Nitrogen oxides emissions from coastal wetland sediments: Experimental assessment of the influence of vegetation and nitrogen input.

Nitrogen oxides (NOx = NO + NO2) have essential impacts on global climate and the environment, making it essential to study the contribution of wetland-generated NOx to environmental problems. With exogenous nitrogen input from anthropogenic activities, wetland sediments become active emission hotspots for NOx. In this study, we conducted field experiments in a typical salt marsh wetland to measure nitric oxide (NO, the primary component of NOx from sediments) exchange fluxes in both mudflat and vegetated sediments. We found that NO fluxes in vegetated sediments (0.40 ± 0.15 × 10-12 kg N m-2 s-1) were relatively higher than in mudflat sediments (-1.31 ± 1.39 × 10-12 kg N m-2 s-1), with this difference occurring only during the vegetation-dying season (autumn). Correlations between sediment NO fluxes and environmental parameters revealed that NO flux variation during the observation period was primarily influenced by sediment respiration, temperature, water content, and substrate availability. However, the influence of these factors on NO fluxes differed between mudflat and vegetated sediments. In-situ data analysis also suggested that tidal horizontal migration, which affects sediment substrate and salinity, may regulate sediment NO emissions. Furthermore, in-situ incubations with nitrogen addition (ammonia, nitrite, and nitrate) were conducted to study the response of sediment NO emissions to exogenous nitrogen. We observed that nitrogen addition caused a 259.7 % increase in NO emissions from vegetated sediments compared to the control during the effective period of nitrogen addition (days 1-3). However, although nitrogen addition markedly stimulated sediment NO emissions, the overall NO production capacity constrained the extent of this increase.

The sulfate assimilation and reduction of marine microalgae and the regulation of illumination.

To examine the sulfate assimilation and reduction process and the regulation of illumination, diatom Phaeodactylum tricornutum and dinoflagellate Amphidinium carterae were selected for continuous simulation incubation under different photon flux densities (PFDs) (54, 108 and 162 µmol photons m-2 s-1), and concentration variations of related sulfur compounds sulfate, dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS) and acrylic acid (AA) in the culture system were observed. The optimal PFD for the growth of two microalgae was 108 µmol photons m-2 s-1. However, the maximum sulfate absorption occurred at 162 µmol photons m-2 s-1 for P. tricornutum and at 54 µmol photons m-2 s-1 for A. carterae. With the increase of PFD, the release of DMSP by P. tricornutum decreased while A. carterae increased. The largest release amount of DMS was 0.59 ± 0.05 fmol cells-1 for P. tricornutum and 2.61 ± 0.89 fmol cells-1 for A. carterae under their optimum growth light condition. The sulfate uptake of P. tricornutum was inhibited by the addition of amino acids, cysteine had a greater inhibitory effect than methionine, and the absorption process was controlled by light. The intermediate products of sulfur metabolism had an up-control effect on the sulfate uptake process of P. tricornutum. However, the addition of amino acids had no obvious effect on the sulfate absorption of A. carterae.