Input, dynamics and loss of reactive nitrogen in a central African tropical mountain forest and Eucalyptus plantation

Next to land use change and climate change, nitrogen (N) deposition is another threat for forest ecosystem functioning. Central Africa contains the second largest area of contiguous moist tropical forests of the world. Tropical forests account for one third of primary production contributing significantly to the terrestrial carbon sink. Currently, there is a huge lack of field-based research in tropical (mountain) forests in Central Africa. Hence, the general objective of this thesis was to investigate biogeochemical processes in a central African pristine tropical mountain forest (Nyungwe) and a nearby Eucalyptus plantation. Nyungwe forest is located in southwestern Rwanda (2°15' – 2°55' S, 29°00'– 29°30' E) in a watershed dividing the Congo basin to the west and the Nile basin to the east and covers an area of approximately 970 km2. The topography is entirely mountainous (1,600 – 2,950 m above sea level) while the climate is humid tropical. For this study, two catchments were selected, one inside pristine Nyungwe forest and another one in a nearby Eucalyptus plantation in the buffer zone of the Nyungwe national park. In each forest type, three experimental plots (20 x 30 m) were selected and marked permanently. This study focused on N input, dynamics and losses, and specifically investigated: 1) litterfall dynamics and leaf litter decomposition rates, 2) N and base cation fluxes via throughfall deposition, litter percolation, soil solution and river water, and 3) soil N dynamics via an in situ15N pool dilution experiment. Litterfall was measured in the Nyungwe pristine forest during two consecutive years and in the nearby Eucalyptus plantation during one year. A 361-days litter decomposition experiment with single and mixed-species leaf litter was carried out with single-species litterbags installed in the pristine forest and mixed-species litterbags in both forest stands. Throughfall, humus percolation and soil solution fluxes and composition were investigated in the Nyungwe pristine forest and in the neighboring Eucalyptus plantation. This study was followed by an investigation on the origin of nitrate in throughfall, humus percolation, soil solution and the river water through use of a V-notch (90°) at the outlet of the pristine forest catchment and stable isotope analyses. Finally, an in situ 15N isotope dilution experiment was carried out in the pristine forest stand, using the ‘virtual soil core’ approach to quantify N dynamics and pathways in the Nyungwe pristine forest soil. Total litterfall amounted to ca. 4 and 2 t ha-1 yr-1 in the Nyungwe pristine forest and Eucalyptus plantation, respectively. The contribution of leaf litter in the pristine forest was ca. 70 and 79% inNyungwe and the Eucalyptus plantation, respectively. Litterfall peaked in the major (July - August) and minor (December -January) dry seasons and at the onset of the rainy season (September - October). In the pristine forest, the initial leaf litter decay rate was highest for Cleistanthus polystachyus leaf litter (0.033 day-1), followed by the forest litter mixture (0.016 day-1),and it was lowest for Parinari excelsa (0.0094 day-1). The final decay rates of Cleistanthus polystachyus, Carapa grandiflora and Eucalyptus litter mixture were similar (0.0014, 0.0013 and 0.0017 day-1) and lower than the final decay rate of forest litter mixture (0.0021 day-1). Decay rates could be related to litter properties such as N, lignin, Ca and polyphenol content. Mixing litter species caused a negative additive effect on the initial decay rate, while a positive additive effect was observed on the final decay rate in the pristine forest stand. Taken together, mixed-species litter showed increased mass loss compared to the expected weighed-based mass loss from the individual litter types in the mixture. Finally, stand type only affected the final decay rate of the forest litter mixture (PE+CP+CG) that was lower in the Eucalyptus than in the pristine forest and is suggested to be caused by reduced forest floor humidity. The average incident rainfall over two years was 2520±23 mm yr-1, but the canopy interception was higher in the pristine forest (43%) than in the Eucalyptus plantation (30%). The annual input of NH4+-N, NO3--N, Na+, K+, Ca2+, Mg2+ and Cl- via rainfall was 2.80, 3.61, 3.84, 12.03, 5.66, 2.08 and 5.07 kg ha-1, respectively. Fluxes of NH4+-N and NO3--N were within the range observed for other mountain rain forests; with NH4+ partly retained by the canopy at both sites, and NO3- released by the pristine forest canopy but retained by the Eucalyptus plantation canopy. Cations (Na+, K+, Ca2+ and Mg2+) were released by both canopies but to a larger extent in the pristine forest than in the Eucalyptus plantation except for Na+. In the rooting zone, NH4+, NO3- and other base cations were absorbed while NO3-was leaching from the top soil. NH4+ was preferentially absorbed above NO3-. Inorganic N losses by leaching were 49% of the total thoughfall input in pristine forest while in the Eucalyptus plantation 60% more than the total thoughfall input was lost, for which NO3-and NH4+represented 94 and 6 % of the total loss in the pristine forest, respectively, and 79 and 21% in the Eucalyptus plantation respectively. The total amount of inorganic N leaving the pristine forest catchment by stream water was 20.8 kg N ha-1 yr-1. Isotope composition measurements showed that NO3- in throughfall was mainly from atmospheric deposition while in humus percolation, soil solution and river water it was mainly originated from soil N processes. 18O-NO3- values in the river water ranged between 10.2 and 20.8‰, confirming that the source of NO3- in the river water was mainly soil N and only partly atmospheric NO3-. High N mineralization is followed by high nitrification rates, with the produced NO3- readily lost to the environment. The 15N labeling experiment showed very rapid 15NO3- enrichment following 15N labeling of the NH4+ pool indicating a fast transfer of NH4+ to NO3-. The investigated tropical forest soil showed two distinct NH4+ oxidations pathways: a slow one by autotrophic nitrifiers and a fast one coupled to iron reduction (Feammox). The gross rate of Feammox was of similar magnitude as nitrification, moreover the obtained Feammox rate approximate that obtained in slurry incubation of tropical forests soils after addition of NH4+ and Fe(III). The forest soil showed a high ratio of nitrification to NH4+immobilization characteristic of an open N cycle with high risk of NO3- losses. Nyungwe pristine forest soil is characterized by an open N cycle in which ammonium (NH4+.) produced by Feammox is a major important N transformation pathway and plant N uptake is dominated by NH4+

Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda

Globally, tropical forest soils represent the second largest source of N2O and NO. However, there is still considerable uncertainty on the spatial variability and soil properties controlling N trace gas emission. Therefore, we carried out an incubation experiment with soils from 31 locations in the Nyungwe tropical mountain forest in southwestern Rwanda. All soils were incubated at three different moisture levels (50, 70 and 90 % water filled pore space (WFPS)) at 17 °C. Nitrous oxide emission varied between 4.5 and 400 μg N m−2 h−1, while NO emission varied from 6.6 to 265 μg N m−2 h−1. Mean N2O emission at different moisture levels was 46.5 ± 11.1 (50 %WFPS), 71.7 ± 11.5 (70 %WFPS) and 98.8 ± 16.4 (90 %WFPS) μg N m−2 h−1, while mean NO emission was 69.3 ± 9.3 (50 %WFPS), 47.1 ± 5.8 (70 %WFPS) and 36.1 ± 4.2 (90 %WFPS) μg N m−2 h−1. The latter suggests that climate (i.e. dry vs. wet season) controls N2O and NO emissions. Positive correlations with soil carbon and nitrogen indicate a biological control over N2O and NO production. But interestingly N2O and NO emissions also showed a positive correlation with free iron and a negative correlation with soil pH (only N2O). The latter suggest that chemo-denitrification might, at least for N2O, be an important production pathway. In conclusion improved understanding and process based modeling of N trace gas emission from tropical forests will benefit from spatially explicit trace gas emission estimates linked to basic soil property data and differentiating between biological and chemical pathways for N trace gas formation

Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda [Discussion paper]

Globally, tropical forest soils represent the second largest source of N2O and NO. However, there is still considerable uncertainty on the spatial variability and soil properties controlling N trace gas emission. To investigate how soil properties affect N2O and NO emission, we carried out an incubation experiment with soils from 31 locations in the Nyungwe tropical mountain forest in southwestern Rwanda. All soils were incubated at three different moisture levels (50, 70 and 90% water filled pore space (WFPS)) at 17 °C. Nitrous oxide emission varied between 4.5 and 400 μg N m−2 h−1, while NO emission varied from 6.6 to 265 μg N m−2 h−1. Mean N2O emission at different moisture levels was 46.5 ± 11.1 (50% WFPS), 71.7 ± 11.5 (70% WFPS) and 98.8 ± 16.4 (90% WFPS) μg N m−2 h−1, while mean NO emission was 69.3 ± 9.3 (50% WFPS), 47.1 ± 5.8 (70% WFPS) and 36.1 ± 4.2 (90% WFPS) μg N m−2 h−1. The latter suggests that climate (i.e. dry vs. wet season) controls N2O and NO emissions. Positive correlations with soil carbon and nitrogen indicate a biological control over N2O and NO production. But interestingly N2O and NO emissions also showed a negative correlation (only N2O) with soil pH and a positive correlation with free iron. The latter suggest that chemo-denitrification might, at least for N2O, be an important production pathway. In conclusion improved understanding and process based modeling of N trace gas emission from tropical forests will not only benefit from better spatial explicit trace gas emission and basic soil property monitoring, but also by differentiating between biological and chemical pathways for N trace gas formation

Soil geochemistry – and not topography – as a major driver of carbon allocation, stocks, and dynamics in forests and soils of African tropical montane ecosystems

The lack of field-based data in the tropics limits our mechanistic understanding of the drivers of net primary productivity (NPP) and allocation. Specifically, the role of local edaphic factors - such as soil parent material and topography controlling soil fertility as well as water and nutrient fluxes - remains unclear and introduces substantial uncertainty in understanding net ecosystem productivity and carbon (C) stocks. Using a combination of vegetation growth monitoring and soil geochemical properties, we found that soil fertility parameters reflecting the local parent material are the main drivers of NPP and C allocation patterns in tropical montane forests, resulting in significant differences in below- to aboveground biomass components across geochemical (soil) regions. Topography did not constrain the variability in C allocation and NPP. Soil organic C stocks showed no relation to C input in tropical forests. Instead, plant C input seemingly exceeded the maximum potential of these soils to stabilize C. We conclude that, even after many millennia of weathering and the presence of deeply developed soils, above- and belowground C allocation in tropical forests, as well as soil C stocks, vary substantially due to the geochemical properties that soils inherit from parent material

Seasonality, drivers, and isotopic composition of soil CO2 fluxes from tropical forests of the Congo Basin

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 mu mol 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 delta C-1(3) 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 C-13 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

Contrasting nitrogen fluxes in African tropical forests of the Congo Basin

The observation of high losses of bioavailable nitrogen (N) and N richness in tropical forests is paradoxical with an apparent lack of N input. Hence, the current concept asserts that biological nitrogen fixation (BNF) must be a major N input for tropical forests. However, well-characterized N cycles are rare and geographically biased; organic N compounds are often neglected and soil gross N cycling is not well quantified. We conducted comprehensive N input and output measurements in four tropical forest types of the Congo Basin with contrasting biotic (mycorrhizal association) and abiotic (lowland-highland) environments. In 12 standardized setups, we monitored N deposition, throughfall, litterfall, leaching, and export during one hydrological year and completed this empirical N budget with nitrous oxide (N2O) flux measurement campaigns in both wet and dry season and in situ gross soil N transformations using N-15-tracing and numerical modeling. We found that all forests showed a very tight soil N cycle, with gross mineralization to immobilization ratios (M/I) close to 1 and relatively low gross nitrification to mineralization ratios (N/M). This was in line with the observation of dissolved organic nitrogen (DON) dominating N losses for the most abundant, arbuscular mycorrhizal associated, lowland forest type, but in contrast with high losses of dissolved inorganic nitrogen (DIN) in all other forest types. Altogether, our observations show that different forest types in central Africa exhibit N fluxes of contrasting magnitudes and N-species composition. In contrast to many Neotropical forests, our estimated N budgets of central African forests are imbalanced by a higher N input than output, with organic N contributing significantly to the input-output balance. This suggests that important other losses that are unaccounted for (e.g., NOx and N-2 as well as particulate N) might play a major role in the N cycle of mature African tropical forests

Leaky nitrogen cycle in pristine African montane rainforest soil

Many pristine humid tropical forests show simultaneously high nitrogen (N) richness and sustained loss of bioavailable N forms. To better understand this apparent upregulation of the N cycle in tropical forests, process-based understanding of soil N transformations, in geographically diverse locations, remains paramount. Field-based evidence is limited and entirely lacking for humid tropical forests on the African continent. This study aimed at filling both knowledge gaps by monitoring N losses and by conducting an in situ N-15 labeling experiment in the Nyungwe tropical montane forest in Rwanda. Here we show that this tropical forest shows high nitrate (NO3-) leaching losses, confirming findings from other parts of the world. Gross N transformation rates point to an open soil N cycle with mineralized N nitrified rather than retained via immobilization; gross immobilization of NH4+ and NO3- combined accounted for 37% of gross mineralization, and plant N uptake is dominated by ammonium (NH4+). This study provided new process understanding of soil N cycling in humid tropical forests and added geographically independent evidence that humid tropical forests are characterized by soil N dynamics and N inputs sustaining bioavailable N loss

Afrotropical secondary forests exhibit fast diversity and functional recovery, but slow compositional and carbon recovery after shifting cultivation

Questions Human disturbance is increasingly affecting forest dynamics across the tropics. Forests can recover via natural secondary succession to pre-disturbance levels of biodiversity, species composition, and ecosystem carbon stocks. Central Africa will be subject to increasingly high shifting-cultivation pressure in the next decades, but succession trajectories of these ecosystem properties are still poorly known for the Congo basin. We addressed two questions: (1) how does taxonomic and functional composition and diversity shift during secondary succession; and (2) how fast do above-ground carbon stocks recover during secondary succession in tropical forests? Location Central Congo basin. Methods We conducted an inventory of trees (diameter at breast height [DBH] >= 10 cm), measured species traits and soil texture and carbon content in 18 plots, located along six secondary succession stages (i.e., from agricultural to old growth forest sites). We measured tree diameter, height for 20% of trees distributed across diameter classes, wood traits from all species, and leaf traits from species that contributed to 85% of the plot basal area. Results We showed that secondary forests recover relatively fast in terms of tree species diversity, alpha functional diversity, and fine-root carbon, with near-old-growth forest values after six decades past disturbance, while floristic composition exhibited slower recovery. Secondary forests only partially shifted from acquisitive to a conservative life history, with shifts in leaf traits being largely decoupled from wood traits. Only 43% of above-ground carbon recovered after 60 years of forest regrowth, potentially through a slow recovery of the large-sized tree stems that dominate carbon stocks of old-growth forests. Conclusions Our findings underline the capacity of Afrotropical forests to recover species and alpha functional diversity after clear-cutting through shifting cultivation. Simultaneously, old-growth forests harbor a particular floristic community and store a large quantity of carbon with much longer recovery trajectories, stressing the need for conservation of these forests in the Congo basin

Soil geochemistry – and not topography – as a major driver of carbon allocation, stocks, and dynamics in forests and soils of African tropical montane ecosystems

The lack of field-based data in the tropics limits our mechanistic understanding of the drivers of net primary productivity (NPP) and allocation. Specifically, the role of local edaphic factors - such as soil parent material and topography controlling soil fertility as well as water and nutrient fluxes - remains unclear and introduces substantial uncertainty in understanding net ecosystem productivity and carbon (C) stocks. Using a combination of vegetation growth monitoring and soil geochemical properties, we found that soil fertility parameters reflecting the local parent material are the main drivers of NPP and C allocation patterns in tropical montane forests, resulting in significant differences in below- to aboveground biomass components across geochemical (soil) regions. Topography did not constrain the variability in C allocation and NPP. Soil organic C stocks showed no relation to C input in tropical forests. Instead, plant C input seemingly exceeded the maximum potential of these soils to stabilize C. We conclude that, even after many millennia of weathering and the presence of deeply developed soils, above- and belowground C allocation in tropical forests, as well as soil C stocks, vary substantially due to the geochemical properties that soils inherit from parent material