Feeding a growing global population requires sustainable, innovative, and cost-effective solutions, especially in light of the environmental damage and nutrient imbalances caused by excessive chemical fertilizer use. Microalgae have gained prominence due to their phylogenetic diversity, physiological adaptability, eco-compatible characteristics, and potential to support regenerative agriculture and mitigate climate change. Functioning as biofertilizers, biostimulants, and bioremediators, microalgae accelerate nutrient cycling, improve soil aggregation through extracellular polymeric substances (EPSs), and stimulate rhizospheric microbial diversity. Empirical studies demonstrate their ability to increase crop yields by 5–25%, reduce chemical nitrogen inputs by up to 50%, and boost both organic carbon content and enzymatic activity in soils. Their application in saline and degraded lands further promotes resilience and ecological regeneration. Microalgal cultivation platforms offer scalable in situ carbon sequestration, converting atmospheric carbon dioxide (CO₂) into biomass with potential downstream vaporization into biofuels, bioplastics, and biochar, aligning with circular economy principles. While the commercial viability of microalgae is challenged by high production costs, technical complexities, and regulatory gaps, recent breakthroughs in cultivation systems, biorefinery integration, and strain optimization highlight promising pathways forward. This review highlights the strategic importance of microalgae in enhancing climate resilience, promoting agricultural sustainability, restoring soil health, and driving global bioeconomic transformation.

Due to soil nutrient depletion and rising food demand from an increasing global population, it is essential to find sustainable ways to boost crop yields, improve soil health, and address the environmental issues induced by agriculture. The most appropriate approach is to consider sustainable amendments, such as biochar and its derivatives, which are vital constituents of soil health due to their affordability, low reactivity, large surface area, and reduced carbon footprint. In this context, biochar and its derivatives in farming systems focus on improving soil structure, nutrient holding capacity, microbial activities, and the perpetuation of soil fertility. Despite its benefits, biochar, if it is used in high concentration, can sometimes become highly toxic, causing soil erosion due to reducing surface area, increasing pH levels, and altering soil properties. This review highlights the production methods and sources of feedstocks, emphasizing their important contribution to the soil’s physicochemical and biological properties. Furthermore, it critically evaluates the environmental applications and their impacts, providing data built upon the literature on contaminant removal from soil, economic factors, heavy metal immobilization, carbon sequestration, and climate resilience. This review emphasizes the main challenges and future prospects for biochar use in comparison to modified biochar (MB) to propose the best practices for sustainable farming systems.

Soil amendments play a critical role in improving soil health and supporting sustainable crop production, especially under declining soil fertility and climate-related stress. However, their impact varies because each amendment influences the soil through different biogeochemical processes rather than a single universal mechanism. This review synthesizes current knowledge on a wide range of soil amendments, including compost, biosolids, green and animal manure, biochar, hydrochar, bagasse, humic substances, algae extracts, chitosan, and newer engineered options such as metal–organic framework (MOF) composites, highlighting their underlying principles, modes of action, and contributions to soil function, crop productivity, and soil carbon dynamics. Across the literature, three main themes emerge: improvement of soil physicochemical properties, enhancement of nutrient cycling and nutrient-use efficiency, and reinforcement of plant resilience to biotic and abiotic stresses. Organic nutrient-based amendments mainly enrich the soil and build organic matter, influencing soil carbon inputs and short- to medium-term increases in soil organic carbon stocks. Biochar, hydrochar, and related materials act mainly as soil conditioners that improve structure, water retention, and soil function. Biostimulant-type amendments, such as algae extracts and chitosan, influence plant physiological responses and stress tolerance. Humic substances exhibit multifunctional effects at the soil–root interface, contributing to improved nutrient efficiency and, in some systems, enhanced carbon retention. The review highlights that no single amendment is universally superior, with outcomes governed by soil–crop context. Its novelty lies in its mechanism-based, cross-amendment synthesis that frames both yield and carbon outcomes as context-dependent rather than universally transferable. Within this framework, humic substances and carbon-rich materials show potential for climate-smart soil management, but long-term carbon sequestration effects remain uncertain and context-dependent.