Impacts of fire and herbivory on soil nutrient cycling and microbial communities in savanna ecosystems

Cvetkovic-Jones, Mary Jennifer and Stevens, Carly and Whitaker, Jeanette and Ostle, Nick (2024) Impacts of fire and herbivory on soil nutrient cycling and microbial communities in savanna ecosystems. PhD thesis, Lancaster University.

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Abstract

Abstract Savanna vegetation dynamics are driven by fire, herbivory, and climate, but their effects on soil properties are poorly understood. Specifically, the potential individual and interactive impacts of these drivers on the soil biogeochemical resilience remains unknown. Savanna ecosystems are increasingly vulnerable to global change and human activity, with predicted future changes in fire, herbivory, and climatic regimes. These drivers have the capacity to significantly alter savanna biodiversity and biogeochemical function, with cascading impacts on human welfare. The consequences of fire-herbivory interactions on soil are difficult to separate due to their synergistic nature, and understanding has been compounded by a lack of fire-herbivory experiments investigating both above and belowground processes. To address this gap, experiments were conducted in the lab and field within Kruger National Park, South Africa, across different rainfall gradients, fire intensities, soil types and herbivore presence. The effects of fire, herbivory and their interactions on soil carbon, nutrient concentrations, and microbial communities were quantified to determine the variability in savanna soil properties and test the biogeochemical resilience of the soils. There was limited evidence that changing fire or herbivory regimes had an effect on soil nutrients or microbial activity, particularly in sandy soils. In contrast, changes in soil nitrate and microbial community composition were observed following a single fire-herbivory event on clay soils. Overall, there was strong evidence that savanna soils are extremely resilient to fire and large mammal herbivory. These finding contribute to unravelling the complex mechanisms underlying fire-herbivory interactions in savanna ecosystems. They also underscore the unique nature of savanna ecosystems compared with temperate ecosystems, emphasizing the necessity for more focussed experiments in arid and tropical savannas to understand how both abiotic and biotic factors influence ecosystem functioning. Such understanding is crucial for predicting how global change will shape the future of savanna ecosystems.