Response of plant and soil microbes to biochar amendment of an arsenic-contaminated soil

• Biochar amendment stimulated soil microbial activity.
• Ryegrass (Lolium perenne L.) germination was not influenced by biochar.
• Root and shoot growth of ryegrass significantly increased in amended soils.
• The fertilizer value of the biochar may have contributed to this enhanced growth.
• Biochar amendment results in significant arsenic uptake in ryegrass shoot.

The historical treatment of livestock with arsenical-based pesticides has resulted in large areas of pastoral land being highly contaminated with arsenic. This study investigated the effect of biochar on soil microbial activity and arsenic phytoextraction in an arsenic-contaminated soil during a 180 d glasshouse experiment. Biochar made from willow feedstock (Salix sp) was pyrolysed at 350 and 550 °C (representing a low- and high-temperature biochar) and amended to soil at rates of 30 t ha−1 and 60 t ha−1 to 30 cm depth (10 and 20 g biochar kg−1 soil, respectively). Ryegrass (Lolium perenne L.) was seeded and plant growth was monitored. Soil microbial activity, quantified using the dehydrogenase activity (DHA) assay, was significantly increased (P < 0.01) under all biochar treatments. This increase was in excess of 100% after 30 d of treatment and was significantly higher (P < 0.05) than the control throughout the trial for 350 °C amended soils. The increase for the 550 °C amended soils relative to the control was greater than 70%. No negative effect of biochar amendments on ryegrass germination was observed. Biochar promoted a 2-fold increase in shoot dry weight (DW) and a 3-fold increase in root DW after 180 d under all biochar amendments and this was attributed, at least in part, to the fertility value of biochar. By increasing dose rates of biochar amendment from 30 t ha−1 to 60 t ha−1 shoot tissue of ryegrass extracted significantly higher (P < 0.05) concentrations of arsenic. Through extrapolation, 350 °C biochar-amended soils were estimated to have the potential to increase ryegrass sward DW growth by 0.68 t ha−1 compared to ryegrass grown on unamended soils. This would correspond to an increase in the extraction of total arsenic by 14,000 mg ha−1 compared to unamended soils and in doing so decreasing soil remediation times by over 50%. This investigation provides insight into the beneficial attributes of biochar in contaminated soil, and specifically that produced from willow wood, and its potential to reduce the time needed to remediate arsenic-contaminated soil. However, more studies are needed to understand the mechanisms through which these benefits are provided.