Dominance of ammonium-derived N2O production in groundwater of the semi-arid Hetao Basin, China

Groundwater enriched with reactive nitrogen is a significant yet poorly quantified source of nitrous oxide (N 2 O). Using 15 N-tracing incubations, we quantified substrate specific N 2 O production rates mediated by ammonium (NH 4 + ) and nitrate (NO 3 − ) along a groundwater flow path with a steep redox gradient (dissolved oxygen [DO] concentrations decreased from 250 to 20 μM) in the semi-arid Hetao Basin, China. Elevated N 2 O concentrations were observed at both oxic upstream and localized reducing downstream sites, with over 90% of samples exhibiting oversaturation and substantial variability (4.6-188 nM) in groundwater with low-DO (20-102 μM). Contrary to expectations that NO 3 − -driven heterotrophic denitrification dominates downstream reducing environments, NH 4 + -driven pathway emerged as the primary source, contributing 66% of gross production. This dominance persisted even under oxygen-depleted conditions, fueled by synergistic NH 4 + -mediated processes, including hydroxylamine oxidation and nitrifier denitrification, particularly in reducing zones enriched with bioavailable organic nitrogen. Concurrently, abundant organic electron donors facilitate complete heterotrophic denitrification, thereby suppressing the accumulation of NO 3 − -derived N 2 O. Despite localized spatial heterogeneity, where NO 3 − -driven heterotrophic denitrification prevailing (> 50%) only at specific high C/N ratio sites, the overall potential for NH 4 + -driven N 2 O production was an order of magnitude higher than that of NO 3 − . Our findings demonstrate that NH 4 + -mediated processes dictate N 2 O dynamics across the redox continuum, highlighting a critical historical underestimation of ammonia's role in subsurface N 2 O sources. Given this study focuses exclusively on the aqueous phase, future research must incorporate sediments-associated biofilm dynamics and specific NH 4 + -driven subpathways to comprehensively refine groundwater N 2 O budgets.