COS 29-2 - Biochar addition alters carbon and nitrogen cycles in soil through stimulation of microbial activity and binding of a volatile carbon source

Abstract: Biochar amendment to soil is an effective strategy for improving soil fertility, and has emerged as effective conservation agricultural management technique. Biochar improves soil fertility directly by increasing levels of soil organic matter, and it has been suggested that it indirectly improves fertility because it is able to sorb low molecular weight organic matter in soil. Primarily, research has focused on the ability of biochar to sorb dissolved organic matter while largely ignoring volatile carbon compounds, which can reach levels equivalent to dissolved organic carbon. In this study we performed a laboratory incubation where soil was treated with one of four biochar feedstock treatments (no biochar, grass, hardwood, or softwood), and one of two methanol treatments (no methanol, methanol). Methanol is a volatile organic compound commonly produced in soil during organic matter decomposition which has been shown to influence soil carbon and nitrogen cycling. We found that biochar and methanol both increased respiration over the course of the experiment, and substrate induced respiration, an indicator of microbial activity. The biochar effect seemed to be driven by the leaching of dissolved organic carbon from the biochar feedstocks. Both biochar and methanol reduced nitrification, likely due to the high carbon:nitrogen content of methanol and biochar leachates. This increase in available C likely lead to increased heterotrophic demand for nitrogen leading to nitrifiers being outcompeted for available NH4-N. Additionally, the biochar with the highest surface area (Softwood) had a strong interaction with methanol addition. This suggests a previously unrecognized mechanism of soil organic matter stabilization where volatile carbon is able to sorb to pyrogenic organic matter such as biochar.