Integrated aggregate turnover and soil organic carbon sequestration using rare earth oxides and 13C isotope as dual tracers

The formation, stabilization and breakdown processes of soil aggregates determine soil organic carbon (SOC) sequestration, in turn, soil aggregate dynamics are mediated by SOC changes. However, the interactions between them remain elusive. Herein, three types of 13C-labelled residues were added to two textured soils. Rare earth oxides (REOs) and 13C isotope were used as dual tracers to simultaneously track aggregate transfer pathways and SOC sequestration during a 56-day incubation period. Residue-derived CO2 followed the sequence of Vetch > Maize > Decomposed maize during the first two weeks. Residue-derived CO2 was significantly negatively correlated with the aggregate turnover time in both investigated soils (P < 0.01), indicating that aggregate turnover was a controlling factor of residue decomposition in addition to its inherent features. Generally, residue addition decreased the aggregate turnover time in the sequence of Vetch < Maize < Decomposed maize. In Red clay soil, macroaggregates attained a higher turnover rate than that of microaggregates, while a similar change pattern was not observed in Sandstone soil with residue application. Aggregates turnover occurred faster in Sandstone soil than in Red clay soil under a given residue application. The aggregate turnover time was significantly reciprocally correlated with the residue-derived C sequestration rate (P < 0.01), suggesting that aggregate turnover was the key factor in C sequestration. A C flow conceptual model was proposed, residue-derived C firstly accumulated in macroaggregates in the formation process, and then relocated from macroaggregates to microaggregates with the breakdown processes at the mid-to-late stage. This study highlights the importance of aggregate turnover in SOC sequestration and demonstrates that these interactions are further affected by residue features and soil texture.