Controls and dynamics of biochar decomposition and soil microbial abundance, composition, and carbon use efficiency during long-term biochar-amended soil incubations

The process-level understanding of biochar-C decomposition and microbial utilization in soils is poorly understood as well as how these dynamics vary across different addition rates and soil types. We present results from a 30-month laboratory incubation, where oak-derived biochar was added at 0, 1, 5, 10, and 20 % rate by weight to four soils varying in soil properties. We determined biochar-C loss using natural abundance C-13 isotope coupled to measurements of CO2 efflux. We measured microbial abundance, composition, and C use efficiency (CUE) using phospholipid fatty acid biomarkers and C-13 labeled glucose, respectively. After 30 months, the amount of the remaining biochar-C was insignificantly different between different biochar addition rates, suggesting that biochar-C loss is proportional to biochar addition rates. In soils with higher C contents (i.e., > 1.5 %), biochar decomposition rates appeared to slow down after initial fast decomposition; while biochar decomposition rates followed one-pool model for soils with lower C. Soil microbial composition significantly changed at 10 or 20 % addition rate after 30 months, which was correlated with the increase in soil C/N. The highest CUE was found at 1 or 5 %, depending on soil types.