Reindeer control over subarctic treeline alters soil fungal communities with potential consequences for soil carbon storage

Abstract

The climate‐driven encroachment of shrubs into the Arctic is accompanied by shifts in soil fungal communities that could contribute to a net release of carbon from tundra soils. At the same time, arctic grazers are known to prevent the establishment of deciduous shrubs and, under certain conditions, promote the dominance of evergreen shrubs. As these different vegetation types associate with contrasting fungal communities, the belowground consequences of climate change could vary among grazing regimes. Yet, at present, the impact of grazing on soil fungal communities and their links to soil carbon have remained speculative. Here we tested how soil fungal community composition, diversity and function depend on tree vicinity and long‐term reindeer grazing regime and assessed how the fungal communities relate to organic soil carbon stocks in an alpine treeline ecotone in Northern Scandinavia. We determined soil carbon stocks and characterized soil fungal communities directly underneath and >3 m away from mountain birches (Betula pubescens ssp. czerepanovii) in two adjacent 55‐year‐old grazing regimes with or without summer grazing by reindeer (Rangifer tarandus). We show that the area exposed to year‐round grazing dominated by evergreen dwarf shrubs had higher soil C:N ratio, higher fungal abundance and lower fungal diversity compared with the area with only winter grazing and higher abundance of mountain birch. Although soil carbon stocks did not differ between the grazing regimes, stocks were positively associated with root‐associated ascomycetes, typical to the year‐round grazing regime, and negatively associated with free‐living saprotrophs, typical to the winter grazing regime. These findings suggest that when grazers promote dominance of evergreen dwarf shrubs, they induce shifts in soil fungal communities that increase soil carbon sequestration in the long term. Thus, to predict climate‐driven changes in soil carbon, grazer‐induced shifts in vegetation and soil fungal communities need to be accounted for.

Reindeer grazing history determines the responses of subarctic soil fungal communities to warming and fertilization

“…These increased temperatures are known to be accompanied by other microclimatic changes, such as decreased wind speed and soil moisture and changes in humidity (Bjorkman et al, 2020). In our experiment, soil temperatures at 3 cm depth (n = 3, EasyLog EL-USB-1; Lascar Electronics, Whiteparish, UK) did not differ depending on the warming treatment in 2013 (Yl€ anne et al, 2020), but the surface temperature in 2012 was c. 1.4°C higher in the warmed plots under both grazing intensities (V€ ais€ anen et al, 2013). Fertilization was implemented by dissolving ammonium nitrate equivalent to 10 g N m À2 to 1 l of local water and applying it on the plots at the time of bud burst (16 June 2010, 9 June 2011, 28 June 2012, 4 June 2013.…”

“…Mack et al, 2004). In 2013, fertilization increased soil mineral N content over seven-fold under LG and four-fold under HG (Yl€ anne et al, 2020).…”

“…Previous studies using the same experimental set-up have shown substantial ecosystem-level impact of experimental treatments that depend on each other and grazing intensity (Yl€ anne et al, 2020). Under both grazing intensities, warming generally decreased soil moisture and soil mineral N availability next to increasing vegetation biomass, and both warming and fertilization increased the activities of several extracellular enzyme activities responsible for catalyzing organic matter degradation (Yl€ anne et al, 2020). In addition, in 2014, 1 yr after the present investigation, warming had decreased soil C by 22% under both grazing intensities in unfertilized plots but had no effect on soil C in fertilized plots (Yl€ anne et al, 2020).…”

“…Under both grazing intensities, warming generally decreased soil moisture and soil mineral N availability next to increasing vegetation biomass, and both warming and fertilization increased the activities of several extracellular enzyme activities responsible for catalyzing organic matter degradation (Yl€ anne et al, 2020). In addition, in 2014, 1 yr after the present investigation, warming had decreased soil C by 22% under both grazing intensities in unfertilized plots but had no effect on soil C in fertilized plots (Yl€ anne et al, 2020). Additionally, soil C stocks decreased as soil fungal : bacterial ratio increased under both grazing intensities (Yl€ anne et al, 2020).…”

“…Under both grazing intensities, fertilization increased soil moisture. Yet, when fertilization was combined with warming under LG it led to drier soils (Yl€ anne et al, 2020), potentially inhibiting saprotrophic activity.…”