| 18 Sep 2025
Seasonal dynamics of dissolved organic matter along an intertidal gradient in semi-arid mangrove soils (New Caledonia)
Abstract. Mangrove ecosystems play a key role in the global carbon cycle, notably through the production, transformation, and export of dissolved organic matter (DOM). If DOM export to adjacent ecosystems is well studied, its dynamics in mangrove soils remain poorly understood. In this study, DOM quantity and quality were investigated in semi-arid mangroves with no external organic matter input, developing along an intertidal gradient: a salt-flat, an Avicennia marina stand, and a Rhizophora stylosa stand. Soil and porewater samples were collected during both wet and dry seasons, and analysed for physicochemical parameters, total and dissolved organic carbon (TOC, DOC), chromophoric and fluorescent dissolved organic matter (CDOM, FDOM), and mineralogical composition. Our results show distinct DOM quantity and quality between habitats. The Rhizophora stylosa stand, characterized by daily tidal immersion and the lowest salinity, presented high and stable DOC concentrations throughout the year. The dominance of one humic-like fluorescent component suggests that soil DOM is primarily mangrove-derived. In this stand, tidal fluctuations are a major cause for continuous Fe reduction-oxidation cycles, which can influence DOM dynamics. In the salt-flat and the Avicennia marina stand, which suffer from water stress due to their position, significant seasonal variations were measured with higher DOC concentrations during the wet and warm season as a result of enhanced microbial activity. In these stands, due to a more open canopy cover, DOM also originates from biological activity, as evidenced by enhanced microbially-derived fluorescent component. In addition, photodegradation can occur. These findings provide new insights into DOM cycling in mangrove soils and highlight the combined effects of zonation and seasons.
General comments
This paper explores the dynamics of dissolved organic matter (DOM) in a semi-arid mangrove of New Caledonia, along a land-sea transect through three habitats during two characteristic seasons (dry, and warm and wet). The field and lab work were considerable, and the study gives many results on various parameters, such as pH, salinity, redox potential, as well as optical properties of the DOM. It identifies two main controlling factors for the DOM dynamics: the first is the habitat (characterized in this case by the distance to the sea and the vegetation), and the second is the season.
The paper stresses the need for more work on this specific type of mangroves, which are less studied than humid tropical mangroves despite their specificities. Thus, it raises many questions and needs for further research, especially to improve the knowledge on the origin of the OM. It was overall very nice to read, although some indices should be introduced more clearly to help the reader interpret the results.
Specific comments
L.137: how did you know the reaction was complete?
L.160: just to clarify, for TOC measurements, you only measured one core per site (same one used for XRD), that’s right?
L.171: we could use a definition of E2 and E3. Same for ‘SUVA’: what does it mean? More generally, when you introduce indices, you could give their range and meaning of extreme values (as you did for HIX).
L.187: I don’t fully understand the difference between what is given by PARAFAC and what is given by CORCONDIA. But it may not be of significant importance for the understanding of the paper.
L.224: how is land-derived OM brought to the salt-flat, as there is not vegetation on it and you state L.96 that Bouraké is “unaffected by external terrigenous sediments and organic inputs, with no direct freshwater input from rivers”? Is it only OM coming from the soil organisms, and not vegetation? Or maybe I am misunderstanding something here.
L.229: in the caption of Figure 2, you could add a word on how to interpret the names of your samples, as you display them on the graph. I guess the beginning stands for Season-Habitat-, then -Depth-Core or -Core-Depth? Also, on the figure, we don’t know yet what a254, a350, a442 stand for: it is only introduced (and only for a350) in L.309: perhaps you could explain it now, or maybe say it will be detailed in section 3.4.
L.259: I think you inverted the seasons: “62.9 ± 1.2% during the dry season vs. 60.7 ± 2.4% during the wet season” --> 62.9 is wet season and 60.7 is dry season, I guess?
L.311: as already said for the Material & Methods L.171, we need more information in the text to understand the results regarding S275-295, E2/E3 and SUVA: what are the min and max values these indices can reach by definition? What do they mean?
L.336: is the photodegradation signal (C4) identical regardless of the origin of the OM that is degraded (terrestrial, mangrove, marine)?
L.342: how do you explain the absence of changes in C3 (marine humic-like fluorescence) between habitats? One would expect it to be higher in R. stylosa > A. marina > salt-flat.
L.399: You stated in your Methods section that ‘HIX values ranged from 0 to 1, with higher values reflecting a greater degree of humification’, but you are now displaying ‘HIX > 30’. Maybe HIX and BIX got mixed up?
L.409: you suggest that a lot of photodegradation happens in the salt-flat: shouldn’t this lead to a higher C4 signal?
L.484: so, C1 and C4 would both indicate photodegradation?
Technical corrections
L.171: you can add comas for better clarity: ‘The SUVA, a proxy for aromaticity and humification, is calculated as […]’.
L.468: typo in ‘adsorbion’
L.583 : ‘regulated’