Changes in soil microbial biomass and community structure of a constructed mangrove wetland for municipal wastewater treatment during 10-yesars operation periods

Constructed wetlands (CWs) have been used for wastewater treatment in many countries, however, their long-term performance and changes in microbial community have seldom been reported. The present study investigated the changes of soil characteristics, microbial biomass carbon (MBC) and microbial community structure in a pilot-scale horizontal subsurface-flow mangrove constructed wetland, which has been used for treating municipal wastewater since 2005. These changes were monitored at different operation periods during 10-years operation. The constructed wetland consisted of two independent belts planted with Aegiceras corniculatum (Ac) and Kandelia obovata (Ko), two dominant mangrove species in South China. Surface soils (0-10 cm) were collected from the two belts in the 2nd, 3rd, 9th and 10th year of operation. The results showed that the contents of soil organic matter and total nitrogen, as well as microbial structure, differed significantly among operation years in both Ac and Ko belts. At the beginning of the operation, MBC was significantly higher than that in the 9th and 10th year of operation; however, microbial respiration in the soil substrate was stable during 10-years operation. Microbial communities based on phospholipid fatty acid analyses (PFLAs) revealed that the ratios of Gram-positive to Gram-negative bacteria, fungi to bacteria, monounsaturated bacteria to saturated bacteria were significantly higher in the 10th year of operation than those at the beginning in both Ac and Ko belts. The highest correlations between the soil microbial community and organic matter, and the soil microbial community and total nitrogen were found in the 10th year of operation, implying that the soil microbial community in this constructed mangrove wetland was more sensitive to changes in soil characteristics after long-term operation, and could be used as an indicator on the performance of CWs.