Decoding the PLFA profiling of microbial community structure in soils contaminated with municipal solid wastes.

This study aimed to assess the influence of municipal solid waste (MSW) disposal on soil microbial communities. Soil samples from 20 different locations of an MSW dumping site contaminated with toxic heavy metals (HMs) and a native forest (as control) were collected for phospholipid fatty acid (PLFA) profiling to predict microbial community responses towards unsegregated disposal of MSW. PLFA biomarkers specific to arbuscular mycorrhizal fungi (AMF), Gram-negative and Gram-positive bacteria, fungi, eukaryotes, actinomycetes, anaerobes, and microbial stress markers-fungi: bacteria (F/B) ratio, Gram-positive/Gram-negative (GP/GN) ratio, Gram-negative stress (GNStr) ratio and predator/prey ratio along with AMF spore density and the total HM content (Cu, Cr, Cd, Mn, Zn, and Ni) were assessed. The results showed that all of the PLFA microbial biomarkers and the F/B ratio were positively correlated, while HMs and microbial stress markers were negatively correlated. The significant correlation of AMF biomass with all microbial groups, the F/B ratio, and T. PLFA confirmed its significance as a key predictor of microbial biomass. With AMF and T. PLFA, Cd and Cr had a weak or negative connection. Among the toxic HMs, Zn and Cd had the greatest impact on microbial populations. Vegetation did not have any significant effect on soil microbial communities. This research will aid in the development of bioinoculants for the bioremediation of MSW-polluted sites and will improve our understanding of the soil microbial community's ability to resist, recover, and adapt to toxic waste contamination.

Biosurfactant from Bacillus sp. A5F Reduces Disease Incidence of Sclerotinia sclerotiorum in Soybean Crop.

The present study was conducted to assess the biocontrol activity of biosurfactants obtained from Bacillus species A5F. The variables significantly influencing the production of biosurfactants under in vitro conditions were further optimized using response surface methodology. Optimal values of selected culture variables, i.e., glucose, soybean oil, and incubation time were 3.5 g l-1, 3.5 ml l-1, and 78 h, respectively, resulting in 2.14-fold enhancement in biosurfactant levels in 5 l fermentor. Identified biosurfactants had a significant effect on chlorophyll content, shoot biomass, number of pods, and seed weight. Biosurfactants also reduced the disease incidence in S. sclerotiorum infected soybean plants and showed antagonistic action against major phytopathogens by disrupting the hyphal cell wall. 16% reduction in ITS gene copy number was observed as compared to control with less non-target effect upon biosurfactant spray on foliar parts of soybean. Thus, the study confirms that biosurfactants from strain A5F can be used as a potent biocontrol agent to control sclerotium wilt on soybean plants.

Novel agrotechnological intervention for soil amendment through areca nut husk biochar in conjunction with vetiver grass.

Soil quality management through effective utilization of agricultural residue is the cynosure of intense global research. Therefore, we have explored the pyrolytic conversion of a locally available agricultural residue, the areca nut husk (AH), into biochar (BC) as a sustainable option towards residue management. The AH was carbonized at 250-400 °C, and residence times of 30-90 min. Subsequent detailed analysis revealed areca nut husk biochar (AHBC) formed at 250 °C with 60 min residence time, had the highest soil organic matter yield index (SOMYI), the lowest H/C and O/C ratio, and an average particle size of 1191.6 nm. Further characterization exposed the highly porous structure of prepared AHBC with oxygenated functional groups attached to its surface. The application of AHBC in conjunction with vetiver (Chrysopogon zizanioides L.) was used as a novel agrotechnological approach to assess soil quality improvement. Various doses of AHBC (5 t ha-1, 10 t ha-1, and 15 t ha-1) were applied in the experimental soils, and the principal component analysis (PCA) revealed that the 15 t ha-1 dose was optimum for the growth of the vetiver. AHBC amendment in soil resulted in increase of plant height and relative water content. This could be attributed to the increase in organic carbon, cation exchange capacity, and nutrients in the soil. Application of AHBC along with vetiver could be a simple, yet effective option, for sustainable agricultural residue and soil management.

Glycoproteins of arbuscular mycorrhiza for soil carbon sequestration: Review of mechanisms and controls.

Glycoproteins, e.g., glomalin related soil proteins (GRSP), are sticky organic substances produced by arbuscular mycorrhizal fungi (AMF). This review summarizes the information on i) the biochemical nature, physical state and origin of GRSP, ii) GRSP decomposition and residence time in soil, iii) GRSP functions, in particular the physical, chemical, and biochemical roles for soil aggregation and carbon (C) sequestration, and finally iv) how land use and agricultural management affect GRSP production and subsequently, organic C sequestration. GRSP augment soil quality by increasing water holding capacity, nutrient storage and availability, microbial and enzymatic activities, and microbial production of extracellular polysaccharides. After release into the soil, GRSP become prone to microbial decomposition due to stabilization with organic matter and sesquioxides, and thereby increasing the residence time between 6 and 42 years. Temperate soils contain 2-15 mg GRSP g-1, whereas arid and semiarid grasslands amount for 0.87-1.7 mg g-1, and GRSP are lower in desert soils. GRSP content is highest in acidic soils as compared to neutral and calcareous soils. Conservation tillage, organic fertilizers and AMF inhabiting crops (e.g. maize, sorghum, soybean, and wheat) increase GRSP production and transform C into stable forms, thereby sustaining soil health and reducing CO2 emissions. Crop rotations with non-mycorrhizal species (e.g. rapeseed) and fallow soils reduce AMF growth and consequently, the GRSP production. The GRSP production increases under nutrient and water deficiency, soil warming and elevated CO2. In the context of global climate change, increased C sequestration through GRSP induced aggregate formation and organic matter stabilization prolong the mean residence time of soil C. Protecting soils against degradation under intensive land use, stable aggregate formation, and prolonging the residence time of C calls for strategies that maximize GRSP production and functions based on reduced tillage, AMF-relevant crop rotations and organic farming.

Effect of High-Temperature Stress on Plant Physiological Traits and Mycorrhizal Symbiosis in Maize Plants.

Increasing high temperature (HT) has a deleterious effect on plant growth. Earlier works reported the protective role of arbuscular mycorrhizal fungi (AMF) under stress conditions, particularly influencing the physiological parameters. However, the protective role of AMF under high-temperature stress examining physiological parameters with characteristic phospholipid fatty acids (PLFA) of soil microbial communities including AMF has not been studied. This work aims to study how high-temperature stress affects photosynthetic and below-ground traits in maize plants with and without AMF. Photosynthetic parameters like quantum yield of photosystem (PS) II, PSI, electron transport, and fractions of open reaction centers decreased in HT exposed plants, but recovered in AMF + HT plants. AMF + HT plants had significantly higher AM-signature 16:1ω5cis neutral lipid fatty acid (NLFA), spore density in soil, and root colonization with lower lipid peroxidation than non-mycorrhizal HT plants. As a result, enriched plants had more active living biomass, which improved photosynthetic efficiency when exposed to heat. This study provides an understanding of how AM-mediated plants can tolerate high temperatures while maintaining the stability of their photosynthetic apparatus. This is the first study to combine above- and below-ground traits, which could lead to a new understanding of plant and rhizosphere stress.

Deciphering the dynamics of glomalin and heavy metals in soils contaminated with hazardous municipal solid wastes.

Heavy metals (HMs) accumulation in the soils poses risks towards the environment and health. Glomalin related soil protein (GRSP) produced by arbuscular mycorrhizal fungi (AMF) has metal-sorption and soil aggregation properties and is critical in the survival of plants and AMF. For the first time, this study attempted to examine the GRSP mediated bio-stabilization of HMs in soils contaminated with municipal solid wastes (MSW). The content and interrelationship of GRSP and HMs, along with soil physicochemical properties were studied in 20 different soil samples from the dumping site. Higher amount of GRSP indicated potential bio-stabilization of HMs at some sites. GRSP exhibited weak positive correlation with essential (Zn, Cu) and toxic HMs (Cd, Ni). Cr and Mn were possibly sequestered in AMF structures and thus found to be negatively correlated with GRSP. The positive correlation observed between GRSP and soil nutrients like N, P and soil organic carbon (SOC) indicating potential of AMF-GRSP in sustaining soil health. Results revealed that AMF residing at contaminated sites produced higher amount of GRSP potentially to bio-stabilize the HMs, and reduce their bioavailability and also facilitate SOC sequestration.

Soybean Processing Mill Waste Plus Vermicompost Enhances Arbuscular Mycorrhizal Fungus Inoculum Production.

This study considered soybean processing mill waste (hulls) as an organic substrate for mass multiplication of indigenous arbuscular mycorrhizal (AM) fungi on sorghum and amaranthus as hosts. In the first experiment, from seven soybean processing mill wastes, three wastes were evaluated for their ability to multiply AM fungi on the two host plants. Among these wastes, hulls were found to be promising for the multiplication of AM fungi and were further examined in a second experiment in combination with vermicompost (VC), a mix of hulls plus vermicompost (SH + VC) amended with soil: sand mix (3:1 v/v) and a soil-sand mix used as a control (SS) in polybags containing the previous two host species. We found that SH blended with VC significantly improved AM fungus production in sorghum polybags assessed through microscopic (spore density in soil, colonization in roots) and biochemical parameters (AM signature lipids in soil: 16:1ω5cis neutral lipid fatty acid (NLFA); phospholipids fatty acid (PLFA) g-1 soil; 16:1ω5cis ester lipid fatty acid (ELFA) g-1 both in soil and roots; and glomalin content in soil. SH + VC contained significantly greater AM fungus populations than the other substrate combinations examined. Principal component analysis (PCA) also identified sorghum as a potential host supporting AM fungus populations particularly when grown under SH + VC conditions. Hence, the combination of soybean hulls and vermicompost was found to be a promising substrate for the mass production of AM fungi using sorghum as a host. These findings have important implications for developing AM fungus inoculum production strategies at the commercial scale.

Application of enzymes as a diagnostic tool for soils as affected by municipal solid wastes.

Assessing the relationship between soil enzyme activities (SEAs) and heavy metals (HMs) without any amendment has rarely been conducted in soils contaminated with municipal solid wastes (MSW). Five soil enzymes [dehydrogenase (DHA), alkaline phosphatase (ALP), acid phosphatase (ACP), urease (UR), and nitrate reductase (NR)] have been assessed for HMs bioremediation using Zea mays L. grown in unamended soils that were contaminated with different types of MSW. Pot experiment was conducted for two seasons with soils collected from seven different locations within the MSW site. Experimental soil samples included a control (CA), contaminated by brick kiln wastes (SA1), kitchen and household wastes (SA2), medical wastes (SA3), mixed wastes (SA4), glass wastes (SA5), and metal scrap wastes (SA6). Rhizospheric soils were collected after the harvest of each season to investigate the impact of HMs on SEAs and physicochemical properties of soil. The results revealed an increase in DHA, ALP, and NR activities by 89.30%, 58.03% and 21.98% in SA1. Likewise, enhanced activities for UR (28.26%) and ACP (19.6%) were observed in SA3 and SA5 respectively. Insignificant increase in the macronutrients and organic carbon (OC) were also noted. The increased microbial count and the relatively higher amount of organic matter (OM) in the rhizosphere indicated the role of OM in HMs immobilization. Principal component analysis (PCA) indicated that DHA and NR are the important soil enzymes, underscored by their active involvement in the C and N turnover in the soil. Likewise, correlation analysis showed that DHA and NR activities were positively correlated with copper (Cu) (0.90, p < 0.01; 0.88, p < 0.01), suggesting its participation as a cofactor in enzymatic activities. In contrast, DHA was negatively correlated with cadmium (Cd) (-0.48, p < 0 0.05). Finally, these results indicated that in the absence of exogenous nutrient amendment, the SEAs were governed by OC, available nitrogen (Avl. N), Cu and Cd respectively. The study also highlighted the need for extensive research on SEAs for its utilization as a bioindicator in various soil bioremediation and quality management practices.

Speciation, contamination, ecological and human health risks assessment of heavy metals in soils dumped with municipal solid wastes.

The main aim of this work is to assess the extent of soil contamination, potential ecological and health risks associated with the disposal of municipal solid waste (MSW) near a Ramsar site in Assam, India. Soil samples were collected and analysed for three heavy metals (HMs), namely, chromium (Cr), manganese (Mn) and zinc (Zn). The sources of HMs and their pollution levels were evaluated using different indices. The results demonstrated that Cr contamination was high near the metal scrap segregations unit within the dumping site, otherwise, the ecological risks associated with Zn and Mn were found to be low. The speciation of Cr and Zn were associated with the Fe-Mn oxide bound (F4) fraction, accounting 44.23% and 30.68%, respectively, whereas Mn (52.55%) was associated with the exchangeable fraction (F2). The fate and origin of HMs were assessed using mobility and enrichment factors and 16 out of the 20 sampling sites fell under the category of heavily polluted category for Cr, while others which were nearby the metal segregation units fell under the strongly to extremely polluted category. In few sites, significant enrichment was observed for Zn and minimal to moderate enrichment for Mn, respectively. Health risk assessment results indicated that Cr posed higher threat to human health through ingestion.

Deciphering the Role of Trehalose in Tripartite Symbiosis Among Rhizobia, Arbuscular Mycorrhizal Fungi, and Legumes for Enhancing Abiotic Stress Tolerance in Crop Plants.

Drought is a critical factor limiting the productivity of legumes worldwide. Legumes can enter into a unique tripartite symbiotic relationship with root-nodulating bacteria of genera Rhizobium, Bradyrhizobium, or Sinorhizobium and colonization by arbuscular mycorrhizal fungi (AMF). Rhizobial symbiosis provides nitrogen necessary for growth. AMF symbiosis enhances uptake of diffusion-limited nutrients such as P, Zn, Cu, etc., and also water from the soil via plant-associated fungal hyphae. Rhizobial and AMF symbioses can act synergistically in promoting plant growth and fitness, resulting in overall yield benefits under drought stress. One of the approaches that rhizobia use to survive under stress is the accumulation of compatible solutes, or osmolytes, such as trehalose. Trehalose is a non-reducing disaccharide and an osmolyte reported to accumulate in a range of organisms. High accumulation of trehalose in bacteroids during nodulation protects cells and proteins from osmotic shock, desiccation, and heat under drought stress. Manipulation of trehalose cell concentrations has been directly correlated with stress response in plants and other organisms, including AMF. However, the role of this compound in the tripartite symbiotic relationship is not fully explored. This review describes the biological importance and the role of trehalose in the tripartite symbiosis between plants, rhizobia, and AMF. In particular, we review the physiological functions and the molecular investigations of trehalose carried out using omics-based approaches. This review will pave the way for future studies investigating possible metabolic engineering of this biomolecule for enhancing abiotic stress tolerance in plants.