Abstract: 【Objective】This study was to explore the changes in topsoil organic matter content and composition of a rice paddy with crop residue return in different forms, in order to provide sound information for crop residue utilization and paddy soil carbon sequestration.【Method】In June, 2015, untreated (CS), manured (CM) and pyrolyzed (CB) maize residue were returned at 10 t C hm^-2, compared with no straw amendment (CK), to a paddy topsoil in Tai Lake plain, China. Topsoil (0-15 cm) samples were collected at rice harvest in November respectively of 2015, 2017 and 2019, and the changes in soil organic carbon (SOC) content and molecular composition were analyzed using ¹³C isotope and biomarker assays. 【Result】Compared to CK, topsoil OC was significantly increased by 8%-36% in 2015 and 2017 with all the residue amendment treatments, but increased by 24% only under CB in 2019,  with the SOC increases mainly from the input biochar. The abundance of lignin phenols in the topsoil under CS and CM peaked after 2-year amendment, and increased by 66% and 115% relative to CK, respectively. After 4 years, molecular abundance of plant-derived lipids increased significantly under all the amendments and the abundance ratio of plant- to microbe-derived lipids (PL/ML) and Shannon diversity of biomarkers (H’) were significantly enhanced. 【Conclusion】Compared to untreated and manured residue, pyrolyzed residue return enhanced SOC sequestration and the molecular diversity of organic matter, through the retention of plant-derived components in the paddy soil.

Key words: paddy soil, crop residue return, soil organic matter, biomarkers, molecular composition, Tai Lake Plain