Changes in biochar physical and chemical properties: Accelerated biochar aging in an acidic soil

Biochar (BC) is increasingly used as soil amendment; however, its stability and alteration in the soil environment are still unclear. Here, we investigated BC decomposition and changes of BC characteristics during a long-term incubation experiment. 13C-depleted BCs were incubated for two years in an acidic Planosol and a calcareous Chernozem, respectively. BC decomposition inferred from the C isotope signature of the incubated materials was marginal. Yet, small angle X-ray scattering showed that the surface roughness of the BCs increased over time. Mid-infrared spectra indicated changes in the BCs' molecular characteristics upon aging. There was an overall increase of infrared bands assigned to H- and O-containing functional groups, especially carboxylic groups. Contact angle measurements revealed that the hydrophobic surfaces of freshly produced BCs became more hydrophilic during soil incubation. Our results suggest that BC aging is strongly influenced by soil traits. In the more acidic Planosol, these aging processes were accelerated.