Differences in soil organic carbon stocks and fraction distributions between rice paddies and upland cropping systems in China

Purpose A large body of research suggests that rice (Oryza sativa L.) cropping facilitates soil organic carbon (SOC) storage, while the stability of the sequestered carbon is still not well understood. The objective of this study was to determine the differences in SOC stocks and fraction distributions between rice paddies and upland cropping fields and their variation in different rice cropping areas. Materials and methods Data from the national soil survey were analyzed to assess the differences in SOC contents between paddy and upland cropping fields at the regional scale. In addition, three pairs of rice and upland cropping systems were selected in Heilongjiang [single rice vs. single corn (Zea mays L.) cropping], Jiangsu [rice-wheat (Triticum aestivum L.) vs. corn-wheat cropping], and Jiangxi (double rice vs. double corn cropping) provinces, representing the major cropping patterns in China. Physical fractionation techniques were used to investigate the differences in SOC stocks and distribution among different pools between rice-based cropping systems and non-rice cropping systems in China. Results and discussion SOC concentrations were, on average, 74.9% higher at the regional scale and 56.8% higher at the field scale in paddy than in upland cropping fields. Carbon proportion of particulate organic matter within microaggregates increased from 14.4% in upland cropping soils to 25.3% in paddy soils at the Heilongjiang site and from 12.4 to 25.5% at the Jiangxi site. Meanwhile, the free silt and clay-associated carbon was significantly greater in paddy than in upland cropping soils at the both sites. Nevertheless, SOC distribution did not markedly differ between paddy and upland cropping fields at the Jiangsu site where rice was rotated with winter wheat annually. Conclusions As compared to upland cropping or rice-upland crop rotation, continuous rice cropping, such as single and double rice cropping, could favor SOC stabilization by occlusion within microaggregates and adsorption to the silt and clay outside microaggregates, which may promote the long-term storage of SOC in paddies.