Fifteen years of conservation tillage increases soil aggregate stability by altering the contents and chemical composition of organic carbon fractions in Mollisols

Mollisols contain a high amount of soil organic carbon (SOC) that is highly susceptible to tillage practices. Conservation tillage (CT) could improve soil aggregate stability, but the predominant factor controlling this stability remains debatable. This study reports on a tillage management experiment established in 2004 to explore the influences of tillage practices on the content and chemical composition of labile and recalcitrant OC fractions, plus with the aggregate stability change and potential mechanisms. CT without straw mulching, reduced tillage (RT), and no tillage (NT) (zero tillage) with straw mulching were set up. Compared with CT, 15‐year RT significantly increased SOC content (by 11.4%) in the surface (0–10 cm) by increasing the amount of aromatic compounds. Simultaneously, RT and NT promoted soil aggregate stability, as verified by the increase in mean weight diameter (MWD) and geometric mean diameter (GMD) of 7%–71.3% at 0–30 cm soil depth. Of importance, RT and NT increased aromatic compound amount of silt + clay fractions (by 12.6%–27.2%) within macroaggregates (>0.25 mm) but decreased that (by 15.6%–32.9%) within free microaggregates (0.053–0.25 mm) than CT at 0–30 cm soil depth. Collectively, conservation tillage controls the SOC stability in Mollisols by regulating the MWD and GMD values and further affects the amount of aromatic compounds in silt + clay fractions within aggregates. Rather than physical and biochemical protection, chemical protection through a mineral association could be the crucial mechanism for aggregate stability under long‐term conservation tillage in Mollisols.