Iron traps terrestrially derived dissolved organic matter at redox interfaces

Results and Discussion

At both study sites (“Zarnekow” site and “Wendewiesen” site), we found dissolved iron concentrations in the anoxic pore waters to be one to three orders of magnitude higher than in the oxic surface waters (Table 1). We observed 79–85% and 58–60% lower DOC concentrations in surface waters at both sites, respectively, compared with the pore waters, which is consistent with previous observations for other rewetted fens (12). For a detailed molecular characterization of DOM, we used electrospray ionization (ESI) Fourier transform (FT) ion cyclotron resonance (ICR) MS, which resolves the molecular composition of DOM at the highest possible level to date. The largest fraction of DOM in the investigated fens is composed of organic acids and neutrals (12) making ESI-FT-ICR-MS run in negative ionization mode an ideal tool with which to study DOM molecular composition. Through this method, we obtained >4,000 molecular formulas of individual compounds per sample. Roughly half of all molecular formulas (45–51%) found at Wendewiesen were organic acids containing only carbon, hydrogen, and oxygen atoms (CHO). About one-third contained one or two nitrogen atoms (CHON₁ and CHON₂, respectively), and 16–19% contained one sulfur atom (CHOS₁) (Table 2). To our knowledge, this is the largest contribution of nitrogen- and sulfur-containing compounds in an environmental DOM sample analyzed by FT-ICR-MS reported to date. The abundance of phosphorus-containing formulas was very low (∼1% and less per sample). Hydrogen-to-carbon (H/C) ratios and oxygen-to-carbon (O/C) ratios of all formulas found varied between 0.3 and 2.7 and between 0.01 and 1, respectively. The relative abundances of H/C and O/C ratios sorted by CHO, CHON₁, CHON₂, and CHOS₁ compounds are shown in Figs. S1 and S2. A complete molecular characterization of the DOM is summarized in Table 2.

Although the O/C ratios of all CHO compounds were similar in surface and pore water, the relative frequency of compounds with ratios below 1–1.5 was lower in the oxic surface water compared with the pore waters (Figs. S1, S2, and S3 A–D ). Similar trends were observed for CHON₁ and CHON₂ compounds [i.e., there were fewer dissolved organic nitrogen compounds with H/C ratios <1 in the surface water compared with the pore waters]. Dissolved organic sulfur showed no clear trend.

The observed differences in H/C ratios indicated that aromaticity played an important role in the cycling of DOM at the redox interface. Using an aromaticity index (AI) (13), we identified about 28% of all formulas (including nitrogen- and sulfur-containing formulas) in the oxic surface water and up to 35% in the pore waters as aromatic (i.e., phenolic compounds indicative of vascular plant remains). Nine percent to 11% were identified as condensed aromatic compounds that are derived from combustion processes. The relative frequency of aromatic CHO, CHON₁, and CHON₂ formulas was higher in the anoxic pore waters compared with the oxic surface water (Fig. 1), which is indicative of selective removal of these compounds at the redox interface. For sulfurized molecules, the trend was less clear.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

Differences in the relative abundance of compounds of different aromaticity at Wendewiesen. All compounds detected in the pore waters at 5–10 cm (PW1) and 55–60 cm (PW14), respectively, that were not detected in the surface water are shown in red. The complete surface water sample is shown in blue for comparison. Aromaticities are indicated as follows: nonaromatic compounds (AI < 0.5, white), aromatic compounds (0.5 < AI < 0.66, light gray), and condensed aromatic compounds (AI > 0.66, dark gray shading). We show normalized abundances to facilitate comparison of the samples. (A) Compounds containing carbon, hydrogen, and oxygen. (B and D) Compounds containing one or two additional nitrogen atoms, respectively. (C) Compounds containing one additional sulfur atom.

A decrease in the abundance of aromatic compounds in the surface water may occur through coagulation with oxidizing Fe (10, 11) diffusing from anoxic pore waters (12), microbial degradation, or photooxidation (14⇓–16). Each of these processes should be identifiable in our data because all leave a distinct molecular fingerprint on the composition of DOM. Photodegradation results in increasing relative abundances of more aliphatic compounds and a shift toward higher O/C ratios because aromatic compounds are transformed into more saturated and oxygenated compounds on irradiation (15, 16). Although we found that aromatic compounds were removed, O/C ratios did not show any significant differences among the samples, except for sulfurized compounds. This suggests that no oxygenation of DOM has occurred (Fig. S2). Because of the molecular evidence, and given that the turbidity of the colored fen waters reduces the penetration of sunlight, we ruled out photodegradation as an important removal pathway above the redox interface.

Microbial degradation may be another possibility. However, microbes tend to degrade more saturated, as well as nitrogen-containing, compounds (17, 18). Because we did not find clear evidence for preferential removal of nitrogen-containing compounds (Table 2), and given that aromatic moieties are generally less susceptible to microbial attack, we may rule this process out as well.

The third possibility is removal of DOM by Fe(III)-induced coagulation (10, 11). At DOC/Fe molar ratios of >10, aromatic DOM is preferentially removed from solutions (10, 11). At lower DOC/Fe ratios, more oxygenated compounds start to disappear (10) as Fe(hydr)oxides form inner-sphere complexes with carboxyl groups, resulting in relatively stable Fe-DOM flocs (19). With pore water DOC/Fe ratios ranging between 10 and 30 (Table 1) and almost unaffected O/C ratios (Table 2 and Fig. S2), our data are therefore consistent with iron-induced removal of aromatic DOM at the redox interface.

This process is relatively rapid. Aeration of anoxic pore water samples from rewetted fens leads to precipitation of Fe(III) and removal of DOM within a few minutes (12), which is much quicker than microbial mineralization of DOM. To estimate the coagulation potential of dissolved Fe and DOM in the pore waters, we repeated the oxygenation experiments of Zak et al. (12) with pore water samples taken at both study sites during several field trips (additional information is provided in Materials and Methods). Initial molar ratios of DOC to dissolved Fe ranged between 9 and 33 (Table S1). After 24 h of aeration 92–99% (mean ± SD: 97 ± 2%, n = 41) of the dissolved Fe and 10–44% (mean ± SD: 27 ± 7%) of the DOC were removed (Table S1), illustrating the efficiency of this retention process.

We further analyzed the effect of oxygenation on the molecular composition of pore water DOM. One aliquot of a pore water sample from Zarnekow was acidified to pH 2 with hydrochloric acid to prevent oxidation of Fe(II) and subsequent precipitation as Fe(III). A second aliquot was aerated and allowed Fe(III)-DOM to coagulate. After Fe had precipitated, the supernatant was sampled, acidified, and further processed. The artificially induced oxidation resulted in differences in molecular composition that were comparable to those observed between surface and pore waters. Aromatic moieties were preferentially removed on aeration (Fig. 2, Fig. S3, and Table 2). These findings are consistent with what would be expected based on previous laboratory studies (10, 11).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Results of the oxygenation experiment at Zarnekow. Compounds removed after coagulation with Fe following aeration are shown in red. The directly acidified pore water sample is shown in blue for comparison. Shaded areas are as in Fig. 1. (A–D) Compound classes sorted as in Fig. 1.

The interactions of Fe with aromatic DOM at the redox interface of the fens described in this study are illustrated in Fig. 3. The reallocation of iron to the oxic surface is well documented in the solid phase because there is threefold more Fe (based on dry mass) in the surface peat layer than in the deeper layers of the fen at Zarnekow (3).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

Schematic diagram of the iron trap at redox interfaces depicting the export of anoxic peat pore water, oxidation of iron at the oxic surface, and coprecipitation of terrestrial DOM with Fe(III). Examples of molecular structures are shown for representative molecular formulas that were selectively retained at the redox interface or passed into the water column. Although many more possible structural isomers exist, the examples illustrate the selective retention of phenolic acids in conjunction with iron precipitation.

A previous study on pore water DOM of a pristine fen characterized with FT-ICR-MS also reported higher degrees of aromaticity in deeper pore waters (20), indicating that the process we observe may also operate in natural fens.

About 4–11% of all molecular formulas in our study had a condensed aromatic structure resulting from thermal alteration of natural organic matter (21). This compound class is termed pyrogenic or black carbon, and represents a relatively stable pool of organic carbon that accumulates in soils (22) and ocean water (23) because of its apparent recalcitrance. Once considered as being inert, recent research has indicated that partial oxidation turns soil black carbon into dissolved black carbon (DBC) through the addition of oxygen functional groups (24, 25). The presence of soluble nitrogen- and sulfur-containing condensed aromatic compounds has not been reported for peat pore waters before and is highly interesting because it raises questions about the origin and the possibility of dissolved black sulfur and dissolved black nitrogen (DBN). FT-ICR-MS provides only limited structural information, such that we cannot unambiguously identify whether N is ring-bound. However, the similar distribution of the AI for CHO, CHON₁, and CHON₂ compounds suggests similar processing of these compounds at redox interfaces. The cycling of both DBC and DBN may therefore be coupled to the redox chemistry of Fe. This is an important finding because the sequestration of black carbon with Fe(hydr)oxides has been postulated before (22) but direct field evidence has been lacking to date. The “rusty carbon sink” (9) appears to be a specific sink for aromatic and condensed aromatic compounds.

Because adsorption of aromatic DOM to Fe(hydr)oxides may be reversible under anoxic conditions (4), the iron trap described here may only temporarily retain DBC. For example, variations in depth of the redox interface following water table fluctuations (induced by climatic variations or human activity) (5) may cause either release or reallocation of DBC within the soil profile. Unless DOM forms a “protective layer” that prevents the reductive dissolution of Fe oxides, we propose that redox interfaces should be considered as intermediate pools for DBC.

Although the importance of iron oxides for the cycling of DOM has long been known (e.g., refs. 26, 27), only recently has the role of sedimentary reactive iron phases for organic carbon storage been estimated to be quantitatively important on a global scale (9). Our study now shows that at redox interfaces, Fe selectively traps vascular plant-derived and pyrogenic DOM. Because redox interfaces are ubiquitous in the environment (e.g., in the pore waters of lakes, estuaries, tidal flats, and marine sediments; in the water column of anoxic basins, such as the Black Sea and Chesapeake Bay), the coupled cycling of iron and terrigenous DOM at oxic-anoxic transitions may explain some of the “missing” terrestrially derived DOM in the oceans (28, 29). Given the reactive nature of iron hydroxides, future studies on the age of the organic carbon associated with iron are needed to assess the potential of this sink for long-term storage.