COS 164-6 - CANCELLED - Mycorrhizal shrubification mediates soil C dynamics in a thawing permafrost tundra

Abstract: Permafrost ecosystems store a third of the world’s soil carbon (C) and are warming three times faster than the global average. Increasing annual temperatures are leading to the decomposition of old C previously stored for millennia. At the same time, many permafrost ecosystems are experiencing vegetation shifts away from non-mycorrhizal sedge dominated communities, towards ericoid and ectomycorrhizal dominated shrub communities, leading to high uncertainty in the future trajectory of C ecosystem balance. Research linking tundra shrubification to soil C storage has primarily focused on the biophysical feedbacks between vegetation on soil temperatures. However, altering the mycorrhizal composition of the plant community may also impact soil C storage via the substrate chemistry of litter inputs, and by altering the decomposer communities. Here, we investigate the effects of fine-scale variation in plant mychorrhizal composition on C cycling in a long-term permafrost warming experiment in a moist acidic tundra. We compared cumulative growing season fluxes of gross primary productivity (GPP), ecosystem respiration (R_eco), and net ecosystem exchange (NEE) from 48 automated chambers with 7 repeated measures of biomass by functional group from 2009-2021. We then compared these measurements to 13 annual estimates of cellulose decomposition from litterbags located at each chamber.

Across plots and years, total plant biomass explained approximately half of the variation in rates of C cycling. We found strong evidence for differences in C cycling by plant functional type. Across years, both GPP and R_eco (per gram biomass) were higher in shrub dominated plots. Plant community effects on NEE varied by year, but shrubbier plots generally had greater growing season C uptake. Further, we found that cellulose decomposition was higher in plots with higher R_eco, suggesting that the higher soil C efflux under shrub communities was due in part to increased soil decomposition, not just autotrophic respiration. Plant community composition was not strongly related to differences in soil temperature across plots, suggesting that broader scale observation of amplified C cycling with shrubification are due in part to plant-microbial feedbacks, not just effects on microclimate.