Biochar-induced Priming Effects Depend on Soil Type and Fresh Carbon Inputs

As greenhouse gas emissions continue to rise, contributing to global warming, land-based CO₂ removal through enhanced carbon storage could help in climate mitigation efforts. Biochar, produced by pyrolysis of biomass under O₂-limited conditions, has gained attention for its potential to stabilise soil carbon. However, in natural ecosystems microbes access multiple carbon sources (e.g. soil organic matter, biochar, plant litter,) meaning that total CO₂ fluxes alone are insufficient to assess biochar effects on soil carbon dynamics. To address this, we investigated how biochar addition affects the fate of multiple carbon sources using isotopic partitioning to improve understanding of priming and source-specific CO₂ emissions. 

Two soils, a fine-textured pasture soil and a coarser-textured arable soil, were incubated with biochar produced from naturally enriched Miscanthus at three pyrolysis temperatures (350, 450, and 700 °C). Control and biochar-treated soils (2% w/w), with and without ¹³C-labelled plant litter, were incubated for 200 days with repeated measurements of CO₂ fluxes and isotopic signatures. Isotopic partitioning separated litter-derived CO₂ from background CO₂ (soil + biochar), and soil- and biochar-derived CO₂ were further partitioned using no-litter treatments. A sensitivity analysis was conducted to assess the robustness of litter-derived CO₂ estimates. 

Biochar pyrolysis temperature strongly influenced total CO₂ emissions: 350 °C biochar substantially increased total cumulative CO₂ emissions in both arable (58%) and pasture (99%) soils, whereas higher-temperature biochar (450, 700 °C) produced smaller increases. Isotopic partitioning showed that biochar-derived CO₂ decreased with increasing pyrolysis temperature in both soils. In arable soil without litter, soil-derived CO₂ changed little (0-8%) across biochar treatments, indicating that higher total CO₂ emissions were driven primarily by biochar-derived CO₂ rather than enhanced native soil carbon decomposition (i.e., priming). In contrast, pasture soil without litter, showed a stronger priming response, with soil-derived CO₂ increasing by 41–45% across biochar treatments relative to the control. Adding litter reduced biochar-induced differences in total CO₂ emissions in pasture from 57–99% to 3–8%, but not in arable soil. Litter-derived CO₂ was not affected by biochar treatments in either soil. 

Overall, our results suggest that, in the short term, biochar effects on total CO₂ emissions are largely driven by priming effects and biochar decomposition rather than the decomposition of new plant material. However, these effects were soil-specific and changed with litter addition, underlining the need for soil- and context-specific evaluation when assessing the stability and climate benefits of biochar additions.