Quantity and quality changes of biochar aged for 5 years in soil under field conditions

To better use biochar as a sustainable soil management and carbon (C) sequestration technique, it is necessary to understand the biochar aging process in soil, as its quantity and quality likely vary over time. In this study, we quantified the biochar amount after five years of application in an arable field that applied fresh biochar at rates of 30, 60 and 90 Mg ha(-1), and measured the physicochemical properties of biochar before and five years after amendment. The aged biochar in soil was quantified by the method of loss on ignition (LOI). After it was isolated from the soil-biochar mixture by hand, biochar structure was observed by scanning electron microscopy (SEM), and specific surface area and pore volume were measured by Brunauer, Emmett and Teller method (BET). Additionally, biochar chemical properties were measured by elemental analysis, solid-state C-13 nuclear magnetic resonance (NMR), Fourier Transform Infrared spectra (FTIR) and analytical pyrolysis (Py-GC/MS). We observed that, after five years, nearly 40% biochar was diminished in the 0-20 cm soil layer for all treatments. The specific surface area (SSA) of aged biochar increased 98-114.3% relative to the fresh biochar. However, the average pore diameter decreased in the aged, relative to fresh biochar, which was proved from the SEM result, indicating new small pores were formed during aging process. Moreover, SEM also showed that aged biochar had a smoother surface than that of the fresh one, suggesting that either labile matter was leached or minerals was dissolved. Additionally, aged biochar C content increased by 2.44% and oxygen (O) content decreased by 2.81%. However, no chemical structural differences were found between the aged biochar and the fresh one, indicating that the biochar used in this study was relatively recalcitrant even in field conditions (wetting/drying, freezing/thawing cycling and tillage). These results improved our understanding of the processes behind carbon sequestration and the potential effects of biochar on soil properties.