Anaerobic urea oxidation is an overlooked but globally relevant nitrogen loss pathway in marine sediments

Sedimentary nitrogen loss is a major sink for nitrogen in marine systems mitigating excess nitrogen inputs, with anammox representing a critical pathway. Conventional understanding holds that anammox bacteria exclusively mediate ammonium oxidation, while their capacity for anaerobic urea oxidation (AUO) has remained largely overlooked. Here we present the direct measurements of AUO rates and contributions across China’s marginal sea sediments, using a revised 15N isotope pairing technique. AUO exhibited a lower rate, accounting for 15 ± 10% of anammox. Moreover, porewater ammonium concentration emerged as a key regulator of AUO contribution due to substrate competition. Extrapolation to a global scale suggests that AUO accounts for ~7% of total sedimentary nitrogen loss, with a disproportionately high contribution in deep-sea sediments. These findings identify AUO as a previously overlooked but globally relevant nitrogen loss pathway, underscoring the need to incorporate AUO and other anaerobic dissolved organic nitrogen transformations into marine nitrogen budgets. Globally, anaerobic urea oxidation accounted for around 7% of total sedimentary nitrogen loss, with a disproportionately high contribution in deep-sea sediment, according to direct isotopic measurements and contributions of anaerobic urea oxidation rates across China’s marginal sea sediments.