Carbon pathways in aggregates and density fractions in Mollisols under water and straw management: Evidence from 13C natural abundance

Mollisols have a high potential to mitigate climate change and play an important role in the global carbon (C) cycle due to their inherently high soil organic matter (SOM). However, little is known about the mechanism of C stabilization in Mollisols, especially subjected to different management effects. To trace C stabilization between aggregates and SOM density fractions in Mollisols, soil samples were collected from different experimental plots: controlled irrigation + rice (Oryza sativa L.) straw removal (CI), flooded irrigation + rice straw removal (FI), controlled irrigation + rice straw return (CI-SR), and flooded irrigation + rice straw return (FI-SR). Each soil sample was separated into three aggregate size classes (>250 μm, 53–250 μm, and <53 μm), then each class individually subjected to density fractionation to obtain free and occluded light fractions (fLF and oLF), as well as dense and mineral-heavy fractions (DF and MF). The overall C content and ¹³C abundance of fractions were measured to interpret C transfer and accumulation among aggregate and density fractions. The highest increase in the soil organic C (9.27–24.88%) was observed in CI-SR compared with the other treatments. Irrigation and straw return primarily affected C accumulation within macroaggregates and the MF, which were the dominant forms in the aggregates and density fractions, respectively. An enrichment in δ¹³C was found from macroaggregates to silt + clay, and from light to heavy fractions, indicating that Mollisols macroaggregates and light fractions acted as the source or initial store of plant residues, and that the C in the silt + clay class and heaviest fraction contained more microbially-transformed C than did the macroaggregates and light fractions. A detailed scheme of C transfer within aggregates and SOM fractions based on the δ¹³C natural abundance revealed the following general sequence: free light → occluded light → dense → mineral fractions, concurring with results reported at upland sites. There is a greater probability of C transfer between SOM density fractions under CI. This contrasted with the results that CI decreases the possibility of C exchange between aggregates than FI, which indicates differences in the C stabilization processes between no-flooded and flooded conditions of Mollisols. In addition, straw returning can reduce the possibility of C exchange both between and within aggregates, which plays a significant role in maintaining the stability of the Mollisols carbon cycle. The present study provides further detailed insights into the C stabilizing mechanisms in Mollisols which depend on management (straw, water status). This finding is conducive to the sustainable use and management of Mollisols toward maintaining or increasing C stock and in realizing the objective of C-neutral agriculture.