Differences in Soil Organic Carbon Fractions Between Paddy Field and Upland Field in Red Soil Region of South China

Although increasing evidences have shown that paddy field soils owned a greater potential for carbon sequestration than that in upland soils, the underlying mechanisms have not been recognized clearly.The objective of this study was to identify the differences in soil organic carbon(SOC)and its fractions between long-term double rice cropping and double corn cropping fields with similar cropping histories.Our results showed that the concentrations of total organic carbon(TOC)and total nitrogen(TN)in the paddy field soil were 2.2 and 2.5 times higher than those in the upland soil, respectively.Compared with the initial level of SOC, long-term rice cultivation increased the concentration of SOC by 30.8%, while no significant effects were found in the upland soil after more than 20 years corn cropping.Furthermore, concentrations of aggregate-associated SOC in the paddy soil were significantly higher than that in the corn field soil.The most difference in aggregate-associated SOC was up to 3 times in the 53～250 μm microaggregates.Concentrations of microaggregate protected C(iPOM_m)on a whole soil basis were 4.2 folds higher in the paddy field than that in the corn field.The proportion of iPOM_m in TOC reached 25.5% in the paddy soil with 2 times higher than that in the upland soil.Moreover, the difference in microaggregate protected C between the paddy field and the upland field could explain 42.8% of the difference in TOC.Hence, it is concluded that above results could enhance the understanding of SOC sequestration in paddy fields, and be beneficial to soil carbon management in paddy soils.