The key role of biochar in amending acidic soil: reducing soil acidity and improving soil acid buffering capacity

Abstract Soil acidification negatively affects plant growth, soil microbial composition and diversity, and the sustainability of agricultural ecosystems. The primary sources of soil acidity include base cations (BCs) leaching loss, the weathering of aluminum-containing minerals, excess cation uptake by plants, and the transformation of nitrogen, carbon, and sulfur compounds by soil microorganisms. Soil acidity reflects the degree of soil acidification and is characterized by the active acidity (intensity factor) and exchangeable acidity (quantity factor), while soil acid buffering capacity (Acid-BC) (capacity factor) controls the degree and rate of soil acidification. Biochar, as an alkaline, porous and carbon-rich matter, not only reduces soil active and exchangeable acidity but also enhances the Acid-BC. Having a variety of BCs and oxygen-containing functional groups (OFGs), biochar improves acidic soil mainly through liming effect, cation exchange, complexation, and precipitation reactions. Thus, to improve acidic soils, the application of biochar may be a better alternative than conventional liming materials. However, further studies into re-acidification of biochar-amended soil due to biochar aging, nitrification of excess fertilizer nitrogen and their interaction are still required by binding the mechanistic comprehension of biochar-to-soil interactions and prolonged field researches. Graphical Abstract