Thorsten Dittmar

Persistence of soil organic matter as an ecosystem property

Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily—and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.

Mangrove production and carbon sinks: A revision of global budget estimates.

results in a conservative estimate of 218 ± 72 Tg C a 1 . When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at 112 ± 85 Tg C a 1 , equivalent in magnitude to 30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.

The biogeochemistry of the river and shelf ecosystem of the Arctic Ocean: a review

The Arctic Ocean is, on a volume basis, the ocean with the highest terrestrial input in terms of freshwater and organic matter. The drainage areas of the Arctic contain more than half of the organic carbon stored globally in soils and are extremely sensitive to climate change. These changes may considerably influence the huge continental flux of water and organic and inorganic constituents to the Arctic Ocean. Because of the immediate global concerns we here review the current knowledge about the biogeochemistry of the Arctic river and shelf ecosystem. Organic matter concentrations in the Arctic rivers are among the highest reported in worlds rivers. Dissolved organic carbon (DOC) reaches concentrations of up to 1000 µM C. The total amount of DOC discharged by rivers into the Arctic Ocean is 18 to 26·1012g C·yr-1 and similar to that of the Amazon. The discharge of particulate organic carbon is much lower with 4 to 6·1012g C·yr-1. Nitrogen and phosphorus are principally discharged as organic compounds. The concentrations of inorganic nutrients are among the lowest worldwide (inorganic nitrogen: 0 to 20 µM; phosphate: 0 to 0.8 µM), with the exception of silicate in some rivers (0.5 to 110 µM).Freshly produced organic matter is labile and its turnover rates are high in the Arctic Ocean. Riverine organic matter, in contrast, is soil-derived and refractory. It seems to behave biogeochemically stable in the estuaries and shelves and therefore does not substantially support the productivity of the Arctic Ocean. Suspended organic matter from the rivers principally settles in the estuaries and on the shelves, hence the terrigenous signature in the sediment decreases with distance from the coast. However, a fraction of terrigenous suspended matter escapes the shelves and is present in considerable amounts even in sediments of the central Arctic Ocean. Terrigenous dissolved organic matter, on the other hand, behaves primarily conservatively in the Arctic Ocean. There are practically no removal mechanisms in the estuaries and shelves. The molecular composition of dissolved organic matter can largely be explained as a mixture of refractory marine and terrigenous compounds. Therefore, the Arctic river discharge plays an important role as a contemporary sink in the global carbon cycle. The few available data on the biogeochemistry of the Russian rivers indicate that the proportion of taiga and tundra in the drainage areas has no considerable influence on the concentration and chemical composition of dissolved organic matter, with the exception of lignin-derived phenols, which can be used as chemotaxonomic tracers. It can therefore be speculated that changes in vegetation due to climate warming may not considerably influence the composition of dissolved organic matter discharged to the Arctic Ocean. The discharge of inorganic nutrients, however, may already has increased in the last decades, as indicated by long-term increases in winter water discharge and the seasonality of nutrient concentrations. For a reliable assessment of future changes long-term and seasonal data of nutrient and organic matter discharge, as well as more detailed biogeochemical information is urgently needed.

Global Charcoal Mobilization from Soils via Dissolution and Riverine Transport to the Oceans

Dissolving Charcoal Biomass burning produces 40 to 250 million tons of charcoal per year worldwide. Much of this is preserved in soils and sediments for thousands of years. However, the estimated production rate of charcoal is significantly larger than that of decomposition, and Jaffe et al. (p. 345; see the Perspective by Masiello and Louchouarn) calculate that a large fraction of the charcoal produced by fires is lost from the land through dissolution and transport to the oceans. A larger-than-assumed fraction of charcoal produced by wildfires leaches out of soils and is transported to the oceans. [Also see Perspective by Masiello and Louchouarn] Global biomass burning generates 40 million to 250 million tons of charcoal every year, part of which is preserved for millennia in soils and sediments. We have quantified dissolution products of charcoal in a wide range of rivers worldwide and show that globally, a major portion of the annual charcoal production is lost from soils via dissolution and subsequent transport to the ocean. The global flux of soluble charcoal accounts to 26.5 ± 1.8 million tons per year, which is ~10% of the global riverine flux of dissolved organic carbon (DOC). We suggest that the mobilization of charcoal and DOC out of soils is mechanistically coupled. This study closes a major gap in the global charcoal budget and provides critical information in the context of geoengineering.

River or mangrove? Tracing major organic matter sources in tropical Brazilian coastal waters

The influence of mangrove-fringed tropical estuaries on coastal carbon budgets has been widely recognised. However, a quantitative differentiation between riverine and mangrove-derived inputs to the dissolved (DOM) and microparticulate organic matter (POM) pool of these environments has been hitherto not possible. Based on lignin-derived phenols and stable carbon isotopes a chemical signature for mangrove, terrestrial and marine-derived organic matter was established for a mangrove estuary in North Brazil. A mixing model was applied to calculate the contribution of each of the three sources to the DOM and POM pool in the estuary throughout 18 tidal cycles in the course of one year. Best source assignment for POM was reached with the yield of lignin phenols and d13C as paired indicators, while the origin of DOM was best identified by the yield of lignin phenols and the acid to aldehyde ratio of vanillyl phenols. Although only about 6 % of the fluvial catchment area is covered by mangroves, their contribution to the estuarine DOM and POM pool generally exceeded several times the terrigenic input from the hinterland. This outwelling of mangrove-derived organic matter was enhanced during the rainy season. DOM and POM were exported from the mangrove to the estuary in similar proportions. Most mangrove-POM was rapidly removed from the water column, while mangrove-DOM behaved conservatively. In contrast, terrestrial DOM was almost entirely removed in the outer part of the estuary, which was accompanied by a concomitant increase in terrestrial POM. This seems to be the result of a geochemical barrier zone for this type of DOM in the estuary. Generally a high proportion of mangrove-DOM was present in the outer part of the estuary, even at high tide. This indicates DOM outwelling from mangroves in adjacent bays or estuaries and points to similar driving forces controlling this process on a regional scale. Mangroves probably play a more important role than rivers for marine carbon budgets along the North Brazilian coast south of the Amazon estuary.

Origin and biogeochemical cycling of organic nitrogen in the eastern Arctic Ocean as evident from D- and L-amino acids

The chemical structure of organic nitrogen and the mechanisms of its cycling in the oceans still remain elemental questions in contemporary marine sciences. The Arctic Ocean provides a model system for studying the fate of terrigenous compounds in the ocean. We chemically characterised and traced the discharge of dissolved organic nitrogen (DON) and its particulate counterpart (PON) from the Russian rivers into the central Arctic Ocean. We focussed on the D- and L-enantiomers of amino acids, the principal organic nitrogen compounds of living biomass. Total dissolved and particulate hydrolysable amino acids (TDAA, PAA) exhibited highest concentrations in the rivers (TDAA: 3.2 µM; PAA: 5.0 µM on average), contributing ~40% to DON and ~60% to PON. In the Arctic Ocean, TDAA and PAA decreased to concentrations of <1 µM, accounting only for ~10% of DON, but ~80% of PON. Dominant amino acids in TDAA were glycine and alanine (in the rivers 35% of TDAA, in deepwater 49%), followed by aspartic acid, glutamic acid and serine. Threonine was also abundant in the rivers, and leucine in deep seawater. Microbial-derived D-enantiomers of aspartic acid, glutamic acid, serine and alanine were found in significant amounts in all river and seawater samples, both dissolved and suspended. In riverine TDAA D-aspartic acid was most abundant (21% of total aspartic acid) and in deep seawater D-alanine predominated (44% of total alanine). The proportions of all D-enantiomers were significantly higher in oceanic versus riverine TDAA and increased with depth in the Arctic Ocean. PAA exhibited much lower proportions of D-enantiomers than TDAA (generally <10% of the respective amino acid).

What’s in an EEM? Molecular Signatures Associated with Dissolved Organic Fluorescence in Boreal Canada

Dissolved organic matter (DOM) is a master variable in aquatic systems. Modern fluorescence techniques couple measurements of excitation emission matrix (EEM) spectra and parallel factor analysis (PARAFAC) to determine fluorescent DOM (FDOM) components and DOM quality. However, the molecular signatures associated with PARAFAC components are poorly defined. In the current study we characterized river water samples from boreal Québec, Canada, using EEM/PARAFAC analysis and ultrahigh resolution mass spectrometry (FTICR-MS). Spearmans correlation of FTICR-MS peak and PARAFAC component relative intensities determined the molecular families associated with 6 PARAFAC components. Molecular families associated with PARAFAC components numbered from 39 to 572 FTICR-MS derived elemental formulas. Detailed molecular properties for each of the classical humic- and protein-like FDOM components are presented. FTICR-MS formulas assigned to PARAFAC components represented 39% of the total number of formulas identified and 59% of total FTICR-MS peak intensities, and included significant numbers compounds that are highly unlikely to fluoresce. Thus, fluorescence measurements offer insight into the biogeochemical cycling of a large proportion of the DOM pool, including a broad suite of unseen molecules that apparently follow the same gradients as FDOM in the environment.

Molecular evidence for lignin degradation in sulfate-reducing mangrove sediments (Amazonia, Brazil)

Abstract —Molecular lignin analyses have become a powerful quantitative approach for estimating flux and fate of vascular plant organic matter in coastal and marine environments. The use of a specific molecular biomarker requires detailed knowledge of its decomposition rates relative to the associated organic matter and its structural diagenetic changes. To gain insight into the poorly known processes of anaerobic lignin diagenesis, molecular analyses were performed in the sulfate-reducing sediment of a north Brazilian mangrove. Organic matter in samples representing different diagenetic stages (i.e., fresh litter, a sediment core, and percolating water) was characterized by alkaline CuO oxidation for lignin composition, element (C, N), and stable carbon isotope analyses. On the basis of these results and on a balance model, long-term in situ decomposition rates of lignin in sulfate-reducing sediments were estimated for the first time. The half-life (T1/2) of lignin derived from mangrove leaf litter (mainlyRhizophora mangle) was ∼150 yr in the upper 1.5 m of the sediment. Associated organic carbon from leaf tissue was depleted to ∼75% within weeks, followed by a slow mineralization in the sediment (T1/2 ≈ 300 yr). Unlike the known pathways of lignin diagenesis, even highly degraded lignin did not show any alterations of the propyl or methoxyl side chains, as evident from stable acid to aldehyde ratios and the proportion of methoxylated phenols (vanillyl and syringyl phenols). Aromatic ring cleavage is probably the principal mechanism for lignin decay in the studied environment. Cinnamyl phenols were highly abundant in mangrove leaves and were rapidly depleted during early diagenesis. Thus, the cinnamyl to vanillyl ratio could be used as a tracer for early diagenesis even under the sulfate-reducing conditions. Syringyl phenols were removed from dissolved organic matter in interstitial water, probably by sorption onto the sediment. Suspended organic matter in a mangrove creek showed a different lignin signature than its source, namely sedimentary organic matter or mangrove litter, with clear evidence for propyl side chain oxidation. This was probably attributable to erosion of aerated thin sediment surface layers during mangrove inundation. Although particulate and dissolved organic matter in the mangrove creek have a common source, their compositional patterns were different, because of different pathways of release, degradation, and transport to the creek.

Iron traps terrestrially derived dissolved organic matter at redox interfaces

Reactive iron and organic carbon are intimately associated in soils and sediments. However, to date, the organic compounds involved are uncharacterized on the molecular level. At redox interfaces in peatlands, where the biogeochemical cycles of iron and dissolved organic matter (DOM) are coupled, this issue can readily be studied. We found that precipitation of iron hydroxides at the oxic surface layer of two rewetted fens removed a large fraction of DOM via coagulation. On aeration of anoxic fen pore waters, >90% of dissolved iron and 27 ± 7% (mean ± SD) of dissolved organic carbon were rapidly (within 24 h) removed. Using ultra-high-resolution MS, we show that vascular plant-derived aromatic and pyrogenic compounds were preferentially retained, whereas the majority of carboxyl-rich aliphatic acids remained in solution. We propose that redox interfaces, which are ubiquitous in marine and terrestrial settings, are selective yet intermediate barriers that limit the flux of land-derived DOM to oceanic waters.

Chemodiversity of dissolved organic matter in lakes driven by climate and hydrology

Despite the small continental coverage of lakes, they are hotspots of carbon cycling, largely due to the processing of terrestrially derived dissolved organic matter (DOM). As DOM is an amalgam of heterogeneous compounds comprising gradients of microbial and physicochemical reactivity, the factors influencing DOM processing at the molecular level and the resulting patterns in DOM composition are not well understood. Here we show, using ultrahigh-resolution mass spectrometry to unambiguously identify 4,032 molecular formulae in 120 lakes across Sweden, that the molecular composition of DOM is shaped by precipitation, water residence time and temperature. Terrestrially derived DOM is selectively lost as residence time increases, with warmer temperatures enhancing the production of nitrogen-containing compounds. Using biodiversity concepts, we show that the molecular diversity of DOM, or chemodiversity, increases with DOM and nutrient concentrations. The observed molecular-level patterns indicate that terrestrially derived DOM will become more prevalent in lakes as climate gets wetter.