[Responses of soil organic carbon content and fractions to land-use conversion from paddy field to upland].

Natural 13C abundance determination method coupled with physical fractionation of soil organic carbon (SOC) was used to evaluate the responses of SOC and its fractions to long-term land-use conversion from paddy field to upland field (corn cultivation). Results showed that land-use conversion from paddy field to upland field led to significant decreases in the contents of SOC and total nitrogen (TN). Concentrations of total organic carbon (TOC) and TN were respectively greater by 76.7% and 47.6% in the paddy field than those in the corn field. Concentrations of occluded particulate organic matter (oPOM) and mineral-associated organic matter (MOM) on a whole soil basis were two times higher in the paddy field than those in the upland field, while no significant difference was found in free particulate organic matter (fPOM). Carbon concentrations of oPOM and MOM fractions on their own weight basis were significantly greater in the paddy field than those in the upland field, especially the oPOM fraction, which was 6 times higher in the former than that in the latter. It could be concluded that SOC protection exerted by soil aggregates in paddy soil was greater than that in upland soil. After a 19-year conversion from paddy field to corn field, delta13C values of SOC fractions significantly increased. Maize-derived carbon (C) accounted for 54.6%, 24.7%, and 19.0% in fPOM, oPOM and MOM, respectively. Mean residence time (MRT) of the initial rice-derived C increased in the order fPOM (24 a) < oPOM (67 a) < MOM (90 a). The above results further indicate that paddy field soil owns greater capability of carbon sequestration than upland soil mainly through increasing the contents of oPOM and MOM in the fractions of SOC.