Denitrification in the periphyton associated with plant shoots and in the sediment of a wetland system supplied with sewage teatment plant effluent

摘要

Seasonal variation in denitrification and major factors controlling this process were detemined in sediment,microbial communites attached to plant shoots (periphton)and in the water of a Phragmites and an Elodea-dominated stand of a constructed wetland system between May 1997 and February 1998.The wetland was supplied with effluent from a sewage treatment plant.The denitrification rate in periphyton on plants shoots (expressed per shoot area)was always considerably hgiher than in the sediment and varied with the chlorophyll-a content of the periphyton in the course of the year.The algae in the periphyton provided attachment surfaces and probably also organic compounds to the denitrifying bacteria.Decreases in periphyton biomass and denitrification rate in the Phragmites and Elodea-dominated stands during the growing season were associated with enhanced shading by Phragmites shoots or a floating layer of macro-algae and Lemna spp.,respectivley.Light availability and the denitrification rate of periphyton increased asgain after the Phragmites shoots were cut in October.Nitrate appeared to limit the denitrification rate in the sediment.Periphyton denitrificaton rates were mostly lower on Elodea shoots than on Phragmites shoots,in spite of the higher living algal biomass on Elodea shoots.This difference was associated with lower nitrate concentratiosn in the Elodea-dominated stand.In the two stands,the daily denitrification rates in periphyton on shoots of Phragmites australis (44.4-121 mg N m~(-2)stand area d~(-1))and Elodea nuttallii (14.8-33.1 mg N m~(-2)d~(-1))were clearly more important than rates in the sediment (0.5-25.5 mg N m~(-2)d~(-1))or the water (0.4-3.9 mg N m~(-2)d~(-1)).The presence of few bacteria attachment sites or low organic carbon availability possibly resulted in low denitrification rates in the water.Denitrification appeared to be a major process in nitrate removal from the through-flowing water in this wetland system.