Sediment export from forests
Tropical forests play an important role in the global carbon balance, assimilating over a third of the global terrestrial gross primary production . They represent a significant CO2 sink  and maintaining this function requires a large and sustained supply of nitrogen (N) . This implies that an understanding of how N losses from tropical soils constrain plant growth and carbon (C) sequestration is critical to assess current and future productivity of tropical forest ecosystems. [4-5]. Despite its importance, a quantified understanding of the different N loss pathways is lacking (ref) [3,6-7]. In contrast to particulate organic C (POC) export, an important pathway of C loss from terrestrial ecosystems, little is know about the magnitude and mechanisms of particulate organic N (PON) export in tropical landscapes, particularly when compared to our understanding of dissolved or gaseous N losses . This is a major knowledge gap because preliminary results have highlighted that lateral particulate export contribute significantly to N balance (N input / N output) in tropical forests that are geomorphologically active [8-10]. PON erosion is therefore an important, yet underappreciated, N loss term that is required to close the N budget of tropical forests .
Here, we outline a project that aims to assess factors controlling PON erosion from tropical forests in the Congo basin and its significance for nutrient availability. To this end, first-order catchments in the three main forests types covering the headwaters of the Congo basin will be monitored to investigate the significance of PON as a loss vector in relation to the geomorphic setting, climate, soils and vegetation characteristics. The focus on particulate fluxes, proposed here, is highly complementary to existing projects in the Congo basin that focus on input and output balances of reactive N (dissolved and gaseous N) currently lead by P. Boeckx of Ghent University via BELSPO and VLIR-UOS funding. To achieve our goals, the specific objectives of the project are to:
1. (i) examine the controls on PON delivery to streams from tropical forest catchments and quantify their rate of transfer
2. (ii) quantify hillslope-scale variation in N transformations and availability in relation to physical erosion
3. (iii) examine the role of N export in relation to other N ecosystem fluxes, whereby we focus on the relative contribution of dissolved versus particulate export fluxes
This project addresses key questions that have been defined in the promoters’ FNRS long-term research strategy (2016-2024). Furthermore, the project will capitalize on an existing collaboration between the promoter and Ghent University (Prof. P. Boeckx): the unique complementarity between physical-based processes (i.e. lateral N fluxes), atmospheric N input and N biogeochemistry will catalyse the international valorisation and dissemination of the expertise of the promoter in this area.