Shrub and Sedge Rhizosphere Communities Display Distinct Affinities Toward Exudates and Soil Organic Matter Degradation: a Quantitative Stable Isotope Probing Analysis

ABSTRACT

Warming temperatures are accelerating permafrost thaw and changing tundra vegetation, where 34 woody shrubs are displacing sedges. Shrubs, such as Betula nana, and sedges, such as 35 Eriophorum vaginatum, exhibit distinct life strategies including unique root-associated, or 36 rhizosphere microbial communities. As permafrost thaws it unlocks previously unavailable 37 carbon and nutrient sources resulting in deeper roots and a translocation of rhizosphere 38 communities. Because permafrost microbial communities contain lower diversity and biomass 39 than rhizosphere communities, the coalescence of rhizosphere and permafrost microbial 40 communities could alter soil organic matter (SOM) degradation rates and increase greenhouse 41 gas emissions. To identify metabolic strategies across distinct rhizosphere and permafrost 42 microbial communities we conducted an isotope tracing incubation experiment. We inoculated 43 thawed permafrost with shrub and sedge rhizosphere communities while adding exudates or 44 water daily and compared this to an uninoculated control. After 46 days, we spiked samples with 18O enriched water or 13 45 C enriched exudates and measured isotope incorporation into microbial 46 DNA with quantitative stable isotope probing (qSIP). Our results indicate that exudate additions 47 had little effect on uninoculated permafrost communities but the addition of exudates and 48 rhizosphere inoculants had a compounding effect on respiration rates. We found that soils 49 inoculated with shrub rhizosphere communities contained a mixture of exudate and SOM 50 degraders while soils inoculated with sedge rhizosphere communities contained mainly SOM 51 degraders. Finally, we found that individual microbial taxa exhibited maximum growth rates in 52 one ‘environment’, which was a combination of microbial inoculant communities and exudate 53 addition treatments. Our results reveal that microbial niches are strongly influenced by substrate 54 preferences and community context, and suggest that a reduction in sedges and an expansion of 55 shrubs may provide a mechanism by which permafrost carbon losses are mitigated through 56 corresponding shifts in microbial communities and their substrate preferences.