No tillage (NT) and straw return (S) collectively affect soil organic carbon (SOC).  However, changes in the organic carbon pool have been under-investigated.  Here, we assessed the quantity and quality of SOC after 11 years of tillage and straw return on the North China Plain.  Concentrations of SOC and its labile fractions (particulate organic carbon (POC), potassium permanganate-oxidizable organic carbon (POXC), microbial biomass carbon (MBC) and dissolved organic carbon (DOC)), components of DOC by fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) and the chemical composition of SOC by ¹³C NMR spectroscopy were explored.  Treatments comprised conventional tillage (CT) and NT under no straw return (S0), return of wheat straw only (S1) or return of both wheat straw and maize residue (S2).  Straw return significantly increased the concentrations and stocks of SOC at 0-20 cm depth but no tillage stratified them with enrichment at 0-10 cm and a decrease at 10-20 cm in comparison to CT, especially under S2.  Labile C fractions showed similar patterns of variation to that of SOC, with POC and POXC more sensitive to straw return and the former more sensitive to tillage.  Six fluorescence components of DOC were identified comprising mostly humic-like substances with smaller amounts of fulvic acid-like substances and tryptophan. Straw return significantly decreased the fluorescence index (FI) and autochthonous index (BIX) and increased the humification index (HIX).  No tillage generally increased HIX in topsoil but decreased it and increased the FI and BIX below the topsoil.  The chemical composition of SOC was: O-alkyl C>alkyl-C>aromatic-C>carbonyl-C.  Overall, NT under S2 effectively increased SOC and its labile C forms and DOC humification in topsoil and microbially-derived DOC below the topsoil.  Return of both wheat and maize straw was a particularly strong factor for promoting soil organic carbon in the plough layer, and the stratification of SOC under no tillage may confer long-term influence on carbon sequestration.