Effect of Volatile Matter Content in Charcoal On Soil Biological Properties.

Volatile matter content is a property of charcoal which describes the degree of thermal alteration, or carbonization.  Results of a series of greenhouse experiments conducted at the University of Hawaii have shown that plant growth is negatively affected by charcoals with high volatile matter content (20-35%) with or without fertilizer supplements, whereas low volatile matter charcoal (6-9%) increased plant growth when combined with fertilizer.  The effect of volatile matter content in corncob charcoal on the overall hydrolytic enzyme activity and carbon and nitrogen dynamics was assessed during a 28-day laboratory incubation of a highly-weathered subsoil, with and without nitrogen additions.  The results showed that high volatile matter charcoal increased the hydrolytic enzyme activity by more than 2-fold in soils without added nitrogen, and by 4-fold in soils receiving nitrogen. Extractable ammonium, nitrate, and organic carbon rapidly declined in high volatile matter charcoal treatments. In contrast, low volatile matter charcoal did not have a significant effect on hydrolytic enzyme activity or nitrogen dynamics, but maintained elevated levels of extractable organic carbon. Our results suggest that high volatile matter charcoal contains a bioavailable carbon source which stimulates microbial activity and inhibits inorganic nitrogen availability, whereas low volatile matter content does not appear to be readily available for microbial consumption. We conclude that volatile matter is an important property of charcoal which causes differential effects on soil biological properties and warrants further investigation. Our findings have considerable agronomic value since the immobilization of nitrogen observed under laboratory conditions serves as a possible explanation for the adverse effect of high volatile matter charcoal on plant growth reported in the previous short-term greenhouse studies.  Further investigation is needed to evaluate whether the observed effects persist in the long-term.