Nitrogen loss from intertidal permeable Wadden Sea sediments

Abstract

In the oceanic nitrogen (N) cycle, the sedimentary N2 production accounts for 50-70 % of global marine N-loss. Coastal regions or continental shelves, where terrestrial riverine systems and the oceansintersect, play a role as a significant N-sink in the marine N-cycle by regulating the fixed-N flow at the land-sea boundary. Although continental shelf sediments cover only 7.5 % of the global marine seafloor, they contribute> 60 % of benthic N-loss. The majority of the seafloor on continental shelves worldwide is covered by permeable sediments. Advection, instead of diffusion, is the predominant mass transport in these permeable sediments.The particle and solute exchanges between water column and sediments under advective conditions exceed those under diffusive conditionsby several orders of magnitude.Advective pore water flowsallow oxygen penetration to greater depths, expanding the biogeochemical oxic zone in permeable sediments. However,so far little is known about N-loss in these sediments, and the impacts of advection on N- loss and N-cycling processes in general. The aim of this thesis is to investigate the extent and mechanisms of N-loss in the Wadden Sea permeable sediments under simulated in situadvective conditions. Spatial and temporal N-loss rates were determined in order to assess the significance of the Wadden Sea permeable sediments, and furthermore these sediments from this worldwide tidal flat system were used as a case study to elucidate the role of permeable sediments in the global marine N-loss. Potential links between N-loss and other N-cycling processes such as nitrification are further explored, especially under the influence of fluctuating oxic-anoxic conditions. Using amodified core 15N-incubation method with one-pulseperfusion to simulate advections, and with simultaneous multiple-sensor measurements, active N-loss via denitrification was found to occur under oxic conditions. Such occurrence was further corroborated by slurry incubations with 15N-labelled substrates by O2microsensor measurement and on-line measurement using membrane inlet mass spectrometry (MIMS). These combined results show that permeable Wadden Sea sediments are characterized by some of the highest denitrification rates (190µmol Nm-2 h-1) under aerobic conditions (with oxygen concentrations of up to 90µM) in the marine environment.This is the first time that the substantial N-loss inpermeable sedimentshas been attributed to aerobic denitrification under oxic-anoxic oscillations driven by advection. To examine the significance of N-loss in permeable Wadden Sea sediments over an annual cycle, N-loss rates were determined across three seasons. The impacts of advection were also evaluated by comparing three incubation methods: (i) intact core incubations simulating diffusive transport, (ii) intact core incubations simulating advective transport conditions, or (iii) slurry incubations. Nitrogen loss ratesunder simulated advective conditions exceeded those under diffusive conditions by 1-2 orders of magnitude, and were comparable to rates determined in slurries. Intensive N loss rates (mean 207± 30 µmol m-2 h-1) were measured in permeable Wadden Sea sediments with little temporal and spatial variation. Furthermore, NOx- fluxes over a full annual cycle were empirically simulated by 2-dimensional model with in situ monitoring data as input parameters, includingtemperatures, bottom current velocities and NOx- concentrations in water column. Combined with actual rate measurements across seasons and sites, theannual N-loss in permeable Wadden Sea sediments was estimated to be 745 mmol N m-2 y-1. These results in the case study of the Wadden Sea verify that permeable sediments, accounting for up to 68 % of the continental shelves, are an important N-sink in the global marine N-cycle. The expansion of the oxic biogeochemical zone in permeable sediments due to advection may favor aerobic processes such as nitrification. Hence, the occurrence of nitrification and its interaction with N-loss processes in permeable Wadden Sea sediments were evaluated using 15N-isotope paring experiments.Net NOx- production was determined under aerobic conditions in these sediments, verifying the active occurrence of nitrification. In addition, the NOx- produced by nitrification could be immediately channeled to N-loss to produce N2. Instead of anammox(at very low rates of <2 µmol N m-2 h-1, and <1 %of total N-loss), aerobic denitrification predominated in these permeable sediments. Moreover, thecoupled nitrification-denitrificationwas found to represent up to 17 % to total N-loss, particularly apparent in surficial (permeable) sediments where the influence from advection was the strongest. This study provides direct and quantitative evidence that nitrification plays a keyrole in linking N-sourcesand N-sinksin permeableWadden Sea sediments.