Abstract: Exploring the effects of straw returning years on soil aggregates and the distribution of organic carbon and total nitrogen in rice-wheat cropping fields is of great significance for improving its soil quality. Field experiment was conducted in 2010 with 9 treatments: no straw returning (NR), 1 year of straw returning (SR1), 2 years of straw returning (SR2), 3 years of straw returning (SR3), 4 years of straw returning (SR4), 5 years of straw returning (SR5), 6 years of straw returning (SR6), 7 years of straw returning (SR7), and 8 years of straw returning (SR8). The results showed that with the increase of straw returning years, the proportion of >2 mm aggregates increased, the proportion of 0.25-2 mm and <0.053 mm aggregates decreased, while the proportion of 0.053-0.25 mm aggregates had little change. The contributing rate of total soil organic carbon and total nitrogen was the highest in >0.25 mm macro-aggregates. Contents of soil organic carbon and total nitrogen were significantly positively correlated with the proportion of >2 mm and >0.25 mm aggregates, while negatively correlated with the proportion of 0.25-2 mm, <0.053 mm and <0.25 mm aggregates. In conclusion, continuous straw returning has positive effects on improving the soil aggregates fraction and promoting the transfer of soil organic carbon and total nitrogen to large aggregates.

Key words:  years of straw returning, water-stable aggregates, organic carbon, total nitrogen, rice-wheat cropping, contribution rate