Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

Abstract

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 CO2 concentrations. 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.

Climate Controls on River Chemistry

“…In places without acid rain and human activities, DOC exhibits elevated concentrations in warmer and drier years, indicating strong climatic influence (Zhi et al., 2020). Dissolved inorganic carbon (DIC) and alkalinity has also seen continued increase (Drake et al., 2020; Kaushal et al., 2013; Najjar et al., 2020; Raymond & Cole, 2003; Zamanian et al., 2018). Low discharge also entails longer residence time (Benettin et al., 2020) and possibly more extensive transformation of nutrients and carbon into greenhouse gases (Campeau et al., 2019).…”

Sediment and soil organic carbon loss during continuous extreme scouring events on the Loess Plateau

“…Researchers have studied the spatial and temporal distribution (Liu, Li, Nie, et al., 2017; Wang et al., 2019), stability, and mineralization of SOC (Novara et al., 2019). During water erosion, SOCL can be influenced by rainfall, terrain, soil, fertilizer use, tillage methods, surface cover, and land use (Drake et al., 2020; Ma et al., 2014). It may be that SOCL reflects the mechanism of soil erosion (Lal, 2005).…”

Heterogeneous Patterns of Aged Organic Carbon Export Driven by Hydrologic Flow Paths, Soil Texture, Fire, and Thaw in Discontinuous Permafrost Headwaters

“…In line with this conclusion, ancient C mobilization into nearby Alaskan lakes has also been shown to be heterogeneous (Bogard et al., 2019; Elder et al., 2018). The mobilization of aged C into Alaskan headwaters due to warming and fire disturbance is somewhat analogous to changes underway in temperate regions experiencing increased loading of aged, OC‐rich soils into aquatic networks in response to hydrologic and land use change (Butman et al., 2015; Drake et al., 2019, 2020; Moore et al., 2013). When aggregated at the global scale, the mobilization of aged DOC observed in the YRB appears to be part of a broader trend toward increased cycling of ancient C in aquatic networks.…”