Soil erosion and nutrient availability in tropical forests of the Congo Basin

The importance of tropical forests in the global climate system is widely known. Through their high productivity they represent a significant global carbon reservoir. Preserving this function needs a continuous supply of nutrients, mainly nitrogen (N) and phosphorus (P). However, the availability and biogeochemical cycling of these nutrients is highly variable in space and time and the understanding of different nutrient loss pathways is still lacking. The influence of soil erosion on the availability of these nutrients in pristine forests of the tropics is not well understood. To gain a better mechanistic understanding of the aquatic loss processes of N and P, we monitored different headwater streams in three distinct tropical forests of the Congo Basin and determined the influence of soil erosion on soil N. This knowledge is important to assess the response of these ecosystems to changing climate and land use. To assess the importance of soil erosion in tropical forests and export of sediments and nutrients from drainage basins, the first part of the thesis focussed on the fundamental understanding of the dynamics and drivers of sediment export in headwater streams of a tropical lowland forest and a deciduous Miombo woodland. We used high temporal resolution turbidity data and analysed the turbidity-discharge hysteresis patterns during storm events. Although these forest systems are situated in very old and stable landscapes, the obtained sediment yields were substantial and comparable to forests in more geomorphic active sites of the tropics. Although the Miombo only experiences a five-month wet season, similar annual sediment export was found as for the lowland forest, due to the lower vegetation cover. Furthermore, results showed that storm events exported 30% and 68% of the annual sediment yield in the lowland and the Miombo, respectively. In Chapter three, it was shown that soil erosion and storm events also play a crucial role in aquatic nutrient export from these forest ecosystems. At both sites, N and P were dominantly exported in particulate or dissolved organic forms and the yields of those nutrient constituents were driven by intense discharge pulses due to storm events. This highlights the need to include samples of those events in a sampling campaign to get reliable annual export yields. This was not possible for the montane forest monitoring site (Chapter four), resulting in a large uncertainty in the sediment and nutrient yield estimates. However, best estimates of the long-term baseflow dataset highlighted the same trend to the importance of particulate and dissolved organic N and P export. Although these losses are significant, there is no indication that erosion leads to local nutrient limitations in the N and P rich montane and lowland forests. The goal of Chapter five was to evaluate if soil erosion influences soil N at the catchment scale. For this, stable isotope signatures of soil N have been analysed as an integrator of N cycling processes. Results showed that only in the Miombo woodland, stable isotopic signatures of soil N were influenced by topography and soil erosion, indicating erosional effects on N cycling. In the last part of this thesis, the influence of land use in the lowlands on aquatic sediment and nutrient export was examined. Results from a field campaign, during which baseflow was intensively sampled, revealed that there was significant higher export of dissolved organic N and particulate inorganic P, while the yields of nitrate were lower in an agricultural catchment compared to the pristine forest catchment. Continuous sensor measurements indicated that our findings most probably underestimated these differences because of the lack of storm event data. The results of this thesis highlight the importance of particulate N and P losses from pristine tropical forests and their inclusion in biogeochemical studies are important, even in so believed low erosive ecosystems. The predicted increase in rainfall intensity and the ongoing loss of primary forests will further enhance the importance of these processes.(SC - Sciences) -- UCL, 202