Abstract
Plant communities affect the soil subsystem and vice-versa. Anthropogenic management activities impact both plants and soil, thereby interfering with the co-dependence between plant, soil, and microbial components. Land management practices are known to interfere with both above and belowground components of terrestrial ecosystems, but we still know very little about
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how land management impacts the combination of aboveground and belowground sub-systems and their linkages with each other. In this thesis, I focus on identifying potential land uses that induce or increase the co-dependence between plants and soil microorganisms by evaluating (1) successional changes in plant community assemblages in both taxonomic and functional composition; and (2) the effects of anthropogenic management on above- and belowground interactions between plant biomass and soil communities in two types of land-use systems in the Amazon forest: natural fallow after slash-and-burn agriculture and agroforestry systems. In this thesis, I took advantage of many different approaches (plant spatial arrangement, estimation of aboveground biomass, analysis of soil physico-chemical properties, and soil metagenomics) to investigate the influence of land use and anthropogenic management on aboveground and belowground communities and their interactions. With respect to plant-plant interactions (Chapter 2), I investigated successional dynamics using a chronosequence approach covering 2-25 years of regeneration after first-cycle shifting cultivation and compare these with mature rainforests. Additionally, I considered the impact of land use intensification with three second-cycle slash-and-burn forests (‘degraded’ sites). The results revealed that land-use intensification (2nd-cycle regrowth) strongly affected all aspects of the spatial organization of secondary vegetation, increasing clustering, co-occurrences of functionally distinct plants, and systematic increase in focal species’ negative impacts on surrounding legume diversity. Thus, land-use intensification affects spatial organization of self-regenerative vegetation far more than first-cycle secondary succession does. Therefore, a second-cycle of slash-and-burn agriculture is enough to induce a stronger clustering of leguminous plants at both univariate and bivariate interactions. When looking into plant-soil relationships (Chapter 3), I evaluated three types of rainforests (young, old, and mature rainforest) and three different agroforestry systems (enriched fallow, commercial plantation, and homegardens). I found a strong impact of agroforestry systems on both soil factors and aboveground biomass. Interestingly, I observed an increasing co-dependence between soil factors and plant biomass in successional systems, but not in the agroforests. These results suggested that agricultural practices in the agroforests disrupt the co-dependence between aboveground-belowground interactions. Finally, I added the soil microbiome to the picture of aboveground-belowground interactions by taking advantage of advanced statistical methods (reviewed in Chapters 4). The results revealed a fungal-driven relationship along succession and in mature forest (Chapter 5). On the other hand, agroforestry systems exhibited a weaker co-dependence between plant biomass and soil factors, and their soil microbiomes appear to promote more specific bacterial populations as opposed to fungal taxa. In conclusion, integrating high-resolution genomic data with complex environmental data in observational studies is an important step towards identifying ecological differentiation and shifts in aboveground-belowground interactions.
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