Do published field experiments inform the longevity of biochar in soil?

The efficiency of biochar application in soil carbon sequestration (SCS) is highly sensitive to biochar longevity. To predict biochar longevity at extended timescales, modelling is essential. It is often expressed that data from field experiments can support this understanding, especially long-term field experiments (LTEs). Our work tests this assertion, using the existing evidence base. A literature search for LTEs of greater than 3-year duration and other criteria was conducted, with the null assumption that biochar C is inert in soil. Observations of soil organic carbon (SOC) after biochar additions from selected LTEs were made against predictions from the Roth C model, assuming no biochar decay.

At the end of July 2022, we found 982 articles describing field experiments that concerned biochar longevity and SCS and initiated within the last two decades. Among them, only 17 reported LTEs matching our screening criteria and providing long-term data suitable for modelling. In these LTEs, a range of SOC dynamics were observed where an acceptable level of fit could not be achieved using a plausible range in parameters. A range of potential reasons for the deviation between measured observations and model predictions were considered, including priming effects on native SOC, migration of biochar particles, sampling and measurement issues, etc. Such factors could not be isolated with sufficient confidence to adjust observational data or model parameters, confounded by the inability to distinguish the dynamics of biochar C and non-biochar C.

Current field data do not enable us to reject the null hypothesis. Reliable parameterization for biochar longevity solely based on field experiments may not be possible on timescales relevant to mitigation of climate change. Instead, alternative strategies for assessing biochar longevity are required, that can be verified in real-time alongside a set of permanent benchmark sites, across different agro-ecological zones with uniform experimental standards, not least in sampling strategy and biochar C and non-biochar C distinction. That will support the incremental adoption of biochar, while providing a robust method for post-hoc adjustment of mitigation benefit.