Charcoal Volatile Matter Content and its Effects on Plant Growth and Biological Properties of an Infertile Tropical Soil.

We conducted laboratory and greenhouse experiments to investigate the effect of flash carbonized corn cob charcoal with varying volatile matter content on soybean (Glycine max, var. Kahala) growth and the biological properties of a weathered Ultisol. Charcoal containing either 35% (HVM) or 11% volatile matter (LVM) was added to pots to achieve 10% w/w with and without a mineral fertilizer blend or chicken manure. High volatile matter content is typically associated with charcoal produced at low temperatures. The addition of HVM charcoal to the soil had a negative impact on soybean dry matter (DM) production with and without fertilizer addition compared with the controls and the LVM treatments. However, LVM charcoal addition had no effect on DM production with or without fertilizer compared with the controls. Nitrogen uptake was significantly lower in the HVM treatment compared with the control whereas N uptake increased with the addition of LVM charcoal. In the laboratory experiment, we incubated the same soil with HVM and LVM charcoal and chicken manure to determine charcoal effects on microbial biomass C (MBC), protease activity, and ammonium (NH₄⁺) production. Incubated soils were analyzed for MBC, protease and inorganic N (NH₄⁺ and NO₃^-) at 2, 7 and 14 days. The results showed that HVM charcoal significantly decreased protease activity at the 2 and 7 day intervals, but by the 14 day interval protease activity in the HVM treated soil was similar to the control. Soil NH₄⁺ concentrations were significantly lower in the HVM treated soil relative to the control at all three time intervals whereas NH₄⁺ concentrations in the LVM treated soil were similar to the control soil.  Results for MBC were more variable, but MBC was significantly lower in the HVM treated soil compared to the control at the 2 day interval. Our results suggest that HVM charcoal has a toxic effect on the soil environment in the short-term adversely affecting N mineralization and N availability to plants.