Climate change, snow cover and microbial diversity in mountain soils: critical processes during snowmelt
Final Report Abstract
In alpine zones, soil microbes are strongly dependent on the annual patterns of snowpack formation: it is under the insulating winter snowpack that the highest microbial biomass is found, and according to current models, it is expected to drastically decrease at thaw. Global warming is already reducing the alpine snow cover, increasing the probability of frozen soils. Understanding the spring dynamics of the soil microbiome and how it is affected by climate warming is essential for establishing predictive scenarios. To this end, we conducted: i) a survey along altitudinal transects in three mountains in the Alps, in spring at snowmelt and in summer, recorded vegetation and topographic, climatic and edaphic parameters for 158 soil samples; ii) a snow manipulation experiment during the winter, to mimic the effects of climate warming by reducing or compacting the snow cover, while measuring the greenhouses gases released in the soil. By using metatranscriptomics, we simultaneously assessed prokaryotic and eukaryotic communities, further classified into nutrition guilds. At thaw, contrary to the current models, we observed an increase of microbial biomass during and immediately after snowmelt, as well as an increase from spring to summer, and no drastic changes in microbial communities. We thus suggest that the continuous growth of soil microbes from spring to summer was due to the soil conditions created by the winter insulating snow pack (most soil temperatures stayed above 0 °C) and supported by increasing amounts of dissolved carbon. Our first published results underline the importance of the insulating effect of the seasonal snow cover for the microbial dynamics in spring, contradicting current models based on soils that were frozen most of the winter. It is on the community dynamics from spring to summer that we have then focussed, revealing: (i) that biotic interactions could explain more variation of the microbial communities than topographic and edaphic variables, more for consumers than for preys, and this effect was stronger in summer than in spring; (ii) a seasonal dynamic in biotic interactions: the consumers' pressure on preys increases from spring to summer, resulting in a higher diversity and evenness of preys. We have shown that in alpine grasslands, consumers effectively contribute to maintain the diverse soil bacterial and fungal community essential for ecosystem functioning. In our snow manipulation experiment, we measured a progressive build-up of greenhouse gases (N2O, CO2 and CH4) during the winter, while O2 levels were low in the plots where the treatment of reducing the snow cover resulted in a frozen soil. Just before and during snowmelt, the values in the plots with removed snow became similar to that of the control plots. Our study stands out by applying metatranscriptomics to a large-scale ecological assessment of the entire soil microbiota. We reached a sequencing depth overcoming the descriptive limits of classical amplicon-based approaches and making inter-domain and intersample comparisons possible. We have complemented our protocol with trait-based approaches to enhance the basic knowledge of the soil food web functioning. We think that we have provided a substantial contribution to the fundamental understanding of spring and spring to summer soil microbial dynamics in the mountains, in particular by highlighting the role of biotic interactions.
Publications
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Biogeochemical dynamics during snowmelt and in summer in the Alps. Biogeochemistry, 162(2), 257-266.
Rindt, Oscar; Rosinger, Christoph; Bonkowski, Michael; Rixen, Christian; Brüggemann, Nicolas; Urich, Tim & Fiore-Donno, Anna Maria
