Biochar as Soil Amendment: What Effect Will It Have on Chemical Availability?.

Large scale agricultural application of pyrolyzed biomass byproduct of biomass-to-fuels technologies (biochar) has the potential to improve soil fertility, while at the same time offset the cost of fossil fuel alternatives and provide a means to sequester carbon. The strong adsorbent properties of char, however, make it imperative to consider possible impacts--beneficial or otherwise--on the mobility and bioavailability of agriculturally important organic chemicals. Bioavailability models assuming either thermodynamic control (biotic response rate-limited by intracellular processes) or kinetic control (biotic response rate-limited by soil sorption-desorption) will be introduced to illustrate the impact of biochar amendment on bioavailability. We have shown that the strong adsorptive properties of char are attenuated over time by interaction with soil substances. Humic substances appear to be the most important component. The degree of attenuation depends on molecular size due to steric exclusion effects at the interface. The strongest suppression of sorption (up to two orders of magnitude) is shown for large molecules (e.g., phenanthrene) when char particles are coated with humic acid or humic-metal ion flocs.