Deep soil ectomycorrhizal communities of beech forests

Ectomycorrhizal fungi (EMF) form symbioses with trees and provide key services related to nutrients

and water uptake that are essential for tree health. The symbiosis is increasingly in focus because of

concerns about forest health due to climate change, in particular tree resilience to drought. Deep

routing plants are typically more resistant to periods of drought and their root distribution patterns

across different soil depths is well understood. In contrast, little is known about EMF species and their

functioning in nutrient and water supply in deep soil layers. Furthermore, EMF functioning is often

compromised due to additional environmental stress, for example nitrogen deposition, soil

acidification or drought. EMF distributions and investigations on environmental effects in deep soil

layers are notoriously difficult to study and therefore, current knowledge is primarily limited to near-

surface horizons, mainly down to 10 cm or 25 cm soil depth. This project is specifically ‘digging deeper’

down to 100 cm and takes advantage of the long-term forest observation monitoring network

consisting of 21 beech forest sites in Switzerland. The network exists since 1984 and covers large

gradients in nitrogen deposition, soil chemistry and drought with a wealth of meta-data, including tree

vitality, for holistic interpretation. The key experimental resource, however, is the archive of 2100 EMF

samples from soil cores collected from layers down to 100 cm. Here, we make use of improvements in

DNA extraction from small amounts of EMF mycelium and highly accurate long-read sequencing to

characterise the EMF communities in deep soils. Half the samples from the archive consist of EMF

mycelium collected from so called ingrowth mesh bags and the other half are root tip samples collected

at the same depths. The ingrowth mesh bags function as ‘enrichment traps’ that enable sampling of

pure fungal mycelium and, therefore, provide a unique opportunity to specifically study deep soil fungi.

With this project we will characterize the deep soil EMF communities at high resolution and uncover

their abundance patterns in response to drought, N deposition and tree physiology. Furthermore, we

make use of the enrichment of EMF DNA within mesh bags to apply shotgun metagenomics. This will

permit novel and direct functional insights into deep soil EMF communities, advancing our

understanding of the role of EMF symbiosis in efficient exploitation of deeply located nutrient and

water resources for improved tree health.