Impacts of the rhizosphere effect and plant species on organic carbon mineralization rates and pathways, and bacterial community composition in a tidal marsh

Microbial Community Shifts in Tea Plant Rhizosphere under Seawater Stress: Enrichment of Beneficial Taxa

Seawater intrusion has a significant impact on the irrigation quality of agricultural water, thereby posing a threat to plant growth and development. We hypothesized that the rhizosphere of tea plants harbors beneficial microorganisms, which may improve the tolerance of tea plants to seawater stress. This study utilized 16s and ITS techniques to analyze microbial community shifts in the tea plant rhizosphere and non-rhizosphere under seawater stress conditions. The findings suggest that seawater stress leads to a reduction in microbial diversity, although the rhizosphere microbial diversity in stressed soils showed a relatively higher level. Moreover, the rhizosphere of the tea plant under seawater stress exhibited an enrichment of plant growth-promoting rhizobacteria alongside a higher presence of pathogenic fungi. Network analysis revealed that seawater stress resulted in the construction of a more complex and stable rhizosphere microbial network compared to normal conditions. Predictions of bacterial potential functions highlighted a greater diversity of functional groups, enhancing resource utilization efficiency. In general, the rhizosphere microorganisms of tea plants are jointly selected by seawater and the host. The microorganisms closely related to the rhizosphere of tea plants are retained and, at the same time, attract beneficial microorganisms that may alleviate stress. These findings provide new insights into plant responses to saline stress and have significant implications for leveraging vegetation to enhance the resilience of coastal saline soils and contribute to economic progress.

Nitrogen fertilization enhances organic carbon accumulation in topsoil mainly by improving photosynthetic C assimilation in a salt marsh

Continuous nitrogen (N) loading alters plant growth and subsequently has the potential to impact soil organic carbon (SOC) accumulation in salt marshes. However, the knowledge gap of photosynthesized carbon (C) allocation in plant-soil-microbial systems hampers the quantification of C fluxes and the clarification of the mechanisms controlling the C budget under N loading in salt marsh ecosystems. To address this, we conducted an N fertilization field observation combined with a 5 h 13C-pulse labeling experiment in a salt marsh dominated by Suaeda. salsa (S. salsa) in the Yellow River Delta (YRD), China. N fertilization increased net 13C assimilation of S. Salsa by 277.97%, which was primarily allocated to aboveground biomass and SOC. However, N fertilization had little effect on 13C allocation to belowground biomass. Correlation analysis showed that 13C incorporation in soil was significantly and linearly correlated with 13C incorporation in shoots rather than in roots both in a 0 N (0 g N m-2 yr-1) and +N (20 g N m-2 yr-1) group. The results suggested that SOC increase under N fertilization was mainly due to an increased C assimilation rate and more efficient downward transfer of photosynthesized C. In addition, N fertilization strongly improved the 13C amounts in the chloroform-labile SOC component by 295.26%. However, the absolute increment of newly fix 13C mainly existed in the form of residual SOC, which had more tendency for burial in the soil. Thus, N fertilization enhanced SOC accumulation although C loss increased via belowground respiration. These results have important implications for predicting the carbon budget under further human-induced N loading.

Climatic zone effects of non-native plant invasion on CH4 and N2O emissions from natural wetland ecosystems

Plant invasion can significantly alter the carbon and nitrogen cycles of wetlands, which potentially affects the emission of greenhouse gases (GHGs). The extent of these effects can vary depending on several factors, including the species of invasive plants, their growth patterns, and the climatic conditions prevailing in the wetland. Understanding the global effects of plant invasion on the emission of methane (CH4) and nitrous oxide (N2O) is crucial for the climate-smart management of wetlands. Here, we performed a global meta-analysis of 207 paired case studies that quantified the effect of non-native plant invasion on CH4 and N2O emissions in tropical/sub-tropical (TS) and temperate (TE) wetlands. The average emission rate of CH4 from the TS wetlands increased significantly from 337 to 577 kg CH4 ha-1 yr-1 in areas where native plants had been displaced by invasive plants. Similarly, in TE wetlands, the emission rates increased from 211 to 299 kg CH4 ha-1 yr-1 following the invasion of alien plant species. The increase in CH4 emissions at invaded sites was attributed to the increase in plant biomass, soil organic carbon (SOC), and soil moisture (SM). The effects of plant invasion on N2O emissions differed between TS and TE wetlands in that there was no significant effect in TS wetlands, whereas the N2O emissions reduced in TE wetlands. This difference in N2O emissions between climate zones was attributed to the depletion of NH4+ and NO3- in soils and the lower soil temperature in temperate regions. Overall, plant invasion increased the global net CH4 emissions from natural wetlands by 10.54 Tg CH4 yr-1. However, there were variations in CH4 emissions across different climatic zones, indicated by a net increase in CH4 emissions, of 9.97 and 0.57 Tg CH4 yr-1 in TS and TE wetlands, respectively. These findings highlight that plant invasion not only strongly stimulates the emission of CH4 from TS wetlands, but also suppresses N2O emissions from TE wetlands. These novel insights immensely improve our current understanding of the effects of climatic zones on biogeochemical controlling factors that influence the production of greenhouse gases (GHGs) from wetlands following plant invasion. By analyzing the specific mechanisms by which invasive plants affect GHG emissions in different climatic zones, effective strategies can be devised to reduce GHG emissions and preserve wetland ecosystems.

Salt-tolerant plant moderates the effect of salinity on soil organic carbon mineralization in a subtropical tidal wetland

Although salinization is widely known to affect cycling of soil carbon (C) in tidal freshwater wetlands, the role of the presence or absence of plants in mediating the responses of soil organic carbon (SOC) mineralization to salinization is poorly understood. In this study, we translocated soils collected from a tidal freshwater wetland to sites with varying salinities along a subtropical estuarine gradient and established unplanted and planted (with the salt-tolerant plant Cyperus malaccensis Lam.) mesocosms at each site. We simultaneously investigated cumulative soil CO₂ emissions, C-acquiring enzyme activities, availability of labile organic C (LOC), and structures of bacterial and fungal communities. Overall, in the planted mesocosm, the soil LOC content and the activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase increased with salinization. However, in the unplanted mesocosm, soil LOC content decreased with increasing salinity, whereas all the C-acquiring enzyme activities did not change. In addition, salinization favored the dominance of bacterial and fungal copiotrophs (e.g., γ-Proteobacteria, Bacteroidetes, Firmicutes, and Ascomycota) in the planted mesocosms. Contrarily, in the unplanted mesocosms salinization favored bacterial and fungal oligotrophs (e.g., α-Proteobacteria, Chloroflexi, Acidobacteria, and Basidiomycota). In both planted and unplanted mesocosms, cumulative soil CO₂ emissions were affected by soil LOC content, activities of C-acquiring enzymes, and microbial C-use trophic strategies. Overall, cumulative soil CO₂ emissions increased by 35% with increasing salinity in the planted mesocosm but decreased by 37% as salinity increased in the unplanted mesocosm. Our results demonstrate that the presence or absence of salt-tolerant plants can moderate the effect of salinity on SOC mineralization in tidal wetland soils. Future C prediction models should embed both planted and unplanted modules to accurately simulate cycling of soil C in tidal wetlands under sea level rise.

Rhizosphere impact bacterial community structure in the tea (Camellia sinensis (L.) O. Kuntze.) estates of Darjeeling, India

India contributes 28% of the world's tea production, and the Darjeeling tea of India is a world-famous tea variety known for its unique quality, flavor, and aroma. This study analyzed the spatial distribution of bacterial communities in the tea rhizosphere of six different tea estates at different altitudes. The organic carbon, total nitrogen, and available phosphate were higher in the rhizosphere soils than the bulk soils, irrespective of the sites. Alpha and beta diversities were significantly (p<0.05) higher in the bulk soil than in the rhizosphere. Among the identified phyla, the predominant ones were Proteobacteria, Actinobacteria, and Acidobacteria. At the genus level, only 4 out of 23 predominant genera (>1% relative abundance) could be classified viz. Candidatus Solibacter (5.36±0.36%), Rhodoplanes (4.87±0.3%), Candidatus Koribacter (2.3±0.67%), Prevotella (1.49±0.26%). The rhizosphere effect was prominent evident from the significant depletion of more ASVs (n=39) compared to enrichment (n=11). The functional genes also exhibit a similar trend with the enrichment of N2 fixation genes, disease suppression, and Acetoine synthesis. Our study reports that the rhizobiome of tea is highly selective by reducing the alpha and beta diversity while enriching the significant functional genes. This article is protected by copyright. All rights reserved.

Root endophyte-enhanced peanut-rhizobia interaction is associated with regulation of root exudates

Root exudates play a crucial role in the symbiosis between leguminous plants and rhizobia. Our previous studies have shown that a fungal endophyte Phomopsis liquidambaris promotes peanut-rhizobia nodulation and nitrogen fixation, but the underlying mechanism are largely unknown. Here, we explore the role of peanut root exudates in Ph. liquidambaris-mediated nodulation enhancement. We first collected root exudates from Ph. liquidambaris-inoculated and un-inoculated peanuts and determined their effects on rhizobial growth, biofilm formation, chemotaxis, nodC gene expression, and peanut nodulation. Our results found a positive effect of Ph. liquidambaris-inoculated root exudates on these characteristics of rhizobia. Next, we compared the root exudates profile of Ph. liquidambaris-inoculated and un-inoculated plants and found that Ph. liquidambaris altered the concentrations of phenolic acids, flavonoids, organic acids and amino acids in root exudates. Furthermore, the rhizobial chemotaxis, growth and biofilm formation in response to the changed compounds at different concentrations showed that all of the test compounds induced rhizobial chemotactic behavior, and organic acids (citric acid and oxalic acid) and amino acid (glutamate, glycine and glutamine) at higher concentrations increased rhizobial growth and biofilm formation. Collectively, our results suggest that root exudates alterations contribute to Ph. liquidambaris-mediated peanut-rhizobia nodulation enhancement.

Rhizosphere effect and its associated soil-microbe interactions drive iron fraction dynamics in tidal wetland soils

It is becoming increasingly clear that plants can affect iron (Fe) dynamics in tidal wetland soils, but whether this is rhizosphere effect-dependent remains unclear. To assess rhizosphere effects on soil Fe cycling, in-situ rhizosphere and bulk soil samples (0-60-cm) were collected from a tidal wetland across plant growth stages (regreening, shooting, and senescence). Changes in Fe fractions, the abundance of Fe-oxidizing/reducing bacteria (16S rRNA gene), root morphology traits, and soil and porewater geochemistry were examined. Overall, the rhizosphere effect decreased soil pH but increased the concentrations of dissolved organic carbon (DOC), porewater Fe²⁺, and bicarbonates (HCO₃^-). Both Fe-oxidizing and Fe-reducing bacteria were more enriched in the rhizosphere than those in the bulk soil. The rhizosphere effect increased the concentrations of amorphous and crystalline Fe(III), and also enhanced the proportion of amorphous Fe(III). The rhizosphere had higher concentrations of non-sulfidic ferrous iron [Fe(II)] but lower concentrations of ferrous sulfide (FeS) and pyrites (FeS₂) than those in bulk soils, suggesting that the rhizosphere effect favors microbial Fe(III) reduction but suppresses microbial sulfate reduction. Moreover, the rhizosphere amorphous Fe(III) levels changed following the patterns of root porosity, which attained peak values at the root tips. The abundance of Fe-reducing bacteria was controlled by both DOC and amorphous Fe(III) concentrations, which were relatively higher during the regreening and shooting stages than those during the senescence stage. Taken together, our findings highlight that the rhizosphere effect transfer Fe from the bulk soil to the rhizosphere and especially redirects it from FeS associations to microbially-mediated Fe redox cycling. This rapid Fe redox cycling could be responsible for buffering soils and organisms from sulfide accumulation and stimulate C mineralization in the tidal wetland ecosystem.