Long-term biochar and soil organic carbon stability – evidence from long-term field experiments in Germany

Organic soil amendments with a long mean residence time (MRT), such as biochar have a high soil organic carbon (SOC) sequestration potential. The highly aromatic structure of biochar reduces microbial decomposition and explains the slow turnover of biochar. This stable aromatic structure indicates a long persistence in soils and thus potential SOC sequestration. However, there is a lack of data on these effects in the long-term and under real field experiment conditions.

To fill this knowledge gap, we sampled two long-term field experiments in Germany, where industrially produced  biochar has been applied nine and eleven years ago. Both locations differ in soil and climate characteristics as well as in the types and amounts of biochar amendments used. High biochar amount additions of 40 Mg ha^-1 combined with digestate, compost or synthetic fertilizer on a very sandy and nutrient-poor soil in northern Germany led to a short-term increase of SOC stocks of 61 Mg ha^-1, 38 Mg ha^-1 dissipated in the following four years, and after nine years the biochar-amended soils showed only slightly higher SOC stocks (+7 Mg ha^-1) than the control soil. Black carbon, which we additionally analysed as a molecular marker for biochar stability, increased in the short and mid-term and decreased almost to the original stock levels after nine years. Biochar amendments of 31.5 Mg ha^-1, pristine, combined with compost or co-composted on a loamy soil in southern Germany led to an SOC stock increase of 38 Mg ha^-1. After eleven years, this stock increase was still stable, thus confirming biochar-induced SOC sequestration. Black carbon stocks on the same soil showed large dispersion, indicating a loss of stability over the long-term.

This study proves that SOC sequestration through the use of biochar amendments is possible. However, it seems to depend on soil and biochar properties such as soil texture whether SOC stocks are stable in the long-term and dissipation can be mitigated, with the loamy soil seemingly offering better sequestration conditions. As considerable biochar dissipation was observed in both soils, further studies need to investigate whether the dissipation is due to lateral and/or vertical particle transport or microbial decomposition. This is an important question for the suitability of biochar as a reliable CO₂ removal technology.

Keywords: Carbon sequestration, biochar dissipation, climate change mitigation, molecular marker

Acknowledgements: Funded by EU grant #101059546