Organic Substrates Research Articles

Performance of ZZ (Zamioculcas zamiifolia) Cultivars on Different Organic Potting Mixture

An experiment was carried out at the research cum instructional farm, Department of Horticulture, School of Agricultural Sciences, Nagaland University, Medziphema, India (2022-2023). The experiment was laid out in completely randomized design (CRD) with 14 treatments and 3 replications. The treatments consisted of organic substrates (T1: FYM, T2: Vermicompost, T3: Used tea leaves, T4: Forest litter, T5: Cocopeat, T6: Perlite and T7: Rice husk) and 2 cultivars (C1: Super Nova and C2: Black Raven). The rooted plants of ZZ cultivars were planted in different organic substrates along with garden soil and sand in the ratio of 1:1:1. Among the cultivars, C1 (Super Nova) was found to be significantly better than Black Raven. Amongst the organic substrates, T5 (Garden soil + sand + cocopeat) was found to be statistically superior in all observed parameters except for plant height which was found to be in maximum in T4 (Garden soil + sand + forest litter). The same treatment of T5 exhibited the highest profit (Rs. 12250) and benefit cost ratio (1.89) on ZZ cultivar Black Raven.

Spin-Restricted Descriptions of Singlet Oxygen Reactions from XMS-CASPT2 Benchmarks.

Reactions of singlet oxygen are numerous, some of which are desired but many are unwanted. Therefore, the ability to correctly predict and interpret this reactivity for complex molecular systems is essential to our understanding of singlet oxygen reactions. DFT is widely used for predicting many reactions but is not suited to degenerate electronic structures; application to isolated singlet oxygen often uses the spin-unrestricted formalism, which results in severe spin contamination. In this work, we demonstrate that spin-restricted DFT can correctly describe the reaction pathway for four prototypical singlet oxygen reactions. By careful benchmarking with XMS-CASPT2, we show that, from the first transition state onward, the degeneracy of the 1Δg state is broken due to differing interactions of the (degenerate) π* orbitals with the organic substrate; this result is well replicated with DFT. These findings demonstrate the utility of using spin-restricted DFT to explore reactions, opening the way to confidently use this computationally efficient method for molecular systems of medium to large organic molecules.

Mechanism of Electrocatalytic H2 Evolution, Carbonyl Hydrogenation, and Carbon-Carbon Coupling on Cu.

Aqueous-phase electrocatalytic hydrogenation of benzaldehyde on Cu leads not only to benzyl alcohol (the carbonyl hydrogenation product), but Cu also catalyzes carbon-carbon coupling to hydrobenzoin. In the absence of an organic substrate, H2 evolution proceeds via the Volmer-Tafel mechanism on Cu/C, with the Tafel step being rate-determining. In the presence of benzaldehyde, the catalyst surface is primarily covered with the organic substrate, while H* coverage is low. Mechanistically, the first H addition to the carbonyl O of an adsorbed benzaldehyde molecule leads to a surface-bound hydroxy intermediate. The hydroxy intermediate then undergoes a second and rate-determining H addition to its α-C to form benzyl alcohol. The H additions occur predominantly via the proton-coupled electron transfer mechanism. In a parallel reaction, the radical α-C of the hydroxy intermediate attacks the electrophilic carbonyl C of a physisorbed benzaldehyde molecule to form the C-C bond, which is rate-determining. The C-C coupling is accompanied by the protonation of the formed alkoxy radical intermediate, coupled with electron transfer from the surface of Cu, to form hydrobenzoin.

Microbiome structure and function in parallel full-scale aerobic granular sludge and activated sludge processes

Aerobic granular sludge (AGS) and conventional activated sludge (CAS) are two different biological wastewater treatment processes. AGS consists of self-immobilised microorganisms that are transformed into spherical biofilms, whereas CAS has floccular sludge of lower density. In this study, we investigated the treatment performance and microbiome dynamics of two full-scale AGS reactors and a parallel CAS system at a municipal WWTP in Sweden. Both systems produced low effluent concentrations, with some fluctuations in phosphate and nitrate mainly due to variations in organic substrate availability. The microbial diversity was slightly higher in the AGS, with different dynamics in the microbiome over time. Seasonal periodicity was observed in both sludge types, with a larger shift in the CAS microbiome compared to the AGS. Groups important for reactor function, such as ammonia-oxidising bacteria (AOB), nitrite-oxidising bacteria (NOB), polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs), followed similar trends in both systems, with higher relative abundances of PAOs and GAOs in the AGS. However, microbial composition and dynamics differed between the two systems at the genus level. For instance, among PAOs, Tetrasphaera was more prevalent in the AGS, while Dechloromonas was more common in the CAS. Among NOB, Ca. Nitrotoga had a higher relative abundance in the AGS, while Nitrospira was the main nitrifier in the CAS. Furthermore, network analysis revealed the clustering of the various genera within the guilds to modules with different temporal patterns, suggesting functional redundancy in both AGS and CAS.Key points• Microbial community succession in parallel full-scale aerobic granular sludge (AGS) and conventional activated sludge (CAS) processes.• Higher periodicity in microbial community structure in CAS compared to in AGS.• Similar functional groups between AGS and CAS but different composition and dynamics at genus level.Graphical abstract

Multiscale Transfer Printing via Shape Memory Polymer with High Adhesion and Modulus Switchability.

Flexible electronics have gained significant attention as an innovative solution to meet the growing need for information collection from the human body and the environment. However, a critical challenge lies in the need for a transfer printing technique that can fabricate rigid and brittle devices on flexible organic substrates. Here, we develop a multiscale transfer printing technique using an ultraviolet-curable shape memory polymer (SMP) that serves as both the stamp and the receiver substrate. The SMP demonstrates exceptional mechanical performance with toughness at room temperature and remarkable flexibility near its glass transition temperature. The SMP material exhibits an impressive shape recovery ratio and remarkable adhesion switchability. We demonstrate robust transfer printing of diverse objects with different materials and morphologies and in situ transfer of multiscale metallic structures. In addition, the in situ fabricated transparent hyperthermia patches with embedded metal grids are demonstrated, offering potential application in the field of sensors, wearable devices, and electronic skin.

Association between organic nitrogen substrates and the optical purity of d-lactic acid during the fermentation by Sporolactobacillus terrae SBT-1

The development of biotechnological lactic acid production has attracted attention to the potential production of an optically pure isomer of lactic acid, although the relationship between fermentation and the biosynthesis of highly optically pure d-lactic acid remains poorly understood. Sporolactobacillus terrae SBT-1 is an excellent d-lactic acid producer that depends on cultivation conditions. Herein, three enzymes responsible for synthesizing optically pure d-lactic acid, including d-lactate dehydrogenase (D-LDH; encoded by ldhDs), l-lactate dehydrogenase (L-LDH; encoded by ldhLs), and lactate racemase (Lar; encoded by larA), were quantified under different organic nitrogen sources and concentration to study the relationship between fermentation conditions and synthesis pathway of optically pure lactic acid. Different organic nitrogen sources and concentrations significantly affected the quantity and quality of d-lactic acid produced by strain SBT-1 as well as the synthetic optically pure lactic acid pathway. Yeast extract is a preferred organic nitrogen source for achieving high catalytic efficiency of d-lactate dehydrogenase and increasing the transcription level of ldhA2, indicating that this enzyme plays a major role in d-lactic acid formation in S. terrae SBT-1. Furthermore, lactate racemization activity could be regulated by the presence of d-lactic acid. The results of this study suggest that specific nutrient requirements are necessary to achieve a stable and highly productive fermentation process for the d-lactic acid of an individual strain.

Heterogeneous Organocatalysts for Light-Driven Reactions in Continuous Flow

Within the realm of organic synthesis, photocatalysis has blossomed since the beginning of the last decade. A plethora of classical reactivities, such as selective oxidation of alcohol and amines, redox radical formation of reactive species in situ, and indirect activation of an organic substrate for cycloaddition by EnT, have been revised in a milder and more sustainable fashion via photocatalysis. However, even though the spark of creativity leads scientists to explore new reactions and reactivities, the urgency of replacing the toxic and critical metals that are involved as catalysts has encouraged chemists to find alternatives in the branch of science called organocatalysis. Unfortunately, replacing metal catalysts with organic analogues can be too expensive sometimes; however, this drawback can be solved by the reutilization of the catalyst if it is heterogeneous. The aim of this review is to present the recent works in the field of heterogeneous photocatalysis, applied to organic synthesis, enabled by continuous flow. In detail, among the heterogeneous catalysts, g-CN, polymeric photoactive materials, and supported molecular catalysts have been discussed within their specific sections, rather than focusing on the types of reactions.

Trihaloisocyanuric Acids: Useful Reagents for Halogenation Reactions and Heterocyclic Scaffold Construction

Abstract: Trihaloisocyanuric acids [1,3,5-trihalo-1,3,5-triazine-2,4,6-(1H,3H,5H)-triones] are commercially available or easily prepared solids. They are highly reactive, stable, easily handled, and have an excellent atom economy, transferring up to three halogen atoms to organic substrates. In these regards, the present review summarizes their synthetic applications as safe and convenient reagents. Therefore, electrophilic halogenation reactions of alkenes, alkynes, arenes, heteroarenes, carbonyl compounds, and heteroatoms, as well as radical halogenation involving saturated substrates and in situ halogenated intermediates for Appel-type reactions are presented and discussed. Remarkably, applications of trihaloisocyanuric acids in processes for the construction of heteroarene scaffolds based on electrophilic halo- and oxidative cyclization, multicomponent reactions, and telescopic reactions are also given.

Optimizing mixing efficiency of anaerobic digesters with high total solids concentrations using validated CFD simulations

In this article, viscosity dependent mixing in anaerobic digestion (AD) towers is analysed using finite volume based computational fluid dynamics (CFD) simulations validated with ultrasound particle image velocimetry (uPIV). The study focuses on improving mixing characteristics for enhanced biogas production resulting from increased organic loading. Mixing of fluids with viscosity ranges found in real-life AD is investigated using the multiple reference frame (MRF) method. The aim is to increase the energetic efficiency of AD by reducing the energy demand required for sufficient mixing such that microorganisms and organic substrates are homogeneously distributed. The distinguishing feature of this work is the implementation of less power-consuming helical ribbon stirring devices (HRSD) for enhanced mixing in different reactor designs. It is demonstrated that HRSD become progressively effective for increasing total solids concentrations (% TS) ranging from 2.5 % TS to 12.1 % TS, particularly in cylindrical reactors. In comparison with conventional propeller-based mixers, innovative HRSD operation reduces dead volume (DV) by up to 22 %. Additionally, HRSD cause less shear forces and require less power due to lower rotational speed. The most energy-efficient mode of operation reduces the power demand by 9.29 % at 12 rpm, while still providing decreased DV of 13.38 % for the most viscous fluid investigated. The outcome is a more efficient and cost-effective mixing approach, which holds strong potential for mixing applications involving highly viscous fluids.

Seasonality and vegetation shape the taxonomic and functional diversity of microorganisms in islands of fertility of a semi-arid region in the Colombian tropics

The islands of fertility or resource islands play a crucial role in combating desertification, as they harbor diverse microbial communities essential for their establishment. The objective of this study was to determine the taxonomic and functional diversity of soil microorganisms in a semi-arid region of the Colombian tropics using a metagenomic approach. To achieve this, we sampled three species of nurse trees as well as bare soil during both the dry and wet seasons. Total DNA was extracted and sequenced extensively. Taxonomic diversity was established by comparing against the SILVA database, while functional diversity was determined using the DIAMOND program and the KEGG database. The analysis unveiled seasonal microbial dynamics influenced by vegetation cover. Islands of fertility exhibited higher diversity and nutritional value compared to bare soil. Wet season conditions boosted microbial diversity, narrowing the gap between vegetated and unvegetated soils. Under drought conditions, Proteobacteria predominated in the resource islands, while Actinobacteria prevailed in the bare soil. Both islands of fertility and the rainy season are significant drivers of microbial communities in these semi-arid environments. The presence of unique genera indicated specialized adaptations, revealing a under-explored microbial diversity. Although complete methanogenesis was absent, methane oxidation pathways were detected. Bacteria demonstrated notable adaptations for nitrate reduction, even under organic substrate limitation. Organic matter and anaerobic microenvironments could play a crucial role in nitrate reduction and denitrification. Sulfate reduction prevailed during the dry season, whereas thiosulfate oxidation was more prominent during the wet season. These findings underscore the remarkable adaptability of microbial communities in challenging environments and provide key insights for addressing desertification in semi-arid zones.

The Influence of Minimal Cultivation Techniques on Growth Rate of Robinia pseudacacia L. Seedlings

The seed provenance, the type of substrate and its properties, as well as the watering regime in the first years after sowing are among the most important characteristics affecting the production of containerized seedlings. The objective of this study was to analyse the growth of black locust (Robinia pseudacacia L.) seedlings on different types of substrate (six mineral and two organic) with different textures and pHs under three different regimes of available water content (no-limiting, medium drought, severe drought) over a period of five months (May–October 2023), using seeds from three sources located in the southeast, south, and west of Romania. The highest seedling emergence rate (73.7%) was obtained for the medium seeds with 18.492 g weight (1000 seed weighs) (Bucharest provenance). Direct sowing in the field and containerized seedlings both showed a trend of growing in height during unlimited water and of growing in diameter during low watering. The lowest survival rate of seedlings (86.7%) occurred in the mineral substrate in the case of rendzina with additional sand (pH 8.70), and for the organic substrate, the lowest survival rate (87.0%) occurred for the peat MKS 3 substrate (pH 4.54).

Impact of Polyethylene-Glycol-Induced Water Potential on Methane Yield and Microbial Consortium Dynamics in the Anaerobic Degradation of Glucose

This study investigated the relationship between water potential (Ψ) and the cation-induced inhibition of methane production in anaerobic digesters. The Ψ around methanogens was manipulated using polyethylene glycol (PEG) in a batch anaerobic reactor, ranging from −0.92 to −5.10 MPa. The ultimate methane potential (Bu) decreased significantly from 0.293 to 0.002 Nm3 kg−1-VSadded as Ψ decreased. When Ψ lowered from −0.92 MPa to −1.48 MPa, the community distribution of acetoclastic Methanosarcina decreased from 59.62% to 40.44%, while those of hydrogenotrophic Methanoculleus and Methanobacterium increased from 17.70% and 1.30% to 36.30% and 18.07%, respectively. These results mirrored changes observed in methanogenic communities affected by cation inhibition with KCl. Our findings strongly indicate that the inhibitory effect of cations on methane production may stem more from the water stress induced by cations than from their direct toxic effects. This study highlights the importance of considering Ψ dynamics in understanding cation-mediated inhibition in anaerobic digesters, providing insights into optimizing microbial processes for enhanced methane production from organic substrates.

Nitrogen-Rich Sewage Sludge Mineralized Quickly, Improving Lettuce Nutrition and Yield, with Reduced Risk of Heavy Metal Contamination of Soil and Plant Tissues

Sewage sludge should primarily find use in agriculture, reducing the quantity directed towards alternative disposal methods like incineration or deposition in municipal landfills. This study evaluated the agronomic value and the risk of soil and plant tissue contamination with heavy metals in sewage sludge obtained from two wastewater treatment plants (WWTP). The experiment was arranged as a 2 × 5 factorial (two sewage sludges, five sanitation treatments), involving lettuce cultivation in pots over two growing cycles. The two sewage sludges were sourced from the WWTPs of Gelfa and Viana do Castelo and underwent five sanitation and stabilization treatments (40% and 20% calcium oxide, 40% and 20% calcium hydroxide, and untreated sewage sludge). The Gelfa sewage sludge, characterized by a higher initial nitrogen (N) concentration, resulted in greater dry-matter yield (DMY) (12.4 and 8.6 g plant−1 for the first and second growing cycles, respectively) compared to that from Viana do Castelo (11.0 and 8.1 g plant−1), with N release likely being a major factor influencing crop productivity. The high N concentration and the low carbon (C)/N ratio of sewage sludge led to rapid mineralization of the organic substrate, which additionally led to a higher release of other important nutrients, such as phosphorus (P) and boron (B), making them available for plant uptake. Alkalizing treatments further stimulated sewage sludge mineralization, increasing soil pH and exchangeable calcium (Ca), thereby enhancing Ca availability for plants, and indicating a preference for use in acidic soils. Cationic micronutrients were minimally affected by the sewage sludge and their treatments. The concentrations of heavy metals in the sewage sludge, soils, and lettuce tissues were all below internationally established threshold limits. This study highlighted the high fertilizing value of these sewage sludges, supplying N, P, and B to plants, while demonstrating a low risk of environmental contamination with heavy metals. Nevertheless, the safe use of sewage sludge by farmers depends on monitoring other risks, such as toxic organic compounds, which were not evaluated in this study.

Electrocatalytic Conversion of Small Molecules Utilizing Concerted Proton-electron Transfer Mediators.

Activation of small molecules such as CO2, N2 or organic substrates and their subsequent transformation into complex value-added chemicals by electrocatalysis, utilizing renewable energy sources under ambient conditions, has gained considerable interest in the last few years. However, activation of these chemically inert molecules is hindered by their intrinsically high activation energy barrier presupposing the development of tailored catalytic systems, often precluding selective transformation to the desired target products. Recent studies have shown that the utilization of concerted proton-electron transfer (CPET) mediators (med-H) may facilitate these challenging electrocatalytic reactions.

Clean and green bioconversion – a comprehensive review on black soldier fly (Hermetia illucens) larvae for converting organic wastes to quality products

Abstract Food security remains a pressing global concern, exacerbated by population growth, diminishing agricultural lands, and climate uncertainties. As the demand for high-quality protein sources like eggs, meat, and milk escalates, conventional feed ingredients face challenges in meeting the burgeoning needs of livestock production. The projected increase in poultry and pig consumption further strains the availability of protein-rich feed sources, necessitating sustainable alternatives. Insects, notably black soldier fly larvae (BSFL), offer numerous advantages, including efficiently converting organic substrates into high-quality protein, fat, minerals, and vitamins. Their rapid reproduction, minimal environmental footprint, and ability to thrive on various organic materials make them an attractive protein source. However, consumer acceptance remains a hurdle, hindering their direct consumption despite their nutritional value. Incorporating BSFL into animal diets, especially poultry and swine, demonstrates promising results regarding growth and production. This review comprehensively overviews BSFL production systems, processing techniques, and nutritional profiles. Various factors influencing BSFL growth and feed quality are discussed, highlighting the importance of optimizing breeding systems and feed formulations. Processing methods are elucidated to ensure the safety and quality of BSFL-based products. Nutritional analysis reveals BSFL as a rich source of essential amino acids, fatty acids, and minerals, making them suitable replacements for soybean meal and fish meal. Despite the economic and environmental benefits of BSFL utilization, challenges persist, including regulatory issues, consumer perceptions, and production scalability. Standardized production protocols and legislative frameworks are needed to facilitate the widespread adoption of BSFL in animal feed industries. In conclusion, integrating BSFL into animal diets presents a promising solution to address protein shortages in livestock production while promoting sustainable resource utilization.

Growth and Cell Size of Microalga Auxenochlorella protothecoides AS-1 under Different Trophic Modes.

Certain microalgal species can grow with different trophic strategies depending on the availability of nutrient resources. They can use the energy from light or an organic substrate, or both, and can therefore be called autotrophs, heterotrophs, or mixotrophs. We recently isolated a microalgal strain from the microplastic biofilm, which was identified as Auxenochlorella protothecoides, AS-1. Strain AS-1 grew rapidly in bacterial culture media and exhibited different growth rates and cell sizes under different trophic conditions. We compared the growth performance of AS-1 under the three different trophic modes. AS-1 reached a high biomass (>4 g/L) in 6 days under mixotrophic growth conditions with a few organic carbons as a substrate. In contrast, poor autotrophic growth was observed for AS-1. Different cell sizes, including daughter and mother cells, were observed under the different growth modes. We applied a Coulter Counter to measure the size distribution patterns of AS-1 under different trophic modes. We showed that the cell size distribution of AS-1 was affected by different growth modes. Compared to the auto-, hetero- and mixotrophic modes, AS-1 achieved higher biomass productivity by increasing cell number and cell size in the presence of organic substrate. The mechanisms and advantages of having more mother cells with organic substrates are still unclear and warrant further investigations. The work here provides the growth information of a newly isolated A. protothecoides AS-1 which will be beneficial to future downstream applications.

Larval Frass of Hermetia illucens as Organic Fertilizer: Composition and Beneficial Effects on Different Crops.

Hermetia illucens has received a lot of attention as its larval stage can grow on organic substrates, even those that are decomposing. Black soldier fly breeding provides a variety of valuable products, including frass, a mixture of larval excrements, larval exuviae, and leftover feedstock, that can be used as a fertilizer in agriculture. Organic fertilizers, such as frass, bringing beneficial bacteria and organic materials into the soil, improves its health and fertility. This comprehensive review delves into a comparative analysis of frass derived from larvae fed on different substrates. The composition of micro- and macro-nutrients, pH levels, organic matter content, electrical conductivity, moisture levels, and the proportion of dry matter are under consideration. The effect of different feeding substrates on the presence of potentially beneficial bacteria for plant growth within the frass is also reported. A critical feature examined in this review is the post-application beneficial impacts of frass on crops, highlighting the agricultural benefits and drawbacks of introducing Hermetia illucens frass into cultivation operations. One notable feature of this review is the categorization of the crops studied into distinct groups, which is useful to simplify comparisons in future research.

Investigating Ligand Sphere Perturbations on MnIII-Alkylperoxo Complexes.

Manganese catalysts that activate hydrogen peroxide carry out several different hydrocarbon oxidation reactions with high stereoselectivity. The commonly proposed mechanism for these reactions involves a key manganese(III)-hydroperoxo intermediate, which decays via O-O bond heterolysis to generate a Mn(V)-oxo species that institutes substrate oxidation. Due to the scarcity of characterized MnIII-hydroperoxo complexes, MnIII-alkylperoxo complexes are employed to understand factors that affect the mechanism of the O-O cleavage. Herein, we report a series of novel complexes, including two room-temperature-stable MnIII-alkylperoxo species, supported by a new amide-containing pentadentate ligand (6Medpaq5NO2). We use a combination of spectroscopic methods and density functional theory computations to probe the effects of the electronic changes in the ligand sphere trans to the hydroxo and alkylperoxo units to thermal stability and reactivity. The structural characterizations for both MnII(OTf)(6Medpaq5NO2) and [MnIII(OH)(6Medpaq5NO2)](OTf) were obtained via single-crystal X-ray crystallography. A perturbation to the ligand sphere allowed for a marked increase in reactivity towards an organic substrate, a modest change in the distribution of the O-O cleavage products from homolytic and heterolytic pathways, and little change in thermal stability.

Noncovalent interaction with a spirobipyridine ligand enables efficient iridium-catalyzed C–H activation

Exploitation of noncovalent interactions for recognition of an organic substrate has received much attention for the design of metal catalysts in organic synthesis. The CH–π interaction is especially of interest for molecular recognition because both the C–H bonds and the π electrons are fundamental properties of organic molecules. However, because of their weak nature, these interactions have been less utilized for the control of organic reactions. We show here that the CH–π interaction can be used to kinetically accelerate catalytic C–H activation of arenes by directly recognizing the π-electrons of the arene substrates with a spirobipyridine ligand. Computation and a ligand kinetic isotope effect study provide evidence for the CH–π interaction between the ligand backbone and the arene substrate. The rational exploitation of weak noncovalent interactions between the ligand and the substrate will open new avenues for ligand design in catalysis.

Organic amendments increased Chinese milk vetch symbiotic nitrogen fixation by enriching Mesorhizobium in rhizosphere.

Symbiotic nitrogen fixation of Chinese milk vetch (Astragalus sinicus L.) can fix nitrogen from the atmosphere and serve as an organic nitrogen source in agricultural ecosystems. Exogenous organic material application is a common practice of affecting symbiotic nitrogen fixation; however, the results of the regulation activities remain under discussion. Studies on the impact of organic amendments on symbiotic nitrogen fixation have focused on dissolved organic carbon content changes, whereas the impact on dissolved organic carbon composition and the underlying mechanism remain unclear. In situ pot experiments were carried out using soils from a 40-year-old field experiment platform to investigate symbiotic nitrogen fixation rate trends, dissolved organic carbon concentration and component, and diazotroph community structure in roots and in rhizosphere soils following long-term application of different exogenous organic substrates, i.e., green manure, green manure and pig manure, and green manure and rice straw. Remarkable increases in rate were observed in and when compared with that in green manure treatment, with the greatest enhancement observed in the treatment. Moreover, organic amendments, particularly pig manure application, altered diazotroph community composition in rhizosphere soils, therefore increasing the abundance of the host-specific genus Mesorhizobium. Furthermore, organic amendments influence the diazotroph communities through two primary mechanisms. Firstly, the components of dissolved organic carbon promote an increase in available iron, facilitated by the presence of humus substrates. Secondly, the elevated content of dissolved organic carbon and available iron expands the niche breadth of Mesorhizobium within the rhizosphere. Consequently, these alterations result in a modified diazotroph community within the rhizosphere, which in turn influences Mesorhizobium nodulation in the root and symbiotic nitrogen fixation rate. The results of the present study enhance our understanding of the impact of organic amendments on symbiotic nitrogen fixation and the underlying mechanism, highlighting the key role of dissolved organic carbon composition on diazotroph community composition in the rhizosphere.