Effects of years of rice straw retention on soil carbon and nitrogen sequestration in rice-wheat system

Growing concerns about environmental conservation and sustainable intensification have raised interests in enhancing soil organic carbon and nitrogen pools. To estimate the effects of straw retention years on soil organic carbon and nitrogen pools, a field experiment initiated from 2010 with 9 straw retention year treatments (no straw retention, NR; 1, 2, 3, 4, 5, 6, 7, 8 straw retention years were SR1, SR2, SR3, SR4, SR5, SR6, SR7, SR8, respectively) was conducted in a rice-wheat double cropping system at Yangzhou University Experimental Station in Yangzhou on the Middle-Lower Yangtze Plain of China. After 9 years of experiment, soil bulk density, soil organic carbon (SOC) and total nitrogen (TN) were determined from 3 soil depths increments. Then the C: N ratio, the stratification ratio and the storage of SOC and TN (using the equivalent soil mass approach) were calculated. This study observed that with the increase of straw retention years, the soil bulk density at 0-20 cm depth decreased gradually. Compared with NR, the soil bulk density at 0-5, 5-10 and 10-20 cm depths showed significant differences (P<0.05) from SR3, SR1 and SR1, respectively. The concentrations of SOC and TN increased gradually with the increase of straw retention years, but the increase amount decreased gradually. There was no significant difference of SOC and TN concentrations between SR6, SR7, and SR8 treatments. Compared to NR, the contents of SOC at 0-5, 5-10 and 10-20 cm depths under straw retention treatments (SR1-SR8) were improved by 6.37%-30.99%, 10.13%-32.04% and 4.15%-24.08%, respectively, while the TN contents were increased by 4.81%-22.00%, 6.86%-29.38%, 5.15%-22.48%, respectively. The soil C: N ratio at 0-5 cm depth increased significantly with the increase of straw retention years in short term (≤3a), however there were no significant difference between treatments at the other soil depths or years of straw retention. With the increase of straw retention years, the stratification ratio of SOC and C: N ratio in 0-5 cm to the other depth rose first and then fell, but that of TN decreased first and then rose. The stratification ratios of SOC, TN and C: N ratio for 0-5:5-10 cm were among 0.95-1.00, 1.02-1.11, 0.87-0.97, respectively, while for 0-5:10-20 cm it was among 1.02-1.11, 1.16-1.21, and 0.85-0.95, respectively. The stocks of SOC at 0-5, 0-10 and 0-20 cm depths under straw retention treatments were 6.37%-30.99%, 8.27%-31.52% and 6.23%-27.85% higher than those of NR, respectively, those of TN stocks were 4.81%-22.00%,5.78%-25.14% and 6.04%-25.66% higher than those of NR, respectively. The SOC and TN stocks increased with the growth of straw retention years at each layer, along with the gradual decrease of increase amount, especially after the 6 years of straw retention. There was no significant difference of SOC and TN stocks between SR6, SR7, and SR8. Above all, straw retention was benefit to SOC and TN sequestration. However, after 6 years of straw retention, the growth rates of soil carbon and nitrogen stocks were significantly reduced, which could be considered to reduce the amount of straw retention.