Adoption of no-tillage alters the pools of SOM with various thermal stability and their chemical composition by changing their sources in Northeast China

The stability and chemical composition of SOM are related to the changes of the proportions of old and young SOM. However, there are few studies that investigated the effects of no-tillage (NT) on the stability, chemical composition, and sources of SOM. In this study, the effects of 9-years of NT on the contents of total, thermally labile and stable SOM, their chemical composition and the contributions from C3 and C4 plants were determined. Before application of NT, the field has been used for C4 maize cultivation under conventional tillage (CT) management about 80 years after initial C3 grassland reclamation. Soil samples were collected in the 0–20 cm soil profile. Under NT treatment, the contents of total SOC, thermally labile and stable SOC, and C4-SOC decreased with soil depths. However, they were greater in the 0–10 cm layers than that in the 10–20 cm layer under CT. The contents of C3-SOC showed no change with depths under both of tillage treatments. NT had greater contents of total SOC, C4-SOC, thermal-labile SOC, and C4-SOCthermal-labile than CT in the 0–5 cm layer, but lower in the subsoil layers. The contents of C3-SOCthermal-labile decreased in the 0–5 cm layer under NT. The alkyl-C and O-alkyl-C were the main fractions of thermal-labile SOM and aromatic-C was the dominant fraction of thermal-stable SOM. C4-SOC (young SOC) was significantly positively correlated with alkyl-C and O-alkyl-C and negatively correlated with aromatic-C. Our results indicated that: (1) compared with continuously CT, applying NT resulted in SOC accumulation in the surface layer, (2) more maize residue input increased the new thermally labile and stable SOM yet lead to decrease the C3-SOCthermal-labile under NT, (3) the contributions of C3 and C4-direved SOM determined the chemical compositions of various SOM pools.