Long-term fertilizer postponing increases soil carbon sequestration by changing microbial composition in paddy soils: A 13CO2 labelling and PLFA study

Soil organic matter (SOM) in paddy soils is critical for sustainably achieving high crop yields, especially in the face of ever-intensifying anthropogenic climate change. Our previous studies showed that long-term fertilizer postponing (FP) sustainably increases rice yields by improving SOM via residual carbon input. However, the effect of C release from living roots on SOM under a long-term FP regimen remains unclear. Therefore, in this study, rice plants were subjected to 13CO2 pulse labelling at the panicle initiation (PI) and heading stage (HS). PIlabelled plants were destructively sampled 6 h after labelling, during spikelet differentiation, and when they reached maturity; on the other hand, HS-labelled plants were sampled 6 h after labelling and during the final harvest. The results showed that FP did not affect the ability of plants to assimilate photosynthetic C at PI and HS; however, it significantly reduced the loss of assimilated C at PI. 13CO2 loss was significantly and positively correlated with the microbial biomass [13C-phospholipid-derived fatty acid (PLFA) content] and microbial community composition. After 6 h of 13CO2 labelling at PI, the total 13C-PLFA content of FP was significantly reduced by 51.2% than that of conventional fertilizer (CF). This was mainly because FP reduced the dominant microbes [i.e., G- (alpha 15:0 and alpha 17:0) and G+ (16:1 omega 7c) bacteria] that utilize assimilated 13C. The 13C-PLFA content of FP was significantly higher than that of CF from 6 h of 13CO2 labelling at HS to harvest, mainly because FP increased the dominant fungi (18:1 omega 9c, 20:1 omega 9c) that utilize assimilated C. Redundancy analysis revealed that microbes using assimilated C at the PI and HS were regulated by soil soluble organic nitrogen and total nitrogen, respectively. Overall, our findings suggest that long-term FP reduced assimilated C loss by reducing the G- and G+ bacterial content at PI and altered the microbial community structure at HS to increase the soil's carbon sequestration potential by increasing the fungal content.