Explosive volcanic eruptions can distribute ash and other rocky fragments (known collectively as tephra) on a continental scale. Tephra deposits constitute a significant natural hazard: as well as threatening human health, tephra smothers vegetation, alters hydrology and slope stability, and interrupts biogeochemical flows. Tephra deposits can remain unconsolidated – and potentially detrimental to ecosystems – for years. Understanding the response of ecosystems to tephra deposition is crucial for hazard mitigation, and long-term ecological restoration. The responses of aboveground (plant) communities to tephra deposition have been studied in some detail. In contrast, the effects on tephra deposition on the belowground communities (soil organisms) that underpin terrestrial ecosystem function are poorly understood. Furthermore, the factors that promote the stabilisation of tephra deposits have not been investigated before. This project addresses these knowledge gaps with an experimental study in Washington State, USA, centred on two key questions.
Q1: How do tephra deposits impact soil biology and biochemistry?
The immediate ecological impact of thick (> 0.5 m) tephra deposits is catastrophic. However, most tephra deposits are thin (< 0.1 m) and their impacts are less predictable. Even thin tephra deposits change the physical properties of land surface, altering surface albedo and hydrology, with knock-on effects for soil temperature and moisture levels. Tephra deposition may impact soil chemistry directly. For example, soils underlying tephra deposits are likely to receive a pulse of carbon (C) and nutrients from the decay of buried vegetation, and a deposit just a few cm thick will isolate the soil from aboveground sources of organic matter. Gas diffusion into the soil could be interrupted, with impacts for soil redox chemistry (e.g., onset of anaerobic conditions and increased production of acidic fermentation products, sulfides and methane). Tephra also has indirect impacts on soil chemistry, as stressful surface conditions will likely reduce net primary productivity (NPP) and belowground C inputs via root exudates.
Soil microbial communities (SMCs, primarily microscopic bacteria and fungi) are sensitive to changes in the soil environment. Thus, it is likely that tephra deposition will change the structure and function of these communities; e.g., the relative proportion of bacteria and fungi and overall microbial diversity. Changes in SMCs are likely to be accompanied by shifts in quantity and quality of soil C and soil respiration, as organic C is consumed by soil organisms, but not replaced by fresh inputs. It is important to understand these changes, because SMCs play a critical role in decomposition and the cycling of C and nutrients in terrestrial ecosystems; they also influence the resilience of these ecosystems to future disturbance.
Q2: What are the biotic and abiotic factors that influence the stabilisation of tephra deposits?
Fresh tephra deposits are often mobilised by the wind and slope processes, endangering human wellbeing and burying terrain unaffected by original deposit. Some tephra deposits can remain mobile for years, whilst others stabilise rapidly and are incorporated into soils without causing further disruption. The stabilisation of tephra deposits is understudied but is probably influenced by a combination of biotic and abiotic factors, e.g. climate, vegetation cover and NPP and tephra characteristics. It is likely that a) fine tephra will be more mobile than coarse material; b) thick deposits will be more mobile than thin layers (because they are less readily fixed by vegetation) and that areas with high NPP will be more favourable to stabilisation, due to the sheltering impact of vegetation and rapid burial by litter. Greater understanding of these factors would enhance efforts to predict (and mitigate) the impact of future tephra-producing eruptions.