Evidence and research needs to identify potential SOC stock increases after biochar application: A literature study and roadmap to understand long-term effects
Authors: Kammann Claudia 
Published: February 2023

The interest in biochar, the persistent product of pyrolysis as a soil amendment, began in the first decade of the 2000s and rooted in research on fertile, anthropogenic Terra preta soils in the Amazon region. Research paper numbers started to rise exponentially from 2007/2008 onwards, resulting in more than 27,434 published papers with the keyword &#8220;biochar&#8221; to date (January 2022) according to Easy Web of Science. Its persistence [1] makes properly produced biochar an interesting approach for carbon dioxide removal (CDR) with added benefits for soil fertility [2]. For the overall soil C sequestration that comes with biochar use, three points are crucial: (1) the persistence of biochar itself with depends largely on the pyrolysis temperature and duration, the soil and climatic conditions; (2) the effect that biochar application may have on the already existing soil organic carbon, where an initial short-lived positive priming seems to switch towards a negative priming after 0,5 &#8211; 2 years when soil-biochar only mixtures are investigated [3]. However, the least well-known item regarding the overall CDR potential of biochar use in soils is the question (3) if and under what circumstances biochar may cause an additional soil organic carbon build-up, above that observed in a control soil/agricultural ecosystem without biochar application when soils receive permanently new C, e.g. via a green cover, crops plus intercropping and other practices. For example Blanco-Canqui et al. [4] observed a significantly higher SOC build-up over 6 years after biochar application on average in three field experiments in the mid-west US of about 7 vs. 2 tons per ha in the biochar versus control plots. Weng et al. [5] demonstrated that, indeed, the maximum soil C concentration ceiling could be lifted by (repeated) biochar applications to a subtropical grassland in Australia. In this contribution, I examine the available experimental evidence and mechanistical understanding with regard to such &#8220;humus-return of biochar investment&#8221; effects, if and under what conditions they can be obtained and what methods are available to investigate this effect in long-term field experiments. The contribution aims to stimulate discussion on a joint methodical framework to investigate such a potentially free &#8220;SOC interest return&#8221; effect of biochar use in agriculture which may be as important as the C sink generated by biochar application itself.Lehmann, J., et al., Biochar in climate change mitigation. Nature Geoscience, 2021. 14(12): p. 883-892.
Schmidt, H.P., et al., Biochar in agriculture - A systematic review of 26 global meta-analyses. Global Change Biology Bioenergy, 2021. 13(11): p. 1708-1730.
Wang, J., Z. Xiong, and Y. Kuzyakov, Biochar stability in soil: meta-analysis of decomposition and priming effects. GCB Bioenergy, 2016. 8(3): p. 512-523.
Blanco-Canqui, H., et al., Soil carbon increased by twice the amount of biochar carbon applied after 6&#160;years: Field evidence of negative priming. GCB Bioenergy, 2020. 12(4): p. 240-251.
Weng, Z., et al., Microspectroscopic visualization of how biochar lifts the soil organic carbon ceiling. Nature Communications, 2022. 13(1): p. 5177.