Pyrogenic carbon soluble fraction is larger and more aromatic in aged charcoal than in fresh charcoal

Recent studies show that pyrogenic matter is one of the most stable compounds in the soil but less inert than previously expected. One potential pathway yielding losses from soil is solubilisation of pyrogenic compounds. In batch experiments, we estimated the proportion and molecular composition of soluble (<0.45 μm) and colloidal fractions (0.45-5 μm) extractable from a freshly pyrolysed charcoal and a 10 year old wildfire charcoal. These fractions represented a very small fraction (<2.7 g kg⁻¹) of chars. The benzene polycarboxylic acids (BPCA) pattern indicated that 40-55 times more condensed structures were released from the aged char than from the fresh char. This study shows that the soluble fraction of the char is small, and tends to increase with the residence time in the soil.