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Abstract

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.