Influence of denitrification in aquatic sediments on the nitrogen content of natural waters

A study was made of microbiological processes, particularly denitrification, leading to the elimination of nitrogen from natural waters. As denitrification is an anaerobic process and natural waters mostly contain dissolved oxygen, this process was suggested to proceed in the anaerobic sediment at the bottom of natural waters. Two widely differing types of aquatic sediments were tested in the laboratory for effects of temperature, oxygen and nitrate in the overlying water, and thickness of the sediment layer on the rate of denitrification. During disappearance of nitrate from the overlying water, by far most of the nitrate was converted to molecular nitrogen by denitrification and only a small part of the nitrate was utilized for cell synthesis (immobilization). Production of gases in the sediment was studied in the presence and absence of nitrate in the overlying water. The sequence detected duringdenitrification in sediments was NO 3-->NO 2-->N 2 O ->N 2 . Oxygen and nitrate diffuse from the overlying water into the sediment. Therefore denitrification proceeded in the sediment below the layer where hydrogen donors were oxidized by oxygen. Redox potentials showed that denitrification shifted deeper into the sediment with time. Mainly heterotrophic denitrifying bacteria of the genera Pseudomonas and Alcaligenes were active in denitrification. Carbohydrates, acetic acid and sulphide were important hydrogen donors for denitrifying bacteria in aquatic sediment. The ultimate effect of denitrification in sediment for the nitrate content of natural waters was tested in an 800-m reach of canal below a discharge. The nitrate content of shallow natural waters decreased permanently and considerably. Two pieces of equipment were devised, allowing simultaneous measurements of the uptake of oxygen and nitrate by completely mixed suspensions of sediments and undisturbed sediment cores.