Effects Of Freezing On The Sediment Environment And Microorganism Around Roots Of Typical Wetland Plants In Agricultural Drainage Ditch

The agricultural drainage ditch is a direct recipient of agricultural surface source pollutants.It is a strip wetland with a unique plant-sediment-microbial system.Studies on wetland plants on aspects such as soil and microbial characteristics around the root system have mainly focused on the growing season of the plants.In contrast,only a few studies have been conducted in autumn and in non-freezing winter conditions in the south.Few studies on the sediment and microbial characteristics of agricultural drainage ditch during the freezing winter period in severe cold area have been reported.In this paper,we investigated the dynamics of the sediment environment and microorganisms around the roots of typical wetland plants Phragmites communis(Phr.)and Typha angustiolia(Typ.)and in the plant-free zone under the low-temperature anaerobic environment during the winter freezing period in a typical agricultural drainage ditch habitat in Northeast China.By analyzing the physicochemical properties and microbial change patterns of the sediment,we investigated the response of microbial community structure to changes in the physicochemical characteristics of the sediment around the root system during the icebound period(pre-icebound,mid-icebound,and deep icebound periods)and the thawing period(pre-ice thawing and late-ice thawing)during the winter freezing period.It can provide scientific basis for rural non-point source pollution control,ecological ditch construction and river restoration,protection and management in cold regions,and also help to further explore the water purification mechanism of low-temperature constructed wetland.The main conclusions are as follows:(1)The mean values of nutrient contents in the sediment during the winter freezing period were in the order of Typ.>Phr.>plant-free zone.Still,the performance was different at different stages.The contents of SOM,DOC,TN,and TP in the sediment around the root system of Typ.were higher than those of Phr.during the icebound period,while the opposite was the case in the late-ice thawing period.The p H of the sediment around the root system of Phr.and in the plant-free zone was greater than that around the root system of Typ..SOM,TN,NH4~+-N,NO3~--N,and TP were significantly different in different sampling periods(P<0.01),and the overall performance was the icebound period>the thawing period around plant roots'sediment.Compared with the pre-icebound period,the nutrient contents of the sediment around the roots of Phr.and Typ.and in the plant-free zone decreased the most in the late-ice thawing period by 78.9%,82.4%,and 63.8%for NH4~+-N,TN,and NO3~--N.Respectively,the sediment in the plant-free zone,the content of SOM,TN,and DOC increased,and SOM increased the most by 49.4%.(2)The overall sediment T PLFAs showed that Typ.>plant-free zone>Phr..Compared with the pre-icebound period,the T PLFAs in the sediment around the root system of Phr.and Typ.and the plant-free zone decreased in the late-ice thawing period by 63.6%,80.7%,and 23.9%,the B PLFAs decreased in the late-ice thawing period by 61.2%?80.2%?25.8%,the sediment in the plant-free zone F PLFAs,Act PLFAs,and AMF PLFAs increased to different degrees.The GP:GN ratios of the sediment showed the icebound period<the thawing period,no significant difference in sediment GP:GN ratios in difficult sediment during the icebound period,and higher GP:GN ratios in the plant-free zone than in areas with plant growth during thawing period.The B:F ratios in the sediment around the root system of Phr.>Typ.>plant-free during the thawing period.The AB:An B ratios of the sediment around the root system of Typ.were greater than those around the root system of Phr.and the plant-free zone.The sediment AB:An B ratio decreased over time.S:M ratio around the root system of Typ.was lower than around the root system of Phr.and the sediment in the plant-free zone.(3)There was no significant difference in the types of bacteria in the sediment around the roots of Phr.and Typ.and the plant-free zone.The?-diversity and richness of the sediment bacterial community around the root system of Phr.and Typ.were greater than those around the root system of Phr.and the plant-free zone.The diversity of bacterial communities showed that the the icebound period>the thawing period.The dominant bacterial phylum in the sediment was all Proteobacteria.The subdominant phylum of sediment bacteria around the root system of Phr.was Chloroflexi or Firmicutes,and the subdominant phylum of the sediment in Typ.,the plant-free zone was Chloroflexi or Bacteroidetes.The relative abundance of Proteobacteria decreased in the icebound stage and increased in the melting ice stage,with the opposite trend of Chloroflexi,Firmicutes,and Bacteroidetes.The dominant sediment phyla were Gamma Proteobacteria,and the subdominant phyla were Bacteroidia or Anaerolineae.(4)SOM,TN,TP,NO3~--N,NH4~+-N,DOC were positively correlated with T PLFAs,B PLFAs,F PLFAs,Act PLFAs,AMFPLFAs,Pro PLFAs.Sediment SOM,NH4~+-N,NO3~--N,DOC around the root system of Phr.were positively correlated with?-diversity of bacterial community.The sediment SOM,TN,and NO3~--N around Typ.roots were significantly and positively correlated with?-diversity of bacterial community.The sediment SOM,TN,and TP in the plant-free zone were significantly and positively correlated with?-diversity of bacterial community.p H and bacterial diversity in the sediment of Phr.and plant-free zone were significantly and negatively correlated.In the sediment around the root system of Phr.,Typ.,Proteobacteria relative abundance was significantly positively correlated with SOM,DOC,TN,TP,NO3~--N,NH4~+-N and negatively correlated with p H.In the sediment around the root system of Phr.,Firmicutes was significantly positively correlated with p H,significantly and negatively correlated with SOM,DOC,TP,NO3~--N.In the sediment around the root system of Typ.,Chloroflexi was significantly positively correlated with p H,significantly and negatively correlated with SOM,DOC,TP,NO3~--N.In the sediment of the plant-free zone,Proteobacteria was significantly and positively correlated with SOM,TN,TP,NH4~+-N,significantly and negatively correlated with p H,Chloroflexi was significantly positively correlated with SOM,TP,NH4~+-N and negatively correlated with p H.