Enhancing post-wildfire soil recovery in northern Arizona using biochar from waste forest biomass

Increasing wildfire severity in the southwestern United States degrades the soil and hinders forest recovery in these ecosystems. The current study evaluates the efficacy of biochar produced from local ponderosa pine (Pinus ponderosa Lawson & C. Lawson: var. arizonica (Engelm.) Shaw) slash using a mobile CharBoss® system for post-wildfire soil restoration. We installed 3 replicates of 3 treatments: (1) Biochar applied and mixed in the soil at 10 t ha^-1 (BC10), (2) biochar applied at 20 t ha^-1 (BC20), and (3) a wildfire-burned control (negative control (NC)) to soils burned in the 2022 Pipeline Fire, near Flagstaff, Arizona, USA. In addition, we installed plots in an unburned positive control (PC). Ponderosa pine seedlings were planted across all treatments, and soil physico-chemical properties, water retention, and seedling survival were monitored for one year. The CharBoss® biochar from pine slash exhibited favorable characteristics for soil amendment, including high fixed carbon (66.17%), low volatile matter (5.48%), and a well-developed porous structure (surface area 185.1 m² g^-1). The application of biochar, especially at 20 t ha^-1, significantly increased soil moisture retention (p < 0.05 at 12 months), with BC20 having the highest gravimetric soil moisture content at the end of the study, exceeding both NC and PC. Biochar also provided some pH buffering, improved the retention of available phosphorus and base cations, and added more stable soil organic carbon. Overall, seedling survival after one year was low due to harsh post-fire conditions, but it was highest in the BC20 treatment (17.8%) compared to the burned control (6.7%) (p = 0.011). These results show that producing and applying biochar from slash and other woody biomass on-site can enhance important soil water and chemical properties and help establish young trees in wildfire-affected semi-arid soils of northern Arizona. Biochar offers a promising, climate-friendly alternative to burning slash piles that helps soil recovery, carbon storage, and reforestation efforts. However, we need longer-term studies to evaluate lasting ecological benefits.