The flux and composition of carbon (C) from land to rivers represents a critical component of the global C cycle as well as a powerful integrator of landscape‐level processes. In the Congo Basin, an expansive network of streams and rivers transport and cycle terrigenous C sourced from the largest swathe of pristine tropical forest on Earth. Increasing rates of deforestation and conversion to agriculture in the Basin are altering the current regime of terrestrial‐to‐aquatic biogeochemical cycling of C. To investigate the role of deforestation on dissolved organic and inorganic C (DOC and DIC, respectively) biogeochemistry in the Congo Basin, six lowland streams that drain catchments of varying forest proportion (12%–77%) were sampled monthly for 1 year. Annual mean concentrations of DOC exhibited an asymptotic response to forest loss, while DIC concentrations increased continuously with forest loss. The isotopic signature of DIC became significantly more enriched with deforestation, indicating a shift in source and processes controlling DIC production. The composition of dissolved organic matter (DOM), as revealed by ultra‐high‐resolution mass spectrometry, indicated that deforested catchments export relatively more aliphatic and heteroatomic DOM sourced from microbial biomass in soils. The DOM compositional results imply that DOM from the deforested sites is more biolabile than DOM from the forest, consistent with the corresponding elevated stream CO2concentrations. In short, forest loss results in significant and comprehensive shifts in the C biogeochemistry of the associated streams. It is apparent that land‐use conversion has the potential to dramatically affect the C cycle in the Congo Basin by reducing the downstream flux of stable, vascular‐plant derived DOC while increasing the transfer of biolabile soil C to the atmosphere.
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Dissolved organic carbon (DOC) flux from rivers in the pan‐Arctic watershed represents an important connection between major terrestrial carbon stocks and the Arctic Ocean. Previous estimates of Arctic carbon flux and dissolved organic matter (DOM) seasonal dynamics have relied predominantly on measurements from the six major Arctic rivers, yet these may not be representative of northern high‐latitude constrained smaller watersheds. Here, we evaluate DOC concentration and DOM composition in the Onega River, a small Arctic watershed, using optical measurements and ultrahigh resolution mass spectrometry. Compared to the six largest Arctic rivers, DOC, absorbance at
- NSF-PAR ID:
- 10420811
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 128
- Issue:
- 5
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Climatic changes are transforming northern high‐latitude watersheds as permafrost thaws and vegetation and hydrology shift. These changes have implications for the source and reactivity of riverine dissolved organic matter (DOM), and thus biogeochemical cycling, across northern high‐latitude systems. In this study, we use a latitudinal gradient from the interior to the North Slope of Alaska to evaluate seasonal and landscape drivers of DOM composition in this changing Arctic environment. To assess DOM source and composition, we used absorbance and fluorescence spectroscopy to measure DOM optical properties, lignin biomarker analyses to evaluate vascular plant contribution to the DOM pool, and Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) to assess DOM compositional changes. We found that seasonal inputs of DOM at elevated discharge during the freshet were typically more aromatic in nature with higher lignin concentrations and carbon‐normalized yields. Landscape characteristics were a major control on dissolved organic carbon (DOC) yields and DOM composition. More northern watersheds, which were steeper, underlain by continuous permafrost, and exhibited a mix of barren and lichen/moss vegetation cover, exported less DOC with relatively more aliphatic DOM compared to more southern basins. Watersheds with deeper active layers exported DOM that was more aromatic with higher polyphenolic and condensed aromatic relative abundances and lignin yields, likely sourced from shallow subsurface flow during high discharge periods. However, contributions from deeper groundwater to streamflow is expected to increase, which would increase interactions of groundwater with mineral soils and decrease aromatic DOM contributions during periods of low discharge.
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Abstract Climate change is dramatically altering Arctic ecosystems, leading to shifts in the sources, composition, and eventual fate of riverine dissolved organic matter (DOM) in the Arctic Ocean. Here we examine a 6‐year DOM compositional record from the six major Arctic rivers using Fourier‐transform ion cyclotron resonance mass spectrometry paired with dissolved organic carbon isotope data (Δ14C, δ13C) to investigate how seasonality and permafrost influence DOM, and how DOM export may change with warming. Across the pan‐Arctic, DOM molecular composition demonstrates synchrony and stability. Spring freshet brings recently leached terrestrial DOM with a latent high‐energy and potentially bioavailable subsidy, reconciling the historical paradox between freshet DOM's terrestrial bulk signatures and high biolability. Winter features undiluted baseflow DOM sourced from old, microbially degraded groundwater DOM. A stable core Arctic riverine fingerprint (CARF) is present in all samples and may contribute to the potential carbon sink of persistent, aged DOM in the global ocean. Future warming may lead to shifting sources of DOM and export through: (1) flattening Arctic hydrographs and earlier melt modifying the timing and role of the spring high‐energy subsidy; (2) increasing groundwater discharge resulting in a greater fraction of DOM export to the ocean occurring as stable and aged molecules; and (3) increasing contribution of nitrogen/sulfur‐containing DOM from microbial degradation caused by increased connectivity between groundwater and surface waters due to permafrost thaw. Our findings suggest the ubiquitous CARF (which may contribute to oceanic carbon sequestration) underlies predictable variations in riverine DOM composition caused by seasonality and permafrost extent.
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Abstract Riverine input of terrestrial dissolved organic matter (DOM) is an important component of the marine carbon cycle and drives net carbon dioxide production in coastal zones. DOM exports to the Arctic Ocean are likely to increase due to melting of permafrost and the Greenland Ice Sheet, but the quantity and quality of DOM exports from deglaciated watersheds in Greenland, as well as expected changes with future melting, are unknown. We compare DOM quantity and quality in Greenland over the melt seasons of 2017–2018 between two rivers directly draining the Greenland Ice Sheet (meltwater rivers) and four streams draining deglaciated catchments that are disconnected from the ice (nonglacial streams). We couple these data with discharge records to compare dissolved organic carbon (DOC) exports. DOM sources and quality differ significantly between watershed types: fluorescence characteristics and organic molar C:N ratios suggest that DOM from deglaciated watersheds is derived from terrestrial vegetation and soil organic matter, while that in glacial watersheds contains greater proportions of algal and/or freshly produced biomass and may be more reactive. DOC specific yield is similar for nonglacial streams (0.1–1.2 Mg/km2/year) compared to a glacial meltwater river (0.2–1.1 Mg/km2/year), despite orders of magnitude differences in instantaneous discharge. Upscaling based on land cover leads to an estimate of total DOC contributions from Greenland between 0.2 and 0.5 Tg/year, much of which is derived from deglaciated watersheds. These results suggest that future warming and ice retreat may increase DOC fluxes from Greenland with consequences for the Arctic carbon cycle.
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