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Syromiatnykov

greenhouse gas emissions, nitrate runoff, and the risk of electron mediator under anaerobic conditions.
eutrophication in aquatic ecosystems. 1-3 Understanding these functional distinctions is essential
Despite these benefits, conventional agricultural for evaluating their synergistic effects when combined
systems have long relied on chemical inputs, often with microbial inoculants in agricultural systems.
with adverse effects on soil health and environmental These substances, including humic acid, fulvic acid,
sustainability. Excessive application of mineral and humin, have been reported to enhance nutrient
fertilizers contributes to groundwater contamination, uptake, stimulate hormonal pathways, and foster
disrupts soil microbial diversity, and degrades soil beneficial rhizosphere microbes. 12-14 When co-applied
structure and fertility. The objective of this review is to with microbial inoculants, humic substances can
4-6
provide a comprehensive synthesis of current research produce synergistic effects: improving nodulation,
on the combined use of nitrogen-fixing bacteria and increasing metabolic activity within nodules, and
humic substances derived from agricultural waste in enhancing nitrogen fixation efficiency. 15-17
soybean cultivation. It aims to elucidate the biological Recent advances in genome-scale metabolic
mechanisms behind their synergy, evaluate agronomic modeling and transcriptomics have further revealed the
and environmental outcomes from both laboratory molecular mechanisms underlying symbiotic nitrogen
and field studies, and identify practical challenges and fixation. Investigations of bacteria such as Sinorhizobium
future research directions for their broader adoption fredii have identified key genetic networks that control
in sustainable agriculture. In response, research is these interactions, providing new opportunities for the
increasingly focusing on biological alternatives, bioengineering of microbial strains adapted to specific
particularly plant growth-promoting bacteria (PGPB), soils and crop genotypes. 18-20
which offer multifunctional benefits. Nonetheless, the practical deployment of these bio-
Microbial partners, such as Rhizobium, based innovations remains challenging, particularly in
Bradyrhizobium japonicum (Bj), and associative low-input or organic farming systems. Factors, such
bacteria like Azospirillum (Az), have been demonstrated as variability in indigenous soil microbiomes, climatic
to enhance nitrogen availability through BNF, strengthen instability, and microbial strain compatibility, limit
plant immunity, and improve stress resilience. efficacy. Addressing these issues requires site-specific
7-9
In particular, inoculation with Bradyrhizobium strategies and comprehensive field validation. 21-23
has consistently demonstrated improved nitrogen This review synthesizes current research on
assimilation and yield gains in diverse agroecological sustainable soybean cultivation, with emphasis on
environments. 10,11 the combined application of nitrogen-fixing bacteria
Concurrently, humic substances—complex organic and humic substances derived from agricultural
compounds formed by the microbial decomposition of by-products. The review also examines their biological
plant and animal residues—are being explored for their mechanisms, agronomic impacts, and ecological roles
agronomic potential. while identifying existing research gaps and potential
Humic substances are heterogeneous organic strategies for broader implementation.
macromolecules resulting from the microbial Amidst the global challenges of climate change
decomposition and humification of plant and animal and environmental degradation, soybean is emerging
residues. They are typically categorized into three as a strategic crop for mitigating agriculture’s carbon
major fractions based on solubility: humic acids, fulvic footprint and restoring soil fertility. The integration of
acids, and humin. Humic acids are soluble in alkaline microbial inoculants with humic substances offers a
conditions but precipitate under acidic pH; they contain promising pathway to enhance BNF, support microbial
high molecular weight aromatic structures and are diversity, and build long-term soil health. These effects
responsible for improving cation exchange capacity are mediated through various mechanisms, such as
and root development. Fulvic acids are soluble at micronutrient chelation, enzymatic activation, and
all pH levels, possess lower molecular weight and improved root architecture, particularly under stress
higher oxygen content, and are more mobile within conditions like drought or salinity. 24-27
plant tissues, contributing to micronutrient transport Notably, iron-enriched artificial humic acids have
and redox buffering. Humin is the most recalcitrant been reported to boost nodulation, nitrogenase activity,
and hydrophobic fraction, insoluble in both acid and and seed yield significantly. Meanwhile, humin, the
alkaline solutions; it plays a role in long-term soil most stable humic fraction, has demonstrated potential
carbon stabilization and may function as an extracellular as an extracellular electron mediator in anaerobic

Volume 22 Issue 6 (2025) 2 doi: 10.36922/AJWEP025230190