In the mostly pristine Congo Basin, agricultural land-use change has intensified in recent years. One potential and understudied consequence of this deforestation and conversion to agriculture is the mobilization and loss of organic matter from soils to rivers as dissolved organic matter. Here, we quantify and characterize dissolved organic matter sampled from 19 catchments of varying deforestation extent near Lake Kivu over a two-week period during the wet season. Dissolved organic carbon from deforested, agriculturally dominated catchments was older (14C age: ~1.5 kyr) and more biolabile than from pristine forest catchments. Ultrahigh-resolution mass spectrometry revealed that this aged organic matter from deforested catchments was energy rich and chemodiverse, with higher proportions of nitrogen- and sulfur-containing formulae. Given the molecular composition and biolability, we suggest that organic matter from deforested landscapes is preferentially respired upon disturbance, resulting in elevated in-stream concentrations of carbon dioxide. We estimate that while deforestation reduces the overall flux of dissolved organic carbon by approximately 56%, it does not significantly change the yield of biolabile dissolved organic carbon. Ultimately, the exposure of deeper soil horizons through deforestation and agricultural expansion releases old, previously stable, and biolabile soil organic carbon into the modern carbon cycle via the aquatic pathway.
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The authors declare that all data supporting the findings of this study other than non-categorized FT-ICR MS data are available within the paper and its supplementary information files. Non-categorized FT-ICR MS formulae data are available from the corresponding author upon request.
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We thank our network of collaborators at the International Institute of Tropical Agriculture and the Université Catholique de Bukavu in DR Congo for their local expertise and logistical assistance. Funding for this research was provided by the Winchester Fund at Florida State University. This work was partially supported by National Science Foundation grants OCE 1464396 to R.G.M.S., and DMR-1157490 and DMR-1644779 to the National High Magnetic Field Laboratory. K.V.O. received funding from FNRS. A.M.H. and S.E.T. received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 695101 (14 Constraint)).
The authors declare no competing interests.
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Supplementary description, Supplementary Figs. 1–6 and Supplementary Tables 1–3.
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Journal of Geophysical Research: Biogeosciences (2019)
Nature Geoscience (2019)