Site characteristics and vegetation drive the abundance and diversity of arbuscular mycorrhiza fungi in selected land degradation surveillance framework study sites in kenya

Background: In tropical agroecosystems, arbuscular mycorrhizal fungi (AMF) is an essential component of soil fertility. Limited number of studies have emphasized the effects of vegetation type and soil physico-chemical characteristics on AMF communities, although several studies have stressed the importance of AMF in agroecosystems. Objective: To evaluate how specific vegetation types and site affect the physico-chemical properties of the soil and the abundance and diversity of AMF spores. Methodology: Three specific locations from the ongoing Kenya Cereal Enhancement Programme Climate Resilient Agricultural Livelihoods (KCEP-CRAL) initiative were chosen to carry out the study. Kubo South in Kwale County, Muminji in Embu County, and Thange in Makueni County were chosen as the study’s site areas. The sites were selected based on the implementation of a land degradation surveillance framework (LDSF), comprising both cultivated and uncultivated plots. In each study site, four vegetation types namely bushland, grassland, cropland, and shrubland were selected. Results: The results showed significant variations in soil physicochemical properties among different sites. Thange exhibited the highest carbon (C) content, pH, and exchangeable bases compared to Muminji and Kubo South. Sand content was higher (57%) in soils from Kubo South compared to that obtained in Muminji (41%) and Thange (27.8%). In contrast, the clay content was higher in Thange (58%) and Muminji (41%) than in Kubo South (27%). Vegetation type had a significant effect on soil pH and C only in Muminji. A higher abundance of AMF spores was recorded in soil from the Muminji site (385.0 spores kg-1 soil) followed by Kubo south (226.0 spores kg-1 soil) and lowest in Thange (67.0 spores kg-1 soil). Muminji had the highest mean taxonomic richness (3.21 species) compared to Kubo South and Thange (2.96 and 1.98 species respectively). Taxonomic diversity as shown by the Shannon diversity index (Hʹ) had a similar trend as richness. However, vegetation type only had a significant effect on AMF richness and diversity. Implication: The findings of this study may especially be important in agroecosystems since AMF play a key role in soil fertility and productivity through soil aggregation process, nutrient cycling, water relations, and in plant nutrition and health which contribute to the overall ecosystem functioning. Conclusion: These findings show that vegetation type and site influence AMF sporulation and diversity and hence may influence the AMF functions in contributing to the reclamation of degraded soil ecosystems.

Land use intensification in a dry-hot valley reduced the constraints of water content on soil microbial diversity and multifunctionality but increased CO2 production

Conversion of abandoned land (mainly savanna) into cropland generally occurs in fragile ecosystems such as dry-hot valleys (DHVs) in southwest China, with the intent of increasing land productivity and conducting ecological restoration. However, the effects of conversion on soil microbial communities and carbon turnover of savanna ecosystems remain unclear, since savannas could be a vital but overlooked carbon sink. To illustrate the ecological consequences of land-use change (LUC) for savanna ecosystems, a 1-year field experiment was conducted in DHVs of southwest China. The soil properties, microbial respiration, and metagenomics from two different land-use types (grassland and mango plantation) were examined to reveal the effects of regional LUC on soil C turnover and microbial traits. Conversion from degraded grassland into cropland increased the contribution of soil microclimate to the microbial community composition, reduced the constraints of soil water content (SWC), and further decreased nutrient availability. LUC reshaped the composition and structure of soil bacterial communities. Specifically, soil dominant microbes that belonged to Actinobacteria and Proteobacteria were significantly enriched by conversion, while rare microbes that belonged to a wider range of phyla were generally depleted, leading to an overall decrease in community diversity. In addition, LUC-induced changes in soil characteristics and microbial communities further decreased soil multifunctionality as well as the carbon use efficiency of microbes. Intensified microbial respiration and a significant increase in the soil CO2 efflux were observed following LUC, which could drive changes in soil microbial community composition and functions (such as growth and regeneration). In summary, through simultaneously reducing constraints on SWC and decreasing nutrient availability, conversion from degraded grassland to cropland in a DHV decreased soil microbial diversity and multifunctionality, and increased microbial respiration and soil CO2 efflux. Our study provides new insights for understanding the role and mechanisms of LUC in soil carbon turnover in ecologically fragile areas such as DHVs.

Polyaspartic acid enhances the Cd phytoextraction efficiency of Bidens pilosa by remolding the rhizospheric environment and reprogramming plant metabolism

The green soil chelator polyaspartic acid (PASP) can enhance heavy metal phytoextraction efficiency, but the potential mechanisms are not clearly understood from the whole soil–plant system. In this study, we explored the effects and potential mechanisms of PASP addition in soils on plant growth and cadmium (Cd) uptake in the Cd hyperaccumulator Bidens pilosa by analysing variations in chemical elements, rhizospheric microbial community, and plant metabolomics. The results showed that PASP significantly promoted the biomass yield and Cd concentration in B. pilosa, leading to an increase in the total accumulated Cd by 46.4% and 76.4% in shoots and 124.7% and 197.3% in roots under 3 and 6 mg kg−1 PASP addition, respectively. The improved soil-available nutrients and enriched plant growth-promoting rhizobacteria (e.g., Sphingopyxis, Sphingomonas, Cupriavidus, Achromobacter, Nocardioides, and Rhizobium) were probably responsible for the enhanced plant growth after PASP addition. The increase in Cd uptake by plants could be due to the improved rhizosphere-available Cd, which was directly activated by PASP and affected by the induced rhizobacteria involved in immobilizing/mobilizing Cd (e.g., Sphingomonas, Cupriavidus, Achromobacter, and Rhizobium). Notably, PASP and/or these potassium (K)-solubilizing rhizobacteria (i.e., Sphingomonas, Cupriavidus, and Rhizobium) highly activated rhizosphere-available K to enhance plant growth and Cd uptake in B. pilosa. Plant physiological and metabolomic results indicated that multiple processes involving antioxidant enzymes, amino acids, organic acids, and lipids contributed to Cd detoxification in B. pilosa. This study provides novel insights into understanding how soil chelators drive heavy metal transfer in soil–plant systems.

Variations in Soil Nutrient Dynamics and Bacterial Communities After the Conversion of Forests to Long-Term Tea Monoculture Systems

The soil microbial community is a key indicator to evaluate the soil health and productivities in agricultural ecosystems. Monoculture and conversions of forests to tea plantations have been widely applied in tea plantation globally, but long-term monoculture of tea plantation could lead to soil degradation and yield decline. Understanding how long-term monoculture systems influence the soil health and ecosystem functions in tea plantation is of great importance for soil environment management. In this study, through the comparison of three independent tea plantations across eastern China composed of varying stand ages (from 3 to 90 years after conversion from forest), we found that long-term tea monoculture led to significant increases in soil total organic carbon (TOC) and microbial nitrogen (MBN). Additionally, the structure, function, and co-occurrence network of soil bacterial communities were investigated by pyrosequencing 16S rRNA genes. The pyrosequencing analysis revealed that the structures and functions of soil bacterial communities were significantly affected by different stand ages, but sampling sites and land-use conversion (from forest to tea plantation) had stronger effects than stand age on the diversity and structure of soil bacterial communities. Soil bacterial diversity can be improved with increasing stand ages in tea plantation. Further RDA analysis revealed that the C and N availability improvement in tea plantation soils led to the variation of structure and function in soil bacterial communities. Moreover, co-occurrence network analysis of soil bacterial communities also demonstrated that interactions among soil bacteria taxa were strengthened with increasing stand age. Our findings suggest that long-term monoculture with proper managements could be beneficial to soil ecosystems by increasing the C and N content and strengthening bacterial associations in tea plantations. Overall, this study provides a comprehensive understanding of the impact of land-use change and long-term monoculture stand age on soil environments in tea plantation.

Resistance of microbial community and its functional sensitivity in the rhizosphere hotspots to drought

Climate change impacts soil microbial communities, activities and functionality. Nonetheless, responses of the microbiome in soil microenvironments with contrasting substrate availability in the rhizosphere to climatic stresses such as drought are largely unknown. To fill this knowledge gap, we coupled soil zymography with site-specific micro-sampling of the soil and subsequent high-throughput sequencing. This helped identify how the bacterial community structure and the genes encoding N-cycling enzymes (leucine aminopeptidase and chitinase) in rhizosphere hotspots and coldspots (microsites with activities in the range of bulk soil but localized within the rhizosphere) of maize respond to drought (20% WHC, two weeks). The elevated activities of leucine aminopeptidase and chitinase in rhizosphere hotspots were caused by the tight collaborative relationships between bacteria and their stable network structure rather than by any significant shift in bacterial community structure or enzyme-encoding gene copies. Despite the similarity in bacterial community structure in soil under drought and optimal moisture, functional predictions indicated the increased relative abundance of genera belonging to Actinobacteria capable of leucine aminopeptidase and chitinase production, especially Streptomyces, Nocardioides, Marmoricola, and Knoellia. Accordingly, the number of gene copies encoded by Actinobacteria for these two enzymes increased by 5.0–17% under drought. Among the bacteria with increased relative abundance under drought, Luedemannella played a crucial role in mediating nutrients and energy fluxes between bacteria. This was reflected in a 35–70% increase in leucine aminopeptidase and chitinase activities under drought. The resistance of enzyme activities to drought was higher in hotpots than that in coldspots. These results revealed that rhizosphere bacterial community composition remained stable, and that the number of gene copies encoded by Actinobacteria responsible for N-cycling enzymes increased under drought. The expected reduction of processes of N cycle was absent. Instead, bacteria increased N mining rate in those hotspots remaining active despite water scarcity. © 2021

Rhizosphere hotspots: Root hairs and warming control microbial efficiency, carbon utilization and energy production

Root hairs proliferation and warming strongly influence exudate release, enzyme activities and microbial substrate utilization. However, how the presence of root hairs regulates those processes in the rhizosphere under elevated temperature is poorly known. To clarify these interactions, a wild type maize (with root hairs) and its hairless mutant were grown for 3 weeks at 20 and 30 °C, respectively. We combined zymography (localize hotspots of β-glucosidase) with substrate-induced respiration and microcalorimetry to monitor exudate effects on enzyme kinetics, microbial growth and heat production in the rhizosphere hotspots in response to warming. Root hairs effects were more pronounced at the elevated temperature: i) β-glucosidase activity of the wild type at 30 °C was 21% higher than that of the hairless maize; ii) temperature shifted the microbial growth strategy, whereas root hairs promoted the fraction of growing microbial biomass; iii) Km and the activation energy for β-glucosidase under the hairless mutant was lower than that under wild maize. These results suggest that microorganisms inhabiting hotspots of the wild type synthesized more enzymes to fulfill their higher energy and nutrient demands than those of the hairless mutant. In contrast, at higher temperature the hairless maize produced an enzyme pool with higher efficiencies rather than higher enzyme production, enabling metabolic needs to be met at lower cost. We therefore conclude that root hairs play an important role in regulating enzyme systems and microbial growth to adapt to climate warming. © 2020 Elsevier Ltd

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