Mobilization of aged and biolabile soil carbon by tropical deforestation

In the mostly pristine Congo Basin, agricultural land-use change has intensified in recent years. One potential and understudied consequence of this deforestationand conversion to agriculture is the mobilization and loss of organic matterfrom soils to rivers as dissolved organic matter. Here, we quantify and characterize dissolved organic mattersampled from 19 catchments of varying deforestationextent near Lake Kivu over a two-week period during the wet season. Dissolved organic carbonfrom deforested, agriculturally dominated catchments was older (14C age: ~1.5 kyr) and more biolabile than from pristine forest catchments. Ultrahigh- resolution mass spectrometryrevealed that this aged organic matterfrom deforestedcatchments was energy rich and chemodiverse, with higher proportions of nitrogen- and sulfur-containing formulae. Given the molecular composition and biolability, we suggest that organic matterfrom deforestedlandscapes is preferentially respired upon disturbance, resulting in elevated in-stream concentrations of carbon dioxide. We estimate that while deforestationreduces the overall flux of dissolved organic carbonby approximately 56%, it does not significantly change the yield of biolabile dissolved organic carbon. Ultimately, the exposure of deeper soil horizonsthrough deforestationand agricultural expansion releases old, previously stable, and biolabile soil organic carbon into the modern carbon cyclevia the aquatic pathway.