Volcanogenic Sulfate and Fluoride deposition: Effects on organic and mineral soil gas fluxes and chemistry

This thesis focuses on the environmental effects of volcanic eruptions such as Eyjafjallajökull (2010) from which volcanic gases and ash particles can impact upon ecosystems located thousands of kilometres from the source. Currently very little is known about the impact of volcanic pollutants such as SO4 and F on the carbon cycle. This study is a first step towards understanding the potential environmental impacts of volcanic eruptions on peatland and mineral soil C gas fluxes.
Ombrotrophic peat mesocosms sampled from the Northern Peninne uplands, UK, were dosed over 20 weeks with concentrations of SO4-S (24.5 kg ha-1) and F (13.5 and 135 kg ha-1) simulating a distal Icelandic tephra deposit. Methane and CO2 gas fluxes were measured at regular intervals, but no significant differences were observed for any of the treatments when compared to the controls. This result contrasts with previous studies, which reported a suppression of CH4 emission with the addition of SO4. It can be explained if CH4 production has remained suppressed in the peat soils as a long-term consequence of heavy SO4 loadings in the Pennines area prior to the reduction of SO2 emissions from industrial sources in the 1970s. The mesocosm study results indicate that F deposition, at rates representative of tephra fallout does not interfere with C gas fluxes in peat soils, despite the well-established toxicity of F in the environment. However, F addition to a pristine peat soil in laboratory slurry experiments showed an increase in potential CH4 production rates thus further research is recommended.
Addition of treatments containing high concentrations of F to peat mesocosms had a significant effect on soil solution chemistry. The addition of F increased the solubility of Al, Fe and acetate resulting in the accumulation of both species in solution near the peat surface. This build up of acetate, Al and Fe over the treatment period suggests that F breaks down organo-metallic compounds causing leaching of organic matter along with metal ions. This may have important implications for microbial communities within the peat that are associated with decomposition of organic matter and carbon cycling.
After the 2010 eruption of Eyjafjallajökull, Iceland, a field experiment was carried out to assess the impact of tephra deposition on soil respiration from a grassland site. The results showed that the chemical effect of ash leaching resulted in a 30% reduction in ecosystem respiration. This study also highlighted the short-term physical effects of tephra deposition on the release of CO2 from soil as the tephra layer impeded CO2 release when wet.
This work provides a useful contribution to the scientific understanding of the effects of volcanic SO4 and F on peatland ecosystems and the physical and chemical effects of ash deposition on soil respiration. Consideration of the impact of volcanic deposition on soil C fluxes in climate models is required in order to be able to fully appreciate how volcanism causes environmental changes.