Interpreting the effect of soil texture on transport and removal of nitrate-N in saline coastal tidal flats under steady-state flow condition

Tidal-flats play important roles in oceanic nitrogen (N) cycles. Particularly, N loss in the tidal-flats depends on soil texture and yet the dominant N removal mechanism in relation to soil texture is not clear. Therefore, the objective of this study was to investigate the effect of soil texture on NO3−-N removal and transport in texturally contrasting tidal-flats of the western coast of Korea [Gangwha (GH, silt) and Saemangeum (SMG, loamy sand) sites]. To interpret the experimental results, we compared the time-course patterns of NO3−-N disappearance during incubation under intertidal and subtidal conditions and the patterns of breakthrough curves (BTCs) of NO3−-N with a conservative tracer (Br−) during miscible displacement experiment. Nitrate disappearance by denitrification was negligible for SMG soils, but was 1.6 and 2.3 mg N kg−1 soil day−1 under intertidal and subtidal conditions for GH soils, respectively. The BTCs of NO3−-N and Br− were identical and followed Gaussian distributions in SMG tidal-flats, while those obtained for GH tidal-flats were broad and asymmetrical. Calculated Pêclet number of Br− in seawater matrix by fitting the CXTFIT model to the measured BTCs was 45.03 for SMG and 4.93 for GH tidal-flats, indicating dominance of advection over dispersion for the former, and vice versa for the latter. From a mass balance of NO3−-N, nearly all of the added NO3−-N (38.8 mg) was recovered in the effluents with a slight unaccounted-for portion (2.8 mg) in SMG system, indicating the possibility of an intense off-shore NO3−-N discharge (leaching) from the tidal-flats. In contrast, a considerably large amount (27.0 mg) of added NO3−-N was not recovered in GH system but nearly one-third (13.5 mg) was recovered in the effluents, suggesting that denitrification dominates over off-shore discharge in NO3−-N removal. Our results showed that the patterns of NO3−-N removal was different depending on soil texture of the tidal-flats and that a dominant mechanism of NO3−-N removal was denitrification in GH tidal-flats and was off-shore discharge in SMG tidal-flats. Therefore, NO3−-N removal characteristics of tidal-flats should be predetermined for site-specific management of waste water N loadings to coastal water systems.