The addition of biomass-derived black carbon or biochar to agricultural soils is attracting attention as a means for sequestering carbon and as a potentially valuable method for improving soil fertility. Recent research has shown, however, that biochar is not completely unreactive in soils as it potentially enhances microbial activity, adsorption of organic contaminants or release of polycyclic aromatic compounds, which may ultimately affect the composition of dissolved organic matter (DOM) leaching from a soil. We have tested this hypothesis using batch and soil column experiments.
Addition of fresh biochar (1 wt. %) to a Gleyic Fluvisol resulted in a 22% reduction in OM mobilization from the soil. Ultrahigh resolution mass spectrometry revealed a marked change in the composition of the OM mobilized. The most saturated and reduced compounds were removed in the solutions leached from the biochar amended soil. On the other hand, while dissolved black carbon-type compounds were already leaching from the un-amended soil, their abundance increased after biochar amendment. We also detected newly-appeared “lignin-type” compounds with relatively high O/C ratio of ca. 0.65, indicating that non-black carbon was also leached from the amended soil and was highly oxygenated DOM. During a flow interruption of 1 week the soil columns turned anoxic. The DOM mobilized after flow interruption was mostly identical to that mobilized under oxic conditions, with the exception of aromatic and polycyclic aromatic compounds being more abundant. These were probably associated with Fe and Mn oxides and released during the reductive dissolution of the oxides.
In summary, the biochar amendment changed the molecular composition of soil-derived DOM. The net effect was (i) a reduction in total OC mobilization and (ii) a shift towards more oxidized and therefore less bioavailable OM that was leached to the subsoil.