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Biochar application has multiple benefits for soil fertility improvement and climate change 42 mitigation. Biochar can act as a source of nutrients and sequester carbon (C) in the soil. The nutrient release capacity of biochar once applied to the soil varies with the composition of the biochar, which is a function of the feedstock type and pyrolysis condition used for biochar production. Biochar has a crucial influence on soil C mineralization, including its positive or negative priming of microorganisms involved in soil C cycling. However, in various cases, biochar application to the soil may cause negative effects in the soil and the wider environment. For instance, biochar may suppress soil nutrient availability and crop productivity due to the reduction in plant nutrient uptake or reduction in soil C mineralization. Biochar application may also negatively affect environmental quality and human health because of harmful compounds such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins, and dibenzofurans (PCDD/DF). In this review, we discuss the linkage between biochar composition and function, evaluate the role biochar plays in soil fertility improvement and C sequestration, and discuss regulations and concerns regarding biochar's negative environmental impact. We also summarize advancements in biochar production technologies and discuss future challenges and priorities in biochar research.

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Biochar promotes the storage of organic carbon (OC) in soils. OC is unevenly distributed in soils among different particle‐size fractions showing different structures, functions, and stability. The objective of this study was to investigate the biochar–soil interactions and the redistribution of soil C in different soil fractions based on a 2‐year field experiment. Fractionation was done by particle sizes including coarse sand (250–2,000 μm), fine sand (53–250 μm), and silt/clay (<53 μm). Integrated spectroscopic techniques were employed to examine physical characteristics of biochar–soil interactions in different soil fractions. Application of biochar increased OC by 37%, 42%, and 76% in soil particle‐size fractions of 53–250, <53, and 250–2,000 μm, respectively. This was supported by X‐ray fluorescence spectroscopy analysis, which showed an increase of C contents by 5–56% with biochar addition. The highest increment in OC was found in coarse sand fraction, and redistribution of OC was detected depending on various soil particle sizes. Results of scanning electron microscopy combined with electron dispersive X‐ray spectroscopy analysis showed the interactions between soil and biochar, which could be attributed to oxidized functional groups (OCO, CO, and CO) captured by the X‐ray photoelectron spectroscopy. The long‐term aged biochar could be beneficial to enhance soil quality by promoting OC storage and facilitating positive biochar–soil interactions.