Abstract: A 16-year field experiment was conducted in a ridge culture paddy field in the hilly region of Sichuan Basin, aimed to investigate the distribution characteristics of stable carbon isotope natural abundance (δ¹³C) in soil humus fractions. The soil organic carbon (SOC) content in the paddy field under different cultivation modes ranked in the order of wide ridge culture ＞ ridge culture ＞ paddy and upland rotation. In soil humus substances (HS), humin (HU) was the main composition, occupying 21%-30% of the total SOC. In the extracted soil carbon, humic acid (HA) dominated, occupying 17%-21% of SOC and 38%-65% of HS. The δ¹³C value of SOC ranged from -27.9‰ to -25.6‰, and the difference of the δ¹³C value between 0-5 cm and 20-40 cm soil layers was about 1.9‰. The δ¹³C value of HA under different cultivation modes was 1‰-2‰ lower than that of SOC, and more approached to the δ¹³C value of rapeseed and rice residues. As for fulvic acid (FA), its δ¹³C value was about 2‰ and 4‰ higher than that of SOC and HA, respectively. The δ¹³C value of HU in plough layer (0-20 cm) and plow layer (20-40 cm) ranged from -23.7‰--24.9‰ and -22.6‰--24.2‰, respectively, reflecting the admixture of young and old HS. The δ¹³C value in various organic carbon fractions was HU＞FA＞SOC＞rapeseed and rice residues＞HA. Long-term rice planting benefited the increase of SOC content, and cultivation mode played an important role in affecting the distribution patterns of soil humus δ¹³C in plough layer and plow layer.

Key words: ridge culture, soil organic matter, humus substances, stable carbon isotope natural abundance (δ¹³C), paddy field