The effects of continuous straw returning and no-tillage on soil aggregate stability and contents of soil organic carbon (SOC) in topsoil (0 - 20 cm) and sub-surface soil (20 - 40 cm) were studied in paddy fields of subtropical China. Soil samples were collected from 8-year experimental field located in Wuxue city

Hubei province. Four treatments including conventional tillage (CT)

straw returning with conventional tillage (CTS)

no-tillage (NT) and straw returning with no-tillage (NTS)

were designed. The results showed that CTS

NT and NTS treatments significantly increased the contents of water-stable aggregates greater than 5 mm and the mean weight diameters (MWD) of water-stable aggregates in the 0 - 20 cm soil layer

and straw returning significantly increased MWD of water-stable aggregates in the 20 - 40 cm layer. Compared with CT

the treatments of CTS

NT and NTS significantly increased the contents of SOC by 20.83%

21.98% and 32.76%

respectively. In addition

CTS and NTS significantly increased the SOC contents in aggregates those > 5

5 ~ 2 and <0.25 mm in diameter

and NT treatment significantly increased the SOC contents in the aggregates with diameter of >5 and 5 ~ 2 mm in the 0 - 20 cm soil layer. Meanwhile

the CTS treatment significantly increased the SOC contents of the aggregates greater than 0.25 mm in the 20 - 40 cm soil layers. Straw returning increased contents of carbon (C)

hydrogen (H)

nitrogen (N) and oxygen (O) in topsoil. In topsoil

no-tillage reduced the content of H

increased the contents of other three elements

but this treatment increased H content in sub-surface soil. Compared with the treatments of CT and CTS

the H/C ratios were reduced under NT and NTS treatments

indicating that the aliphatic components of soil were increasing. The contents of alcohol

phenol

aromatic

aliphatic compounds and carbohydrates in soil increased under straw returning treatment

while the no-tillage mainly increased the contents of aliphatic compounds. The increase of these organic components contributed to enhancement of aggregate stability.