Succession of Dissolved Organic Matter in Paddy Rice Rhizosphere Soil at the Molecular Level

Dissolved organic matter (DOM) is involved in numerous biogeochemical processes, and understanding the ecological succession of DOM is crucial for predicting its response to farming (e.g., fertilization) practices. Although plentiful studies have examined how fertilization practice affects the content of soil DOM, it remains unknown how long-term fertilization drives the succession of soil DOM over temporal scales. Here, we investigated the succession of DOM in paddy rice rhizosphere soils subjected to different long-term fertilization treatments (CK: no fertilization; NPK: inorganic fertilization; OM: organic fertilization) along with plant growth. Our results demonstrated that long-term fertilization significantly promoted the molecular chemodiversity of DOM, but it weakened the correlation between DOM composition and plant development. Time-decay analysis indicated that the DOM composition had a shorter halving time under CK treatment (94.7 days), compared to NPK (337.4 days) and OM (223.8 days) treatments, reflecting a lower molecular turnover rate of DOM under fertilization. Moreover, plant development significantly affected the assembly process of DOM only under CK, not under NPK and OM treatments. Taken together, our results demonstrated that long-term fertilization, especially inorganic fertilization, greatly weakens the ecological succession of DOM in the plant rhizosphere, which has a profound implication for understanding the complex plant-DOM interactions.

Microbial landscapes of the rhizosphere soils and roots of Luffa cylindrica plant associated with Meloidogyne incognita

Introduction

The root-knot nematodes (RKN), especially Meloidogyne spp., are globally emerging harmful animals for many agricultural crops.

Methods

To explore microbial agents for biological control of these nematodes, the microbial communities of the rhizosphere soils and roots of sponge gourd (Luffa cylindrica) infected and non-infected by M. incognita nematodes, were investigated using culture-dependent and -independent methods.

Results

Thirty-two culturable bacterial and eight fungal species, along with 10,561 bacterial and 2,427 fungal operational taxonomic units (OTUs), were identified. Nine culturable bacterial species, 955 bacterial and 701 fungal OTUs were shared in both four groups. More culturable bacterial and fungal isolates were detected from the uninfected soils and roots than from the infected soils and roots (except no fungi detected from the uninfected roots), and among all samples, nine bacterial species (Arthrobacter sp., Bacillus sp., Burkholderia ambifaria, Enterobacteriaceae sp., Fictibacillus barbaricus, Microbacterium sp., Micrococcaceae sp., Rhizobiaceae sp., and Serratia sp.) were shared, with Arthrobacter sp. and Bacillus sp. being dominant. Pseudomonas nitroreducens was exclusively present in the infested soils, while Mammaliicoccus sciuri, Microbacterium azadirachtae, and Priestia sp., together with Mucor irregularis, Penicillium sp., P. commune, and Sordariomycetes sp. were found only in the uninfected soils. Cupriavidus metallidurans, Gordonia sp., Streptomyces viridobrunneus, and Terribacillus sp. were only in the uninfected roots while Aspergillus sp. only in infected roots. After M. incognita infestation, 319 bacterial OTUs (such as Chryseobacterium) and 171 fungal OTUs (such as Spizellomyces) were increased in rhizosphere soils, while 181 bacterial OTUs (such as Pasteuria) and 166 fungal OTUs (such as Exophiala) rose their abundance in plant roots. Meanwhile, much more decreased bacterial or fungal OTUs were identified from rhizosphere soils rather than from plant roots, exhibiting the protective effects of host plant on endophytes. Among the detected bacterial isolates, Streptomyces sp. TR27 was discovered to exhibit nematocidal activity, and B. amyloliquefaciens, Bacillus sp. P35, and M. azadirachtae to show repellent potentials for the second stage M. incognita juveniles, which can be used to develop RKN bio-control agents.

Discussion

These findings provided insights into the interactions among root-knot nematodes, host plants, and microorganisms, which will inspire explorations of novel nematicides.

Different Age-Induced Changes in Rhizosphere Microbial Composition and Function of Panax ginseng in Transplantation Mode

Transplantation is a cultivation mode widely applied in perennial plant growing. This method might be an effective way to alleviate problems associated with continuous cultivation (4-6 years) in ginseng production, but the alleviating mechanism and effects on soil microbial community is unclear. To study this issue, non-transplanted 2-year-old, and 5-year-old (transplantation mode: 2 + 3) and 9-year-old (transplantation mode: 3 + 3 + 3) ginseng rhizosphere soils were analyzed via MiSeq sequencing. The results showed that 9-year-old ginseng rhizosphere soil had lower available nitrogen and the lowest pH, available phosphorus, observed species and community diversity and richness (Chao1, and ACE) among all samples (p < 0.05). The abundances of some bacterial classes (Thermoleophilia, Bacilli, and Nitrospira) and fungal genera (Mortierella, Epicoccum, and Penicillium spp.) and functional richness associated with nutrient element cycles and antifungal activity decreased, while abundances of some fungal genera (Ilyonectria, Tetracladium, and Leptodontidium spp.) increased with increasing age of ginseng plants (p < 0.05 or p < 0.01). However, there was greater similarity between soil samples of 2-year-old and transplanted 5-year-old ginseng plants and the increase in cultivation time from 2 to 5 years did not significantly influence the microbial community, suggesting that transplantation is a viable strategy for suppressing soil-borne diseases in Panax ginseng plants over long growth periods.

Consortium of Plant Growth-Promoting Rhizobacteria Strains Suppresses Sweet Pepper Disease by Altering the Rhizosphere Microbiota

Beneficial microorganisms have been extensively used to make plants more resistant to abiotic and biotic stress. We previously identified a consortium of three plant growth-promoting rhizobacteria (PGPR) strains (Bacillus cereus AR156, Bacillus subtilis SM21, and Serratia sp. XY21; hereafter “BBS”) as a promising and environmentally friendly biocontrol agent. In this study, the effect of BBS on a soil-borne disease of sweet pepper was evaluated. Application of BBS significantly reduced the prevalence of phytophthora blight and improved fruit quality and soil properties relative to the control. BBS was able to alter the soil bacterial community: it significantly increased the abundances of Burkholderia, Comamonas, and Ramlibacter, which were negatively associated with disease severity, relative to the control. A redundancy analysis suggested that BBS-treated soil samples were dominated by Burkholderia, Comamonas, Ramlibacter, Sporichthya, Achromobacter, and Pontibacter; abundance of these genera was related to total organic carbon (TOC), total nitrogen (TN), ammonium nitrogen (AN), total potassium (TP), and available phosphorus (AP) contents. This suggests that BBS treatment shifted the microbe community to one that suppressed soil-borne disease and improved the soil chemical properties.

Assessment of the Diversity of Fungal Community Composition Associated With Vachellia pachyceras and Its Rhizosphere Soil From Kuwait Desert

This research examined the general soil fungi and AM fungal communities associated with a Lonely Tree species (Vachellia pachyceras) existing in the Sabah Al-Ahmad Natural Reserve located at the Kuwait desert. The goals of the study were to describe the general fungal and AM fungal communities present in the rhizospheric, non-rhizospheric soils and roots of V. pachyceras, respectively, as well as local and non-local V. pachyceras seedlings when grown under standard nursery growing environments. Soil and root samples were analyzed for an array of characteristics including soil physicochemical composition, and culture-independent method termed PCR-cloning, intermediate variable region of rDNA, the large subunit (LSU) and internal transcribed spacer (ITS) region sequence identifications. The results reveal that the fungal phylotypes were classified in four major fungal phyla namely Ascomycota, Basidiomycota, Chytridiomycota, and Zygomycota. The largest assemblage of fungal analyses showed communities dominated by members of the phylum Ascomycota. The assays also revealed a wealth of incertae sedis fungi, mostly affiliated to uncultured fungi from diverse environmental conditions. Striking difference between rhizosphere and bulk soils communities, with more fungal diversities and Operational Taxonomic Units (OTUs) richness associated with both the field and nursery rhizosphere soils. In contrast, a less diverse fungal community was found in the bulk soil samples. The characterization of AM fungi from the root system demonstrated that the most abundant and diversified group belongs to the family Glomeraceae, with the common genus Rhizophagus (5 phylotypes) and another unclassified taxonomic group (5 phylotypes). Despite the harsh climate that prevails in the Kuwait desert, studied roots displayed the existence of considerable number of AM fungal biota. The present work thus provides a baseline of the fungal and mycorrhizal community associated with rhizosphere and non-rhizosphere soils and roots of only surviving V. pachyceras tree from the Kuwaiti desert and seedlings under nursery growing environments.

Role of Phenolic Acids from the Rhizosphere Soils of Panax notoginseng as a Double-Edge Sword in the Occurrence of Root-Rot Disease

Chemical agents in the rhizosphere soils of plants might have an influence on root-rot disease, which therefore might reveal the mechanism of root rot in Panax notoginseng (P. notoginseng). With this hypothesis the alterations of phenolic acids (PAs) in the rhizosphere soils of P. notoginseng after pathogen infection were determined. The effects of PAs on the growth of Fusarium oxysporum (F. oxysporum), a fungal pathogenic factor for P. notoginseng, as well as production of fusaric acid, a wilting agent for the plants, were also examined. The results indicate the presence of five PAs (ferulic acid, syringic acid, p-hydroxybenzoic acid, p-coumaric acid, and vanillic acid) in the rhizosphere soils of P. notoginseng, whose contents in the rhizosphere soils of healthy plants are higher than those of the diseased ones. Further we found that individual PA could inhibit the mycelium growth and spore production of F. oxysporum, but stimulate fusaric acid production as well, disclosing the double-edge sword role of PAs in the occurrence of root rot of P. notoginseng and paving the way for the intervention of P. notoginseng root rot via balancing PAs.