Impacts of environmental change on soil nitrogen in emerging and degrading arid ecosystems

  • Thomas C Turpin-Jelfs

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Changes in climate and land use over the past ca. 150 years have led to environmental changes associated with primary succession and land degradation in water-limited ecosystems. As a microbiologically mediated process, the cycling of nitrogen, the nutrient most limiting to primary production in water-limited ecosystems, may be perturbed by changes in external environmental parameters. Yet an understanding of how soil nitrogen responds to environmental changes between trajectories of succession and degradation in water-limited ecosystems is missing. From this research, it was determined that whilst the spatial distribution of soil nitrogen is generally influenced by surface cover, major differences arise between trajectories of succession and degradation. In a water-limited glacier forefield, total nitrogen increases with primary succession from ~85 µg g 1 in recently exposed soils to ~4,400 µg g 1 in 2,000 year old soils, which suggests such ecosystems will become more productive over time. Conversely, the redistribution of total nitrogen (~85 g m 2) from intershrub areas to soils beneath invasive shrub canopies in a water-limited grassland expedites grassland-to-shrubland transitions resulting in reduced pastoral productivity. Despite these differences, >98 % of nitrogen in these systems is organically bound. As this is indicative of nitrogen limitation, the capacity for biological nitrogen fixation should represent a selective advantage. Using the acetylene reduction assay technique, potential rates of asymbiotic biological nitrogen fixation were found to be largely modulated by inorganic nitrogen-to-bioavailable phosphorus ratios and decline from ~0.2 g N m 2 a 1 in a glacier forefield and ~2 g N m 2 a 1 in a shrub-encroached grassland with the accumulation of total nitrogen. The large fraction of organically bound nitrogen suggests the accumulation of organic matter is a significant driver of microbial activity in water-limited ecosystems. When considering the global extent of water-limited environments, changes in soil nitrogen pools within these ecosystems have implications for the global nitrogen cycle.
Date of Award1 Oct 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorKaterina Michaelides (Supervisor) & Alexandre M B Anesio (Supervisor)

Keywords

  • Nitrogen fixtion
  • Nitrogen cycle
  • Glaciers
  • Drylands
  • Environmental change
  • Soil

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