Corrigendum to “Influence of soil properties and feedstocks on biochar potential for carbon mineralization and improvement of infertile soils” [Geoderma 332 (2018) 100–108]

The authors regret that the article omitted important clarifications, particularly concerning the organic carbon balance and certain methodological details. Below, we clarify the key points. We apologize for any inconvenience this may have caused. (1) Biochar Application Rate and Organic Carbon Discrepancy In the original publication, the biochar application rate was stated as 30 t ha⁻¹. To clarify, this corresponds to 22.2 g kg⁻¹, assuming a 10 cm soil incorporation depth and a soil bulk density of 1.35 g cm. Discrepancies were observed between the theoretical carbon content expected from biochar addition and the measured organic carbon content in the soils. We acknowledge that the measured increase in organic carbon was lower than anticipated based on the carbon content of the biochar. This discrepancy may be attributed to several factors: Analytical limitations: The Walkley-Black method, while widely used, has known limitations in recovering biochar-derived carbon due to its resistance to oxidation. Studies have shown that biochar recovery rates using this method can be as low as 23.6 % (Hardy and Dufey, 2017, Kurth et al., 2006). Additionally, factors such as biochar particle size, distribution, stability, and interaction with soil components can influence the efficiency of oxidation during analysis, potentially leading to an underestimation of carbon content. Furthermore, there may be limitations in using NaOH traps to measure CO₂ emissions. The observed increase in organic carbon was lower than theoretically expected. This discrepancy should be interpreted in light of the limitations inherent in the methodology used. Future research should explore alternative analytical techniques to more accurately capture biochar-derived carbon. We also highlight the need for a revised correction factor when calculating organic carbon in soils treated with biochar or in biochar samples. (2) Walkley-Black Method for Organic Carbon Determination The study employed the Walkley-Black method for determining organic carbon, which is widely recognized but has inherent limitations in efficiently oxidizing biochar carbon. Although this method remains an accepted standard in soil science (e.g., Agroscope, 2020; Department of Sustainable Natural Resources, 1990), its recovery rate for biochar-amended soils may be lower than that for conventional organic matter. Several studies have highlighted biochar's resistance to oxidation processes, resulting in a limited recovery rate. For example, Kurth et al. (2006) reported that about 70 % of biochar was resistant to Walkley-Black oxidation. This is especially evident when biochar has not aged sufficiently in the soil, further limiting its recovery rate (Hardy and Dufey, 2017). We acknowledge this limitation and recommend further investigation using complementary techniques.