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). PI-labelled 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− (α15:0 and α17:0) and G+ (16:1ω7c) 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 (18:1ω9c, 20:1ω9c) 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.