Black carbon contribution to stable humus in German arable soils

There is evidence of black carbon (BC) contributing to stable humus in the soil environment but its quantity and fate are poorly examined. We used benzene polycarboxylic acids (BPCAs) as markers to assess the contents and distribution of BC in soils and soil fractions of the long-term field experiments in Halle (Haplic Phaeozem), Bad Lauchstädt (Haplic Chernozem), Rotthalmünster (Haplic Luvisol) and an additional site near Bad Lauchstädt (Haplic Phaeozem), Germany. Black carbon comprised 11.9–13.2% of organic C in the top soils (0–10 cm) of the black soils located around Halle and Bad Lauchstädt while it explained only 2.7% of organic carbon in the Haplic Luvisol of Rotthalmünster. The BPCA pattern at Bad Lauchstädt and Halle suggested that two thirds of BC were of fossil origin. In general, BC contents (in g kg− 1 soil) decreased with increasing soil depth. The C-normalized BC concentrations, however, increased and reached 35 ± 7% of organic carbon at a depth of 87–114 cm. With increasing depth BC was increasingly localized in the coarse silt and sand fractions and the heavy mineral fraction. This indicated that BC was connected to the minerals of this size and preserved there. Inorganic fertilization for > 30 years did not affect BC contents.