Cropping systems affect paddy soil organic carbon and total nitrogen stocks (in rice-garlic and rice-fava systems) in temperate region of southern China.

The accumulation of soil organic carbon (SOC) in agricultural soils is critical to food security and climate change. However, there is still limited information on the dynamic trend of SOC sequestration following changes in cropping systems. Paddy soils, typical of temperate region of southern China, have a large potential for carbon (C) sequestration and nitrogen (N) fixation. It is of great importance to study the impacts of changes in cropping systems on stocks of SOC and total nitrogen (TN) in paddy soils. A six-year field experiment was conducted to clarify the dynamics of SOC and TN stocks in the paddy topsoil (0-20cm) when crop rotation of rice (Oryza sativa L.) -garlic (Allium sativum) (RG) was changed to rice-fava (Vicia faba L.) (RF), and to examine how the dynamics were affected by two N management strategies. The results showed that SOC stocks increased by 24.9% in the no N (control) treatment and by 18.9% in the treatment applied with conventional rate of N (CON), when RG was changed to RF. Correspondingly, TN stocks increased by 8.5% in the control but decreased by 2.6% in the CON. Compared with RG, RF was more conducive to increase the contents of soil microbial biomass C and N. Moreover, changing the cropping system from RG to RF increased the year-round N use efficiency from 21.6% to 34.4% and reduced soil N surplus in the CON treatment from 547kg/ha to 93kg/ha. In conclusion, changes in the cropping system from RG to RF could markedly increase SOC stocks, improve N utilization, reduce soil N surplus, and thus reduce the risk of N loss in the paddy soil. Overall, this study showed the potential of paddy agro-ecological systems to store C and maintain N stocks in the temperate regions.