Long-term in situ permafrost thaw effects on bacterial communities and potential aerobic respiration

Sylvain Monteux,Vincent E J Jassey,James T Weedon,Carolina Olid,Margareta Johansson,Konstantin Gavazov,Gesche Blume-Werry,Frida Keuper,Ellen Dorrepaal

doi:10.1038/s41396-018-0176-z

Abstract

The decomposition of large stocks of soil organic carbon in thawing permafrost might depend on more than climate change-induced temperature increases: indirect effects of thawing via altered bacterial community structure (BCS) or rooting patterns are largely unexplored. We used a 10-year in situ permafrost thaw experiment and aerobic incubations to investigate alterations in BCS and potential respiration at different depths, and the extent to which they are related with each other and with root density. Active layer and permafrost BCS strongly differed, and the BCS in formerly frozen soils (below the natural thawfront) converged under induced deep thaw to strongly resemble the active layer BCS, possibly as a result of colonization by overlying microorganisms. Overall, respiration rates decreased with depth and soils showed lower potential respiration when subjected to deeper thaw, which we attributed to gradual labile carbon pool depletion. Despite deeper rooting under induced deep thaw, root density measurements did not improve soil chemistry-based models of potential respiration. However, BCS explained an additional unique portion of variation in respiration, particularly when accounting for differences in organic matter content. Our results suggest that by measuring bacterial community composition, we can improve both our understanding and the modeling of the permafrost carbon feedback.

Highlights

Northern hemisphere permafrost soils and their overlying active layer store 1035–1580 Pg of organic carbon, about 28% of which is found in peat deposits [1, 2]
We investigated how the combined direct and indirect effects of long-term in situ permafrost thaw through experimental winter-warming modifies the Bacterial community structure (BCS) and potential soil respiration across the active layer and upper permafrost soil, and studied whether and how these responses were linked
Organic matter, and carbon content were unaffected by the deep-thaw manipulation and were lower in the permafrost layer relative to the other two soil layers for both treatments. pH was higher in the permafrost than in the active layer in control soils but not in deep-thaw, while NH4+ was unaffected by depth and treatment (Table 1, SI Table S3, SI Fig. S3)

Summary

Introduction

Northern hemisphere permafrost soils and their overlying active (seasonally thawing) layer store 1035–1580 Pg of organic carbon, about 28% of which is found in peat deposits [1, 2]. This carbon stock is protected from decomposition due to limited microbial activity in frozen soils [3, 4]. Climate change-induced thawing of permafrost soils stimulates decomposition of this carbon, potentially causing a positive feedback to warming [2, 5]. Long-term decreases in soil respiration have variously been attributed to depletion of labile carbon substrates or microbial acclimation [9]

Methods

Results

Conclusion