Biochar application constrained native soil organic carbon accumulation from wheat residue inputs in a long-term wheat-maize cropping system

An understanding of the influence of biochar on soil organic carbon (SOC) formed from different carbon (C) sources, other than biochar, at field scale is required to accurately assess and predict the C sequestration potential of biochar. For this study, we set up a field experiment in 2009, including four treatments (i.e. B0, B30, B60, and B90, where the biochar application rates were 0, 30, 60, and 90 t ha−1, respectively). We then assessed the impact of biochar after five years (i.e. in 2014) on native SOC derived from C3 (wheat) and C4 (maize) crop residues, and also changes in relatively labile and stable SOC fractions. After five years, the content of native SOC derived from crop residues increased by 81% (from 4.32 to 7.84 g kg−1) in the B0 treatment, while the increases of native SOC were relatively lower in the B30 (61%), B60 (43%), and B90 (26%) treatments. Thus biochar decreased the content of native SOC compared to the B0. Additionally, biochar decreased “labile pool I” (first-step, weak acid hydrolysable) of native SOC by 11.2-47.7%, compared to the B0, but did not influence “labile pool II” (second-step, strong acid hydolysable) and “recalcitrant pool” (acid non-hydolysable). Using the natural abundance 13C, our results showed that 62-74% of the native SOC was derived from wheat across all the treatments. Biochar application decreased the contribution of wheat-derived C to native SOC by 14.7, 29.0, and 41.5% in the B30, B60, and B90 treatments, respectively, while the content of maize-derived native SOC did not change, relative to the B0. In conclusion, although wheat-derived native SOC was higher than maize-derived native SOC, biochar application decreased the contribution of wheat residue to native SOC, possibly by enhancing its degradation, thus decreasing wheat-derived native SOC storage in an agricultural system.