Black carbon yields and types in forest and cultivated sandy soils (Landes de Gascogne, France) as determined with different methods: Influence of change in land use

Black carbon (BC) was isolated from sandy soils of a pine forest reference plot and an adjacent plot used for maize cropping since forest clearing 22 years ago. This was performed by: (i) isolation of a refractory organic macromolecular fraction (ROM) using strong hydrolysis followed by chemo-thermal oxidation (CTO) and (ii) direct hand-picking of the untreated soils. Much lower BC contents, ca. ×300, were obtained with the ROM-CTO approach. Experiments on reference chars from the “international BC-ring trial” and high resolution, transmission electron microscopy (HRTEM) observations showed that this large difference was not due to BC component losses resulting from the strong hydrolysis during ROM isolation but was due primarily to complete removal of char/charcoal upon CTO. BC is heavily dominated by char/charcoal and soot only affords a very low contribution in both soils. Calculations showed that BC accounts for a substantial part, ca. 13%, of total ROM and change in land-use resulted in a large loss of BC relative to the forest soil, ca. 60% after 22 years, thus supporting recent questions raised about BC persistence in soil.