Abstract

DNRA is a critical factor contributing to the NH 4 + concentration in groundwater of the Yangtze Estuary. • The DNRA contribution increased with increasing NO 3 − input concentration. • The NH 4 + accumulation rates increase logarithmically with increasing NO 3 − input concentration. • The input of NO 3 − in global DNRA-active areas should be considered. Excessive amounts of ammonium (NH 4 + ) can trigger the occurrence of eutrophication, algal blooms and diseases. Hence, identifying the causes of NH 4 + pollution is crucial for protecting water resources and the ecological environment. Although the causes of high NH 4 + concentrations have been extensively studied, whether the dissimilatory nitrate reduction to ammonium (DNRA)-mediated conversion of nitrate (NO 3 − ) to NH 4 + results in ammonium pollution remains uncertain. In this work, the source of NH 4 + in groundwater of the Yangtze Estuary was identified through the integration of nitrogen isotopic abundance, microbial communities and nitrogen ( 15 N) isotope tracer analysis techniques. The contributions of introduced NO 3 − to NH 4 + via DNRA were simulated through labelling experiments. Our results revealed that the DNRA rates in groundwater ranged from 0.261 to 0.362 μmol/L/h and exceeded those of nitrification, denitrification and anammox. Both the isotopic abundance and microbial communities indicated that DNRA is a critical factor contributing to the NH 4 + concentration in groundwater of the Yangtze Estuary. The results of a simulation experiment revealed that the proportions of the DNRA rate to the total NO 3 − reduction and accumulation rates of NH 4 + at different concentrations of K 15 NO 3 (0.5, 1, 5, 10, 30 and 60 μmol/L) ranged from 67.04% ~ 80.78% and 0.163–0.506 μmol/L/h, respectively. Therefore, introduced NO 3 − could significantly contribute to NH 4 + via DNRA in groundwater of the Yangtze Estuary. Notably, the contribution increased ( y  = 2.318ln( x ) + 70.424, R 2  =