Humic acids as regulators of soil functionality and crop performance in sustainable agroecosystems
Greenhouse gasses emission, depletion of nutrients and soil degradation are major factors that are deteriorating soil health. A global challenge to agriculture is the production of enough crops while reducing the impact of intensive chemical inputs on the environment. Humic acids (HA), a main fraction of humic substances produced through humification of organic matter, are considered as multi-functional biostimulants playing an important role in sustainable and climate-smart crop production systems. Recent developments in molecular spectroscopy, humeomics, and soil–plant systems biology have significantly changed our understanding of HA; from inert recalcitrant macropolymers to supramolecular assemblies that are dynamic and govern soil physicochemical properties, microbial activity and plant physiological responses. This review critically synthesizes the available evidence on structure–function relationships of HA with soil fertility restoration, nutrient-use efficiency, crop productivity and environmental sustainability. Emphasis is placed on mechanisms involved in humic acid-mediated soil aggregation, cation exchange capacity improvement, nutrient chelation and modification of root architecture, hormonal signaling and stress tolerance. Nutrient-use efficiency can be improved through HA applications and lower synthetic fertilizer inputs, as well as long-term stabilization of soil carbon and immobilization of heavy metals through the formation of stable organo-mineral complexes. Despite these benefits, substantial variability in HA sources, extraction protocols and molecular composition as well as adoption methodologies continue to limit their reproducibility and widespread application. Knowledge gaps, such as the standardization of molecular characterization, omics-level validation of the plant and microbial responses, long-term field-scale assessment for carbon (C) permanence and agronomic performance of HA are discussed in this review. It also proposes future research and policy needs to develop humic-based amendments for resilient low-input agricultural systems.