Abstract
Benthic denitrification activities were determined monthly at two locations with different types of riverbeds in Nogawa River, a typical urban stream, by the acetylene inhibition technique during one year from December 1987. The annual mean denitrification rate at the upstream site N-2, 1.6mgN m-2h-1, where the channel of the river was covered with concrete was higher than the rate at site N-3, 0.8 mgN m-2h-1, of which riverbed was composed of sands and gravels. The discrepancy of denitrification activity between the two points is probably due to the structural difference of microbial habitat, which originates mainly from surface conditions of the riverbeds. The average fraction of denitrified nitrogen to inorganic nitrogen load, on the other hand, was lower at the site N-2 as 0.3% than at the site N-3 as 2.3%, when each residence time of river water was taken into consideration. In case that the entire channel of Nogawa River was composed of sands and gravels, the ratio of the denitrified nitrogen within the river sediment to the total inorganic nitrogen flux at the site N-2 should be calculated at 2. 5%. On the contrary, if the whole river bed was covered with concrete, the ratio should drop to 1.3%. Thus, anthropogenic alteration of the riverbed affects in some extent the nitrogen removal by benthic denitrification in Nogawa River.
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