Changes in biochar properties in typical loess soil under a 5-year field experiment

Purpose After biochar is applied to soil as a sustainable soil amendment and a carbon (C) sequestration technique, its physicochemical properties change over time. However, few studies have reported on the changes of biochar properties over the aging process under field conditions. An understanding of such changes can help us to make full use of biochar as a sustainable soil amendment and C sequestration technique. Materials and methods We used apple tree branches as the raw material to produce biochar and studied the changes in the physicochemical properties of the biochar over a 5-year field experiment. Scanning electron microscopy (SEM), specific surface area (SSA) and micropore area, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy (XPS) were performed. Results and discussion After 5 years of aging, the SSA of the biochar had increased by 23.91% relative to that of fresh biochar; however, the SSA initially decreased over the first 3 years and then increased over the next 2 years. The reasons for the initial decrease were the destruction and clogging of the existing pore structure, whereas new micropore formation was responsible for the subsequent increase, as verified by micropore area, SEM and XPS analyses. The C content of the biochar was stable over the 5 years, but the surface O content and quantity of oxygen-containing functional groups increased relative to those of fresh biochar, which impacts the adsorption capacity of the biochar. Conclusions Our findings illustrate that the SSA of the biochar varied with time during the aging process. The stability of the C illustrated the potential of biochar as a C sequestration technique. The increase in oxygen-containing functional groups of the biochar was responsible for the process of nutrient adsorption.