Effects of soil labile carbon fractions and microbes on GHG emissions from flooding to drying in paddy fields

Soil microorganisms and labile soil organic carbon (SOC) fractions are essential factors affecting greenhouse gas (GHG) emissions in paddy fields. However, the effects of labile SOC fractions and microorganisms on GHG emissions from flooding to drying after organic fertilizer replacing for chemical fertilizer remain unclear. Here, a long-term experiment was conducted with four treatments: chemical fertilization only (control), organic fertilizer substituting 25 % of chemical N fertilizer (NM1), 50 % of chemical N fertilizer (NM2), and NM2 combined with crop straw (NMS). GHG emissions were monitored, and soil samples were collected to determine labile SOC fractions and microorganisms. Results revealed the GHG emissions in the NM2 significantly increased by 196.88 % from flooding to drying, mainly due to the higher CO₂ emissions. The GHG emissions per kg of C input in NMS was the lowest with the value of 9.17. From flooding to drying, organic fertilizer application significantly increased the readily oxidizable organic carbon (ROC) contents and C lability; the NM2 and NMS dramatically increased the SOC and non-readily oxidizable organic carbon (NROC). The bacterial communities showed significant differences among different treatments in the flooding, while the significant difference was only found between the NMS and other treatments in the drying. From flooding to drying, changing soil moisture conditions causes C fractions and microbial communities to jointly affect carbon emissions, and the NMS promoted carbon sequestration and mitigated GHG emissions. Our findings highlight the importance of the labile SOC fractions and microorganisms linked to GHG emissions in paddy fields.