Simon Baumgartner

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Baumgartner

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Simon

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0000-0002-4091-9164

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Publications 1 - 10 of 11

Seasonality, drivers, and isotopic composition of soil CO2 fluxes from tropical forests of the Congo Basin
Baumgartner, Simon; Barthel, Matti; Drake, Travis William; et al. (2020)
Biogeosciences
Soil respiration is an important carbon flux and key process determining the net ecosystem production of terrestrial ecosystems. To address the lack of quantification and understanding of seasonality in soil respiration of tropical forests in the Congo Basin, soil CO2 fluxes and potential controlling factors were measured annually in two dominant forest types (lowland and montane) of the Congo Basin over 2 years at varying temporal resolution. Soil CO2 fluxes from the Congo Basin resulted in 3.45 ± 1.14 and 3.13 ± 1.22 µmol CO2 m−2 s−1 for lowland and montane forests, respectively. Soil CO2 fluxes in montane forest soils showed a clear seasonality with decreasing flux rates during the dry season. Montane forest soil CO2 fluxes were positively correlated with soil moisture, while CO2 fluxes in the lowland forest were not. Smaller differences of δ13C values of leaf litter, soil organic carbon (SOC), and soil CO2 indicated that SOC in lowland forests is more decomposed than in montane forests, suggesting that respiration is controlled by C availability rather than environmental factors. In general, C in montane forests was more enriched in 13C throughout the whole cascade of carbon intake via photosynthesis, litterfall, SOC, and soil CO2 compared to lowland forests, pointing to a more open system. Even though soil CO2 fluxes are similarly high in lowland and montane forests of the Congo Basin, the drivers of them seem to be different, i.e., soil moisture for montane forest and C availability for lowland forest.
Stable isotope signatures of soil nitrogen on an environmental-geomorphic gradient within the Congo Basin
Baumgartner, Simon; Bauters, Marijn; Barthel, Matti; et al. (2021)
Soil
Nitrogen (N) availability can be highly variable in tropical forests on regional and local scales. While environmental gradients influence N cycling on a regional scale, topography is known to affect N availability on a local scale. We compared natural abundance of 15N isotopes of soil profiles in tropical lowland forest, tropical montane forest, and subtropical Miombo woodland within the Congo Basin as a proxy to assess ecosystem-level differences in N cycling. Soil δ15N profiles indicated that N cycling in the montane forest is relatively more closed and dominated by organic N turnover, whereas the lowland forest and Miombo woodland experienced a more open N cycle dominated by inorganic N. Furthermore, we examined the effect of slope gradient on soil δ15N within forest types to quantify local differences induced by topography. Our results show that slope gradient only affects the soil δ15N in the Miombo forest, which is prone to erosion due to a lower vegetation cover and intense rainfall at the onset of the wet season. Lowland forest, on the other hand, with a flat topography and protective vegetation cover, showed no influence of topography on soil δ15N in our study site. Despite the steep topography, slope angles do not affect soil δ15N in the montane forest, although stable isotope signatures exhibited higher variability within this ecosystem. A pan-tropical analysis of soil δ15N values (i.e., from our study and literature) reveals that soil δ15N in tropical forests is best explained by factors controlling erosion, namely mean annual precipitation, leaf area index, and slope gradient. Erosive forces vary immensely between different tropical forest ecosystems, and our results highlight the need for more spatial coverage of N cycling studies in tropical forests, to further elucidate the local impact of topography on N cycling in this biome.
In-depth analysis of N2O fluxes in tropical forest soils of the Congo Basin combining isotope and functional gene analysis
Gallarotti, Nora; Barthel, Matti; Verhoeven, Elizabeth; et al. (2021)
The ISME Journal
Primary tropical forests generally exhibit large gaseous nitrogen (N) losses, occurring as nitric oxide (NO), nitrous oxide (N2O) or elemental nitrogen (N2). The release of N2O is of particular concern due to its high global warming potential and destruction of stratospheric ozone. Tropical forest soils are predicted to be among the largest natural sources of N2O; however, despite being the world’s second-largest rainforest, measurements of gaseous N-losses from forest soils of the Congo Basin are scarce. In addition, long-term studies investigating N2O fluxes from different forest ecosystem types (lowland and montane forests) are scarce. In this study we show that fluxes measured in the Congo Basin were lower than fluxes measured in the Neotropics, and in the tropical forests of Australia and South East Asia. In addition, we show that despite different climatic conditions, average annual N2O fluxes in the Congo Basin’s lowland forests (0.97 ± 0.53 kg N ha−1 year−1) were comparable to those in its montane forest (0.88 ± 0.97 kg N ha−1 year−1). Measurements of soil pore air N2O isotope data at multiple depths suggests that a microbial reduction of N2O to N2 within the soil may account for the observed low surface N2O fluxes and low soil pore N2O concentrations. The potential for microbial reduction is corroborated by a significant abundance and expression of the gene nosZ in soil samples from both study sites. Although isotopic and functional gene analyses indicate an enzymatic potential for complete denitrification, combined gaseous N-losses (N2O, N2) are unlikely to account for the missing N-sink in these forests. Other N-losses such as NO, N2 via Feammox or hydrological particulate organic nitrogen export could play an important role in soils of the Congo Basin and should be the focus of future research.
Agricultural Land-Use Increases Carbon Yields in Lowland Streams of the Congo Basin
Drake, Travis W.; Baumgartner, Simon; Barthel, Matti; et al. (2024)
Journal of Geophysical Research: Biogeosciences
As the dominant mode of deforestation in the Congo Basin, shifting agriculture is expected to increase with the projected four-fold population growth for the region by 2,100. To assess how this land-use change will affect the export of carbon (C) to rivers in a typical lowland forest ecosystem, we studied paired watersheds near Kisangani, Democratic Republic of the Congo. Two streams, one draining an intact forest (Forest) and one draining an agricultural landscape (Ag), were gauged, equipped with sensors, and sampled fortnightly for one year. Annual average specific discharge was 1.4 mm d⁻¹ (+76%) higher in the Ag compared to the Forest. Average annual dissolved organic C (DOC) and particulate organic C (POC) concentrations were 5.2 mg L⁻¹ (+163%) and 1.3 mg L⁻¹ (+81%) higher in the Ag stream, which, along with the higher discharge, resulted in 8.3 (+410%) and 2.4 g C m⁻² yr⁻¹ (+97%) larger C yields, respectively. Baseflow dissolved inorganic carbon, carbon dioxide, and methane yields were also higher in the Ag stream. Despite the higher yields of organic C (OC), the composition of OC did not differ significantly. Carbon to nitrogen ratios, along with isotopic signatures, revealed that both streams contained young, semi-degraded organic matter derived from C3 vegetation. Correspondingly, biodegradable DOC (BDOC) proportions did not differ between the streams, although the Ag stream yielded more total BDOC. These results show that agricultural land-use likely exports a greater proportion of Net Primary Productivity (NPP) to aquatic ecosystems, which may affect both C storage in soils and the proportion of gross PP that is ultimately respired.
Increasing calcium scarcity along Afrotropical forest succession
Bauters, Marijn; Janssens, Ivan A.; Wasner, Daniel; et al. (2022)
Nature Ecology & Evolution
Secondary forests constitute an increasingly important component of tropical forests worldwide. Although cycling of essential nutrients affects recovery trajectories of secondary forests, the effect of nutrient limitation on forest regrowth is poorly constrained. Here we use three lines of evidence from secondary forest succession sequences in central Africa to identify potential nutrient limitation in regrowing forests. First, we show that atmospheric phosphorus supply exceeds demand along forest succession, whereas forests rely on soil stocks to meet their base cation demands. Second, soil nutrient metrics indicate that available phosphorus increases along the succession, whereas available cations decrease. Finally, fine root, foliar and litter stoichiometry show that tissue calcium concentrations decline relative to those of nitrogen and phosphorus during succession. Taken together, these observations suggest that calcium becomes an increasingly scarce resource in central African forests during secondary succession. Furthermore, ecosystem calcium storage shifts from soil to woody biomass over succession, making it a vulnerable nutrient in the wake of land-use change scenarios that involve woody biomass export. Our results thus call for a broadened focus on elements other than nitrogen and phosphorus regarding tropical forest biogeochemical cycles and identify calcium as a scarce and potentially limiting nutrient in an increasingly disturbed and dynamic tropical forest landscape.
Substantial Organic and Particulate Nitrogen and Phosphorus Export from Geomorphologically Stable African Tropical Forest Landscapes
Baumgartner, Simon; Bauters, Marijn; Drake, Travis William; et al. (2023)
Ecosystems
Aquatic losses of nutrients are important loss vectors in the nutrient budgets of tropical forests. Traditionally, research has focused mainly on losses of inorganic nutrient forms, whereas the potential contribution of organic and particulate losses to the total nutrient export budget is much less constrained. In this study, we quantified full aquatic nitrogen (N) and phosphorus (P) exports, including inorganic, organic and particulate forms, from a moist tropical lowland forest and a semi-dry Miombo woodland forest within the Congo Basin. While particulate organic N (PON) was the highest N loss vector in the lowland stream (3.34 kg N ha(-1) y(-1); 44% of TN), dissolved organic N (DON) dominated the export in the Miombo stream (1.41 kg N ha(-1) y(.-1); 47% of TN). Aquatic P export was dominated by dissolved organic P (DOP) in both streams, with yields of 0.29 kg P ha(-1) y(-1) (65% of TP) in the lowland and 0.24 kg P ha(-1) y(-1) (69% of TP) in the Miombo. Storm events were driving those losses, exporting disproportionally high N and P loads during short periods of stormflow conditions (32% and 47% of TN and 20% and 40% of TP in the lowland and Miombo, respectively). Our results highlight the need to take particulate and organic forms into account as important loss vectors in the nutrient balance of tropical forests. This finding is of particular importance considering the projected increasing rainfall intensities in many tropical regions which might exacerbate the export of these nutrient forms in the near future.
Low N2O and variable CH4 fluxes from tropical forest soils of the Congo Basin
Barthel, Matti; Bauters, Marijn; Baumgartner, Simon; et al. (2022)
Nature Communications
Globally, tropical forests are assumed to be an important source of atmospheric nitrous oxide (N2O) and sink for methane (CH4). Yet, although the Congo Basin comprises the second largest tropical forest and is considered the most pristine large basin left on Earth, in situ N2O and CH4 flux measurements are scarce. Here, we provide multi-year data derived from on-ground soil flux (n = 1558) and riverine dissolved gas concentration (n = 332) measurements spanning montane, swamp, and lowland forests. Each forest type core monitoring site was sampled at least for one hydrological year between 2016 - 2020 at a frequency of 7-14 days. We estimate a terrestrial CH4 uptake (in kg CH4-C ha−1 yr−1) for montane (−4.28) and lowland forests (−3.52) and a massive CH4 release from swamp forests (non-inundated 2.68; inundated 341). All investigated forest types were a N2O source (except for inundated swamp forest) with 0.93, 1.56, 3.5, and −0.19 kg N2O-N ha−1 yr−1 for montane, lowland, non-inundated swamp, and inundated swamp forests, respectively.
Fire-derived phosphorus fertilization of African tropical forests
Bauters, Marijn; Drake, Travis W.; Wagner, Sasha; et al. (2021)
Nature Communications
Central African tropical forests face increasing anthropogenic pressures, particularly in the form of deforestation and land-use conversion to agriculture. The long-term effects of this transformation of pristine forests to fallow-based agroecosystems and secondary forests on biogeochemical cycles that drive forest functioning are poorly understood. Here, we show that biomass burning on the African continent results in high phosphorus (P) deposition on an equatorial forest via fire-derived atmospheric emissions. Furthermore, we show that deposition loads increase with forest regrowth age, likely due to increasing canopy complexity, ranging from 0.4 kg P ha−1 yr−1 on agricultural fields to 3.1 kg P ha−1 yr−1 on old secondary forests. In forest systems, canopy wash-off of dry P deposition increases with rainfall amount, highlighting how tropical forest canopies act as dynamic reservoirs for enhanced addition of this essential plant nutrient. Overall, the observed P deposition load at the study site is substantial and demonstrates the importance of canopy trapping as a pathway for nutrient input into forest ecosystems.
Aquatic and soil CO₂ emissions from forested wetlands of Congo's Cuvette Centrale
De Clippele, Antoine; Jaeger, Astrid C.H.; Baumgartner, Simon; et al. (2025)
Biogeosciences
Within tropical forest ecosystems, wetlands such as swamp forests are an important interface between the terrestrial and aquatic landscape. Despite this assumed importance, there is a paucity of carbon flux data from wetlands in tropical Africa. Therefore, the magnitude and source of carbon dioxide (CO₂) fluxes, carbon isotopic ratios, and environmental conditions were measured for 3 years between 2019 and 2022 in a seasonally flooded forest and a perennially flooded forest in the Cuvette Centrale of the Congo Basin. The mean surface fluxes for the seasonally flooded site and the perennially flooded site were 2.36±0.51 and 4.38±0.64 µmolm⁻² s⁻¹, respectively. The time series data revealed no marked seasonal pattern in CO₂ fluxes. As for the environmental drivers, the fluxes at the seasonally flooded site exhibited a positive correlation with soil temperature and soil moisture. Additionally, the water level appeared to be a significant factor, demonstrating a quadratic relationship with the soil fluxes at the seasonally flooded site. δ¹³C values showed a progressive increase across the carbon pools, from aboveground biomass to leaf litter and then to soil organic carbon (SOC). However, there was no significant difference in δ¹³C enrichment between SOC and soil-respired CO₂. This lack of enrichment can be attributed to either a significant contribution from the autotrophic component of soil respiration or closed system dynamics. An in-situ-derived gas transfer velocity (k₆₀₀=2.95 cm h⁻¹) was used to calculate the aquatic CO₂ fluxes at the perennially flooded site. Despite the low k₆₀₀, relatively high CO₂ surface fluxes were found due to very high partial pressure of CO₂ (pCO₂) values measured in the flooding waters. Overall, these results offer a quantification of the CO₂ fluxes from forested wetlands and provide insights into the temporal variability of these fluxes and their sensitivity to environmental drivers.
Dynamique des sédiments exportés dans les bassins versants forestier et agricole de la partie ouest de la région du lac Kivu, République Démocratique du Congo
Rukeza, Montfort Bagalwa; Baumgartner, Simon; Lizaga, Ivan; et al. (2025)
Biotechnologie, Agronomie, Société et Environnement
Description du sujet. Réalisée à l’Ouest de la région du lac Kivu, cette étude a évalué les facteurs qui contrôlent les variations spatio-temporelles de la production des sédiments dans deux bassins versants couvrant une superficie de 25 km2. Objectifs. Cette étude vise à étudier la dynamique temporelle d’exportation des sédiments en suspension et à quantifier la charge sédimentaire au sein de bassins versants avec des terres contrastées dans la région du lac Kivu. Méthode. Le débit d’eau journalier et le total des sédiments en suspension ont été mesurés. Les changements temporels de la végétation et d’érosivité des précipitations ont été analysés pour évaluer leur influence sur le débit et le rendement en sédiments. Résultats. Nous avons constaté que certains orages majeurs contribuaient à l’exportation de sédiments, en particulier pour le bassin versant agricole où une superficie d’environ 100 ha couverte de sédiments était présente. Un modèle de régression linéaire peut prédire avec succès l’exportation mensuelle de sédiments en utilisant uniquement l’érosivité des précipitations, la topographie et la couverture végétale (R2 = 0,87***). Nous avons obtenu un rendement annuel en sédiment de 0,5 t·ha-1·an-1 pour Mushuva et 15,3 t·ha-1·an-1 pour Renga. Les valeurs élevées de rendement en sédiments dans le bassin versant agricole sont parmi les plus élevées et sont attribuées à l'application minimale de conservation des sols, à la déforestation accélérée et aux orages fréquents. Conclusions. Il serait intéressant d'utiliser des techniques de protection des sols dans les bassins versants afin de réduire les exportations de sédiments.

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