Leachable soil black carbon after biochar application

Abstract

Black carbon (BC) enters soil via deposition of emissions and nowadays also via application of biochar. Recent studies indicate that a yet not quantified part of BC is lost from soil by leaching, which implies that BC exists in water-transportable form (so-called water-extractable BC; WEBC). We aim at quantifying the amount of WEBC in soil and at elucidating whether it relates to biochar application. The composition of WEBC was further tested to evaluate the degree of BC weathering over time after application, hypothesizing that BC degrades into smaller polyaromatic units. We analysed total BC and WEBC in topsoil (0–10 cm) of a field trial with application of 0, 4.5, and 9 Mg biochar ha−1 using the benzene polycarboxylic acid method. To see if there is a temporal relation between BC input and WEBC formation, we took samples directly after biochar application and three years later. Bulk BC and also WEBC concentrations increased with increasing biochar application rates. In contrast, WEBC portions of bulk BC decreased with increasing amount of applied biochar. On average, we found between 7.56 ± 2.50 and 12.56 ± 1.34 mg WEBC per kg agricultural soil, making up 0.6–1.84% of bulk BC. Over time, no loss or accumulation of either component was observed. However, we found that the composition of WEBC differed from bulk BC; i.e. WEBC consisted of less condensed aromatic structures than bulk BC indicating that de-condensation processes are involved in the formation of leachable BC.