Stable isotope signatures of soil nitrogen on an environmental-geomorphic gradient within the Congo Basin

Abstract. Nitrogen (N) availability can be highly variable in tropical forests on a regional and on a local scale. While environmental gradients influence N cycling on a regional scale, topography is known to affect N availability on a local scale. We compared stable isotope signatures (δ15N) 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. Furthermore, we examined the effect of surface slope angles on δ15N in the same forests to quantify local differences induced by topography. Soil δ15N profiles indicated that the N cycling in in the montane forest is 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, our results show that slope angles only affects the soil δ15N signature in the Miombo forest, which is prone to erosion due to the lower vegetation cover and intense rainfalls 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 N cycling. Values from the montane forest showed high variability in stable isotope signatures, but they were not constrained by topography. A pan-tropical analysis of soil δ15N values (i.e. from our study and the literature) reveals that soil δ15N is best explained by factors controlling erosion, namely mean annual precipitation, leaf area index, and slope angles. The erosive forces vary immensely between different tropical forest ecosystems and our results highlight the need of more spatial coverage of N-cycling studies in tropical forests, to further elucidate the local impact of topography on N cycling in this biome.

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Stable isotope signatures of soil nitrogen on an environmental–geomorphic gradient within the Congo Basin

SOIL ◽

10.5194/soil-7-83-2021 ◽

2021 ◽

Vol 7 (1) ◽

pp. 83-94

Author(s):

Simon Baumgartner ◽

Marijn Bauters ◽

Matti Barthel ◽

Travis W. Drake ◽

Landry C. Ntaboba ◽

...

Keyword(s):

Tropical Forests ◽

Montane Forest ◽

N Cycling ◽

Congo Basin ◽

N Availability ◽

Slope Gradient ◽

Miombo Woodland ◽

Lowland Forest ◽

Stable Isotope Signatures ◽

Soil Δ15n

Abstract. 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.

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Supplementary material to "Stable isotope signatures of soil nitrogen on an environmental-geomorphic gradient within the Congo Basin"

10.5194/soil-2020-70-supplement ◽

2020 ◽

Author(s):

Simon Baumgartner ◽

Marijn Bauters ◽

Matti Barthel ◽

Travis William Drake ◽

Landry Cizungu Ntaboba ◽

...

Keyword(s):

Stable Isotope ◽

Soil Nitrogen ◽

Congo Basin ◽

Stable Isotope Signatures ◽

Supplementary Material

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Seasonality, drivers, and isotopic composition of soil CO<sub>2</sub> fluxes from tropical forests of the Congo Basin

Biogeosciences ◽

10.5194/bg-17-6207-2020 ◽

2020 ◽

Vol 17 (23) ◽

pp. 6207-6218

Author(s):

Simon Baumgartner ◽

Matti Barthel ◽

Travis William Drake ◽

Marijn Bauters ◽

Isaac Ahanamungu Makelele ◽

...

Keyword(s):

Soil Moisture ◽

Soil Respiration ◽

Tropical Forests ◽

Montane Forest ◽

Congo Basin ◽

Co2 Fluxes ◽

Soil Co2 ◽

Montane Forests ◽

Lowland Forest ◽

Lowland Forests

Abstract. 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.

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Nitrogen isotopic composition of plants and soil in an arid mountainous terrain: sunny slope versus shady slope

10.5194/bg-2017-313 ◽

2017 ◽

Author(s):

Chongjuan Chen ◽

Yufu Jia ◽

Yuzhen Chen ◽

Imran Mehmood ◽

Yunting Fang ◽

...

Keyword(s):

Environmental Effects ◽

Regional Scale ◽

Local Environment ◽

Global Scale ◽

Influential Factors ◽

N Cycling ◽

N Availability ◽

Soil N ◽

The Difference ◽

Soil Δ15n

Abstract. Nitrogen cycling is tightly associated with environment. Sunny slope of a given mountain could significantly differ from shady slope in environment. Thus, N cycling should also be different between the two slopes. Since leaf δ15N, soil δ15N and △δ15Nleaf-soil (△δ15Nleaf-soil = leaf δ15N − soil δ15N) could reflect the N cycling characteristics, we put forward a hypothesis that leaf δ15N, soil δ15N and △δ15Nleaf-soil should differ across the two slopes. However, such a comparative study between two slopes has never been conducted yet. In addition, environmental effects on leaf and soil δ15N derived from studies at global scale were often found to be different from that at regional scale. This led to our argument that environmental effects on leaf and soil δ15N could depend on local environment. To confirm our hypothesis and argument, we measured leaf and soil δ15N on the sunny and shady slopes of Mount Tianshan. Remarkable environment differences between the two slopes provided an ideal opportunity for our test. The study showed that leaf δ15N, soil δ15N and △δ15Nleaf-soil on the sunny slope were greater than that on the shady slope although the difference in soil δ15N was not significant. The result confirmed our hypothesis and suggested that the sunny slope has higher soil N transformation rates and soil N availability than the shady slope. Besides, this study observed that the significant influential factors of leaf δ15N were temperature, precipitation, leaf N, leaf C / N and silt / clay ratio on the shady slope, whereas on the sunny slope only leaf C / N was related to leaf δ15N. The significant influential factors of soil δ15N were temperature, precipitation and silt / clay ratio on the shady slope, whereas on the sunny slope MAP and soil moisture exerted significant effects. Precipitation exerted contrary effects on soil δ15N between the two slopes. Thus, this study supported our argument that the relationships between leaf and soil δ15N and environmental factors are local-dependent.

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Seasonality, drivers, and isotopic composition of soil CO<sub>2</sub> fluxes from tropical forests of the Congo Basin

10.5194/bg-2020-133 ◽

2020 ◽

Author(s):

Simon Baumgartner ◽

Matti Barthel ◽

Travis W. Drake ◽

Marijn Bauters ◽

Isaac Ahanamungu Makelele ◽

...

Keyword(s):

Soil Moisture ◽

Soil Respiration ◽

Tropical Forests ◽

Montane Forest ◽

Congo Basin ◽

Co2 Fluxes ◽

Soil Co2 ◽

Montane Forests ◽

Lowland Forest ◽

Lowland Forests

Abstract. Soil respiration is an important carbon flux and key process determining the net ecosystem production of terrestrial ecosystems. To address the enormous 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 for the first time annually in two dominant forest types (lowland and montane) of the Congo Basin during three years at varying temporal resolution. Soil CO2 fluxes from the Congo Basin resulted in 3.69 ± 1.22 and 3.82 ± 1.15 µmol CO2 m−2 s−1 for lowland and montane forests, respectively. Respiration 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. Paired ẟ13C values of 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 were different, i.e. soil moisture for montane forest and C availability for lowland forest.

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