An understanding of the influence of biochar on soil organic carbon (SOC) formed from returned straw carbon (C) source, 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 SOC derived from C3 (wheat) and C4 (maize) crop residues, and also the changes in relatively labile and stable SOC fractions. After 5 years, the content of 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 SOC were relatively lower in the B30 (61%), B60 (43%), and B90 (26%) treatments. Thus biochar decreased the accumulation of straw-C into SOC. Additionally, biochar decreased the labile SOC fraction by 11.2–47.7%, compared to the B0. Using the natural abundance 13C, our results showed that 62–74% of the SOC was derived from wheat across all the treatments. Biochar application decreased the contribution of wheat-derived C to SOC by 14.7, 29.0, and 41.5% in the B30, B60, and B90 treatments, respectively, while the content of maize-derived SOC did not change, relative to the B0. In conclusion, although wheat-derived SOC was greater than maize-derived SOC, biochar application decreased the contribution of wheat straw-C to SOC, possibly by enhancing its degradation, thus decreasing wheat-derived SOC storage in an agricultural system.
Keywords: Biochar, straw return, SOC fraction, field condition, 13C natural abundance.