Carbon Biogeochemistry of Agriculturally Impacted Watersheds of the Congo Basin

The Congo River is the second largest river by discharge after the Amazon. Its diverse network of waterways drains the largest pristine tropical forest-savanna ecosystem in the world (second in size but considerably less degraded than the Amazon Basin). These dense forests, savannas, and lakes connected via the Congo watershed together represent one of the largest contiguous ecosystems and stores of organic carbon (OC) on Earth.
Background and motivation
Carbon exported from terrestrial ecosystems as organic matter or dissolved inorganic gases represents an important component of the global carbon cycle by returning photosynthetically fixed C to the atmosphere and/or transporting it to downstream ecosystems and the ocean. Over the past decade, the estimated quantity of C transferred from land to aquatic systems has grown substantially, largely due to the better modelling of vertical CO2 losses from outgassing. As the estimates for lateral loses of C to aquatic ecosystems grow, the terrestrial sink for C must decrease or be adjusted to balance the C budget.
Once mobilized to aquatic ecosystems, terrestrial carbon is subject to in situ processing from microbial respiration, photo-oxidation, and vertical outgassing, meaning that the form and quantity of C entering the aquatic “pipe” can be fundamentally altered en route from source to ocean. Nevertheless, advances in the environmental application of high-resolution mass spectrometry methods render the composition of dissolved organic matter a powerful tool to fingerprint and integrate landscape-level processes from a single water sample.

Photo: Robert Spencer

Photo: Robert Spencer

As with the Amazon, projected population growth and associated land-use change threatens to disturb and mobilize the vast store of soil and biogenic OC in the Congo. Deforestation, intensifying agriculture, climate-induced desertification, and urbanization all have the potential to dramatically impact the movement and cycling of carbon from land to rivers. Under these growing disturbances, it is assumed that the vast stores of OC in the Congo will become vulnerable to release into aquatic systems and the atmosphere. Despite its importance to the global carbon cycle and the increasing pressures of disturbance, the Congo is a relatively understudied region. Preliminary studies have examined the background carbon chemistry at the mouth of the Congo, which may or may not integrate or convey upstream processes. Much more information is needed on the potential response of rivers and streams to the growing anthropogenic impacts.
Objectives
Our group employs detailed paired-watershed approach to gauge the impacts of disturbance on riverine carbon export and cycling. Specifically, we compare pristine catchments with anthropogenically impacted catchments of comparable soil types and lithologies. Our work focuses on the quantity, quality, sources, and cycling dynamics of organic and inorganic C in these catchments of divergent disturbance regimes. Currently, we work in the montaine forests of eastern Democratic Republic of Congo as well as the lowland forests of Republic of Congo. Our group will make significant headway towards answering the following questions with regard to the Congo Basin:

Photo: Robert Spencer

How much carbon is laterally transported by pristine forest landscapes and what proportion of net ecosystem production does it represent?
1.
How does deforestation and land-use change alter the flux, composition, and fate of this aquatic carbon?
2.
3.
What role do seasonality and hydrology play in the carbon transport dynamics of pristine and impacted catchments?

Photo: Robert Spencer 

Photo: Robert Spencer