Tillage impacts on the fractions and compositions of soil organic carbon

This study aimed to reveal the impacts of tillage systems on the fractions (active, slow, and passive carbon) and chemical compositions of soil organic carbon (SOC). A long-term (26 years) tillage experiment in northeast China examined no-tillage (NT), plowing tillage (PT) and deep loosing (DL). The soil samples (from a depth of 0–20 cm) were wet-sieved into five aggregate classes (1000–2000 μm, 500–1000 μm, 250–500 μm, 50–250 μm, and  250 μm) were assessed by CPMAS 13C NMR. Macro-aggregates contained more SOC concentration than micro-aggregates. However, PT resulted in a greater SOC concentration in 50–500 μm aggregates. NT and DL increased the active C and decreased the slow C in 500–1000 μm aggregates, whereas PT showed the inverse. The 13C NMR spectrum demonstrated that NT increased alkyl-C content, PT obtained a higher carbonyl-C concentration, and DL gained a greater O-alkyl-C concentration. Moreover, evaluating the impacts of tillage systems on the complexity revealed that the most complicated structure was presented in NT, the least in PT, whereas DL had an intermediate effect. Active C contains C2–C6 carbohydrate and anomeric C (C1) polysaccharides, slow C consists of aldehyde-C, ketonic-C and quinone-C, and the passive C is enriched in aromatic-C. In conclusion, long-term tillage systems significantly affected the fractions and compositions of SOC, with NT stabilizing the SOC.