Physicochemical evolution of sugar maple biochar: Insights from a long-term field trial

Biochars are recognized for their ability to sequester carbon, improve soil Ph, and reduce aluminum toxicity in acidic soils. This study investigated the changes in a hardwood (sugar maple, Acer saccharum) biochar after long-term aging in the soil, and its impact on the growth of soybean and on soil pH and available aluminum concentrations. An agricultural soil was treated with five different levels of biochar equivalent to 0, 40, 80, 120, and 160 Mg ha^-1, in separated plots, using a randomized complete block design (RCBD) with five replications across three growing seasons. The ash content and pH of the aged biochar were significantly reduced (72% and 1.2 units, respectively). The decline in soil pH increased soil-available aluminum concentration. Additionally, the surface area and pore volume of aged biochar decreased by 69% and 61%, respectively, while the pore size in the aged biochar increased by 27%. Elemental analysis showed that as biochar aged, it was oxidized, decreasing its H:C ratio and increasing its O:C ratio. Furthermore, increased oxidation caused its surface charge to become more negative, with the zeta potential decreasing from -24 to -39.4 mV. Despite substantial changes to the biochar properties with aging, there was no significant effect on soybean yield. The goal of this project is to understand how biochar changes over time in the soil, its effects on soil health and aluminum toxicity, and whether it continues to benefit crops like soybeans, helping farmers and researchers make sustainable decisions about its long-term use.