Dynamic change of soil organic carbon fractions improved by soil microorganisms at rice critical growth stages under long-term straw return in a double-cropped rice system in southern China

The practice of returning straw enhances the accumulation of soil organic carbon (SOC), dissolved organic carbon (DOC), and microbial communities. Nonetheless, the mechanisms in which microbes affect the variations in SOC and DOC during rice critical growth stages remain unclear. This research examines SOC, DOC, and microbial communities at panicle initiation (PI), heading (HS), and maturity (MS) stages in a paddy field, 13 years after long-term straw incorporation under no chemical fertilizers application with straw removal (CK), straw removal with only chemical fertilizer application (F), straw burning return with some chemical fertilizer application (SBR), and straw return in situ with some chemical fertilizer application (SR). Compared to CK, the other treatments significantly enhanced SOC levels throughout all growth stages. Notably, SR exhibited an average increase of 37.9 % during the early rice season and 41.9 % during the late rice season. Compared to F and SBR, SR significantly increased SOC by 7.1 % at the PI stage of early rice, and by 9.2 % and 10.1 % at all late rice growth stages, respectively. SR also significantly enhanced the DOC level at the late growth stages of both rice seasons. Furthermore, SR enhanced microbial diversity at the PI stage in both rice seasons, aiding SOC accumulation. Although DOC had a positive relationship with microbial diversity in the early rice season, this relationship turned negative in the late rice season, likely due to the influence of the phylum Actinobacteria . Moreover, the significant enrichment of Cyanobacteria under SR at all stages of the late rice season was associated with an increase in SOC content. These results highlight the importance of Actinobacteria and Cyanobacteria in promoting the formation of SOC and DOC during key growth periods under long-term straw return. Future research requires the integration of 13 C labelling with multi-omics to trace microbial-mediated straw carbon sequestration. • Straw return (SR) improved SOC for the critical growth stages in late rice season. • SR notably enhanced soil DOC of most key growth stages in both rice seasons. • SR increased microbial community diversity at panicle initiation stage. • Cyanobacteria enrichment by SR of late rice contributed to the increase in SOC. • Actinobacteria enrichment by SR of early rice facilitated the increase in DOC.