{'fraction_non_alphanumeric': 0.03354890864995958, 'fraction_numerical': 0.01818916734033953, 'mean_word_length': 4.729166666666667, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Sediments underlying the Peruvian OMZ - sink or source for reactive N species

Oxygen minimum zones (OMZs) represent key regions for nitrogen (N) turnover in the water column as well as in the sediments. However, the redox dependent source-sink function of sediments for reactive nitrogen (NO3- + NO2- + NH4+) is not well established. To address this issue, we studied benthic nitrogen cycling under different bottom water oxygen concentrations along a transect traversing the Peruvian OMZ at 11°S. In situ fluxes of nitrogen species across the sediment-water interface were measured using benthic landers and the sediment geochemistry was analyzed in recovered multi-core samples. The fieldwork was conducted through (80 - 400 m water depth) and beyond (700 - 1000 m water depth) the extension of the OMZ. The relative importance of different processes in the benthic nitrogen cycle was investigated using a 1D reaction-transport model tuned to the measured data. The reaction network included the release of ammonium during organic matter mineralization, nitrification and heterotrophic denitrification, as well as anammox. Nitrite was explicitly included in the model as an independent variable. Dissimilatory nitrate reduction to ammonium (DNRA) was also considered where mats of large sulfur bacteria were observed during towed camera deployments (~ 80 - 300 m depth). The model was able to simulate the following features of the benthic N cycle determined from the in situ lander deployments: (i) at the upper fringe of the OMZ (~ 80 - 250 m), the sediments acted as a source of reactive nitrogen due to enhanced ammonium release, (ii) at the lower fringe of the OMZ (300 - 400 m), there was a net uptake of reactive nitrogen of up to ~ 1.9 mmol N m-2 d-1 , and (iii) below the OMZ, reactive nitrogen fluxes into the sediments were low (≤ 0.5 mmol N m-2 d-1 ). The model further predicted that denitrification was the major control on N2 production along the entire transect whereas anammox played a minor role (≤ 10%). At the upper fringe of the OMZ, DNRA was driving ammonium release and determined whether the sediments were a net source or sink for reactive nitrogen species. In contrast, at the lower fringe of the OMZ organic carbon mineralization was the dominant process releasing ammonium. The relative importance of DNRA versus organic carbon mineralization for ammonium release varied systematically between these two end-members along the studied transect.