SEASONAL-VARIATION OF DENTRIFICATION RATE IN SEDIMENT DETERMINED BY USE OF N-15

A laboratory method of determination of sediment denitrification capability is described. The determination of nitrogen gas evolved from 15N-labelled nitrate is based on the principle of isotope dilution. The gas evolved is spiked with unlabelled dinitrogen and the 15N-enrichment of the resulting gas mixture is measured by optical spectrometry. The amount of nitrate reduced to ammonia and organic nitrogen is determined simultaneously. Measurements performed on sediment from a shallow Danish lake indicated first order kinetics of denitrification with respect to nitrate concentration. Great seasonal variation of rate constant was found with maximum value in late August. This variation is probably governed by the pattern of accumulation of decomposable organic matter in sediment. Calculation based on measured rate constants and nitrate concentrations during the year indicate that approximately 10% of the total nitrogen loading of the lake is denitrified. © 1979.