Turnover of soil carbon following addition of switchgrass-derived biochar to four soils

Amending soils with biochar can sequester C and improve soil properties, such as nutrient holding capacity and water retention. While biochars generally have a long residence time in soil, the turnover of biochar C can be influenced by both biochar characteristics and soil properties. Biochar can also potentially alter the rate of decomposition of native soil organic matter (SOM). The turnover of switchgrass (Panicum virgatum L.)-derived biochar C was evaluated in the laboratory using soil from four marginally productive sites in central Pennsylvania. Carbon dioxide emissions from unamended soil, biochar-amended soil, and pure biochar were monitored during 189-d incubations, and data were fit to a two-pool exponential model to estimate the amount and mean residence time (MRT) of C in labile and stable pools. Carbon-13 signatures of emitted CO₂ were also determined to estimate the proportion of emitted CO₂ derived from the biochar. Mixing biochar with each of the soils reduced the apparent MRT of C in both labile and stable pools, but the magnitude of change depended on the soil. Overall, the biochar was largely stable in each soil, with only 1.1 to 2.1% of the added biochar C emitted during incubation. There was no measurable effect of biochar amendment on the turnover of native SOM in any of the soils. Therefore, we conclude that amendment of our soils with switchgrass-derived biochar can effectively increase net C sequestration.