Estimation of ammonification and ammonium assimilation in surficial coastal and estuarine sediments

Ammonification and NH4+ assimilation in slurries of surface sediments from Japanese coastal and estuarine areas were studied with 15NH4+ tracers. In completely aerobic sediments, where NO3− reduction is negligible, the Blackburn 15NH4+ dilution model can be applied to estimate the ammonification rate after a correction for nitrification is made. In semiaerobic sediments, where significant NO3− reduction occurs, a mathematical model based on the change of 15NH4+ and 15NO3− with time was successfully applied to estimate the ammonification rate. The rate of NH4+ assimilation can be obtained directly from the incorporation of 15NH4+ into particles in the sediments after correcting for 15NH4+ dilution during incubation. The rate of ammonification ranged from 5.8 to 220 ng‐atoms N (g sediment)−1 h−1 and that of NH,,’ assimilation ranged from 6.2 to 114, indicating both large seasonal and local variations. We found, however, that both NH4+ assimilation and ammonification were significantly correlated with ATP biomass, NH4+, and organic N contents in sediments. Several lines of evidence also suggested that NH4+ assimilation is primarily an aerobic microbial process.