Changes in soil microbial community composition during Phragmites australis straw decomposition in salt marshes with freshwater pumping

The dynamic changes of soil microorganisms after Phragmites australis straw addition in the incubation tubes were analyzed by phospholipid fatty acid stable isotope probing (PLFA-SIP). After comparing soils from different freshwater pumping areas in the Yellow River Estuary (10-year pumping area, 15-year pumping area and natural salt marsh without pumping), the results showed that the total PLFA contents significantly increased by 59.99%-146.93% after the addition of straw to surface soils (0-10 cm) in the pumping areas, whereas the changes in deeper soils (10-20 cm) were not significant. In particular, the PLFA results showed that bacteria and fungi were significantly increased after 10 days with straw addition. Straw treatment also improved the ratio of fungi to bacteria (F:B) in the surface soils of all sampling sites. The soil microorganisms directly absorbed straw-derived ¹³C, where Gram-negative bacteria (GN) were found to have the highest PLFA-¹³C values during the 40-day decomposition process. Soil characteristics can significantly affect microbial community composition. Accordingly, soil organic carbon (SOC) was found to be significantly positively related to bacterial, fungal and other microbial biomasses, while moisture, electric conductivity (EC) and soil aggregate composition were important factors of influence on the microbial community. The findings indicated that both fungi and bacteria were essential microbial communities in straw decomposition, the significant increase of fungi biomass and the absorption of straw-derived ¹³C by bacteria were the main changes of microbial community. Long-term freshwater pumping can promote straw decomposition by increasing microbial biomass and changing microbial community composition.