Nitrogen cycling in muddy sediments of Great Peconic Bay, USA: Seasonal N reaction balances and multi-year flux patterns

Abstract

To better understand the capacity of sediments to serve as both source and sink of nitrogen (N) and to identify any evidence of evolving changes in sedimentary N cycling, N2production, N remineralization, and N2fixation were studied over a multi-year period (2010–2015) in bioturbated mud of Great Peconic Bay, a temperate northeastern U. S. estuary. Benthic fluxes and rates of organic matter remineralization were measured using in situ and ex situ incubations. Net annual NH+4, NO–3/NO–2, and N2–N fluxes (μ = 1.1, 0.03, and 1.2 mmol m–2d–1) were close to averages for comparable sedi- mentary environments from surveys of published field studies. Net N2fluxes (by membrane inlet mass spectrometry) were influenced in different periods by temperature, oxygenation of sediment, pulsed Corg, and the activity of benthic macrofauna and benthic microalgae, although no single physical or biogeochemical variable showed a strong, direct relationship with net N2fluxes over all sampling periods. In situ measurements sometimes showed more dynamic and higher amplitude diurnal N flux cycles than did ex situ incubations, suggesting ex situ incubations did not fully capture impacts of bioirrigation or benthic photosynthesis.15N tracer experiments indicated anammox was < 7% of total N2production. Acetylene reduction assays demonstrated C2H4production to depths ≥ 15 cm and suggested N2fixation may have approached 25% of gross N2production(3:1 C2H4: N2). Mass balances incorporating independently measured N remineralization estimates were consistent with measured levels of N2fixation. Overall, complex balances of competing processes governed sedimentary N cycling seasonally, and N2production dominated N2fixation. Measured N2fixation was consistent with constraints from N remineralization rates and net N fluxes except in episodic conditions (e. g., algal blooms). There was no indication of progressive changes in N cycling magnitudes or relative N reaction balances over the study period.