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Constructed wetlands(CWs),as a low-cost ecological system,are being more widely used for the treatment of a variety of wastewaters.Although seasonal effects on pollutant removal performances of CWs have been studied in previous work,the underlying mechanisms are poorly understood,especially the interactions between the plants and microorganisms.In this study,the removal performances of NH4+-N and COD in reed CW microcosms were evaluated in summer with unplanted ones as control.The abundance of the amoA gene of the ammonia-oxidizing bacteria(AOB)and the 16S rRNA of the total bacteria were determined using the real-time quantitative polymerase chain reaction(q-PCR).The microbial community was analyzed by pyrosequencing.Our results showed that there were more amoA gene(1.81E3±35.95copies/g)and 16 rRNA(2.39E6±1.8E4copies/g)copies in planted ones than the unplanted ones(4.64E2±17.11copies/g and 2.37E6±2.0E4copies/g)in summer,but there were no significant difference regarding removal performance between them.The pyrosequencing showed that the unplanted ones had obviously more photosythetic bacteria(3205 sequences)and Cynobacteria(3812 sequences)than the planted ones(1608 reads and 862 reads),which are beneficial for the pollutant removal.As the harvest of plants before winter is a conventional operation for CWs in north China,three series of reed CWs microcosms with integrated plant(unharvested),plants being harvested(harvested)and plants being fully cleared(cleared)were then designed to determine the winter performance of the CWs.The results showed a better performance in unharvested one followed by the harvested one.The cleared had the lowest removal efficiency without any plants.The highest microbial population and activity were observed in the unharvested one followed by the harvested and the cleared.The oxygen release(Radial oxygen loss,ROL)from the plants were also identified both in summer and winter(summer:8.9μmol O2/h/plant; winter: unharvested was 0.05 to 0.59μmol O2/h/plant,harvested was 0.01 to 0.52μmol O2/h/plant).Our results indicate that though the oxygen release from plants is obviously high than the winter,the microbial structure had a more significant effect on the pollutant removal in summer.However,in winter,plant effects,especially the ROL reacting with the microbial abundance and activity,play an important role in the performance of CWs based on both chemical and microbial analysis.