Mitigation of carbon dioxide by accelerated sequestration from long-term biochar amended paddy soil

Understanding the process of carbon (C) sequestration under biochar amendment is crucial to mitigate climate change. However, the actual annual C sequestration rates after biochar application and underlying mechanisms at a scale of soil aggregates remain unclear. Thus, the C sequestration rates based on a net ecosystem C budget approach were investigated during a consecutive 6 years paddy field (2012–2018). Further, soil samples were collected in 2018 to identify the effects of long-term biochar (20 or 40 t ha−1 applied in 2012) amendment on 1) C sequestration by determining C hydrolyzing activities and mineralization in aggregate size classes and 2) C sequestration between aggregates using the 13C natural abundance. The results showed that biochar-induced annual C sequestration rate decreased during the first two years, whereas it increased during the following four years. The lower ecosystem respiration (-4%) and higher crop yield (+9%) explained the latter C sequestration, compared to N treatment. Relative to N treatment, six years aged biochar decreased soil total organic C (TOC) mineralization in the bulk soil (-12 %), macroaggregate (MacroA, 250−2000 μm, -38 %) and microaggregate (MicroA, 53−250 μm, -19 %) size classes, but increased in silt (2−53 μm, +5%) and clay (0.1−2 μm, +24 %) size classes. Partial least squares path modeling revealed that the decrease in activities of β-glucosidase (-13 %), α-glucosidase (-20 %), cellobiohydrolase (-17 %) and xylanase (-2.5 %) and the improvement in soil structure increased C accumulation. The 13C natural abundance showed that Δ13C (δ 13C of aggregates – δ 13C of bulk soil) decreased by increasing aggregate size. Biochar increased probability of C flow from MacroA to MicroA, relative to without biochar amended treatment. Thus, biochar protected low molecular weight C compounds in the MacroA, with subsequent fast transfer into MicroA, which having long storage and the highest stable C content among the aggregate size classes. Consequently, increase in C protection via transferring C from MacroA to MicroA and decrease in soil hydrolases activities induced by biochar in MacroA and MicroA contribute to the high C sequestration potential.