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Responses of net primary production and plant-available soil nutrients to elevated nutrient input in a tropical forest on highly weathered soil

Nutrient limitation of net primary production (NPP) in an African tropical forest

dc.contributor.advisorvan Straaten, Oliver Dr.
dc.contributor.authorManu, Raphael
dc.titleResponses of net primary production and plant-available soil nutrients to elevated nutrient input in a tropical forest on highly weathered soilde
dc.title.alternativeNutrient limitation of net primary production (NPP) in an African tropical forestde
dc.contributor.refereeVeldkamp, Edzo Prof. Dr.
dc.description.abstractengGlobally, tropical forests are highly productive ecosystems and play a critical role in sequestering anthropogenic carbon dioxide (CO2) from the atmosphere, accounting for up to half of the terrestrial biosphere’s carbon sink. For reasons not yet fully reconciled, there are indications that the carbon sink strength of these forests is slowly declining, thereby decreasing the buffering capacity that these forests offer in mitigating global climate change. It is recognized that ecosystem nutrient limitations play an important regulatory role in plant growth, therein affecting ecosystem carbon assimilation and specifically net primary production (NPP). Furthermore, the direction and magnitude of these limitations are poorly understood, especially in understudied African tropical forests, on highly weathered soils. This dissertation consists of three studies (Chapters 2−4) aimed at elucidating the mechanistic roles of nitrogen (N), phosphorus (P), potassium (K) and their interactions on different components of NPP (tree stem growth, fine litter production, foliar chemistry, and fine root production) and plant-available soil nutrients. Following a full factorial design, we established 32 (eight treatments × four replicates) experimental plots of 40 m × 40 m each in a semi-deciduous tropical forest in northwestern Uganda. We added N, P, K, their combinations (N+P, N+K, P+K, and N+P+K) and control at the rates of 125 kg N ha−1 yr−1, 50 kg P ha−1 yr−1 and 50 kg K ha−1 yr−1, split into four equal applications. In our first study, we measured tree growth responses among different tree sizes, taxonomic species, leaf habits, and at the community level to nutrient additions. After two years, the response of tree stem growth to nutrient additions was dependent on tree sizes, species, and leaf habits but not community-wide. First, tree stem growth increased under N additions, primarily among medium-sized trees (10−30 cm DBH) and in trees of Lasiodiscus mildbraedii in the second year of the experiment. Second, K limitation was evident in semi-deciduous trees, which increased stem growth by 46 % in +K than –K treatments, following a strong, prolonged dry season during the first year of the experiment. This highlights the key role of K in stomatal regulation and maintenance of water balance in trees, particularly under water-stressed conditions. Third, the role of P in promoting tree growth and carbon accumulation rates in this forest on highly weathered soils was rather not pronounced. Our results underscore the fact that, in a highly diverse forest ecosystem, multiple nutrients and not one single nutrient regulate tree growth and aboveground carbon uptake due to varying nutrient requirements and acquisition strategies of different tree sizes, species and leaf habits. For our second study, we assessed the effect of the nutrient additions on fine litter production and foliar quality. To do this, we placed four-leaf litter collectors (0.75 m × 0.75 m in size) at random locations in each experimental plot, emptied them for dry mass determination every two weeks for three consecutive years (May 2018−April 2021). Our data suggest that: (1) Although annual fine litter production was not significantly affected by nutrient additions in the short-term (3 years), an observed trend towards higher annual fine-litter production in the N addition plots may become stronger with continued nutrient additions. (2) Following a prolonged dry season in the first year of the experiment, leaf litterfall reduced significantly with P and K additions. This observed effects of K in leaf litterfall corroborate the increased stem growth among semi-deciduous trees in our first study and highlights the roles of K as well as P in maintaining water balance in trees, thereby ensuring stress tolerance during water-deficit conditions. (3) Both leaf litter and foliar nutrient contents were affected by the elevated availability of all three nutrients in both positive and negative directions but varied considerably among different tree species. Even though the long-term effects of nutrient perturbation on this ecosystem are yet to be known, the concept of multiple nutrients rather than a single nutrient regulation of litter production and foliar quality was supported in this second study. Our third study evaluated the effects of nutrient additions on below-ground processes including fine root biomass production and plant available soil nutrients. First, we quantified fine root biomass (0−10 cm soil depth) at the end of the first and second years of the experiment by excavating soil monoliths (20 cm × 20 cm) at six random locations within each plot. Next, fine root production in the top 30 cm soil depth was estimated using the sequential coring technique. We found that the addition of N reduced fine root biomass by 35% after the first year of the experiment whereas K addition was associated with reduced fine root production, suggestive of an alleviated N and K limitation in this site as found in our first study. This rapid reduction in fine root biomass in the N and K treatments supports the idea that trees will scale back their energy-intensive root network or production when they have adequate resources available. Additionally, nutrient additions resulted in a cascade of biochemical responses in the soil nutrient availability. Specifically, (1) Net N mineralization and nitrification rates were enhanced by the interaction effects of all three nutrients (N × P × K), highlighting the complementary roles of these nutrients in regulating plant and soil processes in this species-rich ecosystem. (2) Microbial biomass C increased with P additions but was dependent on the season (Wet or dry). Lastly, P additions increased plant-available P by 80%. This large increase indicates high P availability and explains the lack of plant growth response to P additions (as shown in our first study). Overall, this dissertation provides credence to the concept of multiple nutrients (co)regulation of NPP and other ecosystem processes; further substantiating the growing pool of evidence that productivity in tropical forests does not follow Liebig’s Law of the Minimum. Instead, resource and nutrient requirements (and their limitations) vary in different ecological processes or components of NPP in this forest, ranging from tree growth (N and K), leaf litterfall (P and K), root biomass, and production (N and K). These observations are indeed consistent with the multiple resource limitation theory. Considering that most large-scale experimental research to date has focused only on the roles of N and P availability in limiting plant productivity, our data show that other nutrients, specifically K, can be equally important in the functional and biochemical roles related to ecosystem carbon uptake. More such research is undoubtedly needed particularly for the African tropical region, which is the least researched
dc.contributor.coRefereeHölscher, Dirk Prof. Dr.
dc.contributor.thirdRefereeCorre, Marife Dr.
dc.subject.engNutrient limitationsde
dc.subject.engPlant growthde
dc.subject.engFertilization experimentde
dc.subject.engNutrient addition experimentde
dc.subject.engCarbon stockde
dc.subject.engNet primary productionde
dc.subject.engBudongo forestde
dc.subject.engTropical forestsde
dc.subject.engUgandan tropical forestde
dc.subject.engAfrican tropical forestde
dc.subject.engFoliar nutrient concentrationsde
dc.subject.engLeaf litterde
dc.subject.engFine-litter productionde
dc.subject.engFine-root biomassde
dc.subject.engBiomass productionde
dc.subject.engRelative growth ratede
dc.affiliation.instituteFakultät für Forstwissenschaften und Waldökologiede
dc.subject.gokfullForstwirtschaft (PPN621305413)de

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