Nwe, Thu Zar (2023). Disentangling the mechanisms by which nitrogen addition affects soil communities and their functioning. (Thesis). Universität Bern, Bern

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Abstract

This thesis aims to explore the impact of nitrogen enrichment on soil communities and their functional dynamics. Specifically, we have investigated the direct effect of nitrogen enrichment on soil community and functioning and indirect effects mediated by various factors such as plant diversity, functional composition, and fungal pathogens. We addressed these aims with a large-scale grassland PaNDiv experiment. The experiment involved manipulating nitrogen addition, fungicide application to exclude fungal pathogens, and the functional composition and diversity of the plant communities. We collected various measurements, including plant biomass (carbon and nitrogen ratios, calcium content and acid detergent fiber) and leaf traits such as the community weighted mean specific leaf area (CWM SLA) and the community weighted mean dry matter content of the leaf (CWM LDMC), providing information on litter quality, quantity, and diversity, and above- and below-ground biomass (shoot and root), as well as plant cover, to represent microclimate effects. Moreover, we measured foliar fungal infection, soil fungal biomass, soil respiration, and soil available N, as well as feeding rate using bait-lamina strips. Our investigation focuses on three key aspects: 1. The direct and indirect effects of nitrogen addition on the temporal variation and stability of extracellular enzymes associated with the carbon and phosphorus cycles (see Chapter 2). 2. The direct and plant mediated indirect effects of N on the abundance of Collembola (Springtail) and on their mandible traits and feeding strategies (size and feeding mode) (see Chapter 3). 3. The direct and plant mediated indirect effects of N on the abundance and trophic groups of mites (Acari). Here we also addressed various further drivers, such as microclimate and proxies of soil microbial communities (see Chapter 4). In Chapter 2, we investigate the effects of nitrogen enrichment on two soil enzymatic activities produced by plants and microbes: β-1,4-glucosidase (related to carbon cycling) and acid phosphatase (related to phosphorous cycling). The relative importance of the direct effect of N enrichment in the change of enzymatic activities remains unknown, as well as the importance of these various indirect effects mediated by plant diversity, functional composition, or foliar pathogens. We quantified the effects of the richness, functional diversity and composition of plants, together with N addition, and fungicide application on mean levels of enzymatic activities across the year and on the temporal stability of enzymatic activities. In Chapter 3, we investigate the relationship between plant community characteristics and Collembola (springtails) abundance and functional diversity and composition. We measured various parameters related to plants and Collembola, including Collembola mandible traits relating to their feeding strategy and plant biomass and leaf traits, to evaluate trait matching between plants and Collembola. In addition, we tested whether relationships between plant and collembola traits varied depending on changes in microbial communities and nutrient pools, resulting from nitrogen addition or fungicide application. Furthermore, we aimed to understand the organization of Collembola functional traits into feeding strategies. In Chapter 4, we explore the impact of nitrogen and the structure of the plant community on soil mites. We classified mites into trophic groups and analyzed adult and juvenile populations. We used a structural equation model (SEM) to assess the indirect effects of treatment on mite abundance through drivers such as litter quality, belowground plant biomass, plant cover, soil nutrients, and above and belowground microbial abundance. In Chapter 5, the thesis concludes by summarizing the most significant findings and drawing overall conclusions. Nitrogen addition has the potential to increase the abundance of soil communities. However, it may also have a negative effect on their functioning by directly altering soil pH or stoichiometry. We test for these more direct effects by adding N in the PaNDiv experiment. Furthermore, nitrogen addition can alter both the abundance and functioning of soil communities by changing the plant community. Changes in plant functional composition due to nitrogen addition can lead to shifts in soil community abundance. Nitrogen can cause slow-growing (low specific leaf area) species to be replaced by faster-growing (high specific leaf area) species. This shift in plant composition can have an impact on litter-feeding fauna, by altering the type of litter (soft or tough). The specific effect of this shift on soil fauna functioning can vary, resulting in either a positive or negative outcome. We test for this indirect effect by manipulating the fast-slow trait composition of the plant communities in PaNDiv and by measuring multiple aspects of litter quality that might affect soil fauna (shown in the two interlinked circles). A loss of plant diversity following N addition would be expected to reduce the abundance and diversity of soil organisms, as plant diversity should typically benefit soil organisms and their functioning. We test for this effect by manipulating plant species richness and fast-slow functional diversity in PaNDiv. In addition, an increase in plant biomass production above and belowground should increase soil faunal abundance. We test for this by using measuring of shoot and root biomass from the PaNDiv plots in the analyses. Finally, changes in phyllosphere microbial groups following Naddition could also impact the soil fauna, however, these effects are poorly understood. We test for this in PaNDiv by using fungicide to reduce pathogen infection aboveground.

Item Type:	Thesis
Dissertation Type:	Cumulative
Date of Defense:	16 August 2023
Subjects:	500 Science > 570 Life sciences; biology 500 Science > 580 Plants (Botany)
Institute / Center:	08 Faculty of Science > Department of Biology > Institute of Plant Sciences (IPS)
Depositing User:	Sarah Stalder
Date Deposited:	15 Jul 2025 16:05
Last Modified:	15 Jul 2025 22:25
URI:	https://boristheses.unibe.ch/id/eprint/5347

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