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Toke Bang-Andreasen:
Circular resource flow of wood ash to soil

Date: 16-03-2018    Supervisor: Rasmus Kjøller & Regin Rønn

Recycling of wood ash from energy production can counteract soil acidification and return essential nutrients to soils. However, wood ash amendment also affects the soil physiochemical properties that control taxonomic composition and functional expression of the soil microbiome. This potentially poses a risk for soil quality and ecosystem functioning.

This PhD thesis is part of the ASHBACK project (www.ashback.dk) which addresses ecosystem responses to wood ash amendment. The aim of the thesis has been to determine taxonomic and functional dynamics in forest and agricultural soil microbiomes upon wood ash amendment. We investigated microbiome dynamics to increasing doses of wood ash in forest and agricultural soils using 16S rRNA gene amplicon sequencing and total RNA-sequencing (metatranscriptomics). Microbial abundance was determined using agar plating and qPCR on marker genes. We also applied fine-scaled vertical profiling of a top forest soil to investigate prokaryotic responses along a strong vertical wood ash gradient through the soil column.

Wood ash amendment caused strong taxonomic and functional soil microbiome responses. Increased soil pH and electrical conductivity were the strongest explanatory variables for the observed microbiome changes but increased soil concentrations of dissolved organic carbon, phosphate and exchangeable cations were also important. Bacterial numbers increased together with the relative abundance of copiotrophic bacterial groups, when wood ash amendment raised soil pH to neutral or slightly alkaline levels. Furthermore, increased expression of functional genes involved in cell growth and metabolism were observed with these pH increases. The response of fungi was generally less pronounced than that of bacteria. Bacterivorous protozoa became more dominant members of the active soil microbiome with ash amendment, which likely is a response to increased food source availability (i.e. the increased bacterial production).

Extreme wood ash doses (≥90 t ha-1) caused stress- and harmful conditions for the majority of soil indigenous microorganisms. This was illustrated by a community collapse in the agricultural soil (where pH increased to alkaline levels) and in the forest soil by decreased diversity, increased expression of stress response genes and a dramatic community shift towards dominance of endosporeforming Firmicutes.

The normal application of wood ash on the surface of a forest soil surface caused strong gradients of ash-induced changes in soil physiochemical parameters in the very top layers of the soil column.

Consequently, prokaryotic community shifts occurred within these top layers. The strongest community shifts were seen by the end of the experiment, one year after ash application. This suggests that ash amendment can cause persistent microbial responses just below soil surface.

In conclusion, wood ash induces strong taxonomic and functional microbiome responses that are mainly governed by increased soil pH and electrical conductivity. Copiotrophic microbial groups thrive upon wood ash amendment, on the expense of oligotrophic groups, and bacterial numbers are increased. However, extreme ash amendments (≥90 t ha-1) cause adverse effect on the soil microbiome.

Collectively, the work presented in this thesis improves our understanding of soil microbial responses to wood ash amendment, in terms of abundance, community composition and functioning.