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

(2015)
Author
Promoter
(UGent) and Jean Masamba Walangululu
Organization
Abstract
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+.

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Citation

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Chicago
Landry, Cizungu Ntaboba. 2015. “Input, Dynamics and Loss of Reactive Nitrogen in a Central African Tropical Mountain Forest and Eucalyptus Plantation”. Ghent, Belgium: Ghent University. Faculty of Bioscience Engineering.
APA
Landry, C. N. (2015). Input, dynamics and loss of reactive nitrogen in a central African tropical mountain forest and Eucalyptus plantation. Ghent University. Faculty of Bioscience Engineering, Ghent, Belgium.
Vancouver
1.
Landry CN. Input, dynamics and loss of reactive nitrogen in a central African tropical mountain forest and Eucalyptus plantation. [Ghent, Belgium]: Ghent University. Faculty of Bioscience Engineering; 2015.
MLA
Landry, Cizungu Ntaboba. “Input, Dynamics and Loss of Reactive Nitrogen in a Central African Tropical Mountain Forest and Eucalyptus Plantation.” 2015 : n. pag. Print.
@phdthesis{5833365,
  abstract     = {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{\textdegree}15' -- 2{\textdegree}55' S, 29{\textdegree}00'-- 29{\textdegree}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{\textdegree}) 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 {\textquoteleft}virtual soil core{\textquoteright} 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{\textpm}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. \unmatched{f064}18O-NO3- values in the river water ranged between 10.2 and 20.8{\textperthousand}, 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+.},
  author       = {Landry, Cizungu Ntaboba},
  isbn         = {9789059897731},
  language     = {eng},
  pages        = {XIII, 159},
  publisher    = {Ghent University. Faculty of Bioscience Engineering},
  school       = {Ghent University},
  title        = {Input, dynamics and loss of reactive nitrogen in a central African tropical mountain forest and Eucalyptus plantation},
  year         = {2015},
}