NITROGEN TURNOVER RATES IN A RIPARIAN FEN DETERMINED BY N-15 DILUTION

Gross rates of N mineralization, assimilation, nitrification, and NO3- reduction were determined in soil from a wet riparian fen by 1-day incubations of soil cores and slurries with N-15-labelled substrates. N mineralization transformed 0.1% of the total organic N pool daily in the soil cores, of which 25% was oxidized through autotrophic nitrification and 53% - 70% was incorporated into microorganisms. N mineralization and nitrification were markedly inhibited below 5 cm in soil depth. At least 80% of the NO3- reduction in aerated cores occurred through dissimilatory processes. Dissimilatory reduction to NH4+ (DNRA) occurred only below 5 cm in depth. The results show that NH4+ oxidation was limited by available substrate and was itself a strong regulator of NO3--reducing activity. NO3- reduction was significantly increased when the soil was suspended under anaerobiosis; adding glucose to the soil slurries increased NO3- reduction by 2.4 - 3.7 times. Between 3% and 9% (net) of the added NO3- was reduced through DNRA in the soil slurries. The highest percentage was observed in soil samples from deeper layers that were pre-incubated anaerobically.