Impact of reduced rainfall and warming on EMF physiology and forest soil carbon cycles

Global carbon (C) cycling plays a vital role in shaping planetary life and the climate system. One of these carbon pools, soil organic matter (SOM), has been estimated to contain twice the C as stored in terrestrial vegetation and the atmosphere combined. In forest ecosystems, a major contributor to SOM cycling are ectomycorrhizal fungal (EMF) species. EMF form symbiotic relationship with most tree species in the northern hemisphere where in exchange for plant-derived C, the fungi provide the host plants with improved access to nutrients and water. Despite their crucial role in ecosystem C cycling, we poorly understand how EMF functioning and mediation of C cycles will shift under a changing climate. To address this, our study investigated how EMF respiration, production, turnover, and biodiversity shift in response to simulated climate change in two Fagus sylvatica forests in northern Switzerland.

We employed an in-growth mesh approach across the growing season to track EMF physiological and biodiversity responses to experimentally reduced rainfall and increased temperatures, both alone and mixed. The lack of initial C in mesh bags allows us to focus on the growth of EMF hyphae in the bags without attracting other microbes such as saprotrophs that require a source of C to grow. After the incubation period of each mesh bag, we measured in situ respiration, and microbial biomass using phospholipid-derived fatty acids (PLFA). The biomass at different stages of the growing seasons enabled us to estimate fungal production and turnover rates. When integrated with EMF respiration measurements, this allowed us to model the EMF CO₂ flux and carbon use efficiency for the growing season while taking soil moisture and temperature into account. Further, using DNA metabarcoding, the fungal ITS region of samples were sequenced and analysed to provide a better understanding of fungal community structure.

Our initial results show that the climate treatments significantly shift EMF physiology and turnover. Drought had the strongest negative impact on EMF growth and respiration, but this ameliorated by concurrent warming, and it was linked to variation in host plant growth. We further discovered that EMF turnover over the growing season was not steady, with some samples showing signs of greater biomass loss than could be replaced in the later stages of the growing season. This could be due accumulation of necromass or other exudates over time that negatively feedback to impact EMF physiology. In conclusion, EMF have a critical role in forest soil C cycle which direct and indirectly impacts on ecosystem processes, such as their host plant performance under climate change.